National Ignition Facility
Encyclopedia
The National Ignition Facility, or NIF is a large, laser
-based inertial confinement fusion
(ICF) research device located at the Lawrence Livermore National Laboratory
in Livermore, California
. NIF uses powerful lasers to heat and compress a small amount of hydrogen fuel
to the point where nuclear fusion
reactions take place. NIF is the largest and most energetic ICF device built to date, and the first that is expected to reach the long-sought goal of "ignition," producing more energy than was put in to start the reaction. Its mission is to achieve fusion with high energy gain
, and to support nuclear weapon
maintenance
and design by studying the behavior of matter
under the conditions found within nuclear weapons.
Construction began in 1997 but was fraught with problems and ran into a series of delays that greatly slowed progress into the early 2000s. Progress through the 2000s was much smoother, but compared to initial estimates, NIF was completed five years behind schedule and was almost four times more expensive than budgeted. Construction was certified complete on 31 March 2009 by the U.S. Department of Energy, and a dedication ceremony took place on 29 May 2009. The first large-scale laser target experiments were performed in June 2009 and the first integrated ignition experiments were declared completed in October 2010.
and tritium
. The energy of the laser heats the surface of the pellet into a plasma
, which explodes off the surface. The remaining portion of the target is driven inwards, eventually compressing it into a small point of extremely high density. The rapid blowoff also creates a shock wave
that travels towards the center of the compressed fuel from all sides. When it reaches the center of the fuel, a small volume is further heated and compressed to a great degree. When the temperature and density of that small spot are raised high enough, fusion reactions will occur and release energy.
The fusion reactions release high-energy particles, some of which, primarily alpha particle
s, collide with the surrounding high density fuel and heat it further. If this process deposits enough energy in a given area it can cause that fuel to undergo fusion as well. Given the right overall conditions of the compressed fuel—high enough density and temperature—this heating process will result in a chain reaction
, burning outward from the center where the shock wave started the reaction. This is a condition known as "ignition", which will lead to a significant portion of the fuel in the target undergoing fusion and releasing large amounts of energy.
To date most ICF experiments have used lasers to heat the target. Calculations show that the energy must be delivered quickly in order to compress the core before it disassembles. The laser energy must also be focused extremely evenly across the target's outer surface in order to collapse the fuel into a symmetric core. Although other "drivers" have been suggested, notably heavy ions driven in particle accelerator
s, lasers are currently the only devices with the right combination of features.
(TW) flash of light that reaches the target from numerous directions at the same time, within a few picoseconds
. The design uses 192 individual "beamlets", which are amplified in 48 beamlines containing 16 laser amplifiers per line, each one amplifying four of the beamlets.
To ensure that the output of the beamlines is uniform, the initial laser light is amplified from a single source in the Injection Laser System (ILS). This starts with a low-power flash of 1053 nanometers (nm) infra-red light generated in an ytterbium
-doped optical fiber laser known as the Master Oscillator. The light from the Master Oscillator is split and directed into 48 Preamplifier Modules (PAMs). The PAMs pass the light four times through a circuit containing a neodymium
glass amplifier similar to (but much smaller than) the ones used in the main beamlines, boosting the nanojoules of light created in the Master Oscillator to about 6 joules. According to LLNL, the design of the PAMs was one of the major stumbling blocks during construction. Improvements to the design since then have allowed them to surpass their initial design goals.
The main amplification takes place in a series of glass amplifiers located at one end of the beamlines. Before "firing", the amplifiers are first optically pumped
by a total of 7,680 xenon flash lamps (the PAMs have their own smaller flash lamps as well). The lamps are powered by a capacitor
bank which stores a total of 422 megajoules
(MJ) of electrical energy. When the wavefront passes through them, the amplifiers release some of the light energy stored in them into the beam. To improve the energy transfer the beams are sent though the main amplifier section four times, using an optical switch
located in a mirrored cavity. In total these amplifiers boost the original 6 J provided by the PAMs to a nominal 4 MJ. Given the time scale of a few billionths of a second, the power is correspondingly very high, 500 TW.
After the amplification is complete the light is "switched" back into the beamline, where it runs to the far end of the building to the Target Chamber. The target chamber weighs 287,000 pounds (130,000 kg), with a diameter of 10 meters. The total length of the laser from one end to the other is about 1,000 feet (300 meters). A considerable amount of this length is taken up by "spatial filter
s", small telescopes that focus the laser beam down to a tiny point, with a mask
cutting off any stray light outside the focal point. The filters ensure that the image of the beam when it reaches the target is extremely uniform, removing any light that was mis-focussed by imperfections in the optics upstream. Spatial filters were a major step forward in ICF work when they were introduced in the Cyclops laser
, an earlier LLNL experiment. The various optical elements in the beamlines are generally packaged into Line Replaceable Units (LRUs), standardized boxes about the size of a small car that can be dropped out of the beamline for replacement from below.
Just before reaching the Target Chamber the light is reflected off various mirrors in the switchyard in order to impinge on the target from different directions. Since the length of the overall path from the Master Oscillator to the target is different for each of the beamlines, optics are used to delay the light in order to ensure all of them reach the center within a few picoseconds of each other. As can be seen in the layout diagram above, NIF normally directs the laser into the chamber from the top and bottom. The target area and switchyard system can be reconfigured by moving half of the 48 beamlines to alternate positions closer to the equator of the target chamber.
One of the last steps in the process before reaching the target chamber is to convert the infrared light at 1053 nm into the ultraviolet (UV) at 351 nm in a device known as a frequency converter. These are made of thin sheets cut from a single crystal of potassium dihydrogen phosphate. When the 1053 nm (IR) light passes through the first of two of these sheets, frequency addition converts a large fraction of the light into 527 nm light (green). On passing through the second sheet, frequency combination converts much of the 527 nm light and the remaining 1053 nm light into 351 nm (UV) light. IR light is much less effective than UV at heating the targets, because IR couples more strongly with hot electrons which will absorb a considerable amount of energy and interfere with compressing the target. The conversion process is about 50% efficient, reducing delivered energy to a nominal 1.8 MJ.
One important aspect of any ICF research project is ensuring that experiments can actually be carried out on a timely basis. Previous devices generally had to cool down for hours to allow the flashlamps and laser glass to regain their shapes after firing (due to thermal expansion), limiting use to one or fewer firings a day. One of the goals for NIF is to reduce this time to 5 hours, in order to allow 700 firings a year.
The name "National Ignition Facility" refers to the goal of "igniting" the fusion fuel, a long-sought threshold in fusion research. Ignition is considered a key requirement if fusion power
is to ever become practical.
NIF is designed primarily to use the indirect drive method of operation, in which the laser heats a small metal cylinder instead of the capsule inside it. The heat causes the cylinder, known as a hohlraum
(German for "hollow room", or cavity), to re-emit the energy as intense X-ray
s, which are more evenly distributed and symmetrical than the original laser beams. Experimental systems, including the OMEGA and Nova laser
s, validated this approach through the late 1980s. In the case of the NIF, the large delivered power allows for the use of a much larger target; the baseline pellet design is about 2 mm in diameter, chilled to about 18 Kelvin (-255 degrees Celsius) and lined with a layer of solid deuterium
-tritium
(DT) fuel. The hollow interior also contains a small amount of DT gas.
This conversion process is fairly efficient; of the original ~4 MJ of laser energy created in the beamlines, 1.8 MJ is left after conversion to UV, and about half of the remainder is lost in the x-ray conversion in the hohlraum. Of the rest, perhaps 10 to 20% of the resulting X-rays will be absorbed by the outer layers of the target. The shockwave created by this heating absorbs about 140 kJ, which is expected to compress the fuel in the center of the target to a density of about 1,000 g/mL (or 1,000,000 kg/m³); for comparison, lead
has a normal density of about 11 g/mL (11,340 kg/m³). It is expected this will cause about 20 MJ of fusion energy to be released. Improvements in both the laser system and hohlraum design are expected to improve the shockwave to about 420 kJ, in turn improving the fusion energy to about 100 MJ. However, the baseline design allows for a maximum of about 45 MJ of fusion energy release, due to the design of the target chamber. This is the equivalent of about 11 kg of TNT exploding. These output energies are still an order of magnitude less than the 422 MJ of input energy required to charge the system's capacitors (mentioned earlier), whereas an economical fusion reactor would require that the fusion output be at least an order of magnitude more than the input to the capacitors.
NIF is also exploring new types of targets. Previous experiments generally used plastic ablators
, typically polystyrene
(CH). NIF's targets are constructed by coating a plastic form with a layer of sputtered beryllium
or beryllium-copper alloys, and then oxidizing the plastic out of the center. In comparison to traditional plastic targets, beryllium targets offer higher density, high transparency to x-rays, and high thermal conductivity. All of these are advantageous in the indirect-drive mode where the incoming energy is in the form of x-rays. They also have a higher leftover ablative mass compared to the fuel inside, which has the benefit of being less sensitive to instability growth from the roughness of the DT ice (although plastic targets of the same mass also show this effect). A more practical benefit is that the mechanical strength of a Be target is high enough to contain the fuel in gaseous form at room temperature. This could allow the targets to be filled with fuel and stored for periods before being chilled to freeze the DT just before firing. In practice, however, the ice has to be carefully grown from an initial seed
.
Although NIF was primarily designed as an indirect drive device, the energy in the laser is high enough to be used as a direct drive system as well, where the laser shines directly on the target. Even at UV wavelengths the power delivered by NIF is estimated to be more than enough to cause ignition, resulting in fusion energy gains
of about forty times, somewhat higher than the indirect drive system. In this case the value of the Be target is reduced and more traditional plastic targets are more appropriate. A more uniform beam layout suitable for direct drive experiments can be arranged through changes in the switchyard that move half of the beamlines to locations closer to the middle of the target chamber.
It has been shown, using scaled implosions on the OMEGA laser and computer simulations, that NIF should also be capable of igniting a capsule using the so-called polar direct drive (PDD) configuration where the target is irradiated directly by the laser, but only from the top and bottom. In this configuration the target suffers either a "pancake" or "cigar" anisotropy
on implosion, reducing the maximum temperature at the core. However, the amount of energy being dumped into the target by the laser is so high that it ignites anyway. Fusion gains in this configuration are estimated to be anywhere between ten and thirty times less than the symmetrical direct-drive approach, but obtainable with no changes to the NIF beamline layout.
Other targets, called saturn targets, are specifically designed to reduce the anisotropy and improve the implosion. They feature a small plastic ring around the "equator" of the target, which quickly vaporizes into a plasma when hit by the laser. Some of the laser light is refracted through this plasma back towards the equator of the target, evening out the heating. Ignition with gains of just over thirty-five times are thought to be possible using these targets at NIF, producing results almost as good as the fully symmetric direct drive approach.
) or KrF (e.g. Nike laser
, Naval Research Laboratory). By the 1980s the advantage of shorter wavelengths in terms of delivering energy to the interior of the targets had been conclusively demonstrated in LLNL's highly successful Shiva laser
. This put the glass laser approach pioneered at LLNL in the lead for future development.
After the Shiva project, LLNL turned to the 20-beam 200 kJ Nova laser
design which was expected to reach ignition conditions. During the initial construction phase, Nuckolls found an error in his calculations, and an October 1979 review chaired by John Foster Jr. of TRW
confirmed that there was no way Nova would reach ignition. The Nova design was then modified into a smaller 10-beam design that added frequency conversion to 351 nm light, which would increase coupling efficiency. In operation, Nova was able to deliver about 20 to 30 kJ of laser energy, about half of what was initially expected, due to various nonlinear optical effects.
Throughout these efforts, the amount of energy needed to reach ignition had continually risen and it was unclear whether the current 200 kJ estimate was more reliable than earlier ones. The Department of Energy
(DOE) decided that direct experimentation was the best way to settle the issue, and between 1978 and 1988 ran a series of underground experiments at the Nevada Test Site
that used small nuclear bombs to directly illuminate ICF fuel components with high-energy X-rays; LLNL ran their program under the name "Halite", while LANL ran theirs as "Centurion". Initial data were available by mid-1984, and the testing ceased in 1988. Although there is little publicly available data from the Halite-Centurion series, it appears the experiments suggested that an implosion energy of about 20 MJ was required, and that given that about 1/5 of the laser energy is delivered as X-rays, drivers on the order of 100 MJ would be needed.
In 1989/90 the National Academy of Sciences
conducted a second review of the US ICF efforts on behalf of the US Congress. The report concluded that "considering the extrapolations required in target physics and driver performance, as well as the likely $1 billion cost, the committee believes that an LMF [i.e., a Laser Microfusion Facility with yields to one gigajoule] is too large a step to take directly from the present program." Their report suggested that the primary goal of the program in the short term should be resolving the various issues related to ignition, and that a full-scale LMF should not be attempted until these problems were resolved. The report was also critical of the gas laser experiments being carried out at LANL, and suggested they, and similar projects at other labs, be dropped. The report accepted the LASNEX numbers and continued to approve an approach with laser energy around 10 MJ. Nevertheless, the authors were aware of the potential for higher energy requirements, and noted "Indeed, if it did turn out that a 100 MJ driver were required for ignition and gain, one would have to rethink the entire approach to, and rationale for, ICF."
In July 1992 LLNL responded to these suggestions with the Nova Upgrade, which would reuse the majority of the existing Nova facility, along with the adjacent Shiva facility. The resulting system would be much lower power than the LMF concept, with a driver of about 1 to 2 MJ. The new design included a number of features that advanced the state of the art in the driver section, including the multi-pass design in the main amplifiers, and 18 beamlines (up from 10) that were split into 288 "beamlets" as they entered the target area in order to improve the uniformity of illumination. The plans called for the installation of two main banks of laser beamlines, one in the existing Nova beamline room, and the other in the older Shiva building next door, extending through its laser bay and target area into an upgraded Nova target area. The lasers would deliver about 500 TW in a 4 ns pulse. The upgrades were expected to allow the new Nova to produce fusion yields of between 2 and 20 MJ The initial estimates from 1992 estimated construction costs around $400 million, with construction taking place from 1995 to 1999.
led to dramatic changes in defense funding and priorities. As the need for nuclear weapons was greatly reduced and various arms limitation agreements led to a reduction in warhead count, the US was faced with the prospect of losing a generation of nuclear weapon designers able to maintain the existing stockpiles, or design new weapons. At the same time, progress was being made on what would become the Comprehensive Nuclear-Test-Ban Treaty, which would ban all criticality
testing. This would make the reliable development of newer generations of nuclear weapons much more difficult.
Out of these changes came the Stockpile Stewardship and Management Program (SSMP), which, among other things, included funds for the development of methods to design and build nuclear weapons that would work without having to be explosively tested. In a series of meetings that started in 1995, an agreement formed between the labs to divide up the SSMP efforts. An important part of this would be confirmation of computer models using low-yield ICF experiments. The Nova Upgrade was too small to use for these experiments, and a redesign emerged as NIF in 1994.
, National Nuclear Security Administrator, stated "We have prepared a detailed bottom-up cost and schedule to complete the NIF project… The independent review supports our position that the NIF management team has made significant progress and resolved earlier problems." The report revised their budget estimate to $2.25 billion, not including related R&D which pushed it to $3.3 billion total, and pushed back the completion date to 2006 with the first lines coming online in 2004.
Given the budget problems, the US Congress requested an independent review by the General Accounting Office (GAO). They returned a highly critical report in August 2000 stating that the budget was likely $3.9 billion, including R&D, and that the facility was unlikely to be completed anywhere near on time. The report, "Management and Oversight Failures Caused Major Cost Overruns and Schedule Delays," identified management problems for the overruns, and also criticized the program for failing to include a considerable amount of money dedicated to target fabrication in the budget, including it in operational costs instead of development. A follow-up report the next year included all of these items, pushing the budget to $4.2 billion, and the completion date to around 2008.
The public fighting between the various Department of Energy laboratories soon started anew. Los Alamos publicly attacked the facility as ill-conceived. On 25 May Sandia vice president Tom Hunter told the Albuquerque Tribune
that the NIF should be downsized so that it would not "disrupt the investment needed" in other labs. Criticism of the project also came from politicians, government officials and review panels, some going so far as to refer to the project as being "out of control".
LRU installation started in 2005, and has continued at an increasing pace since then. In May 2003, the NIF achieved "first light" on a bundle of four beams, producing a 10.4 kJ pulse of IR light in a single beamline. In 2005 the first eight beams (a full bundle) were fired producing 153 kJ of infrared light, thus eclipsing OMEGA as the highest energy laser (per pulse) on the planet. By January 2007 all of the LRUs in the Master Oscillator Room (MOOR) were complete and the computer room had been installed. By August 2007 96 laser lines were completed and commissioned, and "A total infrared energy of more than 2.5 megajoules has now been fired. This is more than 40 times what the Nova laser typically operated at the time it was the world's largest laser."
Recent reviews of the project have been positive, generally in keeping with the post-GAO Rebaseline schedules and budgets. However, there are lingering concerns about the NIF's ability to reach ignition, at least in the short term. An independent review by the JASON Defense Advisory Group
was generally positive about NIF's prospects over the long term, but concludes that "The scientific and technical challenges in such a complex activity suggest that success in the early attempts at ignition in 2010, while possible, is unlikely."NIF Ignition, JASON Program, June 29, 2005 The group suggested a number of changes to the completion timeline to bring NIF to its full design power as soon as possible, skipping over a testing period at lower powers that they felt had little value.
On 29 May 2009 the NIF was dedicated in a ceremony attended by thousands, including California Governor Arnold Schwarzenegger
and Senator Dianne Feinstein
. The first laser shots into a hohlraum target were fired in late June 2009.
, setting new records for power delivery by a laser, and leading to analysis suggesting that suspected interference by generated plasma would not be a problem in igniting a fusion reaction. Due to the size of the test hohlraums, laser/plasma interactions produced plasma-optics gratings, acting like tiny prisms, which produced symmetric X-ray drive on the capsule inside the hohlraum.
After gradually altering the wavelength of the laser, they were able to compress a spherical capsule evenly, and were able to heat it up to 3.3 million Kelvin
. The capsule contained cryogenically cooled gas, acting as a substitute for the deuterium
and tritium
fuel capsules that will be used later on. Plasma Physics Group Leader Dr. Siegfried Glenzer said they've shown they can maintain the precise fuel layers needed in the lab, but not yet within the laser system.
As of January 2010, the NIF could run as high as 1.8 megajoules. Glenzer said that experiments with slightly larger hohlraums containing fusion-ready fuel pellets would begin before May 2010, slowly ramping up to 1.2 megajoules — enough for ignition according to calculations. But first the target chamber needed to be equipped with shields to block neutrons that a fusion reaction would produce. On June 5, 2010 the NIF team fired lasers at the target chamber for the first time in six months; realignment of the beams took place later in June in preparation for further high-energy operation.
Progress was initially slowed by the potential for damage from overheating due to a concentration of energy on optical components that is greater than anything previously attempted. Other issues included problems layering the fuel inside the targets, and minute quantities of dust being found on the capsule surface.
As the power was increased and targets of increasing sophistication were used, another problem appeared that was causing asymmetric implosion. This was eventually traced to minute amounts of water vapor in the target chamber which froze to the windows on the ends of the hohlraums. This was solved by re-designing the hohlraum with two layers of glass on either end, in effect creating a storm window. Steven Koonin, DOE undersecretary for science, visited the lab for an update on the NIC on 23 April, the day after the window problem was announced as solved. On 10 March he had described the NIC as "a goal of overriding importance for the DOE" and expressed that progress to date "was not as rapid as I had hoped."
NIC shots halted in February 2011, as the machine was turned over to SSMP materials experiments. As these experiments wound down, a series of planned upgrades were carried out, notably a series of improved diagnostic and measurement instruments. Among these changes were the addition of the ARC system, which uses 4 of the NIF's 192 beams as a backlighting source for high-speed imaging of the implosion sequence. NIC runs re-started in May 2011 with the goal of timing the four laser shock waves that compress the fusion target to very high precision. The shots tested the symmetry of the X-ray drive during the first three nanosecond
s. Full-system shots fired in the second half of May achieved unprecedented peak pressures of 50 megabar
s.
According to an article in Science magazine published in October 2011, there are 17 parameters (14 on the laser, and 3 on the hohlraum) that can be tweaked to help the NIF achieve the four identified conditions necessary for ignition. The 4 conditions that need to be achieved for ignition to take place are:
"The imploding fuel must maintain its spherical shape; it must achieve a certain speed; the amount of mixing between the fuel and the capsule material must be kept low; and the entropy of the system must be kept down—in other words, the energy applied needs to be focused on compressing the fuel and not raising its temperature, which would impede compression."
The plan to achieve these four conditions involves: 1) Implosion velocity needs to be increased from 300 km/s to 370 km/s -- this can be tweaked by the pulse shape. 2) The power of the fourth and final burst of the laser pulse has to be 300 times the power of the initial bursts. It was presently at 50x. 3) The hohlraum shape has to be made stubbier so that incoming laser beams are not subject to interference by material blow off 4) The hohlraum material has to be tweaked to avoid the fuel being heated so that compression can be increased by 30 km/s hours more (doubling they achieved when they first switched from using germanium to silicon dopant).
Some similar experimental projects are:
Panorama taken outside the fusion chamber.
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
-based inertial confinement fusion
Inertial confinement fusion
Inertial confinement fusion is a process where nuclear fusion reactions are initiated by heating and compressing a fuel target, typically in the form of a pellet that most often contains a mixture of deuterium and tritium....
(ICF) research device located at the Lawrence Livermore National Laboratory
Lawrence Livermore National Laboratory
The Lawrence Livermore National Laboratory , just outside Livermore, California, is a Federally Funded Research and Development Center founded by the University of California in 1952...
in Livermore, California
Livermore, California
Livermore is a city in Alameda County. The population as of 2010 was 80,968. Livermore is located on the eastern edge of California's San Francisco Bay Area....
. NIF uses powerful lasers to heat and compress a small amount of hydrogen fuel
Hydrogen fuel
An ecologically-friendly fuel which uses electrochemical cells or combusts in internal engines to power vehicles and electric devices. It is also used in the propulsion of spacecraft and can potentially be mass produced and commercialized for passenger vehicles and aircraft.In a flame of pure...
to the point where nuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
reactions take place. NIF is the largest and most energetic ICF device built to date, and the first that is expected to reach the long-sought goal of "ignition," producing more energy than was put in to start the reaction. Its mission is to achieve fusion with high energy gain
Fusion energy gain factor
The fusion energy gain factor, usually expressed with the symbol Q, is the ratio of fusion power produced in a nuclear fusion reactor to the power required to maintain the plasma in steady state. The condition of Q = 1 is referred to as breakeven.In a fusion power reactor a plasma must be...
, and to support nuclear weapon
Nuclear weapon
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter. The first fission bomb test released the same amount...
maintenance
Stockpile stewardship
Stockpile stewardship refers to the United States program of reliability testing and maintenance of its nuclear weapons without the use of nuclear testing....
and design by studying the behavior of matter
Equation of state
In physics and thermodynamics, an equation of state is a relation between state variables. More specifically, an equation of state is a thermodynamic equation describing the state of matter under a given set of physical conditions...
under the conditions found within nuclear weapons.
Construction began in 1997 but was fraught with problems and ran into a series of delays that greatly slowed progress into the early 2000s. Progress through the 2000s was much smoother, but compared to initial estimates, NIF was completed five years behind schedule and was almost four times more expensive than budgeted. Construction was certified complete on 31 March 2009 by the U.S. Department of Energy, and a dedication ceremony took place on 29 May 2009. The first large-scale laser target experiments were performed in June 2009 and the first integrated ignition experiments were declared completed in October 2010.
Background
Inertial confinement fusion (ICF) devices use "drivers" to rapidly heat the outer layers of a "target" in order to compress it. The target is a small spherical pellet containing a few milligrams of fusion fuel, typically a mix of deuteriumDeuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
and tritium
Tritium
Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...
. The energy of the laser heats the surface of the pellet into a plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
, which explodes off the surface. The remaining portion of the target is driven inwards, eventually compressing it into a small point of extremely high density. The rapid blowoff also creates a shock wave
Shock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...
that travels towards the center of the compressed fuel from all sides. When it reaches the center of the fuel, a small volume is further heated and compressed to a great degree. When the temperature and density of that small spot are raised high enough, fusion reactions will occur and release energy.
The fusion reactions release high-energy particles, some of which, primarily alpha particle
Alpha particle
Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...
s, collide with the surrounding high density fuel and heat it further. If this process deposits enough energy in a given area it can cause that fuel to undergo fusion as well. Given the right overall conditions of the compressed fuel—high enough density and temperature—this heating process will result in a chain reaction
Chain reaction
A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events....
, burning outward from the center where the shock wave started the reaction. This is a condition known as "ignition", which will lead to a significant portion of the fuel in the target undergoing fusion and releasing large amounts of energy.
To date most ICF experiments have used lasers to heat the target. Calculations show that the energy must be delivered quickly in order to compress the core before it disassembles. The laser energy must also be focused extremely evenly across the target's outer surface in order to collapse the fuel into a symmetric core. Although other "drivers" have been suggested, notably heavy ions driven in particle accelerator
Particle accelerator
A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: electrostatic and oscillating field accelerators.In...
s, lasers are currently the only devices with the right combination of features.
Driver laser
NIF aims to create a single 500 terawattWatt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...
(TW) flash of light that reaches the target from numerous directions at the same time, within a few picoseconds
1 E-12 s
A picosecond is 10−12 of a second. That is one trillionth, or one millionth of one millionth of a second, or 0.000 000 000 001 seconds. A picosecond is to one second as one second is to 31,700 years....
. The design uses 192 individual "beamlets", which are amplified in 48 beamlines containing 16 laser amplifiers per line, each one amplifying four of the beamlets.
To ensure that the output of the beamlines is uniform, the initial laser light is amplified from a single source in the Injection Laser System (ILS). This starts with a low-power flash of 1053 nanometers (nm) infra-red light generated in an ytterbium
Ytterbium
Ytterbium is a chemical element with the symbol Yb and atomic number 70. A soft silvery metallic element, ytterbium is a rare earth element of the lanthanide series and is found in the minerals gadolinite, monazite, and xenotime. The element is sometimes associated with yttrium or other related...
-doped optical fiber laser known as the Master Oscillator. The light from the Master Oscillator is split and directed into 48 Preamplifier Modules (PAMs). The PAMs pass the light four times through a circuit containing a neodymium
Neodymium
Neodymium is a chemical element with the symbol Nd and atomic number 60. It is a soft silvery metal that tarnishes in air. Neodymium was discovered in 1885 by the Austrian chemist Carl Auer von Welsbach. It is present in significant quantities in the ore minerals monazite and bastnäsite...
glass amplifier similar to (but much smaller than) the ones used in the main beamlines, boosting the nanojoules of light created in the Master Oscillator to about 6 joules. According to LLNL, the design of the PAMs was one of the major stumbling blocks during construction. Improvements to the design since then have allowed them to surpass their initial design goals.
Optical pumping
Optical pumping is a process in which light is used to raise electrons from a lower energy level in an atom or molecule to a higher one. It is commonly used in laser construction, to pump the active laser medium so as to achieve population inversion...
by a total of 7,680 xenon flash lamps (the PAMs have their own smaller flash lamps as well). The lamps are powered by a capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
bank which stores a total of 422 megajoules
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...
(MJ) of electrical energy. When the wavefront passes through them, the amplifiers release some of the light energy stored in them into the beam. To improve the energy transfer the beams are sent though the main amplifier section four times, using an optical switch
Pockels effect
The Pockels effect , or Pockels electro-optic effect, produces birefringence in an optical medium induced by a constant or varying electric field. It is distinguished from the Kerr effect by the fact that the birefringence is proportional to the electric field, whereas in the Kerr effect it is...
located in a mirrored cavity. In total these amplifiers boost the original 6 J provided by the PAMs to a nominal 4 MJ. Given the time scale of a few billionths of a second, the power is correspondingly very high, 500 TW.
After the amplification is complete the light is "switched" back into the beamline, where it runs to the far end of the building to the Target Chamber. The target chamber weighs 287,000 pounds (130,000 kg), with a diameter of 10 meters. The total length of the laser from one end to the other is about 1,000 feet (300 meters). A considerable amount of this length is taken up by "spatial filter
Spatial filter
A spatial filter is an optical device which uses the principles of Fourier optics to alter the structure of a beam of coherent light or other electromagnetic radiation. Spatial filtering is commonly used to "clean up" the output of lasers, removing aberrations in the beam due to imperfect, dirty,...
s", small telescopes that focus the laser beam down to a tiny point, with a mask
Photomask
A photomask is an opaque plate with holes or transparencies that allow light to shine through in a defined pattern. They are commonly used in photolithography.-Overview:...
cutting off any stray light outside the focal point. The filters ensure that the image of the beam when it reaches the target is extremely uniform, removing any light that was mis-focussed by imperfections in the optics upstream. Spatial filters were a major step forward in ICF work when they were introduced in the Cyclops laser
Cyclops laser
Cyclops was a high-power laser built at the Lawrence Livermore National Laboratory in 1975. It was the second laser constructed in the lab's Laser program, which aimed to study inertial confinement fusion ....
, an earlier LLNL experiment. The various optical elements in the beamlines are generally packaged into Line Replaceable Units (LRUs), standardized boxes about the size of a small car that can be dropped out of the beamline for replacement from below.
Just before reaching the Target Chamber the light is reflected off various mirrors in the switchyard in order to impinge on the target from different directions. Since the length of the overall path from the Master Oscillator to the target is different for each of the beamlines, optics are used to delay the light in order to ensure all of them reach the center within a few picoseconds of each other. As can be seen in the layout diagram above, NIF normally directs the laser into the chamber from the top and bottom. The target area and switchyard system can be reconfigured by moving half of the 48 beamlines to alternate positions closer to the equator of the target chamber.
One of the last steps in the process before reaching the target chamber is to convert the infrared light at 1053 nm into the ultraviolet (UV) at 351 nm in a device known as a frequency converter. These are made of thin sheets cut from a single crystal of potassium dihydrogen phosphate. When the 1053 nm (IR) light passes through the first of two of these sheets, frequency addition converts a large fraction of the light into 527 nm light (green). On passing through the second sheet, frequency combination converts much of the 527 nm light and the remaining 1053 nm light into 351 nm (UV) light. IR light is much less effective than UV at heating the targets, because IR couples more strongly with hot electrons which will absorb a considerable amount of energy and interfere with compressing the target. The conversion process is about 50% efficient, reducing delivered energy to a nominal 1.8 MJ.
One important aspect of any ICF research project is ensuring that experiments can actually be carried out on a timely basis. Previous devices generally had to cool down for hours to allow the flashlamps and laser glass to regain their shapes after firing (due to thermal expansion), limiting use to one or fewer firings a day. One of the goals for NIF is to reduce this time to 5 hours, in order to allow 700 firings a year.
NIF and ICF
Fusion power
Fusion power is the power generated by nuclear fusion processes. In fusion reactions two light atomic nuclei fuse together to form a heavier nucleus . In doing so they release a comparatively large amount of energy arising from the binding energy due to the strong nuclear force which is manifested...
is to ever become practical.
NIF is designed primarily to use the indirect drive method of operation, in which the laser heats a small metal cylinder instead of the capsule inside it. The heat causes the cylinder, known as a hohlraum
Hohlraum
In radiation thermodynamics, a hohlraum is a cavity whose walls are in radiative equilibrium with the radiant energy within the cavity. This idealized cavity can be approximated in practice by making a small perforation in the wall of a hollow container of any opaque material...
(German for "hollow room", or cavity), to re-emit the energy as intense X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
s, which are more evenly distributed and symmetrical than the original laser beams. Experimental systems, including the OMEGA and Nova laser
Nova laser
Nova was a high-power laser built at the Lawrence Livermore National Laboratory in 1984 which conducted advanced inertial confinement fusion experiments until its dismantling in 1999. Nova was the first ICF experiment built with the intention of reaching "ignition", a chain reaction of nuclear...
s, validated this approach through the late 1980s. In the case of the NIF, the large delivered power allows for the use of a much larger target; the baseline pellet design is about 2 mm in diameter, chilled to about 18 Kelvin (-255 degrees Celsius) and lined with a layer of solid deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
-tritium
Tritium
Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...
(DT) fuel. The hollow interior also contains a small amount of DT gas.
This conversion process is fairly efficient; of the original ~4 MJ of laser energy created in the beamlines, 1.8 MJ is left after conversion to UV, and about half of the remainder is lost in the x-ray conversion in the hohlraum. Of the rest, perhaps 10 to 20% of the resulting X-rays will be absorbed by the outer layers of the target. The shockwave created by this heating absorbs about 140 kJ, which is expected to compress the fuel in the center of the target to a density of about 1,000 g/mL (or 1,000,000 kg/m³); for comparison, lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
has a normal density of about 11 g/mL (11,340 kg/m³). It is expected this will cause about 20 MJ of fusion energy to be released. Improvements in both the laser system and hohlraum design are expected to improve the shockwave to about 420 kJ, in turn improving the fusion energy to about 100 MJ. However, the baseline design allows for a maximum of about 45 MJ of fusion energy release, due to the design of the target chamber. This is the equivalent of about 11 kg of TNT exploding. These output energies are still an order of magnitude less than the 422 MJ of input energy required to charge the system's capacitors (mentioned earlier), whereas an economical fusion reactor would require that the fusion output be at least an order of magnitude more than the input to the capacitors.
Laser ablation
Laser ablation is the process of removing material from a solid surface by irradiating it with a laser beam. At low laser flux, the material is heated by the absorbed laser energy and evaporates or sublimates. At high laser flux, the material is typically converted to a plasma...
, typically polystyrene
Polystyrene
Polystyrene ) also known as Thermocole, abbreviated following ISO Standard PS, is an aromatic polymer made from the monomer styrene, a liquid hydrocarbon that is manufactured from petroleum by the chemical industry...
(CH). NIF's targets are constructed by coating a plastic form with a layer of sputtered beryllium
Beryllium
Beryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...
or beryllium-copper alloys, and then oxidizing the plastic out of the center. In comparison to traditional plastic targets, beryllium targets offer higher density, high transparency to x-rays, and high thermal conductivity. All of these are advantageous in the indirect-drive mode where the incoming energy is in the form of x-rays. They also have a higher leftover ablative mass compared to the fuel inside, which has the benefit of being less sensitive to instability growth from the roughness of the DT ice (although plastic targets of the same mass also show this effect). A more practical benefit is that the mechanical strength of a Be target is high enough to contain the fuel in gaseous form at room temperature. This could allow the targets to be filled with fuel and stored for periods before being chilled to freeze the DT just before firing. In practice, however, the ice has to be carefully grown from an initial seed
Seed crystal
A seed crystal is a small piece of single crystal/polycrystal material from which a large crystal of the same material typically is to be grown...
.
Although NIF was primarily designed as an indirect drive device, the energy in the laser is high enough to be used as a direct drive system as well, where the laser shines directly on the target. Even at UV wavelengths the power delivered by NIF is estimated to be more than enough to cause ignition, resulting in fusion energy gains
Fusion energy gain factor
The fusion energy gain factor, usually expressed with the symbol Q, is the ratio of fusion power produced in a nuclear fusion reactor to the power required to maintain the plasma in steady state. The condition of Q = 1 is referred to as breakeven.In a fusion power reactor a plasma must be...
of about forty times, somewhat higher than the indirect drive system. In this case the value of the Be target is reduced and more traditional plastic targets are more appropriate. A more uniform beam layout suitable for direct drive experiments can be arranged through changes in the switchyard that move half of the beamlines to locations closer to the middle of the target chamber.
It has been shown, using scaled implosions on the OMEGA laser and computer simulations, that NIF should also be capable of igniting a capsule using the so-called polar direct drive (PDD) configuration where the target is irradiated directly by the laser, but only from the top and bottom. In this configuration the target suffers either a "pancake" or "cigar" anisotropy
Anisotropy
Anisotropy is the property of being directionally dependent, as opposed to isotropy, which implies identical properties in all directions. It can be defined as a difference, when measured along different axes, in a material's physical or mechanical properties An example of anisotropy is the light...
on implosion, reducing the maximum temperature at the core. However, the amount of energy being dumped into the target by the laser is so high that it ignites anyway. Fusion gains in this configuration are estimated to be anywhere between ten and thirty times less than the symmetrical direct-drive approach, but obtainable with no changes to the NIF beamline layout.
Other targets, called saturn targets, are specifically designed to reduce the anisotropy and improve the implosion. They feature a small plastic ring around the "equator" of the target, which quickly vaporizes into a plasma when hit by the laser. Some of the laser light is refracted through this plasma back towards the equator of the target, evening out the heating. Ignition with gains of just over thirty-five times are thought to be possible using these targets at NIF, producing results almost as good as the fully symmetric direct drive approach.
Impetus
LLNL's history with the ICF program starts with physicist John Nuckolls, who predicted in 1972 that ignition could be achieved with laser energies about 1 kJ, while "high gain" would require energies around 1 MJ. LLNL decided early on to concentrate on glass lasers, while other facilities studied gas lasers using carbon dioxide (e.g. Antares laser, Los Alamos National LaboratoryLos Alamos National Laboratory
Los Alamos National Laboratory is a United States Department of Energy national laboratory, managed and operated by Los Alamos National Security , located in Los Alamos, New Mexico...
) or KrF (e.g. Nike laser
Nike laser
The Nike laser at the United States Naval Research Laboratory in Washington, DC is a 56 beam, 4-5 kJ per pulse electron beam pumped krypton fluoride excimer laser which operates in the ultraviolet at 248 nm with pulsewidths of a few nanoseconds. Nike was completed in the late 1980s and is used...
, Naval Research Laboratory). By the 1980s the advantage of shorter wavelengths in terms of delivering energy to the interior of the targets had been conclusively demonstrated in LLNL's highly successful Shiva laser
Shiva laser
The Shiva laser was a powerful 20-beam infrared neodymium glass laser built at Lawrence Livermore National Laboratory in 1977 for the study of inertial confinement fusion and long-scale-length laser-plasma interactions. The device was named after the multi-armed form of the Hindu god Shiva, due...
. This put the glass laser approach pioneered at LLNL in the lead for future development.
After the Shiva project, LLNL turned to the 20-beam 200 kJ Nova laser
Nova laser
Nova was a high-power laser built at the Lawrence Livermore National Laboratory in 1984 which conducted advanced inertial confinement fusion experiments until its dismantling in 1999. Nova was the first ICF experiment built with the intention of reaching "ignition", a chain reaction of nuclear...
design which was expected to reach ignition conditions. During the initial construction phase, Nuckolls found an error in his calculations, and an October 1979 review chaired by John Foster Jr. of TRW
TRW
TRW Inc. was an American corporation involved in a variety of businesses, mainly aerospace, automotive, and credit reporting. It was a pioneer in multiple fields including electronic components, integrated circuits, computers, software and systems engineering. TRW built many spacecraft,...
confirmed that there was no way Nova would reach ignition. The Nova design was then modified into a smaller 10-beam design that added frequency conversion to 351 nm light, which would increase coupling efficiency. In operation, Nova was able to deliver about 20 to 30 kJ of laser energy, about half of what was initially expected, due to various nonlinear optical effects.
Throughout these efforts, the amount of energy needed to reach ignition had continually risen and it was unclear whether the current 200 kJ estimate was more reliable than earlier ones. The Department of Energy
United States Department of Energy
The United States Department of Energy is a Cabinet-level department of the United States government concerned with the United States' policies regarding energy and safety in handling nuclear material...
(DOE) decided that direct experimentation was the best way to settle the issue, and between 1978 and 1988 ran a series of underground experiments at the Nevada Test Site
Nevada Test Site
The Nevada National Security Site , previously the Nevada Test Site , is a United States Department of Energy reservation located in southeastern Nye County, Nevada, about northwest of the city of Las Vegas...
that used small nuclear bombs to directly illuminate ICF fuel components with high-energy X-rays; LLNL ran their program under the name "Halite", while LANL ran theirs as "Centurion". Initial data were available by mid-1984, and the testing ceased in 1988. Although there is little publicly available data from the Halite-Centurion series, it appears the experiments suggested that an implosion energy of about 20 MJ was required, and that given that about 1/5 of the laser energy is delivered as X-rays, drivers on the order of 100 MJ would be needed.
LMF and Nova Upgrade
Nova's partial success, combined with the Halite-Centurion numbers, prompted DOE to request a custom military ICF facility they called the "Laboratory Microfusion Facility" (LMF) that could achieve fusion yields of between 100 and 1,000 MJ. Based on modeling runs using the LASNEX computer program developed at LLNL, it was estimated that LMF would require a driver of about 10 MJ, in spite of the Halite-Centurion test that suggested a higher power. Building such a device was estimated to cost approximately $1 billion. LLNL submitted a design with a 5 MJ 350 nm (UV) driver laser that would be able to reach about 200 MJ yield, which was enough to attain the majority of the LMF goals. The program was estimated to cost about $600 million FY 1989 dollars, and an additional $250 million to upgrade it to a full 1,000 MJ if needed, and would grow to well over $1 billion if LMF was to meet all of the goals the DOE asked for. Other labs also proposed their own LMF designs using other technologies.In 1989/90 the National Academy of Sciences
United States National Academy of Sciences
The National Academy of Sciences is a corporation in the United States whose members serve pro bono as "advisers to the nation on science, engineering, and medicine." As a national academy, new members of the organization are elected annually by current members, based on their distinguished and...
conducted a second review of the US ICF efforts on behalf of the US Congress. The report concluded that "considering the extrapolations required in target physics and driver performance, as well as the likely $1 billion cost, the committee believes that an LMF [i.e., a Laser Microfusion Facility with yields to one gigajoule] is too large a step to take directly from the present program." Their report suggested that the primary goal of the program in the short term should be resolving the various issues related to ignition, and that a full-scale LMF should not be attempted until these problems were resolved. The report was also critical of the gas laser experiments being carried out at LANL, and suggested they, and similar projects at other labs, be dropped. The report accepted the LASNEX numbers and continued to approve an approach with laser energy around 10 MJ. Nevertheless, the authors were aware of the potential for higher energy requirements, and noted "Indeed, if it did turn out that a 100 MJ driver were required for ignition and gain, one would have to rethink the entire approach to, and rationale for, ICF."
In July 1992 LLNL responded to these suggestions with the Nova Upgrade, which would reuse the majority of the existing Nova facility, along with the adjacent Shiva facility. The resulting system would be much lower power than the LMF concept, with a driver of about 1 to 2 MJ. The new design included a number of features that advanced the state of the art in the driver section, including the multi-pass design in the main amplifiers, and 18 beamlines (up from 10) that were split into 288 "beamlets" as they entered the target area in order to improve the uniformity of illumination. The plans called for the installation of two main banks of laser beamlines, one in the existing Nova beamline room, and the other in the older Shiva building next door, extending through its laser bay and target area into an upgraded Nova target area. The lasers would deliver about 500 TW in a 4 ns pulse. The upgrades were expected to allow the new Nova to produce fusion yields of between 2 and 20 MJ The initial estimates from 1992 estimated construction costs around $400 million, with construction taking place from 1995 to 1999.
NIF emerges
Throughout this period, the ending of the Cold WarCold War
The Cold War was the continuing state from roughly 1946 to 1991 of political conflict, military tension, proxy wars, and economic competition between the Communist World—primarily the Soviet Union and its satellite states and allies—and the powers of the Western world, primarily the United States...
led to dramatic changes in defense funding and priorities. As the need for nuclear weapons was greatly reduced and various arms limitation agreements led to a reduction in warhead count, the US was faced with the prospect of losing a generation of nuclear weapon designers able to maintain the existing stockpiles, or design new weapons. At the same time, progress was being made on what would become the Comprehensive Nuclear-Test-Ban Treaty, which would ban all criticality
Critical mass
A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The...
testing. This would make the reliable development of newer generations of nuclear weapons much more difficult.
Out of these changes came the Stockpile Stewardship and Management Program (SSMP), which, among other things, included funds for the development of methods to design and build nuclear weapons that would work without having to be explosively tested. In a series of meetings that started in 1995, an agreement formed between the labs to divide up the SSMP efforts. An important part of this would be confirmation of computer models using low-yield ICF experiments. The Nova Upgrade was too small to use for these experiments, and a redesign emerged as NIF in 1994.
Re-baseline and GAO report
In the wake of these revelations the DOE started a comprehensive "Rebaseline Validation Review of the National Ignition Facility Project" in 2000, which took a critical look at the project, identifying areas of concern and adjusting the schedule and budget to ensure completion. John GordonJohn A. Gordon
General John Alexander Gordon was deputy director of central intelligence, Central Intelligence Agency, Washington, D.C. He served as the President's Homeland Security advisor from 2003 to 2004....
, National Nuclear Security Administrator, stated "We have prepared a detailed bottom-up cost and schedule to complete the NIF project… The independent review supports our position that the NIF management team has made significant progress and resolved earlier problems." The report revised their budget estimate to $2.25 billion, not including related R&D which pushed it to $3.3 billion total, and pushed back the completion date to 2006 with the first lines coming online in 2004.
Given the budget problems, the US Congress requested an independent review by the General Accounting Office (GAO). They returned a highly critical report in August 2000 stating that the budget was likely $3.9 billion, including R&D, and that the facility was unlikely to be completed anywhere near on time. The report, "Management and Oversight Failures Caused Major Cost Overruns and Schedule Delays," identified management problems for the overruns, and also criticized the program for failing to include a considerable amount of money dedicated to target fabrication in the budget, including it in operational costs instead of development. A follow-up report the next year included all of these items, pushing the budget to $4.2 billion, and the completion date to around 2008.
The public fighting between the various Department of Energy laboratories soon started anew. Los Alamos publicly attacked the facility as ill-conceived. On 25 May Sandia vice president Tom Hunter told the Albuquerque Tribune
Albuquerque Tribune
The Albuquerque Tribune was an afternoon newspaper in Albuquerque, New Mexico, founded in 1922 by Carlton Cole Magee as Magee's Independent. It was published in the afternoon and evening Monday through Saturday....
that the NIF should be downsized so that it would not "disrupt the investment needed" in other labs. Criticism of the project also came from politicians, government officials and review panels, some going so far as to refer to the project as being "out of control".
Progress
Recent reviews of the project have been positive, generally in keeping with the post-GAO Rebaseline schedules and budgets. However, there are lingering concerns about the NIF's ability to reach ignition, at least in the short term. An independent review by the JASON Defense Advisory Group
JASON Defense Advisory Group
JASON is an independent group of scientists which advises the United States government on matters of science and technology. The group was first created as a way to get a younger generation of scientists—that is, not the older Los Alamos and MIT Radiation Laboratory alumni—involved in advising the...
was generally positive about NIF's prospects over the long term, but concludes that "The scientific and technical challenges in such a complex activity suggest that success in the early attempts at ignition in 2010, while possible, is unlikely."NIF Ignition, JASON Program, June 29, 2005 The group suggested a number of changes to the completion timeline to bring NIF to its full design power as soon as possible, skipping over a testing period at lower powers that they felt had little value.
Completion
On January 26, 2009, the final line replaceable unit (LRU) was installed, completing one of the final major milestones of the NIF construction project and meaning that construction was unofficially completed. On February 26, 2009, for the first time NIF fired all 192 laser beams into the target chamber. On March 10, 2009, NIF became the first laser to break the megajoule barrier, firing all 192 beams and delivering 1.1 MJ of ultraviolet light, known as 3ω, to the target chamber center in a shaped ignition pulse. The main laser delivered 1.952 MJ of infrared energy.On 29 May 2009 the NIF was dedicated in a ceremony attended by thousands, including California Governor Arnold Schwarzenegger
Arnold Schwarzenegger
Arnold Alois Schwarzenegger is an Austrian-American former professional bodybuilder, actor, businessman, investor, and politician. Schwarzenegger served as the 38th Governor of California from 2003 until 2011....
and Senator Dianne Feinstein
Dianne Feinstein
Dianne Goldman Berman Feinstein is the senior U.S. Senator from California. A member of the Democratic Party, she has served in the Senate since 1992. She also served as 38th Mayor of San Francisco from 1978 to 1988....
. The first laser shots into a hohlraum target were fired in late June 2009.
Buildup to main experiments
On January 28, 2010, the facility published a paper reporting the delivery of a 669 kJ pulse to a gold hohlraumHohlraum
In radiation thermodynamics, a hohlraum is a cavity whose walls are in radiative equilibrium with the radiant energy within the cavity. This idealized cavity can be approximated in practice by making a small perforation in the wall of a hollow container of any opaque material...
, setting new records for power delivery by a laser, and leading to analysis suggesting that suspected interference by generated plasma would not be a problem in igniting a fusion reaction. Due to the size of the test hohlraums, laser/plasma interactions produced plasma-optics gratings, acting like tiny prisms, which produced symmetric X-ray drive on the capsule inside the hohlraum.
After gradually altering the wavelength of the laser, they were able to compress a spherical capsule evenly, and were able to heat it up to 3.3 million Kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
. The capsule contained cryogenically cooled gas, acting as a substitute for the deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
and tritium
Tritium
Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...
fuel capsules that will be used later on. Plasma Physics Group Leader Dr. Siegfried Glenzer said they've shown they can maintain the precise fuel layers needed in the lab, but not yet within the laser system.
As of January 2010, the NIF could run as high as 1.8 megajoules. Glenzer said that experiments with slightly larger hohlraums containing fusion-ready fuel pellets would begin before May 2010, slowly ramping up to 1.2 megajoules — enough for ignition according to calculations. But first the target chamber needed to be equipped with shields to block neutrons that a fusion reaction would produce. On June 5, 2010 the NIF team fired lasers at the target chamber for the first time in six months; realignment of the beams took place later in June in preparation for further high-energy operation.
National Ignition Campaign
With the main construction complete, NIF started working on the "National Ignition Campaign" (NIC), the quest to successfully produce more fusion energy than the beamlines deposit on the target. On October 8, 2010 the first integrated ignition test was announced to have been completed successfully. The 192-beam laser system fired over a million joules of ultraviolet laser energy into a capsule filled with the hydrogen fuel. However, a number of problems slowed the drive toward ignition-level laser energies in the 1.4 to 1.5 million Joule range.Progress was initially slowed by the potential for damage from overheating due to a concentration of energy on optical components that is greater than anything previously attempted. Other issues included problems layering the fuel inside the targets, and minute quantities of dust being found on the capsule surface.
As the power was increased and targets of increasing sophistication were used, another problem appeared that was causing asymmetric implosion. This was eventually traced to minute amounts of water vapor in the target chamber which froze to the windows on the ends of the hohlraums. This was solved by re-designing the hohlraum with two layers of glass on either end, in effect creating a storm window. Steven Koonin, DOE undersecretary for science, visited the lab for an update on the NIC on 23 April, the day after the window problem was announced as solved. On 10 March he had described the NIC as "a goal of overriding importance for the DOE" and expressed that progress to date "was not as rapid as I had hoped."
NIC shots halted in February 2011, as the machine was turned over to SSMP materials experiments. As these experiments wound down, a series of planned upgrades were carried out, notably a series of improved diagnostic and measurement instruments. Among these changes were the addition of the ARC system, which uses 4 of the NIF's 192 beams as a backlighting source for high-speed imaging of the implosion sequence. NIC runs re-started in May 2011 with the goal of timing the four laser shock waves that compress the fusion target to very high precision. The shots tested the symmetry of the X-ray drive during the first three nanosecond
Nanosecond
A nanosecond is one billionth of a second . One nanosecond is to one second as one second is to 31.7 years.The word nanosecond is formed by the prefix nano and the unit second. Its symbol is ns....
s. Full-system shots fired in the second half of May achieved unprecedented peak pressures of 50 megabar
Bar (unit)
The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...
s.
According to an article in Science magazine published in October 2011, there are 17 parameters (14 on the laser, and 3 on the hohlraum) that can be tweaked to help the NIF achieve the four identified conditions necessary for ignition. The 4 conditions that need to be achieved for ignition to take place are:
"The imploding fuel must maintain its spherical shape; it must achieve a certain speed; the amount of mixing between the fuel and the capsule material must be kept low; and the entropy of the system must be kept down—in other words, the energy applied needs to be focused on compressing the fuel and not raising its temperature, which would impede compression."
The plan to achieve these four conditions involves: 1) Implosion velocity needs to be increased from 300 km/s to 370 km/s -- this can be tweaked by the pulse shape. 2) The power of the fourth and final burst of the laser pulse has to be 300 times the power of the initial bursts. It was presently at 50x. 3) The hohlraum shape has to be made stubbier so that incoming laser beams are not subject to interference by material blow off 4) The hohlraum material has to be tweaked to avoid the fuel being heated so that compression can be increased by 30 km/s hours more (doubling they achieved when they first switched from using germanium to silicon dopant).
Similar projects
Other fusion reactor designs could also be potential sources of energy in the future.Some similar experimental projects are:
- DEMOnstration Power PlantDEMODEMO is a proposed nuclear fusion power plant that is intended to build upon the expected success of the ITER experimental nuclear fusion reactor. Whereas ITER's goal is to produce 500 megawatts of fusion power for at least 500 seconds, the goal of DEMO will be to produce at least four times that...
(DEMO) - High Power laser Energy Research facilityHiPERThe High Power laser Energy Research facility , is an experimental laser-driven inertial confinement fusion device undergoing preliminary design for possible construction in the European Union starting around 2010...
(HiPER) - International Fusion Materials Irradiation FacilityInternational Fusion Materials Irradiation FacilityThe International Fusion Material Irradiation Facility, also known as IFMIF, is an international scientific research program designed to test materials for suitability for use in a fusion reactor...
(IFMIF) - ITERITERITER is an international nuclear fusion research and engineering project, which is currently building the world's largest and most advanced experimental tokamak nuclear fusion reactor at Cadarache in the south of France...
- Joint European TorusJoint European TorusJET, the Joint European Torus, is the largest magnetic confinement plasma physics experiment worldwide currently in operation. Its main purpose is to open the way to future nuclear fusion experimental tokamak reactors such as ITER and :DEMO....
(JET) - Laboratory for Laser EnergeticsLaboratory for Laser EnergeticsThe Laboratory for Laser Energetics is a scientific research facility which is part of the University of Rochester's south campus, located in Brighton, New York. The lab was established in 1970 and its operations since then have been funded jointly; mainly by the United States Department of...
(LLE) - Laser MégajouleLaser MégajouleLaser Mégajoule is an experimental inertial confinement fusion device being built near Bordeaux, in France by the French nuclear science directorate, CEA. Laser Mégajoule plans to deliver about 1.8 MJ of laser energy to its targets, making it about as powerful as its US counterpart, the...
(LMJ) - Wendelstein 7-XWendelstein 7-XWendelstein 7-X is an experimental stellarator currently being built in Greifswald, Germany by the Max-Planck-Institut für Plasmaphysik , which will be completed by 2015. It is a further development of Wendelstein 7-AS...
Pictures
Panorama taken outside the fusion chamber.
External links
- How NIF Works
- National Ignition Facility homepage
- NIF Director, Dr Ed Moses, on the progress of the facility, Ingenia magazine, December 2007
- NIF project director Moses says facility is ready to go, SPIE Newsroom, March 23, 2009
- Inside Livermore Lab's Race to Invent Clean Energy Newsweek, November 14, 2009