Nova laser
Encyclopedia
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...
built 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...
(LLNL) in 1984 which conducted advanced 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) experiments until its dismantling in 1999. Nova was the first ICF experiment built with the intention of reaching "ignition", 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....
of 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...
that releases a large amount of energy. Although Nova failed in this goal, the data it generated clearly defined the problem as being mostly a result of magnetohydrodynamic instability
Plasma stability
An important field of plasma physics is the stability of the plasma. It usually only makes sense to analyze the stability of a plasma once it has been established that the plasma is in equilibrium. "Equilibrium" asks whether there are net forces that will accelerate any part of the plasma...
, leading to the design of the National Ignition Facility
National Ignition Facility
The National Ignition Facility, or NIF is a large, laser-based inertial confinement fusion 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...
, Nova's successor. Nova also generated considerable amounts of data on high-density matter physics, regardless of the lack of ignition, which is useful both in fusion power
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...
and 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...
s research.
Background
Inertial confinement fusionInertial 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) 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 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...
. The heat of the laser burns 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 due to Newton's Third Law, eventually collapsing into a small point of very 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. When it reaches the center of the fuel and meets the shock from the other side of the target, the energy in the shock wave further heats and compresses the tiny volume around it. If the temperature and density of that small spot can be raised high enough, fusion reactions will occur.
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 high density fuel around it and slow down. This heats the fuel further, and can potentially 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 can 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 can lead to a significant portion of the fuel in the target undergoing fusion, and the release of significant amounts of energy.
To date most ICF experiments have used lasers to heat the targets. Calculations show that the energy must be delivered quickly in order to compress the core before it disassembles, as well as creating a suitable shock wave. The 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.
History
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. Although this sounds very low powered compared to modern machines, at the time it was just beyond the state of the artState of the art
The state of the art is the highest level of development, as of a device, technique, or scientific field, achieved at a particular time. It also refers to the level of development reached at any particular time as a result of the latest methodologies employed.- Origin :The earliest use of the term...
, and led to a number of programs to produce lasers in this power range.
Prior to the construction of Nova, LLNL had designed and built a series of ever-larger lasers that explored the problems of basic ICF design. LLNL was primarily interested in the Nd:glass laser, which, at the time, was one of a very few high-energy laser designs known. LLNL had decided early on to concentrate on glass lasers, while other facilities studied gas lasers using carbon dioxide (e.g. Antares laser, Los Alamos National Laboratory
Los 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). Building large Nd:glass lasers had not been attempted before, and LLNL's early research focussed primarily on how to make these devices.
One problem was the homogeneity of the beams. Even minor variations in intensity of the beams would result in "self-focusing" in the air and glass optics in a process known as Kerr lensing
Kerr effect
The Kerr effect, also called the quadratic electro-optic effect , is a change in the refractive index of a material in response to an applied electric field. The Kerr effect is distinct from the Pockels effect in that the induced index change is directly proportional to the square of the electric...
. The resulting beam included small "filaments" of extremely high light intensity, so high it would damage the glass optics of the device. This problem was solved 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 ....
with the introduction of the 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,...
ing technique. Cyclops was followed by the Argus laser
Argus laser
Argus was a two-beam high power infrared neodymium doped silica glass laser with a output aperture built at Lawrence Livermore National Laboratory in 1976 for the study of inertial confinement fusion...
of greater power, which explored the problems of controlling more than one beam and illuminating a target more evenly. All of this work culminated in the 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...
, a proof-of-concept design for a high power system that included 20 separate "laser amplifiers" that were directed around the target to illuminate it.
It was during experiments with Shiva that another serious unexpected problem appeared. The infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
light generated by the Nd:glass lasers was found to interact very strongly with the electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s in the plasma created during the initial heating through the process of stimulated Raman scattering
Raman scattering
Raman scattering or the Raman effect is the inelastic scattering of a photon. It was discovered by Sir Chandrasekhara Venkata Raman and Kariamanickam Srinivasa Krishnan in liquids, and by Grigory Landsberg and Leonid Mandelstam in crystals....
. This process, referred to as "hot electron pre-heating", carried away a great amount of the laser's energy, and also caused the core of the target to heat before it reached maximum compression. This meant that much less energy was being deposited in the center of the collapse, both due to the reduction in implosion energy, as well as the outward force of the heated core. Although it was known that shorter wavelengths would reduce this problem, it had earlier been expected that the IR frequencies used in Shiva would be "short enough". This proved not to be the case.
A solution to this problem was explored in the form of efficient frequency multiplier
Frequency multiplier
In electronics, a frequency multiplier is an electronic circuit that generates an output signal whose output frequency is a harmonic of its input frequency. Frequency multipliers consist of a nonlinear circuit that distorts the input signal and consequently generates harmonics of the input signal...
s, optical devices that combine several photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s into one of higher energy, and thus frequency. These devices were quickly introduced and tested experimentally on the OMEGA laser and others, proving effective. Although the process is only about 50% efficient, and half the original laser power is lost, the resulting ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
light couples much more efficiently to the target plasma and is much more effective in collapsing the target to high density.
With these solutions in hand, LLNL decided to build a device with the power needed to produce ignition conditions. Design started in the late 1970s, with construction following shortly starting with the testbed Novette laser
Novette laser
Novette was a two beam neodymium glass testbed laser built at Lawrence Livermore National Laboratory in about 15 months throughout 1981 and 1982 and was completed in January 1983. Novette was made using recycled parts from the dismantled Shiva and Argus lasers and borrowed parts from the future...
to validate the basic beamline and frequency multiplier design. This was a time of repeated energy crises
Energy crisis
An energy crisis is any great bottleneck in the supply of energy resources to an economy. In popular literature though, it often refers to one of the energy sources used at a certain time and place, particularly those that supply national electricity grids or serve as fuel for vehicles...
in the U.S. and funding was not difficult to find given the large amounts of money available for alternative energy
Alternative energy
Alternative energy is an umbrella term that refers to any source of usable energy intended to replace fuel sources without the undesired consequences of the replaced fuels....
and nuclear weapons research.
Design
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 design that added frequency conversion to 351 nm light, which would increase coupling efficiency. The "new Nova" emerged as a system with ten laser amplifiers, or beamlines. Each beamline consisted of a series of Nd:glass amplifiers separated by spatial filters and other optics for cleaning up the resulting beams. Although techniques for folding the beamlines were known as early as Shiva, they were not well developed at this point in time. Nova ended up with a single fold in its layout, and the laser bay containing the beamlines was 300 feet (91.4 m) long. To the casual observer it appears to contain twenty 300 feet (91.4 m) long beamlines, but due to the fold each of the ten is actually almost 600 feet (182.9 m) long in terms of optical path length.
Prior to firing, the Nd:glass amplifiers are first pumped
Laser pumping
Laser pumping is the act of energy transfer from an external source into the gain medium of a laser. The energy is absorbed in the medium, producing excited states in its atoms. When the number of particles in one excited state exceeds the number of particles in the ground state or a less-excited...
with a series of Xenon flash lamps surrounding them. Some of the light produced by the lamps is captured in the glass, leading to a population inversion
Population inversion
In physics, specifically statistical mechanics, a population inversion occurs when a system exists in state with more members in an excited state than in lower energy states...
that allows for amplification via stimulated emission
Stimulated emission
In optics, stimulated emission is the process by which an atomic electron interacting with an electromagnetic wave of a certain frequency may drop to a lower energy level, transferring its energy to that field. A photon created in this manner has the same phase, frequency, polarization, and...
. This process is quite inefficient, and only about 1 to 1.5% of the power fed into the lamps is actually turned into laser energy. In order to produce the sort of laser power required for Nova, the lamps had to be very large, fed power from a large bank of 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...
s located under the laser bay. The flash also generates a large amount of heat which distorts the glass, requiring time for the lamps and glass to cool before they can be fired again. This limits Nova to about six firings a day at the maximum.
Once pumped and ready for firing, a small pulse of laser light is fed into the beamlines. The Nd:glass disks each dump additional power into the beam as it passes through them. After passing through a number of amplifiers the light pulse is "cleaned up" in a spatial filter before being fed into another series of amplifiers. At each stage additional optics were used to increase the diameter of the beam and allow the use of larger and larger amplifier disks. In total, Nova contained fifteen amplifiers and five filters of increasing size in the beamlines, with an option to add an additional amplifier on the last stage, although it is not clear if these were used in practice.
From there all ten beams pass into the experiment area at one end of the laser bay. Here a series of mirrors reflects the beams to impinge in the center of the bay from all angles. Optical devices in some of the paths slow the beams so that they all reach the center at the same time (within about a picosecond), as some of the beams have longer paths to the center than others. Frequency multipliers upconvert the light to green and blue (UV) just prior to entering the "target chamber". Nova is arranged so any remaining IR or green light is focused short of the center of the chamber.
The Nova laser as a whole was capable of delivering approximately 100 kilojoules of infrared light at 1054 nm, or 40-45 kilojoules of frequency tripled light at 351 nm (the third harmonic
Harmonic
A harmonic of a wave is a component frequency of the signal that is an integer multiple of the fundamental frequency, i.e. if the fundamental frequency is f, the harmonics have frequencies 2f, 3f, 4f, . . . etc. The harmonics have the property that they are all periodic at the fundamental...
of the Nd:Glass fundamental line at 1054 nm) in a pulse duration of about 2 to 4 nanoseconds and thus was capable of producing a UV pulse in the range of 16 trillion watts.
Fusion in Nova
Research on Nova was focussed on the "indirect drive" approach, where the laser shine on the inside surface of a thin metal foil, typically made of gold, lead, or another "high-zHeavy metals
A heavy metal is a member of a loosely-defined subset of elements that exhibit metallic properties. It mainly includes the transition metals, some metalloids, lanthanides, and actinides. Many different definitions have been proposed—some based on density, some on atomic number or atomic weight,...
" metal. When heated by the laser, the metal re-radiates this energy as diffuse 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 efficient than UV at compressing the fuel pellet. In order to emit x-rays, the metal must be heated to very high temperatures, which uses up a considerable amount of the laser energy. So while the compression is more efficient, the overall energy delivered to the target is nevertheless much smaller. The reason for the x-ray conversion is not to improve energy delivery, but to "smooth" the energy profile; since the metal foil spreads out the heat somewhat, the anisotropies in the original laser are greatly reduced.
The foil shells, or "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...
s", are generally formed as small open-ended cylinders, with the laser arranged to shine in the open ends at an oblique angle in order to strike the inner surface. In order to support the indirect drive research at Nova, a second experimental area was built "past" the main one, opposite the laser bay. The system was arranged to focus all ten beams into two sets of five each, which passed into this second area and then into either end of the target chamber, and from there into the hohlraums.
Confusingly, the indirect drive approach was not made widely public until 1993. Documents from the era published in general science magazines and similar material either gloss over the issue, or imply that Nova was using the direct drive approach, lacking the hohlraum. It was only during the design of NIF that the topic become public, so Nova was old news by that point.
Nevertheless, Nova remained a useful instrument even in its original form, and the main target chamber and beamlines were used for many years even after it was modified as outlined below. A number of different techniques for smoothing the beams were attempted over its lifetime, both to improve Nova as well as better understand NIF. These experiments added considerably not only to the understanding of ICF, but also to high-density physics in general, and even the evolution of the galaxy and supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
s.
Two beam
Shortly after completion of Nova, modifications were made to improve it as an experimental device. One problem was that the experimental chamber took a long time to refit for another "shot", longer than the time needed to cool down the lasers.In order to improve utilization of the laser, a second experimental chamber was built "past" the original, with optics that combined the ten beamlines into two. Nova had been built up against the older Shiva buildings, with the two experimental chambers "back to back" and the beamlines extending outward from the center target areas. The Two Beam system was installed by passing the beamguides and related optics through the now unused Shiva experimental area and placing the smaller experimental chamber in Shiva's beam bay.
LMF and Nova Upgrade
Nova's partial success, combined with other experimental numbers, prompted Department of EnergyUnited 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...
to request a custom military ICF facility they called the "Laboratory Microfusion Facility" (LMF) that could achieve fusion yield between 100 and 1000 MJ. Based on the LASNEX computer models, it was estimated that LMF would require a driver of about 10 MJ, in spite of nuclear tests that suggested a higher power. Building such a device was within the state of the art, but would be expensive, on the order of $1 billion. LLNL returned 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 access 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 1000 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.
Faced with this enormous project, in 1989/90 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 between 2 and 20 MJ The initial estimates from 1992 estimated construction costs around $400 million, with construction taking place from 1995 to 1999.
For reasons that are not well recorded in the historical record, later in 1992 LLNL updated their Nova Upgrade proposal and stated that the existing Nova/Shiva buildings would no longer be able to contain the new system, and that a new building about three times as large would be needed. From then on the plans evolved into the current National Ignition Facility
National Ignition Facility
The National Ignition Facility, or NIF is a large, laser-based inertial confinement fusion 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...
.
Petawatt
Starting in the late 1980s a new method of creating very short but very high power laser pulses was developed, known as chirped pulse amplificationChirped pulse amplification
Chirped pulse amplification is a technique for amplifying an ultrashort laser pulse up to the petawatt level with the laser pulse being stretched out temporally and spectrally prior to amplification...
, or CPA. Starting in 1992, LLNL staff modified one of Nova's existing arms to build an experimental CPA laser that produced up to 1.25 PW. Known simply as Petawatt, it operated until 1999 when Nova was dismantled to make way for NIF.
The basic amplification system used in Nova and other high-power lasers of its era was limited in terms of power density and pulse length. One problem was that the amplifier glass responded over a period of time, not instantaneously, and very short pulses would not be strongly amplified. Another problem was that the high power densities led to the same sorts of self-focusing problems that had caused problems in earlier designs, but at such a magnitude that even measures like spacial filtering would not be enough, in fact the power densities were high enough to cause filaments
Filament propagation
In nonlinear optics, filament propagation is propagation of a beam of light through a medium without diffraction. This is possible because the Kerr effect causes an index of refraction change in the medium, resulting in self-focusing of the beam....
to form in air.
CPA avoids both of these problems by spreading out the laser pulse in time. It does this by reflecting a relatively multi-chromatic (as compared to most lasers) pulse off a series of two diffraction grating
Diffraction grating
In optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as...
s, which splits them spatially into different frequencies, essentially the same thing a simple prism
Prism (optics)
In optics, a prism is a transparent optical element with flat, polished surfaces that refract light. The exact angles between the surfaces depend on the application. The traditional geometrical shape is that of a triangular prism with a triangular base and rectangular sides, and in colloquial use...
does with visible light. These individual frequencies have to travel different distances when reflected back into the beamline, resulting in the pulse being "stretched out" in time. This longer pulse is fed into the amplifiers as normal, which now have time to respond normally. After amplification the beams are sent into a second pair of gratings "in reverse" to recombine them into a single short pulse with high power. In order to avoid filamentation or damage to the optical elements, the entire end of the beamline is placed in a large vacuum chamber
Vacuum chamber
A vacuum chamber is a rigid enclosure from which air and other gases are removed by a vacuum pump. The resulting low pressure, commonly referred to as a vacuum, allows researchers to conduct physical experiments or to test mechanical devices which must operate in outer space...
.
Although Petawatt was instrumental in advancing the practical basis for the concept of "fast ignition fusion", by the time it was operational as a proof-of-concept device, the decision to move ahead with NIF had already been taken. Further work on the fast ignition approach continues, and will potentially reach a level of development far in advance of NIF at HiPER
HiPER
The 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...
, an experimental system under development in the European Union. If successful, HiPER should generate fusion energy over twice that of NIF, while requiring a laser system of less than one-quarter the power and one-tenth the cost. Fast ignition is one of the more promising approaches to fusion power
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...
.
"Death" of Nova
When Nova was being dismantled to make way for NIF, the target chamber was lent to France for temporary use during the development of Laser MegajouleLaser Mégajoule
Laser 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...
, a system similar to NIF in many ways. This loan was controversial, as the only other operational laser at LLNL at the time, Beamlet (a single experimental beamline for NIF), had recently been sent to Sandia National Laboratory in New Mexico. This left LLNL with no large laser facility until NIF started operation, which was then estimated as being 2003 at the earliest. Work on NIF was not declared formally completed until March 31, 2009.