Flux pumping
Encyclopedia
Flux pumping is a method for magnetising
bulk superconductors to fields
in excess of 15 teslas
. The method can be applied to any type II superconductor and exploits a fundamental property of superconductors. That is their ability to support and maintain currents on the length scale of the superconductor. Conventional magnetic materials are magnetised on a molecular scale which means that superconductors can maintain a flux density orders of magnitude bigger than conventional materials. Flux pumping is especially significant when one bears in mind that all other methods of magnetising superconductors require application of a magnetic flux density at least as high as the final required field. This is not true of flux pumping.
An electric current
flowing in a loop of superconducting wire can persist indefinitely with no power source. In a normal conductor, an electrical current may be visualized as a fluid of electron
s moving across a heavy ion
ic lattice. The electrons are constantly colliding with the ions in the lattice, and during each collision some of the energy
carried by the current is absorbed by the lattice and converted into heat
, which is essentially the vibrational kinetic energy
of the lattice ions. As a result, the energy carried by the current is constantly being dissipated. This is the phenomenon of electrical resistance
.
The situation is different in a superconductor. In a conventional superconductor, the electronic fluid cannot be resolved into individual electrons. Instead, it consists of bound pairs of electrons known as Cooper pair
s. This pairing is caused by an attractive force between electrons from the exchange of phonon
s. Due to quantum mechanics
, the energy spectrum of this Cooper pair fluid possesses an energy gap, meaning there is a minimum amount of energy ΔE that must be supplied in order to excite the fluid. Therefore, if ΔE is larger than the thermal energy
of the lattice, given by kT, where k is Boltzmann's constant and T is the temperature
, the fluid will not be scattered by the lattice. The Cooper pair fluid is thus a superfluid
, meaning it can flow without energy dissipation.
In a class of superconductors known as Type II superconductors, including all known high-temperature superconductors, an extremely small amount of resistivity appears at temperatures not too far below the nominal superconducting transition when an electrical current is applied in conjunction with a strong magnetic field, which may be caused by the electrical current. This is due to the motion of vortices in the electronic superfluid, which dissipates some of the energy carried by the current. If the current is sufficiently small, the vortices are stationary, and the resistivity vanishes. The resistance due to this effect is tiny compared with that of non-superconducting materials, but must be taken into account in sensitive experiments.
according to Faraday's law of induction
. As long as the direction of motion of the magnetic wave is constant then the current induced will always be in the same sense and successive waves will induce more and more current
.
Traditionally the magnetic wave would be generated either by physically moving a magnet
or by an arrangement of coils switched in sequence, such as occurs on the stator of a three-phase motor. Flux Pumping is a solid state method where a material which changes magnetic state at a suitable magnetic ordering temperature is heated at its edge and the resultant thermal wave produces a magnetic wave which then magnetizes the superconductor. A superconducting flux pump should not be confused with a classical flux pump as described in Van Klundert et al.’s review.
The method described here has two unique features:
The system, as described, is actually a novel kind of heat engine in which thermal energy
is being converted into magnetic energy.
When a superconductor is placed in a weak external magnetic field
H, the field penetrates the superconductor only a small distance λ, called the London penetration depth
, decaying exponentially to zero within the bulk of the material. This is called the Meissner effect
, and is a defining characteristic of superconductivity. For most superconductors, the London penetration depth is on the order of 100 nm.
The Meissner effect is sometimes confused with the kind of diamagnetism
one would expect in a perfect electrical conductor: according to Lenz's law
, when a changing magnetic field is applied to a conductor, it will induce an electrical current in the conductor that creates an opposing magnetic field. In a perfect conductor, an arbitrarily large current can be induced, and the resulting magnetic field exactly cancels the applied field.
The Meissner effect is distinct from this because a superconductor expels all magnetic fields, not just those that are changing. Suppose we have a material in its normal state, containing a constant internal magnetic field. When the material is cooled below the critical temperature, we would observe the abrupt expulsion of the internal magnetic field, which we would not expect based on Lenz's law.
The Meissner effect was explained by the brothers Fritz
and Heinz London
, who showed that the electromagnetic free energy
in a superconductor is minimized provided
where H is the magnetic field and λ is the London penetration depth.
This equation, which is known as the London equation, predicts that the magnetic field in a superconductor decays exponentially from whatever value it possesses at the surface.
In 1962, the first commercial superconducting wire, a niobium
-titanium
alloy, was developed by researchers at Westinghouse, allowing the construction of the first practical superconducting magnet
s. In the same year, Josephson
made the important theoretical prediction that a supercurrent can flow between two pieces of superconductor separated by a thin layer of insulator. This phenomenon, now called the Josephson effect
, is exploited by superconducting devices such as SQUID
s. It is used in the most accurate available measurements of the magnetic flux quantum
, and thus (coupled with the quantum Hall resistivity) for Planck's constant h. Josephson was awarded the Nobel Prize for this work in 1973.
is the Bean or Critical State model and variations such as the Kim-Anderson model. However the Bean model assumes zero resistivity
and that current is always induced at the critical current. A more useful model for engineering
applications is the so-called E_J power law in which the Field and the current
are linked by the following equations:
In these equations if n = 1 then the conductor has linear resistivity such as is found in copper
. The higher the n-value the closer we get to the critical state model. Also the higher the n-value then the "better" the superconductor as the lower the resistivity at a certain current. The E_J power law can be used to describe the phenomenon of flux-creep in which a superconductor gradually loses its magnetisation over time. This process is logarithmic and thus gets slower and slower and ultimately leads to very stable fields.
s at cryogenic temperatures makes them particularly attractive for a variety of engineering applications including superconducting magnet
s, magnetic bearings and motors. It has already been shown that large fields can be obtained in single domain samples at 77 K. A range of possible applications exist in the design of high power density electric motors.
Before such devices can be created a major problem needs to be overcome. Even though all of these devices use a superconductor in the role of a permanent magnet and even though the superconductor can trap potentially huge magnetic fields (greater than 10 T) the problem is the induction of the magnetic fields. There are four possible known methods:
Any of these methods could be used to magnetise the superconductor and this may be done either in situ or ex situ. Ideally the superconductors are magnetised in situ.
There are several reasons for this: first, if the superconductors should become demagnetised through (i) flux creep, (ii) repeatedly applied perpendicular field
s or (iii) by loss of cooling then they may be re-magnetized without the need to disassemble the machine. Secondly, there are difficulties with handling very strongly magnetized material at cryogenic temperatures when assembling the machine. Thirdly, ex situ methods would require the machine to be assembled both cold and pre-magnetized and would offer significant design difficulties. Until room temperature superconductors can be prepared, the most efficient design of machine will therefore be one in which an in situ magnetizing fixture is included!
The first three methods all require a solenoid which can be switched on and off. In the first method an applied magnetic field is required equal to the required magnetic field, whilst the second and third approaches require fields at least two times greater. The final method, however, offers significant advantages since it achieves the final required field by repeated applications of a small field and can utilise a permanent magnet.
If we wish to pulse a field using, say, a 10 T magnet to magnetize a 30 mm × 10 mm sample then we can work out how big the solenoid needs to be. If it were possible to wind an appropriate coil using YBCO tape then, assuming an Ic of 70 A and a thickness of 100 μm, we would have 100 turns and 7000 A turns. This would produce a B field of approximately 7000/(20 × 10−3) × 4π × 10−7 = 0.4 T. To produce 10 T would require pulsing to 1400 A! An alternative calculation would be to assume a Jc of say 5 × 108Am−1 and a coil 1 cm2 in cross section. The field would then be 5 × 108 × 10−2 × (2 × 4π × 10−7) = 10 T. Clearly if the magnetisation fixture is not to occupy more room than the puck itself then a very high activation current would be required and either constraint makes in situ magnetization a very difficult proposition. What is required for in situ magnetisation is a magnetisation method in which a relatively small field
of the order of milliteslas repeatedly applied is used to magnetize the superconductor.
s are some of the most powerful electromagnet
s known. They are used in MRI
and NMR
machines, mass spectrometers, and the beam-steering magnets used in particle accelerator
s. They can also be used for magnetic separation, where weakly magnetic particles are extracted from a background of less or non-magnetic particles, as in the pigment
industries.
Other early markets are arising where the relative efficiency, size and weight advantages of devices based on HTS outweigh the additional costs involved.
Promising future applications include high-performance transformer
s, power storage devices, electric power transmission
, electric motor
s (e.g. for vehicle propulsion, as in vactrain
s or maglev train
s), magnetic levitation devices, and fault current limiter
s.
Magnetism
Magnetism is a property of materials that respond at an atomic or subatomic level to an applied magnetic field. Ferromagnetism is the strongest and most familiar type of magnetism. It is responsible for the behavior of permanent magnets, which produce their own persistent magnetic fields, as well...
bulk superconductors to fields
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
in excess of 15 teslas
Tesla (unit)
The tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla...
. The method can be applied to any type II superconductor and exploits a fundamental property of superconductors. That is their ability to support and maintain currents on the length scale of the superconductor. Conventional magnetic materials are magnetised on a molecular scale which means that superconductors can maintain a flux density orders of magnitude bigger than conventional materials. Flux pumping is especially significant when one bears in mind that all other methods of magnetising superconductors require application of a magnetic flux density at least as high as the final required field. This is not true of flux pumping.
An electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
flowing in a loop of superconducting wire can persist indefinitely with no power source. In a normal conductor, an electrical current may be visualized as a fluid of 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 moving across a heavy ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
ic lattice. The electrons are constantly colliding with the ions in the lattice, and during each collision some of the energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
carried by the current is absorbed by the lattice and converted into heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
, which is essentially the vibrational kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
of the lattice ions. As a result, the energy carried by the current is constantly being dissipated. This is the phenomenon of electrical resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
.
The situation is different in a superconductor. In a conventional superconductor, the electronic fluid cannot be resolved into individual electrons. Instead, it consists of bound pairs of electrons known as Cooper pair
Cooper pair
In condensed matter physics, a Cooper pair or BCS pair is two electrons that are bound together at low temperatures in a certain manner first described in 1956 by American physicist Leon Cooper...
s. This pairing is caused by an attractive force between electrons from the exchange of phonon
Phonon
In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, such as solids and some liquids...
s. Due to quantum mechanics
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
, the energy spectrum of this Cooper pair fluid possesses an energy gap, meaning there is a minimum amount of energy ΔE that must be supplied in order to excite the fluid. Therefore, if ΔE is larger than the thermal energy
Thermal energy
Thermal energy is the part of the total internal energy of a thermodynamic system or sample of matter that results in the system's temperature....
of the lattice, given by kT, where k is Boltzmann's constant and T is the temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
, the fluid will not be scattered by the lattice. The Cooper pair fluid is thus a superfluid
Superfluid
Superfluidity is a state of matter in which the matter behaves like a fluid without viscosity and with extremely high thermal conductivity. The substance, which appears to be a normal liquid, will flow without friction past any surface, which allows it to continue to circulate over obstructions and...
, meaning it can flow without energy dissipation.
In a class of superconductors known as Type II superconductors, including all known high-temperature superconductors, an extremely small amount of resistivity appears at temperatures not too far below the nominal superconducting transition when an electrical current is applied in conjunction with a strong magnetic field, which may be caused by the electrical current. This is due to the motion of vortices in the electronic superfluid, which dissipates some of the energy carried by the current. If the current is sufficiently small, the vortices are stationary, and the resistivity vanishes. The resistance due to this effect is tiny compared with that of non-superconducting materials, but must be taken into account in sensitive experiments.
Introduction
In the method described here a magnetic field is swept across the superconductor in a magnetic wave. This field induces currentElectric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
according to Faraday's law of induction
Faraday's law of induction
Faraday's law of induction dates from the 1830s, and is a basic law of electromagnetism relating to the operating principles of transformers, inductors, and many types of electrical motors and generators...
. As long as the direction of motion of the magnetic wave is constant then the current induced will always be in the same sense and successive waves will induce more and more current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
.
Traditionally the magnetic wave would be generated either by physically moving a magnet
Magnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...
or by an arrangement of coils switched in sequence, such as occurs on the stator of a three-phase motor. Flux Pumping is a solid state method where a material which changes magnetic state at a suitable magnetic ordering temperature is heated at its edge and the resultant thermal wave produces a magnetic wave which then magnetizes the superconductor. A superconducting flux pump should not be confused with a classical flux pump as described in Van Klundert et al.’s review.
The method described here has two unique features:
- At no point is the superconductor driven normal; the procedure simply makes modifications to the critical state.
- The critical state is not modified by a moving magnet or an array of solenoids, but by a thermal pulse which modifies the magnetization, thus sweeping vortices into the material.
The system, as described, is actually a novel kind of heat engine in which thermal energy
Thermal energy
Thermal energy is the part of the total internal energy of a thermodynamic system or sample of matter that results in the system's temperature....
is being converted into magnetic energy.
Meissner effect
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
H, the field penetrates the superconductor only a small distance λ, called the London penetration depth
London penetration depth
In superconductors, the London penetration depth characterizes the distance to which a magnetic field penetrates into a superconductor and becomes equal to 1/e times that of the magnetic field at the surface of the superconductor...
, decaying exponentially to zero within the bulk of the material. This is called the Meissner effect
Meissner effect
The Meissner effect is the expulsion of a magnetic field from a superconductor during its transition to the superconducting state. The German physicists Walther Meissner and Robert Ochsenfeld discovered the phenomenon in 1933 by measuring the magnetic field distribution outside superconducting tin...
, and is a defining characteristic of superconductivity. For most superconductors, the London penetration depth is on the order of 100 nm.
The Meissner effect is sometimes confused with the kind of diamagnetism
Diamagnetism
Diamagnetism is the property of an object which causes it to create a magnetic field in opposition to an externally applied magnetic field, thus causing a repulsive effect. Specifically, an external magnetic field alters the orbital velocity of electrons around their nuclei, thus changing the...
one would expect in a perfect electrical conductor: according to Lenz's law
Lenz's law
Lenz's law is a common way of understanding how electromagnetic circuits must always obey Newton's third law and The Law of Conservation of Energy...
, when a changing magnetic field is applied to a conductor, it will induce an electrical current in the conductor that creates an opposing magnetic field. In a perfect conductor, an arbitrarily large current can be induced, and the resulting magnetic field exactly cancels the applied field.
The Meissner effect is distinct from this because a superconductor expels all magnetic fields, not just those that are changing. Suppose we have a material in its normal state, containing a constant internal magnetic field. When the material is cooled below the critical temperature, we would observe the abrupt expulsion of the internal magnetic field, which we would not expect based on Lenz's law.
The Meissner effect was explained by the brothers Fritz
Fritz London
Fritz Wolfgang London was a German theoretical physicist. His fundamental contributions to the theories of chemical bonding and of intermolecular forces are today considered classic and are discussed in standard textbooks of physical chemistry.With his brother Heinz, he made a significant...
and Heinz London
Heinz London
Heinz London was a German Physicist. He worked with his brother Fritz on superconductivity, discovering the London equations when working in Oxford, at the Clarendon Laboratory; these equations gave a first explanation to the Meissner effect...
, who showed that the electromagnetic free energy
Thermodynamic free energy
The thermodynamic free energy is the amount of work that a thermodynamic system can perform. The concept is useful in the thermodynamics of chemical or thermal processes in engineering and science. The free energy is the internal energy of a system less the amount of energy that cannot be used to...
in a superconductor is minimized provided
where H is the magnetic field and λ is the London penetration depth.
This equation, which is known as the London equation, predicts that the magnetic field in a superconductor decays exponentially from whatever value it possesses at the surface.
In 1962, the first commercial superconducting wire, a niobium
Niobium
Niobium or columbium , is a chemical element with the symbol Nb and atomic number 41. It's a soft, grey, ductile transition metal, which is often found in the pyrochlore mineral, the main commercial source for niobium, and columbite...
-titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
alloy, was developed by researchers at Westinghouse, allowing the construction of the first practical superconducting magnet
Superconducting magnet
A superconducting magnet is an electromagnet made from coils of superconducting wire. They must be cooled to cryogenic temperatures during operation. In its superconducting state the wire can conduct much larger electric currents than ordinary wire, creating intense magnetic fields...
s. In the same year, Josephson
Brian David Josephson
Brian David Josephson, FRS is a Welsh physicist. He became a Nobel Prize laureate in 1973 for the prediction of the eponymous Josephson effect....
made the important theoretical prediction that a supercurrent can flow between two pieces of superconductor separated by a thin layer of insulator. This phenomenon, now called the Josephson effect
Josephson effect
The Josephson effect is the phenomenon of supercurrent across two superconductors coupled by a weak link...
, is exploited by superconducting devices such as SQUID
SQUID
A SQUID is a very sensitive magnetometer used to measure extremely weak magnetic fields, based on superconducting loops containing Josephson junctions....
s. It is used in the most accurate available measurements of the magnetic flux quantum
Magnetic flux quantum
The magnetic flux quantum Φ0 is the quantum of magnetic flux passing through a superconductor. The phenomenon of flux quantization was discovered B. S. Deaver and W. M. Fairbank and, independently, by R. Doll and M. Nabauer, in 1961...
, and thus (coupled with the quantum Hall resistivity) for Planck's constant h. Josephson was awarded the Nobel Prize for this work in 1973.E-J Power Law
The most popular model used to describe superconductivitySuperconductivity
Superconductivity is a phenomenon of exactly zero electrical resistance occurring in certain materials below a characteristic temperature. It was discovered by Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum...
is the Bean or Critical State model and variations such as the Kim-Anderson model. However the Bean model assumes zero resistivity
Resistivity
Electrical resistivity is a measure of how strongly a material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electric charge. The SI unit of electrical resistivity is the ohm metre...
and that current is always induced at the critical current. A more useful model for engineering
Engineering
Engineering is the discipline, art, skill and profession of acquiring and applying scientific, mathematical, economic, social, and practical knowledge, in order to design and build structures, machines, devices, systems, materials and processes that safely realize improvements to the lives of...
applications is the so-called E_J power law in which the Field and the current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
are linked by the following equations:
In these equations if n = 1 then the conductor has linear resistivity such as is found in copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
. The higher the n-value the closer we get to the critical state model. Also the higher the n-value then the "better" the superconductor as the lower the resistivity at a certain current. The E_J power law can be used to describe the phenomenon of flux-creep in which a superconductor gradually loses its magnetisation over time. This process is logarithmic and thus gets slower and slower and ultimately leads to very stable fields.
Theory
The potential of bulk melt-processed YBCO single domains to trap significant magnetic fieldMagnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
s at cryogenic temperatures makes them particularly attractive for a variety of engineering applications including superconducting magnet
Superconducting magnet
A superconducting magnet is an electromagnet made from coils of superconducting wire. They must be cooled to cryogenic temperatures during operation. In its superconducting state the wire can conduct much larger electric currents than ordinary wire, creating intense magnetic fields...
s, magnetic bearings and motors. It has already been shown that large fields can be obtained in single domain samples at 77 K. A range of possible applications exist in the design of high power density electric motors.
Before such devices can be created a major problem needs to be overcome. Even though all of these devices use a superconductor in the role of a permanent magnet and even though the superconductor can trap potentially huge magnetic fields (greater than 10 T) the problem is the induction of the magnetic fields. There are four possible known methods:
- Cooling in field;
- Zero field cooling, followed by slowly applied field;
- Pulse magnetization;
- Flux pumping;
Any of these methods could be used to magnetise the superconductor and this may be done either in situ or ex situ. Ideally the superconductors are magnetised in situ.
There are several reasons for this: first, if the superconductors should become demagnetised through (i) flux creep, (ii) repeatedly applied perpendicular field
Field (physics)
In physics, a field is a physical quantity associated with each point of spacetime. A field can be classified as a scalar field, a vector field, a spinor field, or a tensor field according to whether the value of the field at each point is a scalar, a vector, a spinor or, more generally, a tensor,...
s or (iii) by loss of cooling then they may be re-magnetized without the need to disassemble the machine. Secondly, there are difficulties with handling very strongly magnetized material at cryogenic temperatures when assembling the machine. Thirdly, ex situ methods would require the machine to be assembled both cold and pre-magnetized and would offer significant design difficulties. Until room temperature superconductors can be prepared, the most efficient design of machine will therefore be one in which an in situ magnetizing fixture is included!
The first three methods all require a solenoid which can be switched on and off. In the first method an applied magnetic field is required equal to the required magnetic field, whilst the second and third approaches require fields at least two times greater. The final method, however, offers significant advantages since it achieves the final required field by repeated applications of a small field and can utilise a permanent magnet.
If we wish to pulse a field using, say, a 10 T magnet to magnetize a 30 mm × 10 mm sample then we can work out how big the solenoid needs to be. If it were possible to wind an appropriate coil using YBCO tape then, assuming an Ic of 70 A and a thickness of 100 μm, we would have 100 turns and 7000 A turns. This would produce a B field of approximately 7000/(20 × 10−3) × 4π × 10−7 = 0.4 T. To produce 10 T would require pulsing to 1400 A! An alternative calculation would be to assume a Jc of say 5 × 108Am−1 and a coil 1 cm2 in cross section. The field would then be 5 × 108 × 10−2 × (2 × 4π × 10−7) = 10 T. Clearly if the magnetisation fixture is not to occupy more room than the puck itself then a very high activation current would be required and either constraint makes in situ magnetization a very difficult proposition. What is required for in situ magnetisation is a magnetisation method in which a relatively small field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
of the order of milliteslas repeatedly applied is used to magnetize the superconductor.
Applications
Superconducting magnetSuperconducting magnet
A superconducting magnet is an electromagnet made from coils of superconducting wire. They must be cooled to cryogenic temperatures during operation. In its superconducting state the wire can conduct much larger electric currents than ordinary wire, creating intense magnetic fields...
s are some of the most powerful electromagnet
Electromagnet
An electromagnet is a type of magnet in which the magnetic field is produced by the flow of electric current. The magnetic field disappears when the current is turned off...
s known. They are used in MRI
Magnetic resonance imaging
Magnetic resonance imaging , nuclear magnetic resonance imaging , or magnetic resonance tomography is a medical imaging technique used in radiology to visualize detailed internal structures...
and NMR
NMR
NMR may refer to:Applications of Nuclear Magnetic Resonance:* Nuclear magnetic resonance* NMR spectroscopy* Solid-state nuclear magnetic resonance* Protein nuclear magnetic resonance spectroscopy* Proton NMR* Carbon-13 NMR...
machines, mass spectrometers, and the beam-steering magnets used 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. They can also be used for magnetic separation, where weakly magnetic particles are extracted from a background of less or non-magnetic particles, as in the pigment
Pigment
A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...
industries.
Other early markets are arising where the relative efficiency, size and weight advantages of devices based on HTS outweigh the additional costs involved.
Promising future applications include high-performance transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
s, power storage devices, electric power transmission
Electric power transmission
Electric-power transmission is the bulk transfer of electrical energy, from generating power plants to Electrical substations located near demand centers...
, electric motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
s (e.g. for vehicle propulsion, as in vactrain
Vactrain
A vactrain is a proposed, as-yet-unbuilt design for future high-speed railroad transportation. This would entail building maglev lines through evacuated or partly evacuated tubes or tunnels...
s or maglev train
Maglev train
Maglev , is a system of transportation that uses magnetic levitation to suspend, guide and propel vehicles from magnets rather than using mechanical methods, such as friction-reliant wheels, axles and bearings...
s), magnetic levitation devices, and fault current limiter
Fault current limiter
A Fault Current Limiter is a device which limits the prospective fault current when a fault occurs . The term is generally applied to superconducting devices, whereas non-superconducting devices are typically termed Fault Current Controllers...
s.
Sources
- Qiuliang Wang et al., "Study of Full-wave Superconducting Rectifier-type Flux-pumps", IEEE Transactions on Magnetics, vol. 32, No. 4, pp. 2699–2702, Jul. 1996.
- L.J.M. van de Klundert et al., "On fully conducting rectifiers and fluxpumps. A review. Part 2: Commutation modes, characteristics and switches", Cryogencis, pp. 267–277, May 1981.
- L.J.M. van de Klundert et al., "Fully superconducting rectifiers and fluxpumps Part 1: Realized methods for pumping flux", Cryogenics, pp. 195–206, Apr. 1981.
- Kleinert, HagenHagen KleinertHagen Kleinert is Professor of Theoretical Physics at the Free University of Berlin, Germany , at theWest University of Timişoara, at thein Bishkek. He is also of the...
, Gauge Fields in Condensed Matter, Vol. I, " SUPERFLOWSuperfluidSuperfluidity is a state of matter in which the matter behaves like a fluid without viscosity and with extremely high thermal conductivity. The substance, which appears to be a normal liquid, will flow without friction past any surface, which allows it to continue to circulate over obstructions and...
AND VORTEX LINES"; Disorder Fields, Phase TransitionsPhase transitionA phase transition is the transformation of a thermodynamic system from one phase or state of matter to another.A phase of a thermodynamic system and the states of matter have uniform physical properties....
, pp. 1–742, World Scientific (Singapore, 1989); Paperback ISBN 9971-5-0210-0 (also readable online: Vol. I) - Larkin, AnatolyAnatoly LarkinAnatoly Ivanovich Larkin was a Russian theoretical physicist, universally recognised as a leader in theory of condensed matter, and who was also a celebrated teacher of several generations of theorists....
; Varlamov, Andrei, Theory of Fluctuations in Superconductors, Oxford University Press, Oxford, United Kingdom, 2005 (ISBN 0-19-852815-9)