Magnetized target fusion
Encyclopedia
Magnetized target fusion (MTF) is a relatively new approach to producing fusion power
that combines features of the more widely studied magnetic confinement fusion
(MCF) and inertial confinement fusion
(ICF) approaches. Like the magnetic approach, the fusion fuel is confined at lower density by magnetic field
s while it is heated into a plasma
. Like the inertial approach, fusion is initiated by rapidly squeezing the target to greatly increase fuel density, and thus temperature. Although the resulting density is far lower than in traditional ICF, it is thought that the combination of longer confinement times and better heat retention will let MTF yield the same efficiencies, yet be far easier to build. The term magneto-inertial fusion (MIF) is similar, but encompasses a wider variety of arrangements. The two terms are often applied interchangeably to experiments.
MTF is currently being studied mostly by the Los Alamos National Laboratory
(LANL) and Air Force Research Laboratory
(AFRL), and by Canadian startup company, General Fusion
.
, together to form larger ones. Generally the reactions take place at such high temperatures that the atoms have been ion
ized, their electron
s stripped off by the heat; thus, fusion is typically described in terms of "nuclei" instead of "atoms". Nuclei are positively charged, and repel each other due to the electrostatic force. Counteracting this is the strong force that pulls nucleons together, but only at very short ranges. Thus a fluid of nuclei will generally not undergo fusion on its own – the nuclei must be forced together before the strong force can pull them together into stable collections. The amount of energy that needs to be applied to force the nuclei together is termed the Coulomb barrier
or fusion barrier energy. To create needed conditions, the fuel must be heated to tens of millions of degrees, and/or compressed to immense pressures, for a long enough time. The temperature, pressure, and time needed for any given fuel to fuse is termed the Lawson criterion
. Since the criterion contains both pressure and temperature, existing approaches to practical fusion power have generally worked to raise one or another of these values.
Magnetic fusion works to heat a dilute plasma (1014 ions per cm3) to high temperatures, around 20 keV (~200 million C). Ambient air is about 100,000 times denser. To make a practical reactor at these temperatures, the fuel must be confined for long periods of time, on the order of 1 second. The ITER
tokamak
design is currently being built to test the magnetic approach with pulse lengths up to 20 minutes. Inertial fusion works to produce extremely high densities, 1025 ions per cubic cm, about 100 times the density of lead. This causes reactions to occur extremely quickly (~1 nanosecond), which causes confinement time to be extremely short, as the heat of reactions drives the plasma outward. The $3–4 billion dollar National Ignition Facility
(NIF) machine at Lawrence Livermore National Laboratory
(LLNL) will be a definitive test of ICF at megajoule energy levels. Both conventional methods of nuclear fusion are nearing net energy (Q>1) levels now after many decades of research, but remain far from a practical energy-producing device.
In general terms, MTF is an inertial method. The density is increased through a pulsed operation that compresses the fuel, and since temperature is the average energy per unit density, as long as heat is not lost to the surroundings, the temperature of the fuel is raised by a similar amount. In traditional ICF, more energy is added through the lasers that compress the target, energy that leaks away through a variety of processes. No more energy is added in MTF. Instead, a magnetic field is created before compression that confines fuel, and insulates it so less energy is lost to the outside. The result, compared to ICF, is a somewhat-dense, somewhat-hot fuel mass that undergoes fusion at a medium reaction rate, so it only must be confined for a medium length of time.
At first glance it might seem that this approach would have no advantages over traditional ICF methods. All that has changed is a tradeoff between confinement time and density, but the end result is the same. The reason MTF appears to be so much more practical is that the lower density it needs can be formed through a variety of processes that are relatively efficient and inexpensive, whereas ICF demands specialized high-performance laser
s of low efficiency. The cost and complexity of these lasers, termed "drivers", is so great that traditional ICF methods appear to be impractical for commercial energy production. Likewise, although MTF needs magnetic confinement to stabilize and insulate the fuel while it is being compressed, the needed confinement time is thousands of times less than for MCF. Confinement times of the order needed for MTF were demonstrated in MCF experiments years ago.
This is the promise of the MTF approach. Making a pure MCF or ICF device needs extremely high-end engineering that is still being experimented on, with no guarantee that it will ever be practical. But the densities, temperatures and confinement times needed by MTF are well within the current state of the art and have been repeatedly demonstrated in a wide variety of experiments. LANL has referred to the concept as a "low cost path to fusion".
's FRX-L, a plasma is first created at low density by transformer coupling a large electrical current through a gas inside a quartz tube (generally a non-fuel gas for testing purposes). This heats the plasma to about 200 eV (~2.3 million degrees). An arrangement of external magnets keeps the fuel confined within the tube during this period. Plasmas are electrically conducting, allowing a current to be passed through them. This current, like any, will generate a magnetic field that interacts with the current. It is possible to arrange the plasma so that the fields and current will stabilize within the plasma once it is set up, self-confining the plasma. FRX-L uses the field-reversed configuration
for this purpose. Since the temperature and confinement time is much lower than in MCF, by about 100 times, the confinement is relatively easy to arrange and does not need the complex and expensive superconducting magnets used in most modern MCF experiments.
FRX-L is used solely for plasma creation, testing and diagnostics. It uses four high-voltage (up to 100 kV) capacitor banks storing up to 1 MJ of energy to drive a 1.5 MA current in one-turn magnetic-field coils that surround a 10 cm diameter quartz tube. In its current form as a plasma generator, FRX-L has demonstrated densities between 2 and 4 × 1016 cm−3, temperatures of 100 to 250 eV, magnetic fields of 2.5 T, and lifetimes of 10 to 15 µs. All of these are well within an order of magnitude
of what would be needed for an energy-positive machine.
FRX-L was later upgraded to add an "injector" system. This is situated around the quartz tube, and consists of a conical arrangement of magnetic coils. When powered, the coils generate a field that is strong at one end of the tube and weaker at the other, pushing the plasma out the larger end. To complete the system, the FRX-L injector was to be placed above the focus of the existing Shiva Star
"can crusher" at the Air Force Research Laboratory
's Directed Energy Lab at the Kirtland Air Force Base
in Albuquerque, NM.
At some point the plans were changed, and instead a new experiment, FRCHX, has been placed on Shiva Star. Similar to the FRX-L, it uses a generation area and injects the plasma bundle into the Shiva Star liner compression area. Shiva Star delivers about 1.5 MJ into the kinetic energy of the 1 mm thick aluminum liner, which collapses cylindrically at about 5 km/sec. This collapses the plasma bundle to a density around 5x1018 cm−3 and raises the temperature to about 5 keV, producing neutron yields on the order of 1012 neutrons "per shot" using a D-D fuel. The power released in the larger shots, in the MJ, needs a period of resetting the equipment on the order of a week. The huge electromagnetic pulse
(EMP) caused by the equipment forms a challenging environment for diagnostics.
MTF's challenges appear to be similar to those of ICF. To produce power effectively, the density must be increased to a working level and then held there long enough for most of the fuel mass to undergo fusion. This is occurring while the foil liner is being driven inwards. Any mixing of the metal with the fusion fuel will "quench" the reaction (similar problems occur in MCF systems when plasma touches the vessel wall). Similarly, the collapse must be fairly symmetrical to avoid "hot spots" that could destabilize the plasma while it burns.
Problems in commercial development are similar to those for any of the existing fusion reactor designs. The need to form high-strength magnetic fields at the focus of the machine is at odds with the need to extract the heat from the interior, making the physical arrangement of the reactor a challenge. Further, the fusion process emits large numbers of neutron
s (in common reactions at least) that lead to neutron embrittlement
that degrades the strength of the support structures and conductivity of metal wiring. These neutrons are normally intended to be captured in a lithium
shell to generate more tritium
to feed in as fuel, further complicating the overall arrangement.
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...
that combines features of the more widely studied magnetic confinement fusion
Magnetic confinement fusion
Magnetic confinement fusion is an approach to generating fusion power that uses magnetic fields to confine the hot fusion fuel in the form of a plasma. Magnetic confinement is one of two major branches of fusion energy research, the other being inertial confinement fusion. The magnetic approach is...
(MCF) and 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) approaches. Like the magnetic approach, the fusion fuel is confined at lower density by magnetic 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;...
s while it is heated 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...
. Like the inertial approach, fusion is initiated by rapidly squeezing the target to greatly increase fuel density, and thus temperature. Although the resulting density is far lower than in traditional ICF, it is thought that the combination of longer confinement times and better heat retention will let MTF yield the same efficiencies, yet be far easier to build. The term magneto-inertial fusion (MIF) is similar, but encompasses a wider variety of arrangements. The two terms are often applied interchangeably to experiments.
MTF is currently being studied mostly by the 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...
(LANL) and Air Force Research Laboratory
Air Force Research Laboratory
The Air Force Research Laboratory is a scientific research organization operated by the United States Air Force Materiel Command dedicated to leading the discovery, development, and integration of affordable aerospace warfighting technologies; planning and executing the Air Force science and...
(AFRL), and by Canadian startup company, General Fusion
General Fusion
General Fusion is a Canadian company based in Burnaby, British Columbia created for the development of nuclear fusion.They hope to demonstrate their magnetized target fusion technology by 2013...
.
Basic fusion
Fusion reactions combine lighter atoms, such as hydrogenHydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
, together to form larger ones. Generally the reactions take place at such high temperatures that the atoms have been 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...
ized, their 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 stripped off by the heat; thus, fusion is typically described in terms of "nuclei" instead of "atoms". Nuclei are positively charged, and repel each other due to the electrostatic force. Counteracting this is the strong force that pulls nucleons together, but only at very short ranges. Thus a fluid of nuclei will generally not undergo fusion on its own – the nuclei must be forced together before the strong force can pull them together into stable collections. The amount of energy that needs to be applied to force the nuclei together is termed the Coulomb barrier
Coulomb barrier
The Coulomb barrier, named after Coulomb's law, which is named after physicist Charles-Augustin de Coulomb , is the energy barrier due to electrostatic interaction that two nuclei need to overcome so they can get close enough to undergo a nuclear reaction...
or fusion barrier energy. To create needed conditions, the fuel must be heated to tens of millions of degrees, and/or compressed to immense pressures, for a long enough time. The temperature, pressure, and time needed for any given fuel to fuse is termed the Lawson criterion
Lawson criterion
In nuclear fusion research, the Lawson criterion, first derived on fusion reactors by John D. Lawson in 1955 and published in 1957, is an important general measure of a system that defines the conditions needed for a fusion reactor to reach ignition, that is, that the heating of the plasma by the...
. Since the criterion contains both pressure and temperature, existing approaches to practical fusion power have generally worked to raise one or another of these values.
Magnetic fusion works to heat a dilute plasma (1014 ions per cm3) to high temperatures, around 20 keV (~200 million C). Ambient air is about 100,000 times denser. To make a practical reactor at these temperatures, the fuel must be confined for long periods of time, on the order of 1 second. The ITER
ITER
ITER 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...
tokamak
Tokamak
A tokamak is a device using a magnetic field to confine a plasma in the shape of a torus . Achieving a stable plasma equilibrium requires magnetic field lines that move around the torus in a helical shape...
design is currently being built to test the magnetic approach with pulse lengths up to 20 minutes. Inertial fusion works to produce extremely high densities, 1025 ions per cubic cm, about 100 times the density of lead. This causes reactions to occur extremely quickly (~1 nanosecond), which causes confinement time to be extremely short, as the heat of reactions drives the plasma outward. The $3–4 billion dollar 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...
(NIF) machine at 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) will be a definitive test of ICF at megajoule energy levels. Both conventional methods of nuclear fusion are nearing net energy (Q>1) levels now after many decades of research, but remain far from a practical energy-producing device.
MTF approach
While MCF and ICF attack the Lawson criterion problem from different directions, MTF attempts to work between the two. Magnetic fusion confines a dilute plasma at about 1014 cm−3. Inertial fusion works around 1025 cm−3. MTF aims for 1019 cm−3. At this density, the fusion rate is relatively slow, so some confinement time is needed to allow fuel to undergo fusion. Here too, MTF works between the ~1 second times of magnetic methods, and the nanosecond times of inertial, aiming for times on the order of 1 µs. In MTF, magnetic fields are used to slow down plasma losses, and inertial compression is used to heat the plasma.In general terms, MTF is an inertial method. The density is increased through a pulsed operation that compresses the fuel, and since temperature is the average energy per unit density, as long as heat is not lost to the surroundings, the temperature of the fuel is raised by a similar amount. In traditional ICF, more energy is added through the lasers that compress the target, energy that leaks away through a variety of processes. No more energy is added in MTF. Instead, a magnetic field is created before compression that confines fuel, and insulates it so less energy is lost to the outside. The result, compared to ICF, is a somewhat-dense, somewhat-hot fuel mass that undergoes fusion at a medium reaction rate, so it only must be confined for a medium length of time.
At first glance it might seem that this approach would have no advantages over traditional ICF methods. All that has changed is a tradeoff between confinement time and density, but the end result is the same. The reason MTF appears to be so much more practical is that the lower density it needs can be formed through a variety of processes that are relatively efficient and inexpensive, whereas ICF demands specialized high-performance laser
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...
s of low efficiency. The cost and complexity of these lasers, termed "drivers", is so great that traditional ICF methods appear to be impractical for commercial energy production. Likewise, although MTF needs magnetic confinement to stabilize and insulate the fuel while it is being compressed, the needed confinement time is thousands of times less than for MCF. Confinement times of the order needed for MTF were demonstrated in MCF experiments years ago.
This is the promise of the MTF approach. Making a pure MCF or ICF device needs extremely high-end engineering that is still being experimented on, with no guarantee that it will ever be practical. But the densities, temperatures and confinement times needed by MTF are well within the current state of the art and have been repeatedly demonstrated in a wide variety of experiments. LANL has referred to the concept as a "low cost path to fusion".
Devices
In the pioneering experiment, 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...
's FRX-L, a plasma is first created at low density by transformer coupling a large electrical current through a gas inside a quartz tube (generally a non-fuel gas for testing purposes). This heats the plasma to about 200 eV (~2.3 million degrees). An arrangement of external magnets keeps the fuel confined within the tube during this period. Plasmas are electrically conducting, allowing a current to be passed through them. This current, like any, will generate a magnetic field that interacts with the current. It is possible to arrange the plasma so that the fields and current will stabilize within the plasma once it is set up, self-confining the plasma. FRX-L uses the field-reversed configuration
Field-Reversed Configuration
A Field-Reversed Configuration is a device developed for magnetic fusion energy research that confines a plasma on closed magnetic field lines without a central penetration....
for this purpose. Since the temperature and confinement time is much lower than in MCF, by about 100 times, the confinement is relatively easy to arrange and does not need the complex and expensive superconducting magnets used in most modern MCF experiments.
FRX-L is used solely for plasma creation, testing and diagnostics. It uses four high-voltage (up to 100 kV) capacitor banks storing up to 1 MJ of energy to drive a 1.5 MA current in one-turn magnetic-field coils that surround a 10 cm diameter quartz tube. In its current form as a plasma generator, FRX-L has demonstrated densities between 2 and 4 × 1016 cm−3, temperatures of 100 to 250 eV, magnetic fields of 2.5 T, and lifetimes of 10 to 15 µs. All of these are well within an order of magnitude
Order of magnitude
An order of magnitude is the class of scale or magnitude of any amount, where each class contains values of a fixed ratio to the class preceding it. In its most common usage, the amount being scaled is 10 and the scale is the exponent being applied to this amount...
of what would be needed for an energy-positive machine.
FRX-L was later upgraded to add an "injector" system. This is situated around the quartz tube, and consists of a conical arrangement of magnetic coils. When powered, the coils generate a field that is strong at one end of the tube and weaker at the other, pushing the plasma out the larger end. To complete the system, the FRX-L injector was to be placed above the focus of the existing Shiva Star
Shiva Star
Shiva Star, originally just SHIVA, is a high-powered pulsed-power research device located at the Air Force Research Laboratory on the Kirtland Air Force Base in Albuquerque, New Mexico...
"can crusher" at the Air Force Research Laboratory
Air Force Research Laboratory
The Air Force Research Laboratory is a scientific research organization operated by the United States Air Force Materiel Command dedicated to leading the discovery, development, and integration of affordable aerospace warfighting technologies; planning and executing the Air Force science and...
's Directed Energy Lab at the Kirtland Air Force Base
Kirtland Air Force Base
Kirtland Air Force Base is a United States Air Force base located in the southeast quadrant of the Albuquerque, New Mexico urban area, adjacent to the Albuquerque International Sunport. The base was named for the early Army aviator Col. Roy C. Kirtland...
in Albuquerque, NM.
At some point the plans were changed, and instead a new experiment, FRCHX, has been placed on Shiva Star. Similar to the FRX-L, it uses a generation area and injects the plasma bundle into the Shiva Star liner compression area. Shiva Star delivers about 1.5 MJ into the kinetic energy of the 1 mm thick aluminum liner, which collapses cylindrically at about 5 km/sec. This collapses the plasma bundle to a density around 5x1018 cm−3 and raises the temperature to about 5 keV, producing neutron yields on the order of 1012 neutrons "per shot" using a D-D fuel. The power released in the larger shots, in the MJ, needs a period of resetting the equipment on the order of a week. The huge electromagnetic pulse
Electromagnetic pulse
An electromagnetic pulse is a burst of electromagnetic radiation. The abrupt pulse of electromagnetic radiation usually results from certain types of high energy explosions, especially a nuclear explosion, or from a suddenly fluctuating magnetic field...
(EMP) caused by the equipment forms a challenging environment for diagnostics.
Challenges
MTF is not the first "new approach" to fusion power. When ICF was introduced in the 1960s, it was a radical new approach that was expected to produce practical fusion devices by the 1980s. Every approach to date has sooner or later found unexpected problems that greatly increased the difficulty of producing output power. With MCF, it was unexpected instabilities in plasmas as density or temperature was increased. With ICF, it was unexpected losses of energy and difficulties "smoothing" the beams. These have been addressed in modern machines, but only at great expense.MTF's challenges appear to be similar to those of ICF. To produce power effectively, the density must be increased to a working level and then held there long enough for most of the fuel mass to undergo fusion. This is occurring while the foil liner is being driven inwards. Any mixing of the metal with the fusion fuel will "quench" the reaction (similar problems occur in MCF systems when plasma touches the vessel wall). Similarly, the collapse must be fairly symmetrical to avoid "hot spots" that could destabilize the plasma while it burns.
Problems in commercial development are similar to those for any of the existing fusion reactor designs. The need to form high-strength magnetic fields at the focus of the machine is at odds with the need to extract the heat from the interior, making the physical arrangement of the reactor a challenge. Further, the fusion process emits large numbers of neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s (in common reactions at least) that lead to neutron embrittlement
Embrittlement
Embrittlement is a loss of ductility of a material, making it brittle. Various materials have different mechanisms of embrittlement.* Hydrogen embrittlement is the effect of hydrogen absorption on some metals and alloys....
that degrades the strength of the support structures and conductivity of metal wiring. These neutrons are normally intended to be captured in a lithium
Lithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
shell to generate more 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...
to feed in as fuel, further complicating the overall arrangement.
Further reading
- R.E. Siemon, I.R. Lindemuth, and K.F. Schoenberg, Why MTF is a low cost path to fusion, Comments Plasma Physics Controlled Fusion vol 18 issue 6, pp. 363–386 (1999).
- P.V. Subhash et al. 2008 Phys. Scr. 77 035501 (12pp) doi:10.1088/0031-8949/77/03/035501Effect of liner non-uniformity on plasma instabilities in an inverseZ-pinch magnetized target fusion system: liner-on-plasma simulations and comparison with linear stability analysis
External links
- Magnetized Target Fusion at Los Alamos (LANL)
- General Fusion