Magnetized target fusion
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 fields 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.
Fusion reactions combine lighter atoms, such as hydrogen, together to form larger ones. Generally the reactions take place at such high temperatures that the atoms have been ionized, their electrons 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 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.
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 lasers 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".
In the pioneering experiment, Los Alamos National Laboratory'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/s. 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 range of 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 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 neutrons (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.
- Magnetized Target Fusion Experiments at LANL
- J. H. Degnan, J. et al. (1999). "Compression of Plasma to Megabar Range using Imploding Liner". Physical Review Letters 82 (13): 2681. Bibcode:1999PhRvL..82.2681D. doi:10.1103/PhysRevLett.82.2681.
- FRX-L: A Plasma Injector for Magnetized Target Fusion
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- Applications of predictions for FRC translation
- FRCHX Magnetized Target Fusion HEDLP Experiments (IAEA 2008 Fusion Energy Conference)
- 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:10.1088/0031-8949/77/03/035501 Effect 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