Magnetized target fusion

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Magnetized target fusion (MTF) is a relatively new approach to producing fusion power that combines features of the more widely studied magnetic (MCF) and inertial (ICF) approaches. Like the magnetic approach, the fusion fuel is confined at low density by magnetic fields while it is heated into a plasma, but like the inertial approach, fusion is initiated by rapidly squeezing the target in order to dramatically increase the density of the fuel, and thus its temperature. Although the resulting density is much lower than traditional ICF approaches, it is believed that the combination of longer confinement times and better heat retention will allow the MTF approach to provide the same efficiencies, yet be much easier to build.

The MTF approach is currently being studied primarily by the Los Alamos National Laboratory (LANL) and Air Force Research Laboratory (AFRL), and by a Canadian startup company, General Fusion.

Contents

[edit] Basic fusion

Main article: Nuclear fusion

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 thus repel each other due to the electrostatic force. Counteracting this is the strong force which 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 known as the Coulomb barrier or fusion barrier energy.

Generally less energy will be needed to cause lighter nuclei to fuse, as they have less positive charge and thus a lower barrier energy. The best fuel from this standpoint is a one to one mix of deuterium and tritium; both are heavy isotopes of hydrogen. The D-T (deuterium-tritium) mix has a low barrier because of its high ratio of neutrons to protons. The presence of neutral neutrons in the nuclei helps pull them together via the strong force. Tritium has one of the highest ratios of neutrons to protons of any stable or moderately unstable nuclide – two neutrons and one proton. Adding protons or removing neutrons increases the energy barrier.

In order to create the required conditions, the fuel must be heated to tens of millions of degrees, and/or compressed to immense pressures. The temperature and pressure required for any particular fuel to fuse is known as the Lawson criterion. Since the criterion contains both pressure and temperature, existing approaches to practical fusion power have generally focused on increasing one or the other of these values.

The magnetic approach attempts to heat a dilute plasma (1014 ions per cm-3) to high temperatures, around 20 keV (~200 million C). In order 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. The inertial approach instead attempts to produce extremely high densities, 1025 ions per cubic cm (about 100 times the density of lead), in which case the reactions occur extremely quickly (~1 nanosecond). In this case the "confinement" time is extremely short, as the heat of the reactions drives the plasma outward. The $3-4 billion dollar National Ignition Facility (NIF) machine at LLNL will be a definitive test of ICF at megajoule energy levels. Both conventional approaches to nuclear fusion are approaching net energy (Q>1) levels now after many decades of research, but remain far from a practical energy-producing device.

[edit] The MTF approach

While the MCF and ICF approaches attack the Lawson criterion problem from different ends, MTF attempts to sit between the two. Whereas the magnetic approach confines a dilute plasma at about 1014 cm-3 and the inertial approach is around 1025 cm-3, the MTF approach aims for 1019 cm-3.[1] At this density the fusion rate will be relatively slow, so some confinement time is required in order to allow the fuel to undergo fusion. Here too, MFT sits between the ~1 second times of the magnetic approaches, 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.[1]

In general terms, MTF is an inertial approach. 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 increased by a similar amount. In the traditional ICF approach additional energy is added through the lasers that are compressing the target, energy that leaks away through a variety of processes. No additional energy is added in the MTF case, instead a magnetic field is created prior to compression that confines the fuel and "insulates" it so that less energy is lost to the environment. The result, in comparison to ICF, is a somewhat-dense, somewhat-hot fuel mass that undergoes fusion as a medium reaction rate, so it only has to be confined for a medium length of time.

At first glance it might not seem that this approach would have any advantages over the traditional ICF approach; 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 requires can be created though a variety of processes that are relatively efficient and inexpensive, whereas the ICF approach demands specialized high-performance lasers of low efficiency. The cost and complexity of these lasers, known as "drivers", is so great that the traditional ICF approach appears to be impractical for commercial energy production. Likewise, although MTF requires magnetic confinement to stabilize and insulate the fuel while it is being compressed, the required confinement time is thousands of times less than the MCF approach. 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 requires 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 required by the MTF approach are well within the current state of the art and have been repeatedly demonstrated in a wide variety of experiments.[2] LANL has referred to the concept as a "low cost path to fusion".

[edit] MTF devices

In the pioneering experiment, LANL's FRX-L, a plasma is first created at low density by passing an electrical arc through a quartz tube containing a gas (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 the MCF approach, by about 100 times, the confinement is relatively easy to arrange and does not require the complex and expensive superconducting magnets used in most modern MCF experiments.

FRX-L is used solely for plasma creation, testing and diagnostics.[1] 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.[3] 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.[3]

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 5km/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.[4] The power released in the larger shots, in the MJ, requires a period of reseting the equipment on the order of a week. The huge EMP caused by the equipment provides a challenging environment for diagnostics.

General Fusion's approach is similar in concept, and differs primarily in the construction of the compression system. Their reactor design consists of a ~3 m sphere covered with a series of steam-powered mechanical rams, which are triggered in concert to produce a shock wave in the interior. The interior is filled with a liquid lead-lithium mixture. Prior to firing the liquid is spun, forcing it to the outside of the sphere and causing a vertically-oriented cavity to form in the center of the chamber. Two "smoke rings", essentially identical to those in the FRX-L system, are injected into the cavity, one from either end. As the reach the center the rams are fired, collapsing the cavity and compressing the plasma.[5]

Ultimately, the design would fire about once per second, extracting the heat from the lead-lithium mixture using conventional heat exchangers. Tritium would be extracted from the lithium to feed back into the reactor plasma. The reactors would generate about 100 MW each, cost about $50 million to build, and generate electricity for about 4 cents/kWh, competitive with coal. The company is supported purely by venture capital,[6] and is working to have a proof-of-concept system running early in the next decade. Wal van Lierop, CEO of Chrysalix Energy Venture Capital, the primary investor, stated that "Within five years, large companies will start to think about building fusion reactors."[6]

[edit] Challenges

MTF is not the first "new approach" to fusion power — when it was introduced in the 1960s ICF 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 a problem that has dramatically increased the difficulty of producing output power. In the case of MCF it was unexpected instabilities in the plasma as density or temperature was increased, in the case of ICF there were unexpected losses of energy and difficulties "smoothing" the beams. These have been addressed in modern machines, but only at enormous expense.

MTF's challenges appear to be similar to those of ICF. In order to produce power effectively the density has to be increased to a working level and then held there long enough for the majority 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 symmertical to avoid "hot spots" that could destabilize the plasma while it burns.

Problems in commercial development are the similar to those for any of the existing fusion reactor designs. The need to provide 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. Additionally, the fusion process gives off 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 in order to generate more tritium to feed in as fuel, further complicating the overall arrangement.

[edit] References

  1. ^ a b c Magnetized Target Fusion Experiments at LANL
  2. ^ J. H. Degnan; et al. (1999). "Compression of Plasma to Megabar Range using Imploding Liner". PHYSICAL REVIEW LETTERS 82 (13): 2681. 
  3. ^ a b FRX-L: A Plasma Injector for Magnetized Target Fusion
  4. ^ FRCHX Magnetized Target Fusion HEDLP Experiments
  5. ^ General Fusion's Approach
  6. ^ a b Venture capitalist says nuclear fusion is coming

[edit] 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

[edit] External links

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