FESAC Decadal Study: Clean, multi-purpose fusion power from small field-reversed configuration (FRC) reactors

Summary

This paper urges greatly expanded research into scientific and technical areas of aneutronic field-reversed-configuration devices (FRCs), specifically relatively small FRCs whose fusion power is below 50 MW and above 0.5 MW. These properties greatly increase safety, allow rapid exploration of incremental improvements, enable implementation of improved technologies, ameliorate materials issues, reduce financial risk, ease siting and maintenance requirements, simplify incorporation into the electrical grid, reduce need for long-distance transmission lines, expand the potential markets for these reactors, and provide the US and US industry with leadership positions in clean energy. This route to fusion power, first into niche markets not accessible to the mainline (D-T tokamak) approach to fusion, would demonstrate the reliability and safety of fusion and its competitive advantages against established technologies in the electricity-generation market.

I. Fusion research

The majority of funding for research into fusion power comes from national governments and is directed towards multi-GW power plants of the D-T fueled tokamak design. Because of the complexity and radioactivity of these large devices and the scarcity of their fuel, it is estimated that, under a best-case scenario, it will take over a century and an expenditure exceeding 1T$ to produce 1% of the US's current electrical power with D-T tokamak reactors. Similar materials and fuel-supply problems would occur with stellarators. Two private tokamak-centric companies claim they will provide electrical power to the grid within a decade but have not stated how the above-named technical problems will be solved or whether their approach will yield meaningful amounts of reliable power.

Other privately funded companies are examining alternate fusion reactor designs that could accelerate the production of fusion power. Most of these private fusion-reactor research efforts advocate an FRC-device design, enabling reactors to be smaller and cleaner. Smaller size lowers R&D costs and risks, facilitates evolutionary improvements, and makes possible robust distributed power grids and mobile sources. Cleanliness, i.e., reduced radioactivity primarily due to the choice of fuel, improves safety, lessens siting restrictions, and greatly eases materials and other technical problems. Tokamaks are not able to burn aneutronic fuels primarily because of their low β and thin SOL.

From the scientific standpoint, FRC physics differs greatly from that of the mainstream reactor concepts: a) The electrical current in FRCs is perpendicular to the magnetic field while that in tokamaks is primarily parallel. b) FRCs have true magnetic nulls in the plasma, making fluid and gyro-fluid models inapplicable and necessitating the development of far-from-equilibrium kinetic treatments. c) FRCs are inhomogeneous, having a large machine-sized gradient in the magnetic field strength. d) The FRC's rotational transform, q, is zero, predicted to markedly improve confinement. Studies of plasmas with these properties would reveal new phenomena and analyze regions of phase space never explored.

Due primarily to three properties of the FRC – high β, linear geometry, and aneutronic fuels – the technological problems faced by FRCs are far less daunting than those faced by tokamaks.

II. Advances in understanding FRCs

Progress in theoretical and experimental FRC research has improved our understanding of their behavior in critical areas. A 1978 prediction that FRC plasmas would be unstable to the tilt mode was shown to be not valid, particularly for small FRCs of size only a few times larger than the gyro-radii of charged-particle orbits they contain. Notably, experiments on the Princeton FRC device (PFRC) have sustained plasmas for 10⁵ longer than that instability theory predicted. A second critical achievement was the demonstration of classical energy confinement in FRCs designed, built, and operated by TAE Industries. Such excellent energy confinement allows small reactors. Thirdly, TAE studies also documented a new method to eliminate low-frequency instabilities, a problem encountered in nearly every linear plasma device. A fourth example, illustrated by the FRC design espoused by Helion, Inc., describes highly efficient direct conversion to electricity of the energy released by fusion. High efficiency, via direct energy conversion, is only possible with aneutronic fuels.

III. Research topics

There are numerous FRC physics topics (specific to reactors) that demand detailed attention. Some are: 1) efficiency of current drive. For example, will a transport-driven current occur in FRCs, as the bootstrap does in tokamaks; 2) development of fully kinetic modelling techniques, e.g., PIC codes; 3) diagnostics development specialized to the specific properties of FRC, e.g., the presence of nulls in the magnetic field, non-thermal particle distribution functions, the transport of synchrotron radiation through plasmas with strong gradients in magnetic fields; 4) particle flow patterns; 6) practical plasma stabilization methods; 6) the plasma edge, that is, the open field-line region which could be used to extract energy and fusion ash and provide thrust needed for some applications; and 7) plasma heating methods.

Technical topics unique to FRC reactors also demand research. These include: 1) Shielding. In a D-T tokamak, the neutrons deposit 80% of the fusion energy deep into the blanket. Though the neutron generation rate is far lower in aneutronic-fueled reactors, FRC components still require shielding. The material with the best neutron-absorbing properties is 10B. Whether the shielding would be pure B or a compound such as BN or BC4, and whether the shielding would be solid form, or as powder, pellets or liquids, must be analyzed to find the optimum method to shield the components for the lifetime of the reactor. 2) Energy extraction. As noted, aneutronic fuels should allow direct energy conversion to electricity, without going through a thermal cycle. Several direct conversion methods have been proposed; all need testing. Additionally, a fraction of the energy released by fusion will still need to be converted to electricity by thermal processes. Research is required to test methods to increase the efficiency of this conversion from 30% (Carnot cycle) to 60% (Brayton cycle). 3) 3He sources. 3He is rare on the earth. Much is present on the moon; far more is on the gas giant planets, Jupiter and Saturn. Methods to mine these extraterrestrial sources and transport 3He back to earth must be explored. Additionally, an alternate fuel cycle, named 3He-catalyzed D-D fusion, may allow net power generation via 3He and T produced by D-D fusion, though with somewhat more neutron production than the pure D-3He fusion. 4) Mobility. The design of these reactors for a variety of applications that are only possible if the reactor is mobile must be explored. 5) For the sake of brevity we simply state here a few other technological topics that must be studied and could provide, in the short term, products for the commercial non-fusion marketplace: plasma fueling methods, AI reactor controls, and RF systems.

An FRC-reactor development plan would incorporate into FRC reactor designs advances in the above-named research areas, enabling electrical power production at the 0.5-50 MW level from a device near 1-m in diameter and 5 m in length.

IV. Markets

Current terrestrial 3He sources could supply, for decades, fuel for D-3He PFRC reactors that would produce a total power of 100 MW. This is sufficient for reactor development and for their use in a few niche applications. If FRCs operated as conventional power plants, the annual revenue would exceed 500M$ though provide little relief to global warming. Vying with established power-production technologies – such as solar, wind, fission, and fossil – in the electrical-power market is an ill-advised short-term goal; it is the wrong way to proceed. An FRC-specific plan is first to make reactors for customers for whom the cost of electricity is immaterial and for whom no other option exists. Note the similarity of this approach with the route followed by Admiral Rickover who, in the 1950s, focused on nuclear power for submarines, not for civilian power plants. Rickover aimed at developing, improving, and validating nuclear power for a critical use and under intense scrutiny. Admiral Rickover's success promoted the subsequent fast migration of fission power into the civilian electrical-power industry.

A specialized application of small clean PFRC-type reactors is to provide power and propulsion for spacecraft. Fusion-powered rocket engines could shorten trips throughout the solar system and beyond, reduce mission costs by factors exceeding 10, allow missions that would not be possible by conventional chemical rockets, and supply power for station-keeping and communications. This would enable solar system exploration from Mars to Pluto, asteroid interventions, and observations of exoplanets from the 550-AU point. By shortening the duration of the transit and reducing radiation exposure, a single 1-MW fusion rocket engine could save NASA 100M$ in costs for a Mars mission and 300M$ for an orbital mission to Pluto.

Success in space-propulsion endeavors would promote FRC efforts for the terrestrial market with an aim to reduce C emissions. One way is to burn the abundant though lower-energy-density p-11B or D-D fuels. A variation to the D-D method could employ a two-power-plant model in which one power plant breeds 3He and T via 3He-catalyzed D-D fusion and the other burns the 3He produced by T decay. Yet another way to continue with the higher energy-density D-3He fuel is to establish a moon base and mine 3He there. With the growing commercial successes of SpaceX, Blue Origin, Orbital Sciences, Northrup Grumman, Boeing, et al., can a moon base be far away?

The military could use PFRC-type plants for forward deployment or as a means to transition military operations into the fully electric mode. Most US military deaths in Iraq were during the transport of fuel to military bases.

Two non-military critical uses of PFRC-type fusion reactors are to provide clean power at remote locations and at sites of natural disasters, applications that would strongly benefit from the far lesser need for refueling. Note that an array of small clean power plants fits into a distributed power grid which is resilient to terrorist attacks, not susceptible to rolling blackouts, and needs no extensive transmission-line infrastructure.

V. Public/private partnerships

Private public partnerships in FRC research have already been established. Two excellent examples are TAE Industries and Helion. Smaller collaborations are numerous, e.g., Princeton University has licensed to Princeton Satellite Systems several patents in the areas of small, clean, FRC fusion power plants and fusion-powered rocket engines. Strengthening those relationships should be a large part of DOE's program.