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December 14, 2010

Quasi-steady Fusion Reactor based on the Pulsed High Density FRC

The threshold size of a steady state fusion reactor required to achieve ignition, and offer a safe protective shielding will be quite large. Unlike fission, where the first commercial reactor was 50 MW, a DEMO reactor must start operation at multi-gigawatt powers. A Quasi-Steady Fusion reactor (QSFR) based on the FRC provides for a method to significantly reduce the reactor size and thus the associated risk and development time of fusion as an energy source. Quasi-steady here refers to the repetitive regeneration of the fusion plasma with a suitably chosen duty cycle. For a reciprocating burn with a low duty cycle, wall loading can be varied for optimum power loading, and wall erosion can be minimized while maximizing the reactor power density in the blanket. Most importantly, the need for sustainment and auxiliary heating systems, including current drive, are eliminated, which tremendously simplifies reactor operation. With transient burn, the vacuum boundary is much easier to maintain as recycling, fueling and wall gas issues are much easier to handle. Tritium co-deposition, the dominant retention mechanism, is minimized as the tritium residence time in the reactor is only momentary.


The key to a QSFR is the ability to efficiently heat the FRC plasmoid to fusion conditions. The heating mechanism relies on the acceleration and self compression of the FRC. The technologies for the acceleration have been developed and tested in both the IPA and PHDX experiments. The accelerator interaction with the plasmoid is principally through the Lorentz body force acting on the induced currents in the FRC as it passes through each accelerator coil. During acceleration and compression, the only important losses are ohmic losses in the coils and external circuit elements. There is good news here due to the revolution that is occurring in the field of pulse power electronics. With the introduction of high power solid state devices one no longer needs to resort to old spark gap and ignitron technologies for energy transfer. The technology now exists to operate the accelerator at the voltages and currents desired in a repetitive mode at rate up to 60 Hz with devices capable of operating for decades at high efficiency [6]. With recuperative techniques it is believed that the accelerator electrical to plasma kinetic energy efficiency can be 80 to 90%.

The superconducting magnet technology appears to be at hand with the demonstration of a large bore cylindrical, superconducting coil at high field (1.4 m diam., 13 T). For the FRC based QSFR, with the smallest reactor+blanket cross section and simple linear geometry, extending these results to achieve the target field (15 T) and bore (~ 2 m) should not be difficult. An even higher field would be desirable as the fusion gain Q ~ B2.15 with the PHD FRC scaling. In any case, it should be possible to perform all experiments leading up to the final QSFR with conventional resistive coils so that the SC magnet technology is only required at the end.
Since the fusion power density scales as 2B4, an FRC based QSFR will operate at the highest optimal level. The anticipated FRC plasmoid energy is 1-2 MJ in the burn chamber. It is expected that the FRC will drift axially along ~ 5 m of reactor blanket during the millisecond burn time for Q ~ 1. Maintaining an average power of 20 MW (10-20 Hz) the average wall neutron loading would be ~ 3-6 MW/m2, depending on the reactor inner wall radius. The loading can be reduced (increased) by more (less) drift or lower (higher) duty cycle. The same would be true at higher Q. It can be seen that the FRC QSFR offers the possibility of entirely different reactor power regime and range of application for fusion power.

This level of power is particularly well suited for applications such as fissile fuel breeding, as well as space-based applications.

Quasi-steady Fusion Reactor to enable a thorium fuel cycle

By co-locating a molten salt reactor with FRC QSFRs, a waste mitigating closed nuclear cycle is achieved that is highly proliferation resistant. Only non-fissile material enters the plant in the form of thorium. All fuel for the reactor is produced on-site by the FRC QSFR. Only a relatively small fusion power source is required (~ 7% of the fission reactor output) as it is leveraged by the much larger energy yield from the fissile fuel enriched thorium reactor. The fissile fuel doubling time can be as short as 5 years, and essentially all the thorium can be consumed in fission reactions, thus extending the energy reserves from thorium to several thousand years, limited only by the lithium reserves required for DT fusion. Waste from the thorium cycle is orders of magnitude smaller than that of a current PWR, and decays to background levels in less than 500 years – only slightly longer than that from fusion neutron activation. By using the FRC QSFR to enable a thorium based energy cycle, nuclear power can finally deliver what the current uranium based fission can not: abundant, safe, and clean energy. Most importantly, it can be done in a timeframe to allow fusion to play a role in the effort to move from a carbon based energy economy.



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