__ Plasma Current Ream

Figure B.19. Colliding beam fusion reactor space propulsion system from [Cheung et al., 2004].

Magnetic Kick!

mentioned above. Neutral beams are injected to produce a current that sustains the configuration. Electrons are confined by the radial electric field determined by the radial force balance of the fluid. Fusion products escape confinement and, to maintain charge neutrality, extract electrons with sufficiently high energy to climb the electrostatic potential well. This results in the cooling of electrons and a reduction of Bremsstrahlung. The beams tend to thermalize, and this effect must be compensated for by continuous injection which requires a non-negligible amount of re-circulating power (around 50% for the p-11B case).

The CBFR for space propulsion has a chamber 6.9 m long and a 0.6 m radius. The external magnetic field is about 0.5 T. The CBFR generates about 77 MW of fusion power (Pspec « 20 MW/m3) and needs 50 MW of injected power for steady-state operation. A direct energy converter intercepts approximately half of the alpha particles, decelerates them by an inverse cyclotron process, and converts their energy into electricity. The remaining alpha particles are used for direct propulsion. The direct energy converter produces about 38.5 MW of electricity. The remaining 11.5 MW are produced from Bremsstrahlung by a thermoelectric converter (4.6 MWe out of 23 MW). The part that is not converted is passed to a Brayton cycle heat engine that supplies the remaining 7MW. Waste heat (11 MW) is rejected to space.

Mass distribution is shown in Table B.1. The resulting specific power is about 3 kW/kg.

A propulsion system based on the magnetized target fusion approach has been proposed [Thio et al., 1999]. A pair of conical 0-pinches produced a compact torus (either an FRC or a spheromak), which is imploded by a spherically converging plasma liner driven by a number of plasma jets. The liner is compressed to very high density, creating an inner fusion fuel layer producing the main fusion yield, and an external layer, made of hydrogen, that slows down the neutrons and absorbs and converts 95% of their energy to charged particle energy. The spherically expanding plasma produced in this way is tranformed into an axial flow by a pulsed magnetic field. High conversion efficiencies to direct thrust are foreseen.

On paper at least, this system is very compact. Higher radiator efficiencies (up to about 50 kW/kg) have been assumed in this study, leading to a drastic reduction in radiator mass. The reactor weight is estimated at "only" 411 for 25 MW power production: therefore the resulting specific power is astonishingly high (400 kW/ kg, dropping to about 100kW/kg if more conventional figures for radiator mass are employed). The key to such a result is the assumed high fusion power density typical of the MTF approach, and the percentage of conversion of neutron power to charged particle power in the liner, which reduces the amount of power to be radiated away. Clearly, such a proposal is still at the conceptual stage and its feasibility can only be assessed after evaluating future experimental results from other magnetized target fusion facilities, such as FRX-L.

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