Other mirror concepts have been proposed over the years, such as multiple mirrors (a configuration with many identical mirror cell linked together) and the rotating mirror (where plasma rotates around the symmetry axis subject to a radial electric field that induces an E x B drift in poloidal direction). These concepts, still in a preliminary stage of development but based on physics are to a large extent similar to the other mirror concepts, but will not be further considered here.
At present, experiments with multiple mirrors are still carried on GOL-3 at Novosibirsk [Kruglyakov et al., 2002].
516 Appendix B: Assessment of open magnetic fusion for space propulsion B.3.2.6 Mirror studies for space propulsion
Several studies have involved the tandem mirror as a space propulsion system. Here we consider the study in [Kulcinski et al., 1987] for a space orbiting advanced fusion reactor (SOAR; see Figure B.13). Although the system was proposed for energy production only, its features are similar to those of a propulsion system. The electric power is 1 GW and is produced by the D-3He reaction through direct conversion (80% efficiency has been assumed). The fusion power is 1.9 GW with 70 MW in the form of neutrons; about 470 MW are lost through radiation. The shield is designed to absorb all rejected heat, without using a radiator. An optimized LiH blanket is employed as a shield, with a total mass of about 300 t. The length of the central cell is 73 m with a radius of 0.55 m. The total estimated mass of the system is 5001, with an equivalent specific power of 2 kW/kg. The magnet system is axisymmetric and uses (NbTi) superconducting coils for the central cell creating a 7.7 T field, (Nb3Sn) 18 T choke coils, and 12 T end coils. The auxiliary heating power is 70 MW produced with 75% efficiency.
The possibility of using a gasdynamic mirror for space propulsion has been considered in [Kammash et al., 1995b]. The reactor has a central cell 50 m long with a radius of 7 cm and a magnetic field of 15 T. A high-density (k5 x 1022 m-3), low-temperature (T k 6.5 keV) D-T plasma is sustained by the injection of 40 GW of neutral beams with an injection energy of 20 keV. The energy confinement time is about 3 ms. The fusion gain is only Q = 1. The high plasma density produces a very high neutron wall load (K622MW/m2), well above the values considered for terrestrial fusion power plants (<5MW/m2). Taking the already mentioned fluence limit of 15 MWyr/m2, such a neutron wall load would limit the duration of full-power operation to about 9 days. No specific layout is provided. Thrust power is 55 GW. The rest of the power (k36 GW) must be radiated in space and this is the reason for the large radiator mass (k7,200 t), which is the dominant component. For a radiator capable of radiating 5 kW per kilogram of mass, the resulting specific power is about 7 kW/kg. The possibility of using D-3He was also considered in the same study. With the same dimensions and the same gain factor, the magnetic field must be increased
up to 18.5 T, injection energy up to 200 keV, and fusion power to 147 GW. The increase in volume also increases the radiator mass (>300,0001!) but specific power is also increased («80kW/kg). How electricity is produced for the neutral beam power supply is not discussed. All these figures and especially those associated with the energy budget are somewhat inconsistent and should be taken with a pinch of salt.
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