B21 Application of fusion for space propulsion

The starting point is the choice of fuel fusion cycles [Miley, 1987]. The kinetics of candidate fuels is in Table B.2 (see also [Cox et al., 1990]).

The D-T reaction has the largest reactivity and can be ignited at relatively low temperatures («20 keV). However, it has two main associated problems:

— 80% of the energy is produced as energetic (14MeV) neutrons. They require heavy shielding and result in intermediate production of heat (therefore, a radiator is needed);

— to avoid (for safety reasons) large tritium inventories, tritium must be produced in space through the conventional D-T fuel cycle.

The D-D reaction involves deuterium, a very common hydrogen isotope (there are

Table B.2. Fusion reactions.

Reaction Fusion fuel cycles Ignition temperature

33 mg of deuterium in each liter of water); it produces 33% of energy in the form of 2.45 MeV neutrons. Secondary reactions involving D and T produce 14MeV neutrons. Although the neutron problem is somewhat alleviated, the energies of reactants in the range of 100 keV must be achieved for ignition.

The D-3He reaction needs reactant energies in the same range as the D-D reaction, but has the advantage of producing a very limited number of neutrons (<15%) through D-D and secondary D-T reactions. Furthermore, charged reaction products can be used for direct electricity conversion. The main problem here is the lack of 3He on Earth. It is envisaged to produce 3He by lunar mining of Moon dust which has been deposited by the solar wind (estimated reserve in the range of 106 t; see, e.g., [Kulcinski et al., 2000]). Cost would be in the range of $400/g to $1,000/g. For a recent survey of the abundance of noble gases on the Moon, see [Ozima et al., 2005]. Put into perspective, 3He is considered in most studies the most promising fuel for space propulsion.

The p-6Li and p-nB reactions have very low neutron production («5% and «1%, respectively) and are conventionally defined "aneutronic" (although the only truly aneutronic reaction is the 3He-3He reaction). Their main problem is the very stringent requirements to achieve positive fusion gain. Indeed, in a system with equal electron and ion temperature the amount of fusion power never exceeds the power lost via Bremsstrahlung. Thus, even in the ideal case of no losses from heat conduction, the system cannot achieve positive fusion gain except far from thermal equilibrium (different electron and ion temperatures).

Finally, it should be mentioned that the possibility of fusion reactions catalyzed by matter-antimatter reaction has been considered for fusion propulsion systems based on inertial confinement (as mentioned in Chapter 8).

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