which achieves a minimum only as B approaches ~21 Tesla, an impractically high limit. Thus we shall take B=10 Tesla as a practical optimum value and find The mass of the magnet structural supports might be expected to add about 15% to the mass of the magnet and the cryogenic refrigeration system might add another 3% or so. The volume of the LiH shield is given by Direct conversion devices may be considered to be limited by two requirements: (a) that the surface flux not exceed 4 MW m-2 and, (b) for adequate radiative cooling, the aspect ratio of the grid wires is limited to 4 (ref. [10]). Since particles impinge on half the total surface area, the first requirement can be expressed as The mass of the radiator is determined by the surface temperature of the radiator. We assume the temperature to be 800 K (as in the SP-100 design) and estimate an areal mass density of 12 kg m~2. Then we find
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