that ohmic coils could be used instead of superconducting coils, see Chapter 3, (13). This is possible when the acceleration time is less than 250 ms. However, refrigeration of the ohmic bucket coil must be taken into account. Assuming a IKg coil, specific heat of 800 J/Kg °K, and cooling from room temperature to liquid nitrogen temperature 180 KJ of energy is required. If the refrigeration system were only 15% efficient, 1.2 MJ of input energy would be required for every bucket firing. This could be a significant energy requirement compared to that required by the accelerator. Given the less demanding design compared to superconductors this may be a worthwhile price to pay. Failure modes of buckets are now considered. With guidestrips being used for magnetic levitation to support the bucket from the guideway, loss of total bucket coil current would cause catastrophic failure of the mass driver. To overcome this problem the bucket coil could be subdivided into several discrete coils to prevent total loss of coil current if one of these subcoils should fail. With guide strips, loss of drive current would have little effect on guidance. The pull only mass driver design which does away with guide strips relies on active guidance. this is a result of off axis restoring forces generated by the product of dM/dy and bucket and drive coil currents. Theoretical investigation of this effect showed that this restoring force was about an order of magnitude smaller than with guidestrips. A new failure mode is introduced by this scheme since loss of drive current would result in loss of guidance. In a single sector failure total loss of guidance would not occur. But only with the failure of seveal sectors or the entire section would result in total loss of guidance. The removal of guide strips to eliminate a 20% loss in drive force due to shielding and to increase the coupling ratio from 0.8 to 0.9 is not worth the introduction of this single point failure mode.
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