Small, octopole-stabilized tandem mirror reactor

It is shown that the use of octopole stabilization in a tandem mirror allows a large reduction in end-cell length. A novel feature of the method proposed is the placement of the minimum IBI region considerably off axis, thus rendering the core plasma more axisymmetric. The region from the core to the field minimum is bridged by a mirror-confined, hot-electron mantle. Low beta, ideal magnetohydrodynamic stability, as evaluated with the interchange criterion, yields an upper limit to the required mantle peak beta of 78% for a center-cell peak beta of 75% and barrier peak beta of 30%. Estimates for the worst type of classical radial diffusion - with stochastic displacements per bounce - show that such radial losses are negligible for this configuration. First estimates of power balance indicate Q approx. 10 for a reactor producing 500 MW of fusion power and Q approx. 25 for one producing 1000 MW, using conservative assumptions regarding mantle beta requirements and synchrotron radiation losses.