A small, octopole-stabilized tandem mirror reactor

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