The Belt Pinch - A high-beta tokamak with non-circular cross-section

In a tokamak, the attainable beta-values are restricted by the limitations of MHD-stability. The necessary improvement of the beta-value should be possible by going over to a noncircular plasma cross section. In this paper, theoretical conditions for such an improvement are discussed (e.g., volume currents peaked on axis, flattened ends of the elongated cross-section, and diamagnetic plasma). Experimental investigations of this problem have been carried out in the Belt Pinch, where plasma production and heating are achieved by fast magnetic compression. The main result of the Belt Pinch experiments is that MHD-stability requires about the same critical q-value needed in circular-cross-section tokamaks, at least for the present plasma parameters and time scale. Violation of the q-condition results in fast-growing modes which destroy the plasma equilibrium within several microseconds. Compared with such an unstable case, stability for a time scale of 50 microsec means stable behavior for about 20 MHD growth times. Numerical calculations indicate a further improvement in present and future experiments with stronger shock heating and increased temperatures (greater than 100 eV).