Effects of shell configuration on the performance of reversed field pinch plasmas

The experimental results of three different reversed field pinch machines at the Electrotechnical Laboratory in Tsukuba are compared. In particular, the results are summarized in terms of the difference in the configuration of the conductive shell system. The three configurations correspond to TPE-IR.M15 (b/a = 1.18), TPE-IRM20 (b/a = 1.08) and TPE-1RM20mod (b/a = 1.12), where b/a is a measure of the shell proximity, and a and b are the minor radii of the plasma and the inner surface of the shell closest to the plasma, respectively. The magnetic fluctuations, global confinement properties and mode locking events induced by the external field error at the thick shell gap are studied. It is shown that the magnetic fluctuation amplitude increases with b/a, as expected from the linear growth rate of the dominant MHD modes, and that a multilayered, close fitting shell system improves the performance under extreme operating conditions in the high pinch parameter region and when external field errors are considered, while the performance under normal operating conditions is comparable within the shot to shot deviations of the data.