Effect of a resistive vacuum vessel on dynamo mode rotation in reversed field pinches

Locked (i.e., nonrotating) dynamo modes give rise to a serious edge loading problem during the operation of high current reversed field pinches. Rotat ing dynamo modes generally have a far more benign effect. A simple analytic model is developed in order to investigate the slowing down effect of elec tromagnetic torques due to eddy currents excited in the vacuum vessel on th e rotation of dynamo modes in both the Madison Symmetric Torus (MST) [Fusio n Technol. 19, 131 (1991)] and the Reversed Field Experiment (RFX) [Fusion Eng. Des. 25, 335 (1995)]. This model strongly suggests that vacuum vessel eddy currents are the primary cause of the observed lack of mode rotation i n RFX. The eddy currents in MST are found to be too weak to cause a similar problem. The crucial difference between RFX and MST is the presence of a t hin, highly resistive vacuum vessel in the former device. The MST vacuum ve ssel is thick and highly conducting. Various locked mode alleviation method s are discussed. (C) 1999 American Institute of Physics. [S1070-664X(99)016 10-9].