Locked modes in two reversed-field pinch devices of different size and shell system

The behavior of locked modes in two reversed-field pinch devices, the Toroi dal Pinch Experiment (TPE-RX) [Y. Yagi , Plasma Phys. Control. Fusion 41, 2 552 (1999)] and Extrap T2 [J. R. Drake , in Plasma Physics and Controlled N uclear Fusion Research 1996, Montreal (International Atomic Energy Agency, Vienna, 1996), Vol. 2, p. 193] is analyzed and compared. The main character istics of the locked mode are qualitatively similar. The toroidal distribut ion of the mode locking shows that field errors play a role in both devices . The probability of phase locking is found to increase with increasing mag netic fluctuation levels in both machines. Furthermore, the probability of phase locking increases with plasma current in TPE-RX despite the fact that the magnetic fluctuation levels decrease. A comparison with computations u sing a theoretical model estimating the critical mode amplitude for locking [R. Fitzpatrick , Phys. Plasmas 6, 3878 (1999)] shows a good correlation w ith experimental results in TPE-RX. In Extrap T2, the magnetic fluctuations scale weakly with both plasma current and electron densities. This is also reflected in the weak scaling of the magnetic fluctuation levels with the Lundquist number (similar to S-0.06). In TPE-RX, the corresponding scaling is similar to S-0.18. (C) 2000 American Institute of Physics. [S1070-664X(0 0)03810-6].