Control of the Locked Mode in a Reversed-Field Pinch Plasma Using a Rotating Toroidal Field

Abstract

The locked mode (LM), which is locked both in phase and to the wall, is successfully controlled using a rotating toroidal field (RTF) in TPE-RX (large reversed-field pinch machine with R/a = 1.72 m/0.45 m and plasma current Ip up to 480 kA) at a low Ip (∼200 kA) and high filling pressure (∼0.8 mTorr). It is demonstrated that the onset of the LM can be delayed or even eliminated by applying the RTF. Only the Fourier component with m=0/n=1 of the magnetic fluctuation exhibits rotation with the RTF, but other modes do not exhibit systematic movement synchronizing with the m=0/n=1 mode rotation. The phase relation between the m=0/n=1 mode and the other modes becomes random, which probably prevents the phases of the other modes from locking and results in the elimination of the LM. It is found, however, after the LM has taken place, it cannot be controlled by the RTF and hence an early start and fast rotation of the RTF are required for control. It is not possible to control the LM at a high Ip (\\gtrsim250 kA), since the LM takes place from the early stage of discharge. Compared with discharges without the LM, plasma performance is degraded by the LM. The performance can be recovered by eliminating the LM with the RTF. However, the RTF itself degrades the plasma performance when applied to LM-free discharges with a RTF value comparable to that necessary for controlling the LM.