Progress in low-aspect-ratio RFP research in RELAX

RELAX (A=R/a=0.5m/0.25m) is a low-A RFP machine to study optimization of the RFP configuration, with exploration of the potential advantages of the low-A RFP. The discharge and plasma parameters in RELAX to date are as follows: plasma current of up to 125kA, discharge duration of up to 2-2.5ms, electron density in the range from 0.2 to 2.0 $\times$ 10$^{19 }$m$^{-3}$, and central electron temperature of 100-150eV (at Ip $\sim $ 100kA) from Thomson scattering. Recent modification of the insulated gap flanges has improved axisymmetry of the average toroidal field. The QSH duration has also been improved as a result of the improved axisymmetry of the toroidal field. The vacuum vessel has been covered with 64 saddle coils (4 polidally $\times$ 16 toroidally) for the RWM stabilization, and initial experiments have been started to stabilize a specific mode by using limited number of the power supplies. The experimental results will be compared with a non-linear 3-D MHD simulation result which has revealed that the QSH state with experimentally observed m/n=1/4 structure can be realized for A=2 configuration. Some scenarios for high current (Ip$>$100kA) discharges with sufficient controllability of the density will also be discussed.