Progress in low-aspect-ratio RFP studies in RELAX

Equilibrium analyses have shown that the aspect ratio A is one of the important parameters for optimization of the RFP configuration because it affects the q profile or fraction of the pressure-driven bootstrap current. RELAX (A=R/a=0.5m/0.25m) is a low-A RFP machine to explore the potential advantages of low-A configurations. The discharge and plasma parameters in RELAX to date are as follows: plasma current up to 100kA, discharge duration of up to 2.5ms, electron density in the range from 0.2 to 2.0$\times$ 10$^{19}$ m$^{-3}$, and electron temperature $<$100eV from double-filtered soft-X ray (SXR) measurements. Characteristic plasma behaviors in very shallow reversal and extremely deep reversal regions have been reported. A non-linear 3-D MHD simulation for A=2 RFP configurations has revealed that the quasi-single helicity (QSH) state with experimentally observed m/n=1/4 structure can be realized in shallow reversal cases. New diagnostics are being installed in RELAX: a soft-X ray (SXR) imaging system for the study of 3-D structural studies on MHD instabilities, Thomson scattering system, and 140GHz interferometer for higher density regimes. The vacuum vessel has been covered with 64 saddle coils for RWM stabilization but with limited number of power supplies at present.