Optimization for low-aspect-ratio RFP configuration 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 10$^{19}$ m$^{-3}$ from interferometer, and electron temperature $\sim$ 100eV from double-filtered soft- X ray (SXR) measurements. We will describe two extreme discharge regions, i.e., extremely deep reversal and very shallow reversal regions, attained in RELAX. Extremely high $\Theta$ (up to $\sim$ 3.5), deep reversal (F down to $\sim$ - 1.5 ) regions have been attained in RELAX without discrete relaxation events. Edge magnetic fluctuation level is lower for the discharges in the extremely deep reversal region. On the other hand, in very shallow-reversal region where F $\sim$ 0, helically deformed hot core has been identified using SXR imaging diagnostics. Behavior of the MHD instabilities in these two interesting regimes will be discussed.