Experimental Study of Potential Profile Formation in Large Helical Device

The radial profile of potential, , in the core region of the Large Helical Device (LHD) was measured with a Heavy Ion Beam Probe (HIBP) in wide parameter area, in which radial electric field, Er, could not be measured in previous experiments. The radial profiles of Er were estimated from the differentiation of the experimental data of HIBP. These Er profiles were compared with the calculation result from neoclassical theory, and the theoretical prediction almost coincided with the experimental data of Er in the core region of LHD plasmas. 1. Instroduction In toroidal magnetized plasmas, the radial electric field Er is a very important parameter in transport phenomena. In the low collisional regime of helical devices, ripple induced diffusion loss is predicted from the neoclassical theory. This ripple-induced diffusion loss is non-ambipolar, so the radial electric field is formed. Diffusion loss in low collisional regime can be suppressed by the radial electric field, Er, which is determined by the ambipolar condition. In previous experiments in LHD, charge exchange recombination spectroscopy (CXRS) was used to study Er formation. However, most of data were obtained in the outer region (>0.6) of plasmas, in which helical ripple is large, because of insufficient emission from the impurities in the core region. In order to clarify physics of Er formation in the core region, where helical ripple is relatively small, the radial profile of plasma potential was measured with Heavy Ion Beam Probe (HIBP) [1-7]. In addition, we investigated 0 0.1 0.2 0.3 0.4 0.5 0 0.2 0.4 0.6 0.8 1 Rax=3.60m Rax=3.75m Rax=3.90m