Experimental check of neoclassical predictions for the radial electric field in a stellarator

Investigations of the radial electric field have been carried out at the Stellarator Wendelstein 7-AS using charge-exchange spectroscopy with high spatial resolution based on the high-energy Li beam diagnostic (Li-CXS). To evaluate the electric field, the radial force balance equation is used together with the measured profiles of poloidal velocity, density and the temperature of carbon impurities. Results of experiments are compared with predictions of the neoclassical theory. We also use a simple analytic approximation, in which the radial electric field is proportional to the pressure gradient of H+ ions of the bulk plasma. This approximation agrees reasonably well with both the measurements and the accurate neoclassical calculations and, therefore, can be used for a fast estimation of the radial electric field.