Density control and plasma edge characterisation of ECRH heated plasmas in the TJ-II stellarator

In the 1999 experimental campaign, the Spanish stellarator, TJ-II (R = 1.5 m, a < 0.22 m, B 0 < 1 T) has been operated under a broad range of parameters, including changes in the magnetic configuration, working gas (H 2 vs He), microwave heating power (100-600 kW, two independent lines at 53.2 GHz, second harmonic X-mode) and plasma-wall interaction conditions (wall conditioning, poloidal vs toroidal limiter). Although a close coupling between the plasmas and the TJ-II vacuum vessel is naturally present in most conditions, a good control of central plasma values has been achieved for both atomic species even under conditions close to the highest power density (n e0 < 1.7 x 10 13 cm 3 , T e0 < 1.3 keV). For this purpose, a careful control of wall conditions has been required. In addition, the low electron density and temperature of TJ-II edge plasmas have led to a significant reduction of the expected plasma-wall interaction. In this work, the issues of plasma density control and edge characteristics for the different plasma species and heating power are addressed. Results of new edge diagnostics, as a thermal lithium beam and a supersonic helium beam, among others are presented.