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(c0) < 1.7 X 10(13) cm(-3), T-e0 < 1.3 keV). For this purpose, a cartful 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. (C) 2001 Elsevier Science B.V. All rights reserved.