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, B0<1 T) has been operated under a broad range of parameters, including changes in the magnetic configuration, working gas (H2 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 (ne0<1.7×1013 cm−3, Te0<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.