Properties, control and ICR-heating of the plasma in TEXTOR

The accessible parameter range, the impurity contents and the susceptibility to ICRH power input depend on conditioning procedures and on the surface materials of limiter and liner. The respective combinations stainless steel and inconel, "carbonized" stainless steel and "carbonized" inconel as well as "carbonized" graphite and "carbonized" inconel have been compared. For the latter pulses of up to 3.8 sec duration and loop voltages below 1 V could be achieved and the accessible operation regime in the Hugill-Diagramme could be significantly extended. The metal concentrations were reduced to the order of 10^-5, whereas the total concentration of carbon and oxygen impurities was typically around two to three percent, with a falling tendency in the course of repetitive baking treatments. Results on j(r) derived from polarimetry are presented and discussed: in many cases the stationary value of q(0) was found to be ≳ 30 % below unity. No sawteeth on the j(r) profile could be observed. ICR heating has been extended to the 2 MW level and to the minority/second harmonic mode. The τ-scaling still remains comparable to that of the L-regime. With the Ansatz E = _OHP′_OH + τ_AUXP _RF the best fit gives τ_AUX[ms] = (60-15)I _P[MA]. The degradation of τ_P is similar to that of τ_E; the fluxes of neutral hydrogen, carbon and oxygen and the value of n_e close to the limiter increase with the RF power. Pronounced savteeth are observed in the H_α-signal at the main limiter and at the wall and also in n_e close to the limiter with amplitudes of up to 50 %. The application of ICRH increases the particle removal by the pump limiter ALT-1 as a consequence of the reduced τ_P and the resulting increased plasma RF into the entrance throat. This effect compensates potential difficulties for pump limiter application from the heat load on the limiter head vhich was found to increase more than linearly with P_RF. First: experiments with localized perturbation coils for boundary "ergodization" resulted in a pronounced stationary island pattern in the boundary layer; they influenced the global plasma confinement and, in some cases, led to a quench of the internal m=2 mode.