H. Wedler

Vacuum insulated antenna feeding lines for ICRH at ASDEX upgrade

Abstract Since ICRH antennas and their coaxial feeding lines are necessarily vacuum insulated, their RF electric strength depends mainly on the residual gas pressure, which can be severely impeded during high power RF application. This interaction between RF, vacuum and electric strength and the consequences on design and operation are discussed in general and in the case of ICRH at ASDEX Upgrade, whose feeding design is described in detail.

High power ICRF heating on the divertor tokamak ASDEX

First ICRF experiments on ASDEX have been performed at 67 MHz, corresponding to 2 Omega CH-heating of a hydrogen plasma at B0=2.2 T. Despite divertor operation ICRH is accompanied by a significant increase of impurity production which can drastically be reduced by means of wall carbonisation. RF power up to 2.3 MW is routinely coupled to the plasma for pulse lengths of up to 1 sec. The RF heating is found to depend strongly on plasma preheating. In combination with neutral beam injection the ICRF heating efficiency is even higher than the one of NI. Confinement degrades with ICRH to values in between NI-L-type and OH confinement.

Analysis of the loading resistance for ICRF heating experiments in ASDEX

The loading resistance has been analyzed for ICRF heating experiments in ASDEX plasmas, and some interesting features have been observed regarding the loading resistance in three particular cases. (1) In H-minority heating experiments, the loading resistance decreases sharply at the L-H transition. However, during the H-phase, which lasts for a few hundred milliseconds, the increasing density induces a variation in the loading resistance. This is reasonably accounted for in terms of an eigenmode effect, and is in agreement with theoretical predictions. (2) In long-pulse second-harmonic heating experiments a slowly changing isotope concentration (H concentration from 70% to 85%) is accompanied by a gradual decrease in loading resistance. (3) In 2 Omega CH heating experiments with repetitive pellet refueling, in which two confinement phases have been identified, it has been observed that the temporal behaviour of the loading resistance is quite different for the two phases.