F. Hofmeister

ICRF system enhancements at ASDEX Upgrade

The ion cyclotron range of frequency (ICRF) heating system at the tokamak ASDEX Upgrade has been in operation since 1991. It still continues to be modified in order to improve its efficiency, flexibility and reliability and to meet changing physics demands. Recently the radio frequency (RF) transmission line feeding system of the antennas has been altered to allow for plasma heating at different frequencies and for current drive at one fixed frequency to be chosen before the shot. A matching system using transmission lines with premagnetized ferrites has been successfully tested. It is planned to use such a system in a feedback loop to allow matching even of fast changes of antenna loading during plasma operation.

The RF system and matching procedure for ASDEX and ASDEX upgrade

Abstract The ICRH system outline of both the former ASDEX and the present ASDEX Upgrade experiment is described with regard to the matching procedure. The matching experience gained from the operation of ICRH on ASDEX lead to improvements in the ICRH operation on ASDEX Upgrade. The matching process envisaged for ASDEX Upgrade and future matching options are described.

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.

ICRF antenna resistance and the H‐mode: Experimental and theoretical parameter dependence in ASDE and optimization for ASDEX upgrade

The antenna resistance in ASDEX was calculated using the FELICE 3‐dim antenna code with the mesured temperature and density profiles and the plasma position. The change of antenna resistance at the H‐code transition was modelled by varying the density gradient at the boundary. The theoretical estimates are in reasonable agreement with the measured values. During the H‐phase, the excitation of eigenmodes complicates the picture.The magnitude of the sudden decrease of the antenna resistance at the transition is experimentally observed to be a weak function of plasma position and density, in agreement with theoretical estimates.

ACQUISITION OF TECHNICAL DATA AND MATCHING PROCEDURES FOR THE ICRH SYSTEM ON ASDEX UPGRADE

In spring this year the ICRH system for ASDEX Upgrade was ready for operation. The system is designed to deliver up to 8 MW from 4 generators to 4 antennas with 2 loops each (ref. 1). n nIn the first experimental phase a total power of approx. 2 MW with pulse lengths of up to 2 sec. was achieved. n nH-minority heating both in helium and in deuterium (30 MHz) and second harmonics heating of hydrogen (60 MHz) showed good results with a system configuration feeding the 2 antenna loops with opposite phase. n nAntenna coupling resistances calculated with reference to a line with 25 Ω characteristic impedance were in the order of 2 Ω for H-minority / 30 MHz operation and 5 Ω in the 60 MHz second harmonics heating regime.

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.

ICRF heating system for W VII-AS

The ICRF heating system for the W VII‐AS modular stellarator is described, with emphasis on antenna design and the choice of heating scenarios for a stellarator.

First Results of Ion Cyclotron Resonance Heating on ASDEX Upgrade

ASDEX Upgrade is equipped with an ICRH system consisting of 4 generators of 2 MW power each and 4 double loop antennas. The generators, tuneable in frequency from 30 to 120 MHz, cover several heating scenarios over a wide range of magnetic fields (1 T<Bt<3.9 T): minority heating of H and He3 and second harmonic heating of H and D. ICRH‐heated discharges in ASDEX Upgrade were so far carried out mainly at 30 MHz and a magnetic field of 2 T (H minority in D and He). Peak powers of 2.4 MW and pulse length up to 2.5 s were achieved (total energy 3.75 MJ). In L‐mode, the density on turn‐on of the ICRH stays constant, or even decreases. The ratio of radiated power to total input power is unchanged (60% in an unboronized machine, 30% in a freshly boronized machine) between Ohmic and ICRH phases. The electron temperature increases with 0.9 MW from 1 to 1.25 keV, the loop voltage drops. Transitions to the H‐mode were easily and reliably achieved with ICRH alone (necessary ICRH power as low as 0.9 MW) and the length of the ELMy H‐mode phases was limited only by the applied ICRH pulse length (ELMy H‐mode phases of up to 2 s were achieved). The paper presents further results on heating and confinement in L and H‐mode, antenna and edge studies and on divertor measurements. Preliminary experiments, performed with a combination of H minority heating (30 MHz) and H second harmonic (60 MHz) in 600 kA He and D discharges (H minority in the 5 to 20% range) at 2 T, and with non‐resonant heating (30 MHz and 60 MHz at 1.35 T) are briefly discussed.