W. Becker

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.

Heating and Confinement in the Ion Cyclotron Range of Frequencies on the Divertor Tokamak ASDEX

The paper summarizes the experiments performed with ion cyclotron resonance heating (ICRH) on ASDEX, from November 1984 until March 1986; the most interesting results are reported and discussed in detail. Heating and confinement studies using the hydrogen second harmonic scheme and the hydrogen minority scheme (PIC < 2.6 MW, tIC < 1.5 s) show a typical L-mode behaviour, i.e. a power dependent confinement degradation, which is rather similar to that found with neutral beam injection (NBI) heating. ICRH is accompanied by a slightly improved particle and energy confinement compared with that of NBI; this is also true for a combined ICRH + NBI scheme, up to Ptot ≈ 4.5 MW, absorbed in the plasma. Particular efforts have been devoted to investigations of the second harmonic regime in H/D plasmas with nH/ne ≈ 0.1 - 1, with a view to heating mixtures in reactor relevant plasmas. The achievement of H-mode transitions with ICRH alone in the hydrogen minority scheme at an absorbed RF power of about 1.1 MW supports the assumption of common confinement properties in auxiliary heated tokamaks, since they appear to be widely independent of the additional heating method. ICRH specific impurity problems, such as the strong release of iron from the vessel walls, have been overcome by applying extensive in situ wall carbonization. The mechanisms responsible for impurity generation have partly been identified and analysed; however, the problem still remains to be solved. Impurities preferentially released from the ICRH antenna do not pose problems.

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.

ICRF Operation during H-Mode with ELMs Development Status at ASDEX Upgrade

Fast plasma boundary variations like ELMs result in heavy antenna coupling changes, strongly affecting the operation and the reliability of the ICRF devices. At ASDEX Upgrade it is tried to overcome these interactions by electronic means and by the use of hybrids. The strategy and status of this development are described.

Status and development of the ICRF antennas on ASDEX Upgrade

The 3 dB couplers on the ICRF system of ASDEX Upgrade allow operation of the ICRF system, even in the presence of strong ELMs. The refurbishing of the power tetrodes has restored the nominal power of the generators. Under those conditions, the limiting factor on the power to the plasma can become the voltage stand‐off of the antennas. The paper reports the status, progress and plans in this area. By doubling the radius of the sides of the antenna straps (from 5 mm to 10 mm) the edge electric field of the antenna straps was reduced. Experiments in a vacuum test set‐up confirmed an increase of the voltage stand‐off of the antenna, from typically 27 kV to 41 kV for 10 s pulses and from 42 kV to 58 kV for 1 s pulses. The effect of plasma scrape‐off layer variations on the voltage stand‐off was investigated with a high voltage RF probe of a simple geometry inserted in the plasma edge. The voltage stand‐off of the probe is reduced during the period of the edge density increase correlated with an ELM. The reduct...

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.