F. Wesner

The ASDEX upgrade ICRH antenna

Abstract The ICRH antenna, of which four are installed in ASDEX Upgrade, is designed to operate over a wide range of frequencies (30–80 MHz). Each antenna consists of two toroidally spaced loops, which can be operated in-phase or out-of-phase. The loops are folded in the poloidal direction such that even at the high frequency of 80 MHz the current varies in the poloidal direction by only 20%. The mechanical design is flexible: the major components of the antenna can be reutilized when the antenna is adapted to a single-null, double-null or limiter plasma configuration.

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.

Retention of gaseous and target produced impurities in the ASDEX divertor chamber

This letter reports on two experiments undertaken to evaluate the retention of gaseous and target produced impurities in the ASDEX divertor. The retention for gaseous impurities was determined by puffing Ar into the main chamber and simulating the time behaviour of the Ar XVI line intensity with a time dependent impurity transport code including a simple divertor model. During Ohmic heating a factor of 3 and 4.5 enhancement of impurity retention if found relative to the vacuum time constant (90 ms) of the divertor chamber, for ne = 2 × 1013 cm−3 and ne = 3.5 × 1013 cm−3, respectively, while a drastic breakdown of the retention occurs during high power NI heating. – To deduce the retention of impurities generated at the divertor plates, a segment (3.5%) of the plates was covered with copper, a metal previously not used in ASDEX. By measuring the Cu influx at the target plates and the line intensity of the Cu XX line (11.38 A) in the core plasma and by using the transport code, it is found that during NI heating (ne ≤ 2 × 1013 cm−3) Cu atoms originating from the target plates have a ≤ 3.5 times higher probability to penetrate into the core plasma than if they had when originating from the main chamber walls.

Influence of the plasma on ICRF antenna voltage limits

Abstract An ion cyclotron range of frequencies (ICRF) probe [F.W. Baity, G.C. Barber, V. Bobkov, R.H. Goulding, J.-M. Noterdaeme, D.W. Swain, in: 14th Topical Conference on Radiofrequency Power in Plasmas, Oxnard 2001, AIP Conference Proceedings 595, AIP, Melville, NY, 2001, p. 510] has been implemented to study voltage stand-off of the ICRF antennas on ASDEX Upgrade (AUG). The probe was operated at first in a test stand where features of high RF voltage operation in vacuum and plasma created by an ion source of the Hall type [Plasma Sources Sci. Technol. 8 (1999) R1] were studied. Vacuum arcs as well as ignition of high voltage glow discharge are candidate processes to explain voltage limits of the ICRF antennas. The setup on AUG was used to expose high RF voltages in real conditions of the tokamak scrape-off layer which are faced by the ICRF antennas. It is found that high voltage breakdown on the ICRF antenna is often correlated with ELM activity. The maximal RF voltage increased from shot to shot, i.e. the conditioning effect is observed. For the good-conditioned ICRF probe it was shown experimentally that the voltage limit can be increased while the rectified current is suppressed at the same time.

ICRF Heating and H-Mode/ Operational Experience and Results at ASDEX Upgrade

The coupling properties of ICRH antennas depend on the plasma boundary characteristics, which are modified by the H-mode. This strongly affects the operation and reliability of the technical heating device. At ASDEX Upgrade these interactions were studied and a method to overcome the most critical effect of ELMs was tested.

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.

Influence of boronization and carbonization on ICRF-heated plasmas in ASDEX

Abstract The properties of ICRF heating applied to a boronized or carbonized machine are compared. Results from radiation, impurity production, density changes and heating are presented and discussed. The experimental observation that more ICRH power could be applied to an ohmic plasma with boronization is related to edge and divertor observation.