H. Schütz

Status of the new multi-frequency ECRH system for ASDEX Upgrade

Summary form only given. The first two-frequency GYCOM gyrotron Odissey-1 has been installed and put into operation in the new multi-frequency ECRH system at the ASDEX Upgrade tokamak experiment. It works at 105 GHz and 140GHz with output power 610kW and 820kW respectively at a pulse length of 10s. A further extension of the system with 3 more gyrotrons is underway. These gyrotrons will be step-tunable and operate at two additional intermediate frequencies between 105 and 140GHz. Such gyrotrons will require broadband vacuum windows. Construction and cold tests of a first broadband double-disc toms window are completed. The transmission to the tonis is in normal air, through corrugated aluminum waveguides with I.D.=87mm over a total length of about 70m. Calorimetric measurements gave a total transmission loss of only 12% at 105GHz and 10% at 140GHz. The variable frequency will significantly extend the operating range of the ECRH system, e.g. allow for central heating at different magnetic fields. Other experimental features, like the suppression of neoclassical tearing modes (NTM), require to drive current on the high field side without changing the magnetic field. The stabilization of NTMs requires a very localized power deposition such that its center can be feedback controlled, for instance to keep it on a resonant q-surface. For this reason fast movable launchers have been installed.

Plans for a new ECRH system at ASDEX Upgrade

Abstract A new ECRH system is being constructed for ASDEX Upgrade with a total power of 4 MW, generated by four gyrotrons, and a pulse duration of 10 s. Particular features are the use of gyrotrons which can work at various frequencies in the range 104–140 GHz and correspondingly broad band transmission components. The transmission will be at normal air pressure, and at the torus we will have a tunable double disk vacuum window. A further aim is the installation of fast moveable mirrors for a feedback controlled localized power deposition.

Extension of the ECRH operational space with O2 and X3 heating schemes to control tungsten accumulation in ASDEX Upgrade

ASDEX Upgrade has been operated with tungsten-coated plasma-facing components for several years. H-mode operation with good confinement has been demonstrated. Nevertheless, purely neutral beam injection-heated H-modes with reduced gas puff, moderate heating power or/and increased triangularity tend to accumulate tungsten, followed by a radiative collapse. Under these conditions, central electron heating with electron cyclotron resonance heating (ECRH), usually in X2 polarization, changes the impurity transport in the plasma centre, reducing the central tungsten concentration and, in many cases, stabilizing the plasma. In order to extend the applicability of central ECRH to a wider range of magnetic field and plasma current additional ECRH schemes with reduced single-pass absorption have been implemented: X3 heating allows us to reduce the magnetic field by 30%, such that the first H-modes with an ITER-like value of the safety factor of q95 = 3 could be run in the tungsten-coated device. O2 heating increases the cutoff density by a factor of 2 allowing higher currents and triangularities to be addressed. For both schemes, scenarios have been developed to cope with the associated reduced absorption. In the case of central X3 heating, the X2 resonance lies close to the pedestal top at the high-field side of the plasma, serving as a beam dump. For O2, holographic mirrors have been developed which guarantee a second pass through the plasma centre. The beam position on these reflectors is controlled by fast thermocouples. Stray-radiation protection has been implemented using sniffer probes.

Present Status of the New Multifrequency ECRH System for ASDEX Upgrade

A new multifrequency electron cyclotron resonance heating system is under construction for the Axially Symmetric Divertor Experiment (ASDEX) Upgrade tokamak experiment. For the first time in a fusion device, this system employs multifrequency gyrotrons that are step-tunable in the range 105-140 GHz. In its final stage the system will consist of four gyrotrons with a total power of 4 MW and a pulselength of 10 s. The first two gyrotrons, working at 105 and 140 GHz, were installed and tested. Transmission line elements such as corrugated waveguides, polarizer mirrors and vacuum windows are designed to cope with this frequency band. The system includes fast steerable launchers at the front end that will allow for localized feedback-controlled power deposition in the plasma.

The new multifrequency electron cyclotron resonance heating system for ASDEX upgrade

A new multifrequency electron cyclotron resonance heating (ECRH) system is currently under construction at the ASDEX Upgrade tokamak experiment. This system will, for the first time in a fusion device, employ multifrequency gyrotrons, step-tunable in the range 105 to 140 GHz. In its final stage the system will consist of four gyrotrons with a total power of up to 4 MW and a pulse length of 10 s. The variable frequency will significantly extend the operating range of the ECRH system both for heating and current drive. The matching optics unit includes a set of phase-correcting mirrors for each frequency as well as a pair of broadband polarizer mirrors. The transmission line consists of nonevacuated corrugated HE11 waveguides with inner diameter of 87 mm and has a total length of ˜70 m. A fast steerable launcher enables the steering of the beam over the whole plasma cross section poloidally. The first two-frequency gyrotron has been installed recently. It is equipped with a single-disk diamond window. The next gyrotrons will be step-tunable with two additional frequencies between 105 and 140 GHz. They will require a broadband output window, which will be either a Brewster or a double-disk window.

Experience with the ECRH System of ASDEX Upgrade

The 140 GHz ASDEX-Upgrade ECRH system uses four gyrotrons to generate a total power of 2 MW in a Gaussian beam for 2 s. External magnetic perturbations, originating from the poloidal stray magnetic field of the tokamak air core transformer, and also from the cryomagnet of the adjacent gyrotron, influence the startup of the oscillation and the electron beam deposition in the collector. The transmissions are partly quasioptical and partly corrugated HE-11 waveguide lines. Their losses have been determined calorimetrically.

Progress and First Results With the New Multifrequency ECRH System for ASDEX Upgrade

A multifrequency electron cyclotron resonance heating (ECRH) system is currently under construction at the ASDEX Upgrade tokamak experiment. The system employs depressed collector gyrotrons, step tunable in the range of 105-140 GHz, with a maximum output power of 1 MW and a pulse length of 10 s. One two-frequency GYCOM gyrotron has been in routine operation at ASDEX Upgrade since 2006. A further extension of the system with three more gyrotrons is underway. An in situ calibration scheme for the broadband torus window has been developed. The system is equipped with fast steerable mirrors for real-time MHD control. The gyrotron and the mirrors are fully integrated into the discharge control system. The ECRH system turned out to be essential for the operation of H-modes after covering the plasma facing components of ASDEX Upgrade with tungsten. Deposition of ECRH inside rhotor < 0.2 is necessary to prevent accumulation of W in plasmas with high pedestal temperatures. With respect to the limited loop voltage available in ITER, the use of ECRH for neutral-gas preionization to facilitate plasma breakdown and its application during the current ramp-up to increase the conductivity in order to save transformer flux have been demonstrated successfully for 105 GHz, 3.2 T (O1-mode) and 140 GHz, 2.2 T (X2-mode), corresponding to 170 GHz at ITER with the full and half values of its foreseen toroidal field of 5.3 T.

Operation Experience with the ASDEX Upgrade ECRH System

Abstract In 1989 the planning for a 140-GHz, 2-MW, 2-s electron cyclotron resonance heating system for ASDEX Upgrade started. These plans were finally approved in 1993. The system comprises four gyrotrons with four separate transmission lines and launchers. Although a 0.5-s test gyrotron was already installed in autumn 1994, it was only in summer 1997 when the first gyrotron of the final system was ready for use in the experiments, and in spring 2000 the system was completed with all four gyrotrons. This paper reviews the experience gained in construction and operation of this system. In particular, we describe how we solved problems with external magnetic fields affecting gyrotron operation. These fields originate both from the tokamak and from the cryomagnet of adjacent gyrotrons. We also report about the gyrotron performance, our techniques for the alignment of the transmission lines, the calibration of the polarizer mirrors, and the power calibration.

Feed Forward Polarization Control during ECRH discharges at ASDEX Upgrade

Abstract The new electron cyclotron resonance heating (ECRH) system at the ASDEX Upgrade tokamak allows for an adjustment of the polarization of the injected ECRH beam during plasma discharges. Three sniffer probes for millimeter wave stray radiation, with broad and polarization insensitive radiation characteristics, have been installed around the torus to monitor nonabsorbed radiation. The influence of varying ECRH-beam polarization on the detected stray radiation is studied. For perpendicular X2 heating the minimum detectable amount of wrong (O2-mode) polarization is found to be 5%. The system also allows full change of polarization from X2 to O2 mode, as it is useful for O2 heating above the X2-mode cutoff. These experiments show a high directivity of the stray radiation due to the toroidally inclined O2-mode injection.

The ECRH system of ASDEX Upgrade

Abstract The ECRH system of ASDEX Upgrade is now completed. Four gyrotrons generate a total Gaussian beam power of 2 MW/2 s at 140 GHz. The power is transmitted partly quasioptically and partly via corrugated HE-11 waveguides. The transmission line losses, determined calorimetrically, are about 12%. The four focused beams lead to a very localised power deposition in the plasma and can be steered in both poloidal and toroidal directions.