C. Lechte

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.

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.

Investigation of the Scattering Efficiency in Doppler Reflectometry by Two-Dimensional Full-Wave Simulations

Two-dimensional full-wave simulations of Doppler reflectometry in slab and curved geometries have been carried out with the finite-difference time-domain code IPF-FD3D. The goal is to find the instrument response function of the reflectometer that allows the recovery of the poloidal plasma-density-fluctuation-wavenumber spectrum by Doppler reflectometry at different angles of incidence. Apart from nonlinearities caused by high fluctuation levels, the scattering process is found to be strongly dependent on the angle of incidence on the cutoff layer, the beam parameters, and the plasma-density gradient length. In addition, the code was applied to the actual geometry of an ASDEX-Upgrade plasma.

Experimental turbulence studies for gyro-kinetic code validation using advanced microwave diagnostics

For a comprehensive comparison with theoretical models and advanced numerical turbulence simulations, a large spectrum of fluctuation parameters was measured on the devices ASDEX Upgrade, TCV, and Tore-Supra. Radial profiles of scale-resolved turbulence levels in H-mode discharges are measured and compared with GENE simulations in the transition range from ion-temperature-gradient to trapped-electron-mode turbulence. Correlation reflectometry is used to study the microscopic structure of turbulence and GAMs in discharges where poloidal flow damping was varied by means of variations of the shape of the poloidal plasma cross-section and collisionality. Full-wave codes and synthetic diagnostics are applied for the interpretation of the data.

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.

ECRH and W7-X: An intriguing pair

The construction of the W7-X basic machine is almost completed and the device is approaching the commissioning phase. W7-X operation will be supported by ECRH working at 140 GHz in 2nd harmonic X- or O-mode with 10 MW cw power. Presently the activities at W7-X concentrate on the implementation of wall-armour, in-vessel components and diagnostics. The ECRH-system is in stand by with 5 out of 10 gyrotrons operational. The status of both, the W7-X device and the ECRH system is reported. Further R&D activities concentrate on extending the launching capability for sophisticated confinement investigations with remote steering launchers in a poloidal plane with weak magnetic field gradient.

Synthetic Diagnostics In The European Union Integrated Tokamak Modelling Simulation Platform

Abstract The European Union Integrated Tokamak Modelling Task Force (ITM-TF) has developed a standardized platform and an integrated modeling suite of codes for the simulation and prediction of a complete plasma discharge in any tokamak. The framework developed by ITM-TF allows for the development of sophisticated integrated simulations (workflows) for physics application, e.g., free-boundary equilibrium with feedback control, magnetohydrodynamic stability analysis, core/edge plasma transport, and heating and current drive. A significant effort is also under way to integrate synthetic diagnostic modules in the ITM-TF environment, namely, focusing on three-dimensional reflectometry, motional Stark effect, and neutron and neutral particle analyzer diagnostics. This paper gives an overview of the conceptual design of ITM-TF and preliminary results of the aforementioned synthetic diagnostic modules.

140 GHz, 1 MW, CW gyrotron development for the ECRH system of the stellarator Wendelstein 7-X

A 10 MW, 140 GHz ECRH system with a pulse duration of 30 minutes is currently under construction for the stellarator W7-X at Greifswald. The RF power will be provided by 10 gyrotrons. A European collaboration has been established to develop and build 9 (out of 10) tubes each with an output power of 1 MW for continuous wave (CW) operation. This contribution reports on recent results with the series gyrotrons.

Simulation of a Plasma-Filled Horn Antenna With Improved Radiation Pattern

The gain increase of a shortened horn antenna filled with a metamaterial structure was confirmed in numerical and experimental investigations reported in recent papers. All described embedded structures are multilayer grids or wires, optimized for one single frequency of operation. This paper presents first numerical studies of a plasma-filled horn. A strong focusing effect was observed.

Experimental results on high-power gyrotrons for the stellarator W7-X

This paper reports on the status of the 140 GHz, 1 MW, CW series gyrotrons for the stellarator W7-X. High power experiments have been performed at pulse length of up to 1800 s and an output power of up to 1.05 MW.