A. Meier

CVD diamond windows studied with low- and high-power millimeter waves

As part of long-pulse high-power gyrotron development low- and high-power millimeter-wave characterization has been performed on bare and brazed chemical vapor deposition (CVD) diamond disks. The dielectric property measurements performed with low-power open resonator studies demonstrate the availability of large area CVD diamond disks fulfilling the requirements for high-power windows. In brazed components, additional surface losses are put to evidence. Their contribution to enhanced dielectric absorption differ characteristically between existing brazing techniques. A particular acid and hydroxide treatment for reducing the observed surface terms was studied for brazed windows at different stages of their integration into gyrotron tubes, such as premounting, onsite, and post-dismantling. Video inspections of the output and transmission windows in high-power tests gave evidence of stationary light emission phenomena from irregularly distributed spot-like centres in the output window. The absence of a similarly stationary phenomenon in all disks studied in transmission rules out typical defect structures such like nondiamond like phase inclusions or specifically terminated surface bonds as a critical source. Local temperature peaks and the presence of only singular light emission events in the transmission windows hint at contamination effects by particles which are only stationary in vacuum conditions.

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.

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.

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.

Development of frequency step tunable 1 MW gyrotrons in D-band

Gyrotrons are widely used in electron-cyclotron-plasma heating of fusion installations. Modern and future installations require microwave sources with power of at least 1 MW. There are two main directions in the development of megawatt gyrotrons at GYCOM. The first line is an enhancement of parameters for conventional gyrotrons. The main efforts are applied now to implementing of CVD diamond windows into the gyrotrons and providing gyrotron operation in CW (or very long pulses of tens seconds) regime.

Experimental and theoretical thermal analysis of CVD diamond window units for the ITER upper launcher

An ITER torus window prototype with CVD diamond disks and corrugated waveguides are being investigated by using IR imaging and temperature measuring technique during high power RF microwave loading up to 1 MW at a frequency of 170 GHz at the JAEA gyrotron facility. To evaluate the cooling efficiency of the window design the temperature distribution over the diamond disk area is measured and compared with a theoretical thermal FEM analysis.

Tunable double disk window for ECH&CD system of ASDEX-U

A double disk window was designed which fulfils the broadband transmission requirements for the torus window in a step-tunable ECH&CD system for ASDEX-U. Thermo-hydraulic as well as thermo-mechanical modelling were performed in order to analyse the cooling layout and the stress in the window under operation conditions. Specific issues of the window fabrication are presented.

Design and performance tests of a high power torus window for a remotely steered EC launcher

For the ITER remotely steered (RS) EC launcher which is designed to stabilise the neoclassical tearing modes (NTM) at the q=3/2 and q=2/1 surfaces by inducing off-axis current drive, a high performance CVD torus window with edge cooling has been designed and manufactured to allow off-axis transmission of up to 2 MW mm-wave power at 170 GHz. Its compatibility with the operation parameters of ITER cooling systems was demonstrated and its performance in a remotely steered beamline (steering range: -12/spl deg/ to 12/spl deg/) was studied by low power tests which are to be complemented by high power, short pulse tests presently being initiated at the Forschungszentrum Karlsruhe.

Preliminary design of the ITER ECH upper launcher

The design of the ITER electron cyclotron launchers recently reached the preliminary design level -the last major step before design finalization. The ITER ECH system contains 24 installed gyrotrons providing a maximum ECH injected power of 20 MW through transmission lines towards the tokamak. There are two EC launcher types both using a front steering mirror; one Equatorial Launcher for plasma heating and four Upper Launchers (UL) for plasma mode stabilization (neoclassical tearing modes and the sawtooth instability). A wide steering angle of the ULs allows to focus on magnetic islands which are expected on the rational magnetic flux surfaces q = 1 (sawtooth instability), q = 3/2 and q = 2 (NTMs).

The ITER ECH & CD Upper Launcher: Steps towards final design of the first confinement system

The ITER Electron Cyclotron Heating and Current Drive (ECH&CD) Upper Launcher, whose preliminary design was approved in 2009, is on its way towards the final design. The design work is being done by a consortium of several European research institutes in tight collaboration with F4E. The main focus is the finalization of the design of all components for the First Confinement System (FCS), which forms the vacuum and Tritium barrier. The FCS comprises structural components as well as the external waveguide components in the port cell. Structural components of the FCS include the flange seal, backend frame and closure plate. The external waveguide components include the isolation valve, CVD diamond windows, miter bends and straight waveguides. Because finalizing of the design of these components is directly influenced by the layout of many in-vessel components, the design work includes also further development of the entire launcher. This paper summarizes the most recent status of the design work on the structural components of the launcher FCS, which are the support flange, the socket, the closure plate and feed-throughs for waveguides and cooling pipes. The design work includes the engineering layout of these components in accordance with system requirements, load specifications and Quality and Safety classification. An outline of the overall design of the launcher will be presented. The design progress was based on a set of related analyses, of which particular results are given. Also the integration of the associated mm-wave components, assembly strategies, neutronic aspects and the design of the shielding components will be described.