Project Microwave Heating for Wendelstein 7-X (PMW)
Dr. Volker Erckmann, Dr. Hermann Hunger
In recent years, electron cyclotron resonance systems have been established as standard means for localised heating (Electron Cyclotron Resonance Heating, ECRH) or current drive in fusion relevant plasmas.
Thus, ECRH will provide the basic day-one heating system for the stellarator W7-X, which is currently under construction at IPP Greifswald. In the first stage, W7-X will be equipped with a 10 MW ECRH system operating at 140 GHz in continuous wave (CW). The complete ECRH system will be provided by KIT, which established, together with EU partners, the 'Projekt Mikrowellenheizung für W7-X’ (PMW) in 1998, covering the design, development, construction, installation and integrated tests of all components required for stationary plasma heating on site at IPP Greifswald. PMW also coordinates the contributions from the Institut für Plasmaforschung (IPF) of the University of Stuttgart, which is responsible for the microwave transmission system and part of the HV system, and from the team at IPP Greifswald, which is responsible for the in-vessel components and for the in-house auxiliary systems. PMW benefits also from the collaboration with Centre de Recherche de Physique des Plasmas (CRPP) in Lausanne, Commissariat à l´Energie Atomique (CEA) in Cadarache and Thales Electron Devices (TED) in Vélizy.
Fig. 1: ECRH Building with 10 Gyrotrons and Transmission Line
A contract between KIT and the industrial company TED had been settled to develop and build the continuously operating series gyrotrons. The first step of this collaboration was the development of a prototype gyrotron for W7-X with an output power of 1 MW for CW operation at 140 GHz. This step has been finished successfully.
Seven series gyrotrons have been ordered from TED. First operation and long pulse conditioning of these gyrotrons will take place at the test stand at KIT, where pulses up to 180 s at full power are possible (factory acceptance test, FAT), 30 minutes shots at full power are possible at IPP (site acceptance test, SAT). Ten gyrotrons will be available for W7-X, including the pre-prototype tube, the prototype tube and a 140 GHz CPI-tube. To operate these gyrotrons, eight superconducting magnetic systems have been ordered at Cryomagnetics Inc., Oak Ridge, USA, in addition to the Oxford Instruments and Accel magnets.
Further progress was made towards the completion of the project. All of the components of the transmission system, HV-systems and in-vessel-components have been ordered, manufactured, delivered and are ready for operation at IPP Greifswald. A part of the existing ECRH system is already used to test new concepts and components for ECRH. Some delay arose in the project during the last years due to unexpected difficulties in the production of the series gyrotrons.
The first TED series gyrotron SN1 has been tested successfully at KIT and IPP in 2005 (920 kW/1800 s). It fulfilled all the specifications; no specific limitations were observed during the acceptance test. This gyrotron has been sealed in order to keep the warranty; the two prototype gyrotrons are routinely used for experiments.
The next series gyrotrons showed a somewhat different behaviour with respect to parasitic oscillations excited in the beam tunnel region. During a pause a new beam tunnel was developed. The first W7-X gyrotron, which is equipped with an improved beam tunnel, was delivered and tested at KIT in the middle of 2010. In the meantime production of gyrotrons is continued.
The transmission line consists of singlebeam waveguide (SBWG) and multibeam waveguide (MBWG) elements. For each gyrotron, a beam conditioning assembly of five single-beam mirrors is used. Two of these mirrors match the gyrotron output to a Gaussian beam with the correct beam parameters, two others are used to set the appropriate polarization needed for optimum absorption of the radiation in the plasma. A fifth mirror directs the beam to a plane mirror array, the beam combining optics, which is situated at the input plane of a multi-beam wave guide (MBWG). This MBWG is designed to transmit up to seven beams (five 140 GHz beams, one 70 GHz beam plus an additional spare channel) from the gyrotron area (entrance plane) to the stellarator hall (exit plane). To transmit the power of all gyrotrons, two symmetrically arranged MBWGs are used. At the output planes of the MBWGs, two mirror arrays (beam distribution optic, BDO) separate the beams again and distribute them via two other mirrors and CVD-diamond vacuum barrier windows to individually movable antennas (launchers) in the torus. The BDOs and the successive mirrors are mounted in so-called towers with "pinnacles" on top.
The manufacturing and installation of the components of the basic transmission system are finished now. For each gyrotron beam characterization and the subsequent design and manufacturing of the surfaces of the two matching mirrors are performed. As in the past years, the ECRH system is used for test of special components.
For two of the N-ports of W7-X, "remote-steering" (RS) launchers are foreseen. This is due to the fact that front steering launchers as used in the A and E ports will not fit into these narrow ports. The remote-steering properties are based on multi-mode interference in a square waveguide leading to imaging effects. For a proper length of the waveguide, a microwave beam at the input of the waveguide (with a defined direction set by a mirror system outside of the plasma vacuum) will exit the waveguide (near the plasma) in the same direction. For W7-X, the vacuum window, a vacuum valve as well as a mitre bend must be incorporated into the 4.6 m long waveguide.
A conceptual design for the two RS-launchers was performed. Low-power experimental investigations on a prototype waveguide have been started to benchmark the calculations.
For the operation of gyrotrons with depressed collector, a precisely controlled beam acceleration voltage is necessary, which is supplied by the body-voltage modulator. The beam current of the gyrotrons is controlled by the cathode heater supply, which is on cathode potential (about -55 kV). In case of arcing inside the gyrotron, a thyratron crowbar protects the tubes from being damaged.
All ten body-voltage modulators and the protection units are now ready for operation. With growing experience with the complete system, some final optimization issues concerning the system diagnostics in case of a gyrotron fault were implemented.
The four ECRH-plug-in launchers were successfully tested for vacuum tightness in the large MISTRAL vacuum chamber of IPP. In addition, extensive mechanical tests of the mirror drive mechanism were performed successfully.
The N-Port-launcher design was refined. Production drawing could now be provided, if the decision for manufacture is done.