G. Gantenbein

Electron Cyclotron Heating for W7-X: Physics and Technology

The Wendelstein 7X (W7-X) stellarator (R = 5.5 m, a = 0.55 m, B < 3.0 T), which at present is being built at Max-Planck-Institut für Plasmaphysik, Greifswald, aims at demonstrating the inherent steady-state capability of stellarators at reactor-relevant plasma parameters. A 10-MW electron cyclotron resonance heating (ECRH) plant with continuous-wave (cw) capability is under construction to meet the scientific objectives. The physics background of the different heating and current drive scenarios is presented. The expected plasma parameters are calculated for different transport assumptions. A newly developed ray-tracing code is used to calculate selected reference scenarios and optimize the electron cyclotron launcher and in-vessel structure. Examples are discussed, and the technological solutions for optimum wave coupling are presented. The ECRH plant consists of ten radio-frequency (rf) modules with 1 MW of power each at 140 GHz. The rf beams are transmitted to the W7-X torus (typically 60 m) via two open multibeam mirror lines with a power-handling capability, which would already satisfy the ITER requirements (24 MW). Integrated full-power, cw tests of two rf modules (gyrotrons and the related transmission line sections) are reported, and the key features of the gyrotron and transmission line technology are presented. As the physics and technology of ECRH for both W7-X and ITER have many similarities, test results from the W7-X ECRH may provide valuable input for the ITER-ECRH plant.

Development of a 140 GHz, 1 MW, continuous wave gyrotron for the W7-X stellarator

The development of high power gyrotrons in continuous wave (CW) operation for heating of plasmas used in nuclear fusion research has been in progress for several years in a joint collaboration between different European research institutes and industrial partners. A recent RD program aims at the development of 140 GHz gyrotrons with an output power of 1 MW in CW operation for the 10 MW ECRH system of the new stellarator plasma physics experiment Wendelstein 7-X at IPP Greifswald, Germany. The work is performed under responsibility of FZK Karlsruhe in collaboration with CRPP Lausanne, IPF Stuttgart, IPP Garching and Greifswald, CEA Cadarache and TED Velizy. The gyrotron operates in the TE28.8 mode and is equipped with a diode type magnetron injection electron gun, an improved beam tunnel, a high-mode purity low-ohmic loss cavity, an optimized non-linear up-taper, a highly efficient internal quasi-optical mode converter, a single-stage depressed collector and an edge-cooled, single disk CVD-diamond window. RF measurements at pulse duration of a few milliseconds yielded an RF output power of 1.15 MW at a beam current of 40 A and a beam voltage of 84 kV. Depressed collector operation has been possible up to decelerating voltages of 33 kV without any reduction of the output power, and an efficiency of 49 % has been achieved. Long pulse operation of the gyrotron was possible with an output power of 1 MW at a pulse length of 10 s without any signs of a limitation caused by the tube. For this output power the efficiency of the tube could be increased from about 30 % without depression voltage to about 50% with depression voltage. At an output power of 640 kW, a pulse length of 140 s could be achieved.

170 GHz, 2 MW Coaxial Cavity Gyrotron - investigation of the parasitic oscillations and efficiency of the RF-output system -*

The mechanism of excitation of parasitic LF oscillations inside a coaxial gyrotron has been studied. At present a new coaxial insert with a modified profile on the cathode side of the resonator is under test. A suppression or at least increase of the starting current for the excitation of the LF oscillations is expected. As a first step to increase the Gaussian content of the RF output beam a launcher with only a 3rd order azimuthal perturbation was fabricated. Tests at low power have already been performed and tests with the experimental pre-prototype gyrotron are in progress. The results are presented and discussed.

First experimental results from the EU 2 MW coaxial cavity ITER gyrotron prototype

The EU is working towards providing 2 MW, coaxial-cavity, CW, 170 GHz gyrotrons for ITER. Their design is based on results from an experimental pre-prototype tube in operation at FZK for several years, having a pulse length of several milliseconds. The first industrial prototype tube is designed for CW operation, but, in a first phase, will be tested out to Is at the European Gyrotron Test Facility in Lausanne, Switzerland as part of a phased testing/development program (1s, 60 s, CW). It is known that RF beam profile shaping, stray radiation handling, and collector cooling at these high power levels are three issues for the gyrotron. The gyrotron, magnet and body power supply have been delivered and successfully installed at the test stand, hosted by the CRPP. The main high voltage power supply delivery is delayed, so one of the power supplies dedicated to 3 of 9 gyrotrons in the TCV EC system is being used as a backup power source (all 3 TCV power sources can be interfaced with the test stand). Cathode conditioning began in November 2007 followed by collector conditioning in December. Parasitic low frequency oscillations have not hindered operation, and the tests have progressed to conditioning out to 0.14 s pulses by March 2008. During this period, the performance concerning microwave generation has been characterised and the RF beam profile has been measured at several planes to allow reconstruction of the phase and amplitude profile at the gyrotron window and to provide the necessary information permitting proper alignment of the compact RF loads prior to pulse extension. The power will be measured, according to the pulse length, using either a very-short pulse (<0.01 s) load on loan from FZK, or short-pulse (<0.2 s) or long-pulse (CW), spherical, calorimetric loads developped as part of this program by CNR. This paper presents the preliminary results of these operations.

1 MW, 140 GHz, CW gyrotron for Wendelstein 7-X

A continuous wave, 1 MW, 140 GHz gyrotron oscillator has been designed and constructed as a joint collaboration between FZK Karlsruhe, CRPP Lausanne, IPF Stuttgart, CEA Cadarache and TTE Velizy for the 10 MW ECRH system of the new stellarator plasma physics experiment, Wendelstein 7-X, at IPP Greifswald, Germany. The tube is equipped with a diode MIG gun, an improved beam tunnel, a high-mode purity low-ohmic loss cavity, an optimized nonlinear up-taper, a highly efficient internal quasioptical mode converter, a single stage depressed collector for energy recovery and a large-aperture, edge-cooled, single disk CVD-diamond window and will be operated in the TE/sub 28.8/ mode. The gyrotron design and the teststand at the Forschungszentrum Karlsruhe are described; first experimental results for short pulse operation are presented.

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.

Experimental results on the 140 GHz, 1 MW, CW gyrotrons for the stellarator W7-X

The development of high power gyrotrons (118 GHz, 140 GHz) in continuous wave operation (CW) has been in progress for several years in a joint collaboration between different European research institutes and industrial partners. In this frame, two 140 GHz prototype gyrotrons for CW operation had been constructed and tested at the Forschungszentrum Karlsruhe. According to the results of the prototypes, seven 140 GHz CW gyrotrons were ordered. The first tube was operated at the Forschungszentrum Karlsruhe. A power of 950 kW at efficiency of 43 % (with energy recovery) could be obtained for pulse lengths of 180 s (limited by the available high-voltage power supply). A 30 minute pulse was performed with an output power of 540 kW. During this pulse almost no decrease in performance was found, especially the tube pressure only increased in the range of 10/sup -9/ mbar.

140 GHz, 1 MW, CW gyrotron for fusion plasma heating

Gyrotrons at high frequency with high output power are mainly developed for microwave heating and current drive in plasmas for thermonuclear fusion. For the stellarator Wendelstein 7-X now under construction a 10 MW ECRH system is foreseen. A European collaboration has been established to develop and build the 10 gyrotrons each with an output power of I MW for continuous wave operation (30 min). The design parameters of the gyrotron are summarized.

The 10 MW ECRH and CD system for W7-X

Electron Cyclotron Resonance Heating (ECRH) is the main heating method for the Wendelstein 7‐X Stellarator (W7‐X), which is the next step device in the stellarator line of IPP and is presently under construction in the Greifswald branch of IPP. The experiment aims at demonstrating the inherent steady state capability of stellarators at reactor relevant plasma parameters. W7‐X (major radius 5.5 m, minor radius 0.55 m) is equipped with a superconducting coil system operating at 3 T for steady state operation and a divertor for 10 MW steady state heat removal. A 10 MW ECRH plant with CW‐capability at 140 GHz is under construction to meet the scientific objectives. The microwave power is generated by 10 gyrotrons with 1 MW each. A European R&D program aiming at the development of a prototype gyrotron for W7‐X has been successfully terminated by fall of 2002. A prototype gyrotron with the same specifications was developed for W7‐X at CPI (USA). Test results and limitations are reported. The distinct microwave ...

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.