W. Kasparek

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.

EU megawatt-class 140 GHZ CW gyrotron

The first series tube of the gyrotrons for the 10-MW electron cyclotron resonance heating system of the stellarator W7-X was tested at Forschungszentrum Karlsruhe (FZK) and yielded a total output power of 0.98 MW, with an efficiency of 31% (without a single-stage depressed collector) in short-pulse operation and of 0.92 MW in pulses of 180 s (efficiency of almost 45% at a depression voltage of 29 kV). The Gaussian mode output power was 0.91 MW. The pulselength at full power (1 MW) is limited at FZK by the available power supply. At a reduced electron beam current, it is possible to operate at longer pulselengths. At an output power of 0.57 MW (electron beam current of 29 A), the pulselength was increased to 1893 s. There was no physical reason for a limitation of this pulse: The pressure increase during the pulse was less than a factor of two and ended up at a very low value in the 10-9 mbar range. The tube was delivered to Max-Planck-Institut fuumlr Plasmaphysik Greifswald for tests at full power and up to 30-min pulselength. The Gaussian mode RF output power, measured in a calorimetric load after a 25-m-long quasi-optical transmission line (seven mirrors), was 0.87 MW at a total output power of 0.92 MW in 30-min pulses. Again, no indications for a limitation in pulselength were found. The second series tube was tested in short-pulse operation and showed a strange behavior concerning a mode hopping which has not yet been understood. The third series gyrotron delivers up to now 0.65 MW at a pulse duration of 180 s. Preliminary operation of the prototype tube as a two-frequency gyrotron delivered 0.41 MW in 10-s pulses at 103.8 GHz (TE21,6 mode)

ECRH and ECCD with high power gyrotrons at the stellarators W7-AS and W7-X

Electron cyclotron resonance heating (ECRH) plays a key role in stellarator research, because it provides net current free plasma start up and heating toward reactor relevant plasma parameters. ECRH was extensively used and investigated in the stellarator experiments at IPP Garching, i.e., the W7-A and the W7-AS stellarators. These experiments provide a solid physics and technological basis for the 10 MW, CW ECRH system, which is under construction for the superconducting next step stellarator W7-X and will become operational in 2005. We briefly describe some of the major stellarator specific physics results on ECRH and electron cyclotron (EC)-current drive from W7-A and W7-AS. The scientific goals and the design of W7-X are outlined together with the demands for the ECRH system, which is the main heating system in the first stage of the experiment. The present status of the ECRH engineering design including the gyrotrons, all auxiliary systems, the transmission line, and the launching system are presented.

Magnetic island localization for NTM control by ECE viewed along the same optical path of the ECCD beam

Abstract Neoclassical tearing modes (NTMs) deteriorate high-pressure tokamak plasma confinement and can be suppressed by electron cyclotron current drive (ECCD). In order to obtain efficient suppression, the ECCD power needs to be deposited at the center of an NTM magnetic island. To enhance efficiency, this power also needs to be synchronized in phase with the rotation of the island. The problem is that of real-time detection and precise localization of the island(s) in order to provide the feedback signal required to control the ECCD power deposition area with an accuracy of 1 to 2 cm. Existing schemes based on mode location, equilibrium reconstruction, and plasma profile measurements are limited in positional and temporal accuracy and moreover will become very complex when applied to ITER. To overcome these limitations, it is proposed to provide the feedback signal from electron cyclotron emission (ECE) measurements taken along the identical line of sight as traced by the incident ECCD millimeter-wave beam but in reverse direction. Experiments on TEXTOR have demonstrated a proof of principle. These measurements motivate the further development and the implementation of such an ECCD-aligned ECE system for NTM control in larger fusion machines. Possible implementation of such a system on ASDEX-Upgrade, based on waveguides equipped with a fast directional switch, is presented in this paper. Possible further development for ITER is also discussed.

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.

Simulation and experimental study of a remote steering system for ECRH/ECCD antenna beams

Abstract The present design for the ITER electron cyclotron wave launcher is based on individual circular corrugated waveguides running up to the vacuum vessel. At each waveguide end near to the plasma, a pair of movable mitre bends provides steering of the beam between 0° and 40°. An alternative to this concept could be corrugated square or rectangular waveguides. These waveguides show imaging characteristics, which can be used for remote scanning of the beam, thus avoiding movable parts near to the plasma. To obtain a safe data base for the realisation of this concept, theoretical and experimental studies were carried out. The calculations show that a scanning range of more than ±10° is possible with negligible loss into side lobes. Furthermore, concepts to improve the scanning range can be derived from the calculations. Measurements of amplitude and phase distribution in the output plane of the waveguide and in the far-field show very good agreement with theory for a beam polarisation perpendicular to the scanning direction. For beams polarised parallel to the scanning direction, where the propagation in the waveguide is determined mainly by the grooved walls, a reduced beam quality is measured, which can be attributed to imperfect machining of the grooves. The results show also, that the imaging characteristics are in principle maintained after the introduction of a pair of mitre bends with the bending plane perpendicular to the scanning plane. Finally, the application to ITER is discussed and concepts to improve the scanning range are given.

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.

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.

On electron cyclotron resonance heating and current drive in the W VII‐AS stellarator

The design of the Advanced Stellarator Wendelstein W VII‐AS is based on optimization of the vacuum magnetic field configuration. Essential features of the underlying concept are the reduction of the Pfirsch‐Schluter currents and of the neoclassical heat transport losses. This optimization in general leads to a non‐axisymmetric complex magnetic field configuration, which was realized by a set of modul twisted coils. Plasma operation started in October 1988. In the first experimental campaign the experiments concentrated on ECRH alone, combination with NBI is foreseen as the next step.

First tests of the new multi-frequency ecrh system at ASDEX upgrade

Summary form only given. The power deposition in ECRH (electron cyclotron resonance heating) of fusion plasmas is primarily determined by the magnetic field. For a single frequency ECRH system this has the consequence that for central heating the magnetic field is no longer a free parameter. However, for tokamak plasmas with different plasma currents or different equilibria, the magnetic field should be a free parameter in order to operate at a reasonable edge safety factor q(a). Furthermore, in a plasma with given parameters, some experimental features, like suppression of neoclassical tearing modes (NTM), require to drive current on the high field side without changing the magnetic field. These requests can be satisfied if the gyrotron frequency is variable. A new broadband ECRH system is currently under construction at the ASDEX Upgrade tokamak at IPP Garching. This system will employ multi-frequency gyrotrons which are step-tunable in the frequency range 105-140 GHz. In its final stage the system will consist of 4 gyrotrons with a total power of 4 MW and a pulse length of 10 s. It employs a fast steerable launcher in the plasma vessel for feedback controlled power deposition that allows for poloidal steering of 10 deg, within 100 ms. Transmission line elements, such as corrugated waveguides, polarizer mirrors and vacuum windows, are designed to cope for this frequency band. The first two-frequency gyrotron, operating at 105 GHz and 140 GHz, is currently being put into operation at ASDEX Upgrade