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Anomalous scattering of high power millimetre waves from gyrotrons at 140 and 110 GHz is investigated for plasma with rotating islands at TEXTOR. The magnetic field and plasma density influence the spectral content of the scattered waves and their power levels significantly. Anomalous strong scattering occurs in two density regimes, one at low densities and one at high densities, that also depend on the magnetic field. The two regimes are separated by a quiescent regime without anomalous scattering. Investigations suggest that scattering in the high-density regime is generated at the low-field side intersection of the gyrotron beam and the island position. The transition from the quiescent regime to the high-density regime occurs when the gyrotron frequency is twice the upper hybrid frequency at this position. There is some evidence that the scattering in the low-density regime is generated near the plasma centre. Under this assumption all the observed scattering is generated when the gyrotron frequency is near or below twice the upper hybrid frequency.

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A CW capable inline electron cyclotron emission (ECE) separation system for feedback control, featuring oversized corrugated waveguides, is commissioned on ASDEX upgrade (AUG). The system is based on a combination of a polarization independent, non-resonant, Mach-Zehnder diplexer equipped with dielectric plate beam splitters [2, 3] employed as corrugated oversized waveguide filter, and a resonant Fast Directional Switch, FADIS [4, 5, 6, 7] as ECE/ECCD separation system. This paper presents an overview of the system, the low power characterisation tests and first high power commissioning on AUG.

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Electron cyclotron resonance heating (ECRH) systems for next-step large fusion devices operate in continuous wave power in the multimegawatt range. The unique feature of narrow and well-localized power deposition assigns a key role to ECRH for different tasks, such as plasma start-up, electron heating, current drive, magnetohydrodynamic (MHD) control and profile shaping. The integration of high-power microwave diplexers in the transmission lines will improve the flexibility and efficiency while simultaneously reducing the complexity of large ECRH systems. They can serve as power or beam combiners, as slow and fast directional switches to toggle the power from continuously operating gyrotrons between two launchers, and as discriminators of low-power electron cyclotron emission (ECE) signals from high-power ECRH using a common transmission line and antenna. Among various design options a resonant diplexer with a narrow resonance was selected for application at ASDEX Upgrade. The design is driven by the specific physics requirements for MHD control experiments and possible use for line-of-sight ECE. The compact, waveguide-compatible design features a feedback-controlled mirror drive for tracking of the resonator to the gyrotron frequency. High-power, long-pulse tests were performed with the 140-GHz ECRH system for the stellarator W7-X. Results on the transmission characteristics, power combination, and stationary and controlled distribution of the input power to two outputs are presented. The qualification for in-line ECE was investigated.

Lit.opg., bijl.

Lit.opg., bijl.

A demonstration of real-time feedback control for autonomous tracking and stabilization of m/n = 2/1 tearing modes in a tokamak using Electron Cyclotron Resonance Heating and Current Drive (ECRH/ECCD) is reported. The prototype system on TEXTOR combines in the same sight-line an Electron Cyclotron Emission (ECE) diagnostic for tearing mode sensing and a steer-able ECRH/ECCD antenna. The mode location is retrieved from the ECE measurements and serves as input for a control loop, which aligns the ECRH/ECCD deposition with the tearing modes by steering of a launcher mirror. The alignment is achieved by matching the mode location in the sensor spectrum with the fixed ECRH/ECCD actuator frequency. The control response is dominated by the response of the mechanical launcher. Analysis of the launcher dynamics receives special emphasis in the control design. In addition, the ECRH/ECCD power is modulated in phase with the rotation frequency of the O-point of the tearing modes using a feedback loop, which extracts the modes frequency and phase from the ECE data. The 'line-of-sight' concept has advantages over conventional tearing mode suppression schemes as it provides information of the modes location and phase in the direct vicinity of the EC-deposition. It avoids the use of ray-tracing in the control loop and guarantees accurate alignment in the presence of disturbances. The experimental results demonstrate the capabilities of the control system to track and suppress tearing modes in real-time. A relatively simple control design suffices to meet the performance requirements demanded for effective tearing mode suppression. In addition, a control-oriented model for the tearing mode control problem is being developed. This model includes tearing mode and plasma dynamics, models for the actuators, diagnostics, data-processing and control algorithms. Simulation results for typical TEXTOR conditions will be discussed and validated with experimental results.

In tokamak plasmas with a tearing mode, strong scattering of high power millimeter waves, as used for heating and noninductive current drive, is shown to occur. This new wave scattering phenomenon is shown to be related to the passage of the O point of a magnetic island through the high power heating beam. The density determines the detailed phasing of the scattered radiation relative to the O-point passage. The scattering power depends strongly nonlinearly on the heating beam power. ©2009 The American Physical Society

ITER employs Electron Cyclotron Waves (ECW) for heating of the plasma and to stabilize MHD modes including the neoclassical tearing modes (NTM). To cover all the significant ITER plasma scenarios, the upper port ECW launcher needs a beam steering mechanism to reach the magnetic q-surfaces where NTM instabilities can grow. Two alternative approaches are being studied in Europe under EFDA tasks: Front Steering (FS), which has rotating mirrors situated near the plasma, and Remote Steering (RS), which has the mirror steering mechanism situated in the secondary vacuum. The steering mirrors in the RS system project the mm-wave beam onto a Square Corrugated Waveguide (SCW) of a length such that the input beam profile is imaged at the output, producing a steered output beam. The advantage of RS is that the steering mechanism is shielded from direct plasma exposure and neutron bombardment and it also allows simpler maintenance. One of the drawbacks of RS is that the output steering range is inversely coupled to the focusing. The optimization of the quasi-optical system is done for a short (4.3 m) and a long (9.1 m) SCW. The results are evaluated for their effectiveness in NTM stabilization by calculating the parameter ? which represents the ratio between the EC wave driven current and the bootstrap current, preferably ? should exceed 1.2. The performance is also evaluated in terms of beam focusing properties and power loading on the mirrors. The ? achieved so far meets the requirements in the ITER reference scenarios for q=2/1 and partly for q=3/2. Additional tools under investigation for optimization are: non spherical mirror curvatures, tapered SCWs and the use of general astigmatic beams. This work has been carried out as part of the EFDA work program Task TW6-TPHEECHULB for the development of the ITER ECRH Upper Port Launcher system, in collaboration with the Euratom Associations IFP Stuttgart, IPP Garching, FZK, IFP/CNR and CRPP.

One of the main goals of the ITER Electron-Cyclotron (EC) Upper Launcher is the stabilisation of Neoclassical Tearing Modes (NTMs). The fulfilment of this objective is connected with the possibility of generating a current inside the island by EC waves, thus replacing the missing bootstrap current driving the mode. To achieve this, the EC current profile should be as narrow as possible, to increase the current density in the island region, thus reducing the total injected power necessary to suppress the mode. A design option for the ITER Upper Launcher is based on the so-called Remote Steering (RS) concept [1], in which the steering mechanism is placed behind the waveguides, thus avoiding direct neutron bombardment from the plasma. In this paper, the performance of the latest RS layout is evaluated. Generally astigmatic beams [2] are employed to minimise the beam broadening in the plasma due to diffraction, which has a negative impact on the width of the current profile. The stabilisation efficiency is estimated in terms of the figure of merit ?NTM=JCD/Jbs (JCD being the EC driven current and Jbs the equilibrium bootstrap current at a given surface), for (2,1) and (3,2) modes for three reference ITER scenarios. The criterium for complete mode suppression is given as ?NTM >1.2. The EC current profile is calculated by means of the TORBEAM code [3], which allows a straightforward modelling of generally astigmatic beams. The performance of a "long waveguide" design of the RS launcher is also discussed.

General astigmatism treatment has been included in the description of the resulting beams for the latest design concepts proposed for the Remote Steering (RS) Launcher. The inclusion of general astigmatism is intended to provide the key parameters of the resulting beams and perform realistic beam tracing calculations in the plasma. It is in fact well known that the many requirements foreseen for the ITER ECRH Upper launcher force the beams to be injected in the plasma with a sufficient steering capability aiming at the stabilization of neoclassical tearing modes (NTM) through localized deposition of EC waves. To do this, double curvature mirrors are used in the latest RS quasi-optical systems. The mirrors are oriented at an angle with respect to each other and with respect to the directions of astigmatism of the incoming launched beam. The resulting beams are found to be generally astigmatic beams [1]. The correct description of the beam parameters resulting from this kind of complex launching system can in principle be used to optimize the RS quasi-optical system (in terms of localized heating and current drive efficiency), acting for example on the curvatures of the last mirror. The curvature in the direction determined by the steering plane should not be considered a free parameter, since the necessary output steering range has to be granted, but the curvature in the direction orthogonal to the steering plane could be modified to improve the performances of the launcher. Starting from the design proposed [2], the analysis presented here includes beam optics calculations. This approach was agreed within affiliate institues to optimise the efficiency of the RS Upper launcher design. The work was carried out under EFDA Task TW6-TPHE-ECHULB.

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Voordrachten uitgesproken naar aanleiding van het afscheid van Prof. Mörzer Bruyns, Landbouwhogeschool, 1978

Voordrachten uitgesproken naar aanleiding van het afscheid van Prof. Mörzer Bruyns, Landbouwhogeschool, 1978