B. Plaum

Overview of the ITER EC upper launcher

The ITER electron cyclotron (EC) upper port antenna (or launcher) is nearing completion of the detailed design stage and the final build-to-print design stage will soon start. The main objective of this launcher is to drive current locally to stabilize the neoclassical tearing modes (NTMs) (depositing ECCD inside of the island that forms on either the q = 3/2 or 2 rational magnetic flux surfaces) and control the sawtooth instability (deposit ECCD near the q = 1 surface). The launcher should be capable of steering the focused beam deposition location to the resonant flux surface over the range in which the q = 1, 3/2 and 2 surfaces are expected to be found for various plasma equilibria susceptible to the onset of NTMs and sawteeth. The aim of this paper is to provide the design status of the principal components that make up the launcher: port plug, mm-wave system and shield block components. The port plug represents the chamber that provides a rigid support structure that houses the mm-wave and shield blocks. The mm-wave system comprises the components used to guide the RF beams through the port plug structure and refocus the beams far into the plasma. The shield block components are used to attenuate the nuclear radiation from the burning plasma, protecting the fragile in-port components and reducing the neutron streaming through the port assembly. The design of these three subsystems is described; in addition, the relevant thermo-mechanical and electro-magnetic analyses are reviewed for critical design issues.

A fast switch, combiner and narrow-band filter for high-power millimetre wave beams

A fast directional switch (FADIS) is described, which allows controlled switching of high-power microwaves between two outputs. A possible application could be synchronous stabilization of neoclassical tearing modes (NTMs). Generally, the device can be used to share the installed EC power between different types of launchers or different applications (e.g. in ITER, midplane/upper launcher). The switching is performed electronically without moving parts by a small frequency-shift keying of the gyrotron (some tens of megahertz), and a narrow-band diplexer. The device can be operated as a beam combiner also, which offers attractive transmission perspectives in multi-megawatt ECRH systems. In addition, these diplexers are useful for plasma diagnostic systems employing high-power sources due to their filter characteristics. The principle and the design of a four-port quasi-optical resonator diplexer is presented. Low-power measurements of switching contrast, mode purity and efficiency show good agreement with theory. Preliminary frequency modulation characteristics of gyrotrons are shown, and first results from high-power switching experiments using the ECRH system for W7-X are presented.

Fast switching and power combination of high-power electron cyclotron wave beams: principles, numerical results and experiments

To combine powers from a set of gyrotrons and to switch the combined wave beam between a number of output channels, various kinds of multiplexers can be used. Especially, narrow-band frequency diplexers in connection with small frequency-shift keying of gyrotrons can be used to switch the millimeter-wave power between two output channels. This technique can, for example, be used for fast beam steering for synchronous stabilization of rotating neoclassical tearing modes in tokamaks. Beam steering can be performed by a multistage multiplexer, provided that phase-controlled sources are available. In the paper, various concepts for fast directional switches as well as their integration into transmission lines are discussed. Calculations and low-power measurements of prototypes are presented. A resonant diplexer experiment is at present being prepared to be tested at high power in the electron cyclotron resonance heating (ECRH) system for Wendelstein 7X (W7-X). Requirements and techniques for frequency control of the gyrotrons are discussed, and the results of preliminary frequency modulation experiments are shown. Finally, future prospects for the application of diplexers in large ECRH systems are discussed.

Diplexers for Power Combination and Switching in High Power ECRH Systems

Electron Cyclotron Resonance Heating (ECRH) systems for next step large fusion-devices operate at a Continuous Wave (CW) power well beyond 10 MW generated by a large number of gyrotrons with typically 1 MW power per unit. The combination of the power of two (or more) gyrotrons and switching of the power between different launchers for different physics applications is an attractive feature for such systems. The combination of beams from different gyrotrons would reduce the number of transmission lines and the requirements on port space. Fast switching between two antennas synchronously with the Magneto-Hydro Dynamic (MHD) modes frequency would increase the efficiency of mode stabilization. Both combination and switching as well as power sharing between different ports can be performed with high-power four-port diplexers using small frequency differences or small frequency-shift keying of the gyrotrons, respectively. Fast directional switches (FADIS) and beam combiners (BC) can be designed on the basis of different physical mechanisms: some selected design variants were investigated and the results are presented. Considerations on the integration of FADIS/BCs into large ECRH systems and their use in test arrangements are presented.

Optimization of oversized waveguide components using a genetic algorithm

Abstract A method for optimizing bends for oversized waveguides using a genetic algorithm is described. A program was developed to optimize the curvature function and – for corrugated waveguides – the corrugation depth of a bend. After a description of the algorithm and its implementation, the results of the optimization of a 28 GHz TE 01 bend are shown. Starting with a constant curvature, the algorithm produced a bend with a peak power of spurious modes inside the bend of about 12% and a spurious mode content at the bend end of less than 0.1%. A comparison shows, that the optimized bend is much better than bends of the same size with analytical curvature functions (sin, sin 2 ).

Objectives, physics requirements and conceptual design of an ECRH system for JET

A study has been conducted to evaluate the feasibility of installing an electron cyclotron resonance heating (ECRH) and current drive system on the JET tokamak. The main functions of this system would be electron heating, sawtooth control, neoclassical tearing mode control to access high beta regimes and current profile control to access and maintain advanced plasma scenarios. This paper presents an overview of the studies performed in this framework by an EU-Russia project team. The motivations for this major upgrade of the JET heating systems and the required functions are discussed. The main results of the study are summarized. The usefulness of a 10 MW level EC system for JET is definitely confirmed by the physics studies. Neither feasibility issues nor strong limitations for any of the functions envisaged have been found. This has led to a preliminary conceptual design of the system.

Optimization of waveguide bends and bent mode converters using a genetic algorithm

Waveguide bends must be designed such that they have only a little mode conversion. A small spurious mode content inside the bend can be tolerated if the modes are converted back to the main mode at the end of the curved section. A program was developed to optimize bends and bent mode converters for oversized waveguides using a genetic algorithm.

A 32-element frequency-steered array antenna for reflectometry in W-band

In fusion plasma experiments, Doppler reflectometry is used as a contactless diagnostic to evaluate density fluctuations and plasma rotation profiles. It demands a steerable beam that provides a defined spot-size for the wave plasma interaction and multiple frequencies to probe plasma regions of different densities. In this paper, the suitability of a frequency-scanned phased array as a front-end for a Doppler reflectometry system is demonstrated.

Far Field Calculations and Measurements of an S-Bend Deformed Oversized HE11-Waveguide Antenna

The mode conversion in an s-bend deformed HE11 waveguide is calculated. With the mode mixture at the end of the waveguide, the corresponding far-fields for different waveguide lengths are calculated and compared with measurements. Optimisations of the total waveguide length and diameter are performed.

Optimization of a Frequency Diplexer Based on the Talbot Effect in Oversized Rectangular Waveguides

Calculations of a rectangular waveguide frequency diplexer for high power applications based on the Talbot effect are presented. For the calculation, a method based on analytical approximations, which is valid in highly oversized waveguides, as well as a more accurate mode analysis method are described. The latter allows the integration into an optimization code, which reduces mode conversion losses while maintaining practicable geometrical dimensions.