Microwave engineering in fusion plasmas

Project:           Control of microwave heating of the TCV fusion plasma.
Assignment: Polarization matching.
Level:              Master graduation project

1. Introduction

In ITER, the world’s largest nuclear fusion experiment that is presently under construction in Cadarache (France), a plasma confined by magnetic fields will be heated to some 200 million C. One of the heating methods is the injection of high-power microwaves, at a frequency tuned to the gyro motion of the electrons in the magnetic field. For optimal coupling the polarization of the injected waves must match that of the waves emitted by the electrons themselves (electron cyclotron emission:ECE). Ref. a.

2. Description

The problem posed is to match the polarisation of the injected microwave beam to the polarisation that is emitted by the electrons. The effect of a mismatch in polarization must be studied and a specification drawn up to what degree the polarization should be matched and how fast. A scheme must be devised on how to obtain the full polarisation state of the ECE, bearing in mind that there is likely to be a difference in polarisation between the sightline at which the ECE is measured and the sightline at which the power is launched. Finally, the desired  polarization to launch should be communicated to a control system capable of steering the polarisation optics of the microwave power source. The research will initially focus on the – fast and accurate - determination of the polarization in a realistic geometry, and on the evaluation of the effect of a mismatch of the polarization. A potential scheme for the polarization measurement consists of two antenna’s with different polarization direction connected to a downconversion mixer and a phase and amplitude detector (Figure 1). For experimental verification of the developed method we will focus on measurements at 60 GHz.

3. Practical arrangements

The overall project is a collaboration between the groups of three faculties: i) Control Systems Technology of Mechanical Engineering, ii) the Fusion group in Applied Physics and iii) Mixed-signal Microelectronics (MsM) in Electrical Engineering. The assignment will initially be carried out in the laboratory of MsM under supervision of both Fusion and MsM. The MsM supervisor will be Marion Matters-Kammerer and the Fusion supervisor will be Hans Oosterbeek. Once the laboratory work is mature enough it is foreseen that measurements are taken at an actual fusion experiment, the tokamak "TCV" in Switzerland.

4. References

a) Hans-Jürgen Hartfuss and Thomas Geist, Fusion Plasma Diagnostics with mm-Waves, Wiley-VCH, 2013, ISBN 978-3-527-41105-4
b) Constantine A. Balanis, Antenna Theory: Analysis and Design, ISBN: 978-0-471-66782-7