Microwave heating and antennas

Two microwave systems are used to heat the TJ-K plasma. A magnetron at 2.45 GHz and a power of 6 kW is used for the low-magnetic-field operation at 70 mT. At the high magnetic field of 0.3 T, a traveling-wave-tube (TWT) amplifier with a power of 1.2 kW is available. The TWT can be modulated in frequency and power. The frequency can be tuned in the range 7.9-8.4 GHz.

The array antenna was built for operation with the TWT. In the frequency range from 7.9 to 8.4 GHz, the emitted wave vector can be swept from -45 to +45 °. Calculations of the O-X-B conversion process have shown that the optimal angle of incidence of the microwave with respect to the magnetic field is about -45 °.

The figure depicts the pattern of the array-antenna for a frequency of 7.9 GHz. The measured characteristics fits rather well the theoretical one. The full width at half maximum of the shown pattern is at about 20 °. The power level of the next maxima are about 20 dB below the main maximum. The main antenna loop is at an angle of about -42 °. This is close enough to the optimum angle for the O-X-B conversion process

In order to gain flexibility with electron-cyclotron-resonance heating (ECRH) in fusion plasmas with respect to magnetic field and density, the frequencies at the second and third harmonic of the resonance are of importance (X2 and O3 heating). However, the absorption ate higher harmonics is not complete. In order to reflect the not-absorbed part of the microwave beam back into the plasma - and this in the desired polarisation - a holographic mirror has been designed and built. The mirror will be installed into the ASDEX upgrade tokamak and used for experiments.

This picture showsAlf Köhn-Seemann

Alf Köhn-Seemann

Research associate, Plasma Dynamics and Diagnostics

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