Microwave Receivers for Fast-Ion Detection in Fusion Plasmas

The main objectives of this thesis are to determine fundamental properties of a millimeterwave radiometer used to detect radiation associated with dynamics of fast ions and to investigate possibilities for improvements and new designs. The detection of fast ions is based on a principle called collective Thomson scattering (CTS).

The Danish CTS group has been involved in fusion plasma experiments for more than 10 years and the future plans will most probably include the International Thermonuclear Experimental Reactor (ITER). Current CTS systems designed by the Danish group are specified for the frequency range from 100 to 110 GHz.

In this thesis we follow the path of the radiation from a fusion plasma to the data acquisition unit. Firstly, the scattered radiation passes through the quasi-optical system. Quasi-optical elements required to be installed on the high field side (HFS) on the ITER are assessed. For the ITER HFS receiver we have designed and measured the quasioptical components that form a transmission link between the plasma and the radio frequency (RF) electronics.

This HFS receiver is required to resolve the near parallel velocity components created by the alpha particles. Secondly, the radiation will encounter the RF part. This part is not yet designed for ITER, but instead the solution is addressed to the CTS receiver installed at ASDEX Upgrade (AUG).We have put effort to thoroughly examine and evaluate the performance of the receiver components and the receiver as an assembled unit.

We have measured and analyzed all the receiver components starting from the two notch filters to the fifty square-law detector diodes. The receiver sensitivity is calculated from the system measurements and compared with the expected sensitivity based on the individual component measurements. Besides the system considerations we have also studied improvements of two critical components of the receiver.

The first component is the notch filter, which is needed to block strong probing radiation coming from a gyrotron. The newly designed notch filters within the scope of this thesis are superior to their predecessors and are installed in the CTS receiver. A filter was subsequently designed, built, and tested by the CTS group and installed by the German ECE group at AUG.

Our filter enables the ECE group to make measurements in the frequency range corresponding to the gyrotron operation. The second component is the mixer. The conversion loss of the mixer, together with loss in waveguide components and quasi-optic parts, is the main contributor to the noise and thereby degrades the signal to-noise ratio.

The architecture of the mixer is a subharmonic type, optimized to be driven by a double local oscillator (LO) frequency in order to downshift the RF to intermediate frequency (IF). The simulated results are presented for the case of 140 GHz, which is relevant for a number of fusion plasma diagnostics such as ECE and interrogation of neo-classical tearing modes (NTM).

Finally, conclusions are drawn and future aspects presented. This study seeks to give insights towards new solutions and improvements of the existing CTS receiver architecture.