Contact-less charged particles density measuremnment possibility is one of the main demands on hot plasma diagnostics of tokamaks. On CASTOR tokamak there was used hommodynamical microwave interferometer working on $λ=4$ mm (frequency $f=75$ GHz), with dispersive conduction of relative length about $\frac{L}{λ} = 3000$ (in reality created by waveguide transporting probing wave from generator to transmitting antenna and waveguide transporting the wave from before and thus bearing the information about the plasma density, back to the receiver).
For wave probing generator, there was used Gunn diode with fast ($f_{IF} = 500$ kHz) frequency wobbling, which enables to monitor the plasma density changes over the time on $f_{IF}$ interfrequency and thus with appropriate time resolution converging to 10 microseconds. At stated length of dispersion conducting, it is necessary for formation of interfrequency sinus-like signal on receiver (for obtaining phase difference between referencing and measuring wave on receiver of 2π) relative small frequental wobbling generator ($\frac{Δf}{f} = \frac{λ}{L}$ about $Δf = 25$ MHz). This electronic wobbling was created by saw-like shaped voltage, lead to varactor, which then, according to ammount of voltage, changes the frequency of generated wave. According to experiences with operations of interferometer on CASTOR, for its demanded function it is needed the power of at least 100 mW. The designated garant for this section is Ing. František Žáček. CSc.
After the tokamak has been moved to FNSPE CTU, the former interferometer was never successfully installed. An attempt to install the interferometer at GOLEM tokamak has been made in 2011 by a group of students consisting of O. Grover, T. Odstrcil and M. Odstrcil. Although they eventually did not make the interferometer reliably operational, a lot of useful work has been done. To replace the interferometer circuit, an alternative way of evaluating the phase shift was devised. The idea was to digitally sample the sawtooth and the signal at the detecting diode, and calculate their mutual phase shift by a computer script. This script based on the Fourier transform algorithm was written and used for further tests. The interferometer was moved to GOLEM, vacuum anges with antennas were mounted on the tokamak vessel and the plywood board with the crutial microwave parts was installed on the top shelf in far left corner of the tokamak room. The sawtooth generator, selective amplifier and Gunn oscillator power supply was positioned on the left side of the same shelf and connected to power supply switched by tokamak relays. This allows to turn the device on and off automatically, without the need to actually manipulate the switches. There was some unused space on the right side of the shelf that accommodated four-channel Tektronix DPO3014 oscilloscope to monitor or save the interferometer output. Figure below shows the final setup. The next step was to connect the device to the tokamak by long dispersion line.
Old interferometry setup on CASTOR tokamak. Shown selective amplifier and phase detector are stored for backup or planned second line.
Ukrainian Gunn generator and its power supply.
Tokamak vacuum flange and antennas.
Waveguides to the tokamak.
The PCB mounted inside the box and its front panel.
Final state of the interferometer shelf.
Analog phase detector circuit
##Reports
* [2017 Michal F.: Závislost hustoty na ostatních parametrech plazmatu na tokamaku GOLEM](http://golem.fjfi.cvut.cz/wiki/TrainingCourses/FTTF/2016-2017/MichFarn/index_MF)
* [2016 Lukáš M.: Diploma thesis](http://golem.fjfi.cvut.cz/wiki/Library/GOLEM/MastThesis/15MatenaLukas.pdf)
© Lukáš Matěna
Edited by M.Farník