Interferometry

Basic description

#Interferometry in the 4 mm zone on the CASTOR tokamak

Basic description

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.

Obr. 1: Scheme of the Wharton interferometer used in the GOLEM tokamak

Current status of the section on the GOLEM tokamak

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.

Calibration

With the interferometer in final configuration, it is now necessary to properly calibrate it. The calibration consists of several easy steps. See section 2.7 of this thesis to understand them. The trick is to maximize the amplitude at the amplifier output.

Display the sawtooth and selective amplifier output on an oscilloscope.
Adjust the sawtooth generator frequency until the amplifier output reaches maximum amplitude This ensures that the sawtooth has the same frequency as the center frequency of the selective amplifier. It should therefore happen at 522 kHz.
Set the sawtooth amplitude to minimum and gradually increase it, until the amplifier output reaches maximum amplitude. This step ensures that the beats on the diode have the same frequency as the sawtooth, because the modulation results in phase shift of exactly 2π.
Adjust the attenuator in the reference line so that the amplifier is close to saturation (but not saturated)

Density evaluation

Because the old analog interferometer circuit was out of order, another way to evaluate interferometer output had to be developed. Because all data at GOLEM are processed by computer, an obvious solution is to sample the two signals and evaluate their phase shift by a computer script - digital density evaluation. The sawtooth and sine signal are sampled and base carrier frequency is determined as maximum frequency in the spectrum, which is calculated by Fourier transform of the sine signal. Complex exponential with this frequency is obtained by inverse Fourier transform after all other frequencies are eliminated. In the next step, the signals are multiplied by this exponential and their phase is evaluated as an argument of the resulting complex number.

While the digital density evaluation described in previous chapter works well, there is a disadvantage with this method. It is necessary to sample two channels at once and in order for the algorithms to work reliably, they have to be sampled at sampling rate of 1 MS/s as a minimum. A closely related issue is higher demand for hard drive space to save data from the two channels. Another disadvantage is that the algorithms need some time to finish. A decision was made to get rid of these issues by designing an electronic circuit to evaluate phase shift - analog density evaluation. This way there is no need to sample the two signals and save them. Instead, only one channel at arbitrarily low sampling rate could be used to sample density data. A printed circuit board (PCB) has beed designed to accommodate the power supply, new sawtooth generator, copy of the old selective amplifier and phase detecting circuit

For most up-to-date information, please contact the professional garant for this section - Ing. František Žáček. CSc.