Relative Frequency Calibration for Fast Frequency Sweep Microwave Reflectometry

Abstract

A frequency-modulated reflectometer with a frequency band of 26.5 to 40 GHz has been constructed and installed in the TST-2 spherical tokamak to measure fast density profile evolution. In order to calibrate the instantaneous frequency for various frequency sweep rates, a waveform matching method has been proposed and applied to the reflectometer. This method compares the interference patterns (i.e., waveforms) of a fixed target reflection, which do not depend on the sweep rate, and obtains the instantaneous frequency by fitting the target waveform time to the time for a reference sweep waveform. This allows evaluation of output frequency stability. The overall frequency error, including reproducibility, is around 0.1 GHz. Sweep rates up to 20 µs were used to measure the density profile evolution during rf heating.

Parametric decay instability during high harmonic fast wave heating experiments on the TST-2 spherical tokamak

“…A microwave (25.85 GHz) reflectometer [5] is used to detect the oscillation of the cutoff layer caused by the RF wave. This reflectometer can also be used to measure the density profile by using a swept-frequency oscillator [6]. Visible light emission is also used to detect oscillations due to the RF wave [7,8].…”

Real-time detection and correction of frequency sweeping non-linearities of FMCW reflectometry microwave sources

“…The herein presented methods to overcome these challenges are based on solutions long implemented on ASDEX Upgrade's broadband reflectometers [5,6]. More recently, different strategies have also been put to test in newer reflectometers and in different tokamaks [7][8][9], however, not with the intent or the ability of producing single sweep on-line full source recalibration or linearity verification.…”

The response of microwave reflectometry under generalized configuration

“…In microwave reflectometry, a microwave is launched into the plasma, and the wave reflected from the cutoff layer in the plasma is detected [1,2]. Since the phase of the reflected wave is determined by the optical distance, the electron density profile can be derived from the phase variation along the beam path [3][4][5][6][7][8][9][10][11][12][13] and the density fluctuation can be derived from the phase fluctuation [14][15][16][17][18][19][20]. Although wave behavior is well understood in a one-dimensional system [21][22][23], two-or three-dimensional configurations are complicated because of diffraction effects, and it is not straightforward to obtain the relationship between the phase fluctuations and the measured complex electric field.…”