Frequency-modulated Reflectometer Research Articles

Measurement of axial phase difference of density fluctuations owing to spontaneously excited waves by using microwave reflectometer on GAMMA 10/PDX.

In the GAMMA 10/PDX tandem mirror, plasma with strong ion-temperature anisotropy is produced by using the ion cyclotron range of frequency waves. This anisotropy of ion temperature causes several Alfvén-Ion-Cyclotron (AIC) waves to spontaneously excite in the frequency range just below the ion cyclotron frequency. In addition, difference-frequency (DF) waves are excited in the radial inner region of the plasma by wave-wave coupling among the AIC waves. The radial density profiles were measured at multi-axial positions using a frequency-modulation reflectometer with an axial array of microwave antennas, and an axial variation of the density was found to be significant. In addition, a relative phase difference of the DF wave between axially separated two points was first obtained by finely choosing the probing frequency of the reflectometers with a maximum coherence used as a measure, indicating that the DF wave is a propagating wave, while the pump AIC waves are standing waves in the axial region of measurement.

The electron density profile measurement with a fast time-resolution in the KSTAR tokamak

A frequency-modulation reflectometer is installed in the KSTAR tokamak to measure the plasma density profile. The reflectometer operates in the frequency range of 32.3–107.3 GHz. A plasma density up to 7 × 1019 m−3 can be measured when the toroidal magnetic field is 2 T. The reflectometer measures the electron density profile with a time resolution of 25 μs for 0.16 s. This fast time-resolution is essential for the study of the transport barrier formation during the L-H transition. In this paper, the measured time evolutions of the sawtooth oscillation, the density increase due to supersonic molecular beam injection (SMBI), and the pedestal relaxation due to an edge-localized mode (ELM) are presented. The spatial profile of slow fluctuations (< 10 kHz) may be extracted from these fast profile measurements. In addition, the dependences of the pedestal structure, such as its width and height, on the plasma parameters are characterized.

Graphics processing unit-assisted density profile calculations in the KSTAR reflectometer.

Wavelet transform (WT) is widely used in signal processing. The frequency modulation reflectometer in the KSTAR applies this technique to get the phase information from the mixer output measurements. Since WT is a time consuming process, it is difficult to calculate the density profile in real time. The data analysis time, however, can be significantly reduced by the use of the Graphics Processing Unit (GPU), with its powerful computing capability, in WT. A bottle neck in the KSTAR data processing exists in the data input and output (IO) process between the CPU and its peripheral devices. In this paper, the details of the WT implementation assisted by a GPU in the KSTAR reflectometer are presented and the consequent performance improvement is reported. The real time density profile calculation from the reflectometer measurements is also discussed.

Development of frequency modulation reflectometer for Korea Superconducting Tokamak Advanced Research tokamak

Frequency modulation reflectometer has been developed to measure the plasma density profile of the Korea Superconducting Tokamak Advanced Research tokamak. Three reflectometers are operating in extraordinary polarization mode in the frequency range of Q band (33.6-54 GHz), V band (48-72 GHz), and W band (72-108 GHz) to measure the density up to 7 × 10(19) m(-3) when the toroidal magnetic field is 2 T on axis. The antenna is installed inside of the vacuum vessel. A new vacuum window is developed by using 50 μm thick mica film and 0.1 mm thick gold gasket. The filter bank of low pass filter, notch filter, and Faraday isolator is used to reject the electron cyclotron heating high power at attenuation of 60 dB. The full frequency band is swept in 20 μs. The mixer output is directly digitized with sampling rate of 100 MSamples/s. The phase is obtained by using wavelet transform. The whole hardware and software system is described in detail and the measured density profile is presented as a result.

Development of frequency modulation reflectometer for KSTAR tokamak: Data analysis based on Gaussian derivative wavelet

A frequency modulation reflectometer has been developed to measure the density profile of the KSTAR tokamak. It has two channels operating in X-mode in the frequency range of Q band (33-50 GHz) and V band (50-75 GHz). The full band is swept in 20 μs. The mixer output is directly digitized at the sampling rate of 100 MSamples∕s. A new phase detection algorithm is developed to analyze both amplitude and frequency modulated signal. The algorithm is benchmarked for a synthesized amplitude modulation-frequency modulation signal. This new algorithm is applied to the data analysis of KSTAR reflectometer.

Relative Frequency Calibration for Fast Frequency Sweep Microwave Reflectometry

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.

High-resolution dual-polarization frequency modulated reflectometer density profile measurements on DIII-D

A novel dual-polarization technique has been applied to the frequency-modulated electron density profile reflectometer on DIII-D. In this application, both O- and X-mode waves are simultaneously launched from a common high-performance solid-state source, and simultaneous measurements of both O- and X-mode reflections from corresponding cutoffs serve to expand the density coverage. The system consists of both Q-band (33–50 GHz) and V-band (50–72 GHz) subsystems, their combination covers a density range from 0 to 6.4×1019 m−3, with a time resolution of ⩾10 μs and spatial resolution of ⩾4 mm. The dual-polarization technique was implemented successfully in the Q-band subsystem during 2003 and detailed information for H-mode pedestal physics studies was provided. It has also recently been applied to the V-band subsystem, and extended contributions to H-mode pedestal physics and particle transport studies are expected during the 2004 DIII-D campaign.

Design of an X-mode swept frequency modulation reflectometer for the measurement of KSTAR plasma density profiles (invited)

An X-mode swept frequency modulation (FM) reflectometry system has been designed to measure the electron density profiles of the “initial” KSTAR plasma. Fast swept HTO oscillators are employed to avoid density fluctuation effects, and frequency quadruplers are utilized to expand the HTO frequency range of 8–18 GHz to completely cover the X-mode cutoff frequency range of 33–66 GHz. The system can also be utilized to measure the edge profiles of the “Day One” KSTAR plasma by either switching from X- to O-mode reflectometry or by employing higher frequency millimeter-wave components and retaining the X-mode reflectometry configuration. To facilitate engineering design and optimization, a 3D drawing tool is utilized to effectively deal with any technical problems that may happen under actual KSTAR conditions. Details of the KSTAR FM reflectometry system are described together with important design issues.

The effects of density fluctuation on density profile measurements using a broadband frequency modulated reflectometer

Numerical simulations of frequency modulation (FM) reflectometry are carried out for the effects of density fluctuations on phase measurements. It is found that the fluctuation amplitude, the fluctuation frequency, the sweep time of a source, and the beat frequency of a mixer output play the important roles. The reliability of density profile measurement is investigated for various sweep times and local path lengths. It is demonstrated that the reconstructed density profiles seem to be improved when the sweep time is faster than 100 μs and the local path length is shorter than 1 m. The numerical simulations are compared with the experimental results obtained from a broadband FM reflectometer applied to the GAMMA 10 tandem mirror.

Reflectometric density profile measurement using an ultrafast sweep oscillator

A broad band frequency modulation reflectometer system has been installed and operated on the GAMMA 10 tandem mirror. A fast sweep hyperabrupt varactor-tuned oscillator (HTO) is used as a source, which can be swept over the full frequency band (12–18 GHz) in less than 5 μs. The high frequency fringes due to the change in the cut-off layer can be distinguished from those due to the existence of density fluctuations, since the fluctuation level with frequency components larger than the ion cyclotron range of frequency (10 MHz) is much smaller than the average density (10 −4 or less). The reliability of density profile measurement is investigated for various sweep times of HTO from 5 μs to 5 ms. It is observed that the reliability is greatly improved and the reconstructed density profiles agree well with that obtained from an interferometer when the sweep time is faster than 10 μs. The present reflectometer is routinely applied to determine the density profiles of the central cell plasma in GAMMA 10.

Development of a fast solid-state high-resolution density profile reflectometer system on the DIII-D tokamak

A new fast-sweep, broadband frequency-modulated reflectometer on the DIII-D tokamak has produced routine, reliable density profiles with high spatial (⩽1 cm) and temporal resolution (∼100 μs). The system utilizes a solid-state microwave source and an active quadrupler, covering the full Q-band (33–50 GHz) and providing high output power (20–60 mW). The rf source frequency is linearized using an arbitrary function generator and the temperature of the source is actively controlled to reduce rf frequency drifts. The system hardware allows the rf frequency to be swept fullband in 10 μs, but, due to digitization limits, the sweep time used currently is 75–100 μs. The reliability of the reconstructed profiles was improved by a combination of fast frequency sweep, which reduces density fluctuation effects on the measurements, and advanced signal analysis based on digital complex demodulation, which improves phase accuracy. The fast-sweep system has resolved fast-changing edge density profiles during edge localized modes with unprecedented resolution.

Ultrafast broadband frequency-modulation reflectometer for density profile measurements in GAMMA 10

A broadband microwave reflectometer was applied to the GAMMA 10 tandem mirror using a fast-sweep hyperabrupt varactor-tuned oscillator (HTO) that can be swept over the full band (12–18 GHz) in less than 5 μs. Clear fringes are observed during a plasma shot when the resultant frequencies from the reflectometer output are much larger than the ion cyclotron range of frequency (3–10 MHz). The density profile measurement is performed for various sweep times of HTO from 1 ms to 5 μs in which two reconstruction methods are utilized, zero-cross counting of the fringes and frequency analysis using the maximum entropy method. The reliability of the profile measurement seems to be improved when the sweep time is faster than 10–30 μs. The integrated value of many reconstructed density profiles agrees well with the profile obtained from a scanning interferometer. The fast-sweep reflectometer is also applied to the measurement of density fluctuations such as plasma movement as well as density profiles.

Resolution of a frequency-modulated reflectometer

Resolution of a frequency-modulated reflectometer