Fringe jump analysis and electronic corrections for the Tore Supra far infrared interferometer

Abstract

On the Tore Supra tokamak, the ten-channel far infrared interferometer consists of a double color (119 and 195 microm) system with two detectors for each channel to measure the plasma density. The phase measurement is obtained by combining a 100 kHz shifted reference beam with the probing beam that has crossed the plasma. The achieved precision--a few percent of a fringe--is very good compared with the expected variations due to plasma, which are on the order of several fringes. However, the counting of the fringe variations can be affected when the signal is perturbed by electromagnetic interferences or when it deviates in the presence of strong plasma refraction changes occurring during ICRH breakdowns, pellet injections, or disruptions. This induces a strong decrease in the reliability of the measurement, which is an important concern when the diagnostic is used for density control. We describe in this paper the renewing of the electronics that has been achieved to reduce and correct the number of the so-called fringe jumps. A new zero crossing method for phase measurement is used, together with a field programmable gate array semiconductor integration, to measure the phase and activate the algorithm of corrections every 10 micros. Comparisons between a numerical oscilloscope analysis and the corrected acquired data in the case of laboratory amplitude modulation tests and in the case of real plasma perturbations are also discussed.

Robust phase estimation for signals with a low signal-to-noise-ratio

“…For concreteness we describe the signal processing presently applied at ASDEX Upgrade [13] only. However, the general methodology is very similar at most tokamak and stellarator devices [14]. The detector signals at the tokamak experiment ASDEX Upgrade are digitized using analog-to-digital converters (ADC) with a sampling rate of 1 MHz with 14 bit resolution, providing an oversampling factor of 100 of the beat-wave signal.…”

Line integrated density measurements on the Versatile Experiment Spherical Torus (VEST) using frequency sweep interferometer

“…Phase jumps appear as rapid density changes in the measured LID. Assuming that the plasma density changes slowly in a stable equilibrium state, the phase jumps can be compensated for using algorithms implemented in software [13] or hardware [14,15]. However, in general, for example in the case of a disruption event, it is impossible to distinguish whether the sudden phase change is a phase jump or due to an actual rapid density change.…”

Phase jump detection and correction based on the support vector machine

“…As a well-performing electron density diagnostics system, the interferometer has been extensively used in numerous tokamaks. However, the line-integrated electron density measurement is frequently disturbed by fringe jumps, which has been observed in several fusion devices, such as ASDEX Upgrade [2,3], JET [4][5][6][7][8], KSTAR [9], LHD [10], NSTX [11], Tore Supra [12], and SUNIST [13]. Consequently, several methods have been attempted to detect the fringe jump and eliminate the disturbance from the fringe jump [2][3][4][5][6][7][8][9][10][11][12].…”

“…Many electronic hardware and offline software systems for automatic fringe jump detection and correction have been developed to compensate for the impact of fringe jump. The electronic correction system can realize high corrections frequency and has achieved satisfying results on LHD [10], Tore Supra [12], and JET [7,8]. However, the detection efficiency of fringe jump requires improvement, and the rapid change of density, for example, caused by pellet injection, may result in overcorrection of the phase [12].…”