Real-time dispersion interferometry for density feedback in fusion devices
JOURNAL OF INSTRUMENTATION(2018)
摘要
Interferometry as one of the most common core fusion diagnostics has traditionally suffered from incomplete vibration compensation. Dispersion interferometry promises a more complete compensation of vibrations. For this reason it is being employed in an increasing number of experiments. However, thus far none of them have shown reliable real-time low-latency processing of dispersion interferometry data. Nonetheless this is a necessity for most machines when trying to do density feedback control, most notably in long discharges like the ones planned at the W7-X stellarator and ITER. In this paper we report the development of a new phase extraction method specifically developed for real-time evaluation using field programmable gate arrays (FPGA). It has been shown to operate reliably during the operation phase OP1.2a at W7-X and is now routinely being used by the W7-X density feedback system up to very high densities above 1.4 x 10(20) M-2 without showing 2 pi-wraps and exhibits increased wrap stability by double-data-rate sampling. A rigorous error analysis has been conducted shedding insights into the signal composition of a dispersion interferometer have been gained. This includes the environmental effects, most notably air humidity, on the phase measurement and the correction thereof.
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关键词
Plasma diagnostics-interferometry,spectroscopy and imaging,Digital signal processing (DSP),Analysis and statistical methods,Pattern recognition, cluster finding, calibration and fitting methods
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