A High-Speed Optical Diagnostic that uses Interference Filters to Measure Doppler Shifts
Authors: S.F. Paul, C. Cates, M. Mauel, D. Maurer, G. Navratil, and M. Shilov
Date of PPPL Report: August 2004
Presented at: the 15th APS Topical Conference on High-temperature Plasma Diagnostics, 19-22 April 2004, San Diego, CA; proceedings to be published in the Review of Scientific Instruments.
A high-speed, non-invasive velocity diagnostic has been developed for measuring plasma rotation. The Doppler shift is determined by employing two detectors that view line emission from the identical volume of plasma. Each detector views through an interference filter having a passband that varies linearly with wavelength. One detector views the plasma through a filter whose passband has a negative slope and the second detector views through one with a positive slope. Because each channel views the same volume of plasma, the ratio of the amplitudes is not sensitive to variations in plasma emission. With suitable knowledge of the filter characteristics and the relative gain, the Doppler shift is readily obtained in real time from the ratio of two channels without needing a low throughput spectrometer. The systematic errors -- arising from temperature drifts, stability, and frequency response of the detectors and amplifiers, interference filter linearity, and ability to thoroughly homogenize the light from the fiber bundle -- can be characterized well enough to obtain velocity data with ±1 km/sec with a time resolution of 0.3 msec.