Bench testing of a heterodyne CO2 laser dispersion interferometer for high temporal resolution plasma density measurements

T Akiyama; M A Van Zeeland; R L Boivin; T N Carlstrom; J A Chavez; C M Muscatello; R C O'Neill; J Vasquez; M Watkins; W Martin; A Colio; D K Finkenthal; D L Brower; J Chen; W X Ding; M Perry

doi:10.1063/1.4969055

Bench testing of a heterodyne CO₂ laser dispersion interferometer for high temporal resolution plasma density measurements

Rev Sci Instrum. 2016 Dec;87(12):123502. doi: 10.1063/1.4969055.

Authors

T Akiyama¹, M A Van Zeeland², R L Boivin², T N Carlstrom², J A Chavez², C M Muscatello², R C O'Neill², J Vasquez², M Watkins², W Martin², A Colio³, D K Finkenthal³, D L Brower⁴, J Chen⁴, W X Ding⁴, M Perry⁵

Affiliations

¹ National Institute for Fusion Science, 322-6 Oroshi-cho, Toki-shi, Gifu 509-5292, Japan.
² General Atomics, P.O. Box 85608, San Diego, California 92186-5608, USA.
³ Palomar College, San Diego, California 92069, USA.
⁴ Department of Physics and Astronomy, University of California Los Angeles, Los Angeles, California 90095, USA.
⁵ California State University, San Marcos, San Marcos, California 92096, USA.

PMID: 28040946
DOI: 10.1063/1.4969055

Abstract

A heterodyne detection scheme is combined with a 10.59 μm CO₂ laser dispersion interferometer for the first time to allow large bandwidth measurements in the 10-100 MHz range. The approach employed utilizes a 40 MHz acousto-optic cell operating on the frequency doubled CO₂ beam which is obtained using a high 2nd harmonic conversion efficiency orientation patterned gallium arsenide crystal. The measured standard deviation of the line integrated electron density equivalent phase resolution obtained with digital phase demodulation technique, is 4 × 10¹⁷ m^-2. Air flow was found to significantly affect the baseline of the phase signal, which an optical table cover was able to reduce considerably. The heterodyne dispersion interferometer (DI) approach is found to be robustly insensitive to motion, with measured phase shifts below baseline drifts even in the presence of several centimeters of retroreflector induced path length variations. Plasma induced dispersion was simulated with a wedged ZnSe plate and the measured DI phase shifts are consistent with expectations.