A heterodyne dispersion interferometer for wide bandwidth density measurements on DIII-D

• Published in: Review of scientific instruments 2018-10, Vol.89 (10), p.10B105-10B105
• Main Authors: Akiyama, T., Van Zeeland, M. A., Boivin, R. L., Carlstrom, T. N., Chavez, J. A., Muscatello, C. M., O’Neill, R., Vasquez, J., Watkins, M., Martin, W., Colio, A., Finkenthal, D. K., Brower, D. L., Chen, J., Ding, W. X., Perry, M.
• Format: Article
• Language: English
• Subjects: Acousto-optics; Carbon dioxide; Carbon dioxide lasers; Density; Dispersion; Interferometers; Laser applications; Magnetohydrodynamics; Scientific apparatus & instruments; Temporal resolution; Variation
• ISSN: 0034-6748; 1089-7623
• DOI: 10.1063/1.5037997

In order to improve both the density and particularly the temporal resolution beyond previous dispersion interferometers (DIs), a heterodyne technique based on an acousto-optic (AO) cell has been added to the DI. A 40 MHz drive frequency for the AO cell allows density fluctuation measurements into the MHz range. A CO2 laser-based heterodyne DI (HDI) installed on DIII-D has demonstrated that the HDI is capable of tracking the density evolution throughout DIII-D discharges, including disruption events and other rapid transient phenomena. The data also show good agreement with independent density measurements obtained with the existing DIII-D two-color interferometer. The HDI line-integrated density resolution sampled over a 1 s interval is ∼9 × 1017 m−2. Density fluctuations induced by MHD instabilities are also successfully measured by the HDI.