Collective optical Thomson scattering in pulsed-power driven high energy density physics experiments (invited)

• Published in: Review of scientific instruments 2021-03, Vol.92 (3)
• Main Authors: Suttle, L. G., Hare, J. D., Halliday, J. W. D., Merlini, S., Russell, D. R., Tubman, E. R., Valenzuela-Villaseca, V., Rozmus, W., Bruulsema, C., Lebedev, S. V.
• Format: Article
• Language: English
• Subjects: High energy density physics; Optical fibers; OTHER INSTRUMENTATION; Plasma flows; Plasma temperature; Thomson scattering; Velocity measurement
• ISSN: 0034-6748; 1089-7623

Optical collective Thomson scattering is used to diagnose magnetized high energy density physics experiments at the Magpie pulsed-power generator at Imperial College London. The system uses an amplified pulse from the 2nd harmonic of a Nd:YAG laser (3 J, 8 ns, 532 nm) to probe a wide diversity of hightemperature plasma objects; with densities in the range of 1017-1019 cm-3 and temperatures between 10 eV and a few keV. The scattered light is collected from 100 μm-scale volumes within the plasmas, which are imaged onto optical fiber arrays. Multiple collection systems observe these volumes from different directions, providing simultaneous probing with different scattering K-vectors (and different associated α-parameters, typically in the range 0.5 – 3) allowing independent measurements of separate velocity components of the bulk plasma flow. The fiber arrays are coupled to an imaging spectrometer with a gated ICCD. The spectrometer is configured to view the ion-acoustic waves (IAWs) of the collective Thomson scattered spectrum. Fits to the spectra with the theoretical spectral density function S(K,ω) yield measurements of the local plasma temperatures and velocities. Fitting is constrained by independent measurements of the electron density from laser interferometry, and the corresponding spectra for different scattering vectors. Furthermore, this TS diagnostic has been successfully implemented on a wide range of experiments, revealing temperature and flow velocity transitions across magnetized shocks, inside rotating plasma jets and imploding wire arrays, as well as providing direct measurements of drift velocities inside a magnetic reconnection current sheet.