Abstract
Optical collective Thomson scattering (TS) 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 second harmonic of a Nd:YAG laser (3J, 8ns, 532nm) to probe a wide diversity of high-temperature plasma objects, with densities in the range of 1017-1019cm-3 and temperatures between 10eV 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 of 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 intensified charge coupled device. The spectrometer is configured to view the ion-acoustic waves 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. 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.
Highlights
Thomson scattering (TS) is a powerful diagnostic tool in high energy density physics (HEDP) experiments as it allows local timeresolved measurements of key plasma parameters
We have described a TS diagnostic used in pulsedpower driven HEDP experiments
It offers flexibility to reconfigure the scattering geometry between experiments to (1) probe with scattering vectors that select the flow velocity components that are of most interest, (2) set the TS α-parameter to select the TS regime, and (3) maximize the spectral resolution through the magnitude of the K-vector
Summary
Thomson scattering (TS) is a powerful diagnostic tool in high energy density physics (HEDP) experiments as it allows local timeresolved measurements of key plasma parameters. At the Magpie pulsed-power generator, TS is used to diagnose HEDP experiments studying the interactions of supersonic flows of magnetized, high-temperature, and high-atomic number plasmas (average ionization Z ≫ 1). The electron densities in these plasmas typically range between 1017 and 1019 cm−3 and are accompanied by embedded magnetic fields of several tesla (originating from the drive current), which are advected with the flows. The interactions of these flows with either stationary obstacles or counter-propagating flows generate shock structures with associated plasma heating (reaching temperatures between 10 eV and several keV) and in some cases reconnection of the embedded magnetic fields.
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