Abstract
A grid image refractometer (GIR) has been implemented at the Nike krypton fluoride laser facility of the Naval Research Laboratory. This instrument simultaneously measures propagation angles and transmissions of UV probe rays (λ = 263 nm, Δt = 10 ps) refracted through plasma. We report results of the first Nike-GIR measurement on a CH plasma produced by the Nike laser pulse (∼1 ns FWHM) with the intensity of 1.1 × 10(15) W/cm(2). The measured angles and transmissions were processed to construct spatial profiles of electron density (ne) and temperature (Te) in the underdense coronal region of the plasma. Using an inversion algorithm developed for the strongly refracted rays, the deployed GIR system probed electron densities up to 4 × 10(21) cm(-3) with the density scale length of 120 μm along the plasma symmetry axis. The resulting n(e) and T(e) profiles are verified to be self-consistent with the measured quantities of the refracted probe light.
Highlights
Knowing plasma conditions in the underdense coronal region is crucial to understanding physical processes of laser plasma instabilities (LPIs)1 in laser fusion research
We report a new implementation of the grid image refractometer (GIR) system on the Nike krypton fluoride (KrF) laser facility and its application for measuring profiles of density and temperature in a plasma created for LPI research in the Naval Research Laboratory (NRL) KrF laser fusion program
The spatial distribution of the electron density was constructed in three steps: deflection angles of the probe light were extracted from the beamlet locations recorded in the GIR images, optical path differences (OPDs) of the individual beamlets were calculated from the angle data, and the density profile was built by Abel-inverting3 the OPDs for the cylindrically symmetric plasma structure
Summary
Knowing plasma conditions in the underdense coronal region is crucial to understanding physical processes of laser plasma instabilities (LPIs) in laser fusion research. In large scale plasmas relevant to laser fusion, two plasmon decay (TPD) and stimulated Raman scattering (SRS) draw particular attention among various LPIs for their low thresholds and hot electron generation that preheat the target These instabilities exist in the underdense coronal region near or lower than the quarter critical density (nec/4), where nec is the critical density of the driver laser light. The GIR method naturally provides a way to determine the electron temperature profile by simultaneously measuring beamlet transmissions (or absorptions) through the plasma. We report a new implementation of the GIR system on the Nike KrF laser facility and its application for measuring profiles of density and temperature in a plasma created for LPI research in the NRL KrF laser fusion program.
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