Measurements of Electron Density Profiles of Plasmas Produced by Nike KrF Laser for Laser Plasma Instability (LPI) Research

Knowing spatial profiles of electron density ($n_e$) in the underdense coronal region ($n < n_c/4$) of plasma is essential to understanding LPI initiation in inertial confinement fusion research. In the recent Nike LPI experiment, a side-on grid imaging refractometer (GIR)\footnote{R.S. Craxton, et al, Phys. Fluids B 5, 4419 (1993)} was deployed for measuring the underdense plasma profiles. Plasmas were produced from flat CH targets illuminated by Nike KrF laser with total energies up to 1 kJ of 0.5 $\sim$ 1 nsec FWHM pulses. The GIR resolved $n_e$ up to $3\times 10^{21}/cm^3$ in space taking 2D snapshot images of probe laser ($\lambda = 263 nm$, $\Delta t = 10 ps$) beamlets ($50\mu m$ spacing) refracted by the plasma at a selected time during the laser illumination. The individual beamlet transmittances were also measured for $T_e$ estimation. Time-resolved spectrometers with an absolute-intensity-calibrated photodiode array and a streak camera simultaneously detected light emission from the plasma in spectral ranges relevant to Raman (SRS) and two plasmon decay instabilities.\footnote{J. Oh, et al, GO5.4, APSDPP (2010)} The measured spatial profiles are compared with simulation results from the FAST3D radiation hydrocode and their effects on the LPI observations are investigated.