Abstract
The physics of laser-plasma interaction is studied on the Shenguang III prototype laser facility under conditions relevant to inertial confinement fusion designs. A sub-millimeter-size underdense hot plasma is created by ionization of a low-density plastic foam by four high-energy (3.2 kJ) laser beams. An interaction beam is fired with a delay permitting evaluation of the excitation of parametric instabilities at different stages of plasma evolution. Multiple diagnostics are used for plasma characterization, scattered radiation, and accelerated electrons. The experimental results are analyzed with radiation hydrodynamic simulations that take account of foam ionization and homogenization. The measured level of stimulated Raman scattering is almost one order of magnitude larger than that measured in experiments with gasbags and hohlraums on the same installation, possibly because of a greater plasma density. Notable amplification is achieved in high-intensity speckles, indicating the importance of implementing laser temporal smoothing techniques with a large bandwidth for controlling laser propagation and absorption.
Highlights
A comprehensive understanding of parametric instabilities and the possibility of controlling them in the context of inertial confinement fusion (ICF) remains a challenging task
We have studied stimulated scattering of intense laser pulses and hot-electron production in an underdense and hot preformed plasma on the Shenguang III prototype (SGIII-P) facility
By using a low-density foam target heated by multi-kilojoule laser beams, we succeeded in creating a plasma with a temperature of about 2 keV and a density up to 0.2ncr extending over more than a 1000 laser wavelengths
Summary
A comprehensive understanding of parametric instabilities and the possibility of controlling them in the context of inertial confinement fusion (ICF) remains a challenging task. The details of the absorption processes and the detrimental effects of hot electrons on the implosion process are of great importance for both direct and indirect implosion schemes. There are no reliable methods of controlling parametric instabilities and there is a serious risk of their adverse effects preventing ignition conditions from being reached. We present results of studies of the excitation of parametric instabilities and generation of hot electrons in an underdense preformed plasma under conditions relevant to ICF implosion experiments with spatial scales of 300 μm or more and electron temperatures in the region of 2 keV. The experiments are performed on the Shenguang III prototype (SGIII-P) laser facility delivering about 8 kJ energy at a wavelength of 0.35 μm and a pulse duration of a few nanoseconds.
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