On the use of Thomson scattering and laser-plasma interaction to characterize the plasmas in indirect drive ICF experiments

On the megajoule class laser facilities, the features of laser-produced plasmas have evolved toward unprecedented high electron temperatures reached in the environment of a cm-scale indirect-drive Hohlraum for ~ ten nanoseconds. In this context, the need for in situ experimental characterization of the plasma parameters becomes critical in order to test hydrodynamics simulations in these novel conditions. We will present the recent studies performed at CEA in order to investigate the options for plasma characterization on the Laser Mégajoule (LMJ).



In a first step, we have performed an extensive study of the laser probe beam required parameters in order to achieve Thomson scattering measurements with a 263 nm probe [1]. This study mainly compared the computed levels of signals for the 351 nm LMJ beams to the computed levels of signal of the probe. This study is based on the classical thermal levels expected for the Thomson scattering contributions and completely ignore contributions from stimulated waves.

In a second step, we have designed and performed experiments on the Omega laser facility in order to investigate the adequacy of these previsions and the risk of collecting contributions from laser plasma interaction in the Thomson scattering diagnostic for multiple positions of the collecting optics with respect to the 351 nm laser beams. We will show how the Omega facility has been used to mimic the possible LMJ configurations as well as the results acquired in these Omega experiments [2]. We will further illustrate how laser-plasma interaction may also be used for the purpose of plasma characterization.

[1] S. Depierreux, et al., Rev. Sci. Instrum. 91, 083508 (2020).

[2] C. Neuville, et al., Phys. Rev. Lett. 116, 235002 (2016) ; S. Depierreux, et al., Plasma Phys. Control. Fusion 62, 014024 (2019).