Speaker
Description
Low density porous materials are considered in the inertial confinement fusion studies as a promising material for smoothing laser beam intensity modulations and creation of spherical targets filled with a liquid deuterium-tritium fuel. However, the role of intrinsic structural foam inhomogeneities on seeding instabilities is not known. Experimental studies and modelling of the time of foam ionization and homogenization are of prime importance in that context.
We present here the results of a recent experiment on the PALS laser installation dedicated to measurements of foam ionization and plasma characterization. Three types of low-density porous targets were irradiated by intense sub-nanosecond laser pulses on the 3rd and 1st harmonics of iodine laser at laser intensities in the range 1014 – 1015 W/cm2: a) plastic TMPTA targets of average density 10 mg/cc doped with 8 weight percent of chlorine, b) 3D graphene targets of average density about 7 mg/cc and c) 3D printed regular porous targets of average density 8 mg/cc composed of plastic wires of radius 2.2 m (Figure 1). While the pore size of TMPTA and graphene foams is typically 2 m, the pore size is of order 50 m for the printed target. The thickness of all targets was in the 600 – 800 m range and a copper foil was attached to the rear side of all targets for measurements of the hot electron production. All targets were underdense for the 3rd harmonic and overdense for the basic laser frequency.
Figure 1. SEM image of 3D printed low average density plastic target.
We measured the speed of ionization wave propagation into the low-density porous matter via X-ray streak. Laser energy transformation into fast electrons was detected via time-integrated spatially resolved absolutely K emission from the copper foil. Energy and spectrum of laser radiation scattered back under angle 30 was registered. LYSO scintillators detected a very weak signal of hard X-rays. Chlorine emission spectra (Figure 2) from chlorine doped TMPTA foams were used for measurement of electron and ion temperatures. Electron temperature was estimated from the ratio of Ly- and He- lines. Ion temperature in the ionized material was derived from the Doppler broadening of chlorine He-y intercombination line.
Figure 2. Time-integrated chlorine emission spectra plotted at depths 90 and 225 m inside chlorinated TMPTA foam. PALS third harmonic of energy 200 J and pulse duration 293 ps was focused on the front surface of the foam
Experimental results are compared with the results of fluid simulations using our novel sub-grid model [1] of laser interaction with low density porous matter incorporated into PALE and FLASH hydrodynamic codes.
References
[1] L. Hudec et al., A hybrid (ablation-expansion) model for low-density foams, contribution to ECLIM 2022
Acknowledgements
The authors acknowledge financial support from the European Regional Development Fund through the projects CAAS (CZ.02.1.01/0.0/0.0/16_019/0000778) and ADONIS (CZ.02.1.01/0.0/0.0/16 019/0000789) and from the Ministry of Education, Youth and Sports of the CR via project LM2018114. This work has been partially carried out within the framework of the EUROfusion Consortium and has received funding from EUROfusion project CfP-FSD-AWP21-ENR-01-CEA-02.
