Supersonic Heat Wave Propagation in Laser-Produced Underdense Plasmas

Intense, multi-keV X-ray sources are required for radiographic applications in laboratory astrophysics and ICF. Low-density targets are favorable for efficient laser-to-X-ray conversion because supersonic energy deposition leads to volumetric heating with low hydrodynamic losses. We report on recent X-ray generation experiments at the GEKKO XII laser. Ti-doped SiO$_2$ aerogel-filled (3.3 mg/cm$^3$, 3-6 atom\% Ti) Be or CH cylinders were irradiated with nine laser beams with a total of 1 kJ energy in a 2.5~ns square pulse, at 351~nm wavelength. Laser irradiance at the entrance of cylinder was 1.4${\times}$10$^{14}$ W/cm$^2$. The observed heat wave clearly shows two different phases in terms of propagation velocities. The measured heat-front propagation velocity was 1.4$\times$10$^8$ cm/s, which is a Mach number of 10 for the given conditions. Electron temperature in the heated target was derived from time-resolved X-ray spectra. By changing observation points, electron temperature profiles of the heat wave along the cylinder axis were obtained at different times.