Minimum condition of target gas material at an ionization-stage control scheme in a laser-plasma electron acceleration

Mori, Michiaki; Kando, Masaki; Kotaki, Hideyuki; Hayashi, Yukio; Kiriyama, Hiromitsu; Okada, Hajime; Pirozhkov, A. S.; Bulanov, S. V.; Kondo, Kiminori; Bolton, P.

We report on the appropriate and inappropriate gas materials to generate energetic electrons. The 4-TW peak power and 40-fs pulse duration laser beam illuminated the gas-jet target with intensity of 910W/cm measured in vacuum. We investigated energetic electron beam generation using neon and argon. Energetic electron beam was observed in argon at the lowest neutral gas density of 510cm. However, no energetic electrons ( 1 MeV, 1 pC) were observed in neon although neutral gas density is increased from510cm to 510cm. By considering ionization stage at such an intensity, the maximum plasma density is reached to be a quarter critical plasma density, at which the maximum growth-rate of laser-plasma instability is expected. On the other hand, propagation of the pumping laser was observed in neon and argon by using optical probing. Significantly different images were observed. The structure of the laser channel for energetic electron beam generation that observed in argon was absent in neon. These results imply that the additional increase of the plasma density due to ionization cannot explain the electron generation. The analysis including the propagation of a laser in ionizing gas would be necessary.