Figure 4. Electron energy spectra from 120 TW-level laser pulses.

Spectra obtained from 4 mm gas jet of (a) pure He and (b) 99.5% He and 0.5% N₂ gas mixture. Spectra obtained from 1 cm gas jet of (c) pure He and (d) 99.7% He and 0.3% N₂ gas mixture. (e) and (f) are deconvoluted energy spectra of the images in (b,d), respectively. The laser-plasma parameters are given in the text.