Effects of ambient pressure on Cu Kα X-ray radiation with millijoule and high-repetition-rate femtosecond laser

Hard Cu K alpha X-ray radiation was generated with a millijoule and high-repetition-rate Ti: sapphire laser in air, helium or vacuum (2.7-1.3x10(4) Pa) ambient. The characteristic X-ray was obtained by focusing the 0.06-1.46 mJ/pulse, 100 fs, 1 kHz repetition femtosecond laser onto a solid copper target to a spot 4.8 mu m in diameter. The relationship between K alpha X-ray conversion efficiency and atmospheric conditions was explained with a simple electron collision model that suggested that the electron mean free path is an important parameter in the generation of ultrafast pulsed X-rays in any ambient condition. We also demonstrated a high-intensity X-ray source working in helium at atmospheric pressure.