with millijoule high-repetition-rate femtosecond laser

High intensity Cu Kα X-ray was generated in helium at atmospheric pressure (760 torr) using a commercial millijoule high-repetition rate Ti: sapphire laser. The characteristic Kα X-ray was generated by focusing the 0.06 mJ–1.46 mJ, 100 fs and 1 kHz repetition femtosecond laser onto a solid Cu target to a spot with a 5 μm diameter. We obtained the characteristic Kα X-ray of 5.4 × 109 photos/s into 2π sr at a 1 kHz repetition rate with 5.0 × 10-6 conversion efficiency. Kα X-ray intensity can be enhanced with 4–8 % prepulse plasma into 7.7 × 109 photos/s/sr with 6.8 × 10-6 conversion efficiency. The X-ray intensity and conversion efficiency in helium achieved the almost the same level as in vacuum. Such vacuum-free femtosecond X-ray source with a tabletop laser can be a promising and easy accessible tool for time-resolved X-ray diffraction and other radiographic applications. The time-resolved X-ray diffraction experiment would reveal the ultrafast dynamics in crystalline materials, such as coherent phonon vibrations and ultrafast phase transitions