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A 0. 5-TW, 120-fs Ti: sapphire laser, when focused to greater than 10^18 W/cm^2 onto a solid target, creates a plasma which emits radiation that extends beyond 1 meV. The x-ray yield increases as the 3/2 power of the incident laser energy, reaching 0. 3% energy conversion to radiation above 20 keV at 40 mJ of laser energy on target. An x-ray spectral distribution of 1/E fits the data for most of the radiation, falling faster at higher photon energies.
Kmetec et al. (Mon,) studied this question.