MeV x-ray generation with a femtosecond laser

A 0.5-TW, 120-fs Ti:sapphire laser, when focused to greater than ${10}^{18}$ W/${\mathrm{cm}}^2$ 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.