PICOSECOND X-RAYS FROM SUBPICOSECOND-LASER-PRODUCED HOT-DENSE MATTER

Short-pulse, high-intensity laser-plasma interactions are investigated experimentally with temporally and spectrally resolved soft x-ray dia gnostics. The emitted x-ray spectra from sold targets with various ato mic numbers are characterized for a laser pulse width tau(l) similar t o 400 fs. With low contrast (10(5)), the x-ray spectrum in the lambda = 40-100 Angstrom spectral region is dominated by line emission, and t he x-ray pulse duration is found to be characteristic of a long-scale- length, low-density plasma. Bright, picosecond, broadband emission, ch aracteristic of a short-scale-length, high-density plasma, is produced only when a high laser contrast (10(10)) is used. It is demonstrated experimentally that the pulse width of laser-produced x-ray radiation may be varied down to the picosecond time scale by adjusting the incid ent ultrashort-pulse laser flux. The results are found to be in qualit ative agreement with the predictions of both a code-independent model of radiation from a collisionally dominated two-level ion and a hydrod ynamics code coupled to a detailed-configuration atomic physics model. X-ray film measurements of conversion efficiency, along with pinhole camera measurements of the emission region, reveal very high x-ray bri ghtness.