Relativistic positron creation using ultraintense short pulse lasers.

We measure up to 2x10;{10} positrons per steradian ejected out the back of approximately mm thick gold targets when illuminated with short ( approximately 1 ps) ultraintense ( approximately 1x10;{20} W/cm;{2}) laser pulses. Positrons are produced predominately by the Bethe-Heitler process and have an effective temperature of 2-4 MeV, with the distribution peaking at 4-7 MeV. The angular distribution of the positrons is anisotropic.

Ultrafast x-ray Thomson scattering of shock-compressed matter.

Spectrally resolved scattering of ultrafast K-alpha x-rays has provided experimental validation of the modeling of the compression and heating of shocked matter. The elastic scattering component has characterized the evolution and coalescence of two shocks launched by a nanosecond laser pulse into lithium hydride with an unprecedented temporal resolution of 10 picoseconds. At shock coalescence, we observed rapid heating to temperatures of 25,000 kelvin when the scattering spectra show the collective plasmon oscillations that indicate the transition to the dense metallic plasma state.

Direct experimental evidence of back-surface ion acceleration from laser-irradiated gold foils.

Au foils were irradiated with a 100-TW, 100-fs laser at intensities greater than 10(20) W/cm2 producing proton beams with a total yield of approximately 10(11) and maximum proton energy of >9 MeV. Removing contamination from the back surface of Au foils with an Ar-ion sputter gun reduced the total yield of accelerated protons to less than 1% of the yield observed without removing contamination. Removing contamination from the front surface (laser-interaction side) of the target had no observable effect on the proton beam.