Physics of laser-driven relativistic plasmas energetic X-rays, proton beams and relativistic electron transport in Petawatt laser experiments.

摘要

Experiments investigating laser-matter interactions, where the laser power extends into the Petawatt (1015 Watt) regime, are presented. The focused (f/3) laser intensities (∼500 Joules at .5 pico-seconds) as high as 3 1020 W/cm2 are reached for the first time and drive fully relativistic motions of the electrons found at the laser-matter interface. We report on the experimental measurements of the radiation phenomena characteristic of these super-intense laser pulses. Significantly, the discovery of laser-accelerated intense proton beams is presented. We summarize the extensive studies into the proton beam characteristics and acceleration mechanisms of the proton beam physics. These energetic beams carry currents into the Meg-Amp range and have peak energies as high as 48 MeV with multiple-slope temperatures of ∼3 and ∼50 MeV and usually exhibit a high-energy cut-off. They are accelerated primarily off the rear surfaces of our thin foil targets and have ballistic trajectories normal to the emission surface. The proton acceleration mechanism (Target Sheath Normal Acceleration) is found to be a very efficient process. Laser energy to proton beam energy conversion ratios of 10–30% are inferred in the data. The driving force behind the proton acceleration mechanism is ultra-high current relativistic electron transport in solid density plasma. These electrons participate in many effects but notably in Bremsstrahlung radiation and the Petawatt laser performance to create extraordinarily bright X-Ray sources is investigated in detail. We report the most intense forward driven x-ray fluxes yet measured in laser experiments, with peak irradiance as high as 2 Rads at 1 meter. Overall yields into high energy x-rays of 11 Joules imply laser absorption mechanisms with 45–55% efficiency. A Monte Carlo Ponderomotive Kinematics (MPK) code is developed and is used to analyze the laser relativistic electron interaction, transport and high-energy x-ray relationships. These experiments required the development of new laser-plasma diagnostics. Radio-chromic film detectors were developed for the first time for use in high-energy plasma diagnosis. Development of a large array of thermo-luminescent x-ray detectors is also covered. Novel techniques of nuclear activation are employed for the first time in laser plasma studies and are used to identify the unique features of laser-driven proton beams.