Radiation Generation from Ultra Intense Laser Plasma Interactions with Solid Density Plasmas for Active Interrogation of Nuclear Materials.

Abstract

The development of short pulse high power lasers has led to interest in laser based particle accelerators. Laser produced plasmas have been shown to support quasi-static TeV/m acceleration gradients which are more than four orders of magnitude stronger than conventional accelerators. These high gradients have the potential to allow compact particle accelerators for active interrogation of nuclear material. In order to better understand this application, several experiments have been conducted at the HERCULES and lambda^3 lasers as the Center for Ultrafast Optical Science at the University of Michigan.

Electron acceleration and bremsstrahlung generation were studied on the lambda^3 laser. The scaling of the intensity, angular, and material dependence of bremsstrahlung radiation from an intense (I>10^18W/cm^2) laser-solid interaction has been characterized at energies between 100 keV and 1 MeV. These were the first high resolution (lambda/d_lambda>100) measurements of bremsstrahlung photons from a relativistic laser plasma interaction. The electron populations and bremsstrahlung temperatures were modeled in the particle-in-cell code OSIRIS and the Monte Carlo code MCNPX and were in good agreement with the experimental results.

Proton acceleration was studied on the HERCULES laser. The effect of three dimensional perturbations of electron sheaths on proton acceleration was investigated through the use of foil, grid, and wire targets. Hot electron density, as measured with an imaging Cu Kalpha crystal, increased as the target surface area was reduced and was correlated to an increase in the temperature of the accelerated proton beam.

Additionally, experiments at the HERCULES laser facility have produced directional neutron beams with energies up to 16.8(+/-0.3) MeV using (p,n) and (d,n) reactions. Efficient (d,n) reactions required the selective acceleration of deuterons through the introduction of a deuterated plastic or cryogenically frozen D2O layer on the surface of a thin film target. The measured neutron yield was up to 1.0(+/-0.5)x10^7 neutrons/sr with a flux 6.2(+/-3.7) times higher in the forward direction than at 90 degrees. This demonstrated that femtosecond lasers are capable of providing a time averaged neutron flux equivalent to commercial D-D generators with the advantage of a directional beam with picosecond bunch duration.