All-laser-driven, MeV-energy X-ray source for detection of SNM

Abstract

A quasi-monoenergetic MeV X-ray source based on laser-driven electron acceleration and Thomson scattering is under development at the Extreme Light Laboratory at the University of Nebraska, Lincoln. Reported are experimental results on the generation of high- brightness, nearly monoenergetic 300-MeV energy electron beams with the high power, short-pulse DIOCLES laser system. The laser system produces > 100 TW of maximum peak power per pulse. The maximum pulse energy is 3.5 J with a temporal duration of 30 fs. Energetic electron beams are produced by focusing a laser pulse with 40-50-TW peak-power on a supersonic helium nozzle to drive a relativistic plasma wave (laser wakefield). Electron beams with energies of 320 + 2.0 MeV are accelerated over a distance of 3 mm. The beam has an angular spread of 5 mrad with a charge of 100 pC. The use of a stable and well-characterized laser system - in conjunction with high temporal contrast and adaptive optics correction on the amplified beam (to obtain ideal focal spots) - has enabled generation of very reproducible electron beams, both in terms of energy and pointing stability. It is found that electron acceleration is most efficient, beam brightness is highest and reproducibility is best in the resonant regime, where the temporal duration of the laser pulse equals the plasma period. A beamsplitter is used after compression to generate two pulses in the ratio of 80% to 20%. The higher power pulse drives the laser wakefield to produce the energetic electron beams while the lower power pulse is transported through an independent line and focused on the electron beam to generate X-rays. Experiments are currently in progress to observe and characterize the X-ray beam. Theoretical predictions indicate that 1~2 plusmn 10% - MeV X-ray photons can be produced in a well-collimated beam. The expected photon flux is 10 photons per laser shot. Characterization of such a high-flux high energy X-ray beam is in progress. Quasi-monoenergetic X-rays offer significant advantages for the detection of sensitive nuclear materials using techniques such as nuclear resonance fluorescence. A systematic effort is also in progress to further improve the characteristics of laser produced electron beams with regard to monochromaticity, divergence and stability and also permit easy tunability of the X-ray source. The design of a compact system capable of being deployed in the field will also be discussed as part of a long-term solution to the critical requirement for an efficient cargo-scanning system.