Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

High-power lasers that fit into a university-scale laboratory can now reach focused intensities of more than 10 ¹⁹ W cm ^-2 at high repetition rates. Such lasers are capable of producing beams of energetic electrons, protons and γ-rays. Relativistic electrons are generated through the breaking of large-amplitude relativistic plasma waves created in the wake of the laser pulse as it propagates through a plasma, or through a direct interaction between the laser field and the electrons in the plasma. However, the electron beams produced from previous laser-plasma experiments have a large energy spread, limiting their use for potential applications. Here we report high-resolution energy measurements of the electron beams produced from intense laser-plasma interactions, showing that-under particular plasma conditions-it is possible to generate beams of relativistic electrons with low divergence and a small energy spread (less than three per cent). The monoenergetic features were observed in the electron energy spectrum for plasma densities just above a threshold required for breaking of the plasma wave. These features were observed consistently in the electron spectrum, although the energy of the beam was observed to vary from shot to shot. If the issue of energy reproducibility can be addressed, it should be possible to generate ultrashort monoenergetic electron bunches of tunable energy, holding great promise for the future development of 'table-top' particle accelerators.

Abstract:

The investigation of proton and heavy ion acceleration was carried out in ultrahigh intensity laser plasma interactions using VULCAN lasers. The first spatially integrated measurement of proton and heavy ion acceleration was performed with nuclear activation techniques. High-intensity laser-plasma interactions provide a unique and potentially important source of nuclear radiation for radioisotope production. By controlling the target conditions and the accelerated ion beam properties, the production of radioisotopes can be controlled effectively.

Abstract:

Solid-state dynamics experiments at very high pressures and strain rates are becoming possible with high-power laser facilities, albeit over brief intervals of time and spatially small scales. To achieve extreme pressures in the solid state requires that the sample be kept cool, with T-sample < T-melt. To this end, a shockless, plasma-piston "drive" has been developed on the Omega laser, and a staged shock drive was demonstrated on the Nova laser. To characterize the drive, velocity interferometer measurements allow the high pressures of 10 to 200 GPa (0.1 to 2 Mbar) and strain rates of 10(6) to 10(8) s(-1) to be determined. Solid-state strength in the sample is inferred at these high pressures using the Rayleigh-Taylor (RT) instability as a "diagnostic." Lattice response and phase can be inferred for single-crystal samples from time-resolved X-ray diffraction. Temperature and compression in polycrystalline samples can be deduced from extended X-ray absorption fine-structure (EXAFS) measurements. Deformation mechanisms and residual melt depth can be identified by examining recovered samples. We will briefly review this new area of laser-based materials-dynamics research, then present a path forward for carrying these solid-state experiments to much higher pressures, P > 10(3) GPa (10 Mbar), on the National Ignition Facility (NIF) laser at Lawrence Livermore National Laboratory.