Professor Justin Wark

Abstract:

When a laser pulse of intensity 1019 W cm−2 interacts with solid targets, electrons of energies of some tens of MeV are produced. In a tantalum target, the electrons generate an intense highly directional γ-ray beam that can be used to carry out photonuclear reactions. The isotopes 11C, 38K, 62,64Cu, 63Zn, 106Ag, 140Pr, and 180Ta have been produced by (γ, n) reactions using the VULCAN laser beam. In addition, laser-induced nuclear fission in 238U has been demonstrated, a process which was theoretically predicted at such laser intensities more than ten years ago. The ratio of the 11C and the 62Cu β+ activities yields shot-by-shot temperatures of the suprathermal electrons at laser intensities of ∼1019 W cm−2. © 2000 The American Physical Society.

Abstract:

We present high-resolution spectroscopic measurements of the optically thick hydrogen-like Al Ly-α line shape from a cylindrically expanding plasma. The cylindrical expansion is produced by symmetrically irradiating a 120 μm diameter coated wire target with six beams of the VULCAN Nd:glass laser at an irradiance of 2×1014Wcm-2. Small shifts in the line position and changes in the line shape can be attributed to radiation emitted from different regions of the plasma and passing through different gradients in plasma density, temperature, and velocity. The experimental profiles are compared to a time-dependent hydrodynamics code incorporating a Sobolev escape probability treatment of the radiative transport. © 2000 Elsevier Science B.V.

Abstract:

Time-resolved x-ray diffraction with picosecond temporal resolution is used to observe scattering from impulsively generated coherent acoustic phonons in laser-excited InSb crystals. The observed frequencies and damping rates are in agreement with a model based on dynamical diffraction theory coupled to analytic solutions for the laser-induced strain profile. The results are consistent with a 12 ps thermal electron-acoustic phonon coupling time together with an instantaneous component from the deformation-potential interaction. Above a critical laser fluence, we show that the first step in the transition to a disordered state is the excitation of large amplitude, coherent atomic motion. © 2000 The American Physical Society.