Jennifer Rassuchine

Recent experiment on fast electron transport in ultra-high intensity laser interaction

We performed an experiment with cone targets in planar geometry devoted to the study of fast electron generation, propagation, and target heating.. This was done at LULI with the 100 TW laser at intensities up to 10 19 W/cm 2 . Fast electrons penetration, with and without cones, was studied with different diagnostics (Kα imaging, Kα spectroscopy, visible emission) for ω or 2ω irradiation. At ω, the pre-plasma generated by the laser pedestal fills the cone and prevents the beam from reaching the tip.

WE‐E‐330D‐01: The Production of Ultrafast Bright K‐Alpha X‐Rays From Laser Produced Plasmas for Medical Imaging

Purpose: To show the potential of improving image quality with a cleaner, brighter, quasi‐monochromatic X‐ray micro‐source via laser produced plasmas(LPP).Method and Materials: First generation targets consisting of ten micron thick gold formed into free‐standing pyramids have been built. PIC (Particle‐In‐Cell) simulations have been performed in order to validate this target geometry. Preliminary experiments with a Ti‐Sapphire CPA laser have been achieved with these targets. Second generation parabolic cone targets with an optimal angle for electron transport have also been built. This new nano‐fabricated target could optimize X‐ray source characteristics. Results:PIC (Particle‐In‐Cell) simulations show that conical targets optically guide laser light resulting in a higher density of hot electrons at the apex These simulations show a possible ten times augmentation in hot electron density and a three times increase in electron temperature with a conical verses flat target. This increase in collimated suprathermal electrons boosts total photon yield as well as possibly enhancing line emission verses the bremsstrahlung continuum. Preliminary experiments demonstrate a three‐fold higher X‐ray yield and a two‐fold reduction in focal spot with the pyramidal verses the flat target. Furthermore, the geometry of the conical targets not only reduces focal spot size to a few microns and pulse duration to a couple picoseconds, but allows the particles to escape the target perpendicular to the surface resulting in a particle‐free, ultra‐short X‐ray micro‐beam. Conclusion: Comparing LPP X‐ray source parameters to that of a standard X‐ray tube shows substantial improvements in focal spot size, photon flux, spectral range and emission duration. Focusing on target design can provide a cleaner, brighter, quasi‐monochromatic X‐ray source that could improve image quality in any medicaldiagnostic regime. Such advancements show promising applications in mammography and angiography. Conflict of Interest: Research sponsored by DOE/NNSA under University of Nevada Reno grant #DE‐FC52‐01NV14050.

Enhanced energy localization and heating in high contrast ultra-intense laser produced plasmas via novel conical micro-target design

We report new experiments showing enhanced laser-target coupling and energy localization using nano-fabricated micro-conical Cu targets performed at the 100 TW CPA laser at LULI. A comparison was made between 1ω (λ = 1.057 μm, I = 1019 W/cm2) and 2ω (λ = 0.53 μm, I = 4-8 × 1018 W/cm2) irradiation to determine the effect of ASE induced preformed plasma filling the cone, using as principal diagnostics 2D Cu Kα imaging (transverse and rear-side), and high-resolution conical crystal spectroscopy of the Cu Kα bands. The 2ω irradiation exhibits laser absorption up to 50 μm deeper into the cone tip (versus at 1 ω), with a commensurately smaller Kα emission zone. Spectroscopy indicates a higher average charge state for the Cu emission at 2ω, with some shots exhibiting up to at least O-like emission. We deduce that micro-cone targets have similar performance in terms of material heating as a 50 μm diameter reduced mass target, despite a 900-fold larger mass. The observed enhancement in energy localization and heating in the cone geometry is supported by 2D collisional PIC simulations which indicate the presence of self-generated resistive magnetic field structures (≥ 10 MG) which confine the energetic electrons to the tip region