Angular distribution of fast electrons and protons in short pulse laser target interaction Hui Chen Yuan Ping, Ronnie Shepherd, Jim Dunn- LLNL; Dustin Offermann, Anthony Link, Linn Van Woerkom - OSU; James King, Farhat Beg - UCSD; Cliff Chen - MIT; Lee Elberson, Windell Hill -U. Maryland Modeling by Andreas Kemp and Scott C. Wilks LLNL Lawrence Livermore National Laboratory, P. O. Box 808, Livermore, CA 94551 This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344

Angular distribution of electrons and protons measurements are important for understanding the laser target interaction Hot electron measurements often have been at single position/angle relative to target Proton measurements have concentrated on the back of the target where they are most energetic and beam-like Highly non-uniform distributions of hot electrons have been found by several groups The group of Zhang et al. (2000-06) looked at hot electrons at intensities up to mid-1018 W/cm2 Malka and Miquel (1996) did first Thot at three angles at upper - 1018 W/cm2 Chen et al. APS DPP 2006 Electrons, 2D PIC, Wilks We will attempt to answer the following questions with detailed experiments: What is the correlation between the electron dose and its distribution for all angles? What is the correlation between the spatially resolved hot electrons and the protons?

The measurements were performed on the Callisto laser with peak intensity of 3x1018 to 6x1019W/cm2 Chamber center photo Exp. setup Lasers: E=1 - 10 J, 130 fs, FWHM focus~5um Diagnostics: Proton ring with radiochromic film (RCF) Two Proton spectrometers using image plates Electron spectrometers using image plate(4 - 7) Ultra-thin thermoluminescence dosimeters TLDs (20 - 40) Targets: 1 to 50 um thick foils of two sizes: A) 2 x 10 mm2 flat foils of CH, Al, Cu, Ag B) ~ 0.2 x0.2 mm2 foil reduced mass targets of Cu Mounted flag-style on glass fiber P-spec

Full angular coverage of TLDs was complemented by calibrated E-spectrometers at several positions TLDs: electron dose for E>350 keV (red) electron dose for E>1 MeV (blue) Especs: spectra for 0.1 - 4.2 MeV at many  Laser E > 1 MeV E > 350 KeV Dose Abs Calibration Angle E-spec #2 TLD: ~6e-7 J for E>350 keV E-spec: 2e-6 J for E>100 keV Quantitatively consistent diagnostics Tanaka et al, 2004 Chen et al, 2007

Electron distributions and temperatures from two laser intensities for the same target condition 0.5 MeV 0.7 MeV Target front 3x1018 W/cm2 3x1019 W/cm2 ~Laser direction 0.5 MeV 0.6 MeV Back normal 0.8 MeV

2D PIC modeling shows hot electron behavior similar to that observed in the experiments Electrons from back are hotter than Those from front of target Electrons are accelerated mostly In the forward direction Electrons are accelerated mostly In the forward direction Black:all particles gb x (c/w0) 5e19 W/cm2 5e19 W/cm2 Forward direction Forward direction Laser 2D PSC modeling by Andreas Kemp Under-dense Over-dense Electron temperatures are higher at the back of the target than at the front of the targets

The proton spatial distribution was recorded by the RCF ring (E>1 The proton spatial distribution was recorded by the RCF ring (E>1.3 MeV) and proton spectrometers (0.1 - 4 MeV) Proton dose Target edge Front normal Film label Target edge Back Normal Spectra from 2 proton spectrometers at front and back of target normal positions Target back normal Laser Target Target front normal The Ring Noise level

The proton distributions for large and small targets are dramatically different, so are the proton doses Beam like protons from large foils of metal and CH 10 um Ag, CH 2x10 mm2 5e19 W/cm2 Broader proton distributions in reduced mass targets 10 um Cu 0.2 mm2 5e19 W/cm2 1e19 W/cm2 Electrons: 5e19 W/cm2 More than a factor of 2 higher conversion from laser to protons are found in reduced mass target

The near isotropic proton acceleration in reduced mass targets can be explained by their unique E-fields 2D PSC modeling by Andreas Kemp Reduced Mass Targets accelerate protons not only from front and back, but all sides of target, due to near equal electric field strengths everywhere around the target.

Conclusions Measurements were performed on the Callisto laser with a peak intensity of 3x1018 to 6x1019W/cm2 with full angular coverage using Multiple charged particle detectors. We found: The electron angular distributions are highly anisotropic. Electron temperatures in the forward directions are hotter than the backward direction. The proton distributions for reduced mass targets tend to be more isotropic than distributions from large targets. Less difference was observed for hot electrons for these two target types. The 2D collisional PIC simulations of the electric field for reduced mass targets agree with observations