Session XO6: Ultraintense Lasers and Particle Generation

Measurements of Proton Generation with Intense, Kilojoule Laser Pulses on OMEGA EP

The scaling of energetic-proton generation by intense, high-energy laser pulses with laser energy will be presented. Energetic protons have been generated by target-normal sheath acceleration in intense laser--solid interactions on OMEGA EP with up to 2100-J, 10-ps-long laser pulses. Nuclear activation of copper-film stacks is used to determine the energy spectrum of the forward-accelerated protons. The results show that the maximum proton energy is proportional to $E^{0.5}$, with maximum proton energy greater than 50 MeV for a typical 1000-J, 10-ps shot. The conversion efficiency from laser energy to total proton beam energy is proportional to the laser energy, with about 2{\%} of the laser energy converted into protons with energies greater than 4 MeV at 1000 J. The experimental data extends previous work to higher laser energies and is in good agreement with it. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

Above-60-MeV proton acceleration with a 150 TW laser system

Laser-accelerated proton beams can be used in a variety of applications, e.g. ultrafast radiography of dense objects or strong electromagnetic fields. Therefore high energies of tens of MeV are required. We report on proton-acceleration experiments with a 150 TW laser system using mm-sized thin foils and mass-reduced targets of various thicknesses. Thin- foil targets yielded maximum energies of 50 MeV. A further reduction of the target dimensions from mm-size to 250 $\times$250$\times$25 microns increased the maximum proton energy to $>$65 MeV, which is comparable to proton energies measured only at higher-energy, Petawatt-class laser systems. The dependence of the maximum energy on target dimensions was investigated, and differences between mm-sized thin foils and mass-reduced targets will be reported.   

Narrow energy spread proton and ion spectra from high-intensity laser interactions

Experiments were performed to investigate proton and ion acceleration from thin foil targets, using a high contrast, ultra-short laser pulse from the HERCULES laser at the Univ. of Michigan. Experiments were performed with $30$ TW, $32$ fs pulses after two plasma mirrors with an $F\slash 1$ off-axis parobolic mirror to attain an intensity of $>10^ {21} \; \rm{Wcm}^{-2}$ on $Si_{3}N_{4}$ and Mylar targets of thicknesses ranging 50 nm to 13 $\mu$m with ASE contrast of $10^ {-13}$. Using a short prepulse, proton beams with energy spreads below 75$\% ~\Delta E \slash E$ were observed from all thicknesses, with a maximum energy of 10 MeV and a minimum energy spread of 25$\%$. Similarly narrow energy spreads were observed for O, N, and C ions for $Si_{3}N_{4}$ thickness of 50 nm, with energies up to 2 MeV per nucleon and energy spread of 23$\% $, with energy spread increasing with increased thickness. Maximum energies were confirmed with CR39 track detectors, while a Thomson ion spectrometer was used to measure energy spectra. Two dimensional particle-in-cell simulations were also performed and will be presented.   

Three-dimensional dynamics of break-out afterburner ion acceleration using high-contrast short-pulse laser and nano-scale targets

Ultra-intense laser interaction with solid density carbon targets is examined in 3D VPIC simulations. It is shown that a linearly polarized laser pulse at $>10^{20}$ W/cm$^2$ intensity will turn a solid density, nm-scale target relativistically transparent and begin an epoch of dramatic acceleration of ions. Called the Break-Out-Afterburner (BOA) [L. Yin, et al., Phys. Plasmas 14, 056706 (2007)], this mechanism leads to order-of-magnitude greater ion energy and beam currents. The BOA lasts until the electron density in the expanding target reduces to the non-relativistic critical density. A striking feature of the BOA mechanism is that the ion beam symmetry is broken, with the production of lobes in the direction orthogonal to the laser polarization and propagation directions, along which the highest ion beam energy is observed. These ion beam lobes have been measured on recent Trident experiments. An analytic theory for the production of ion beam lobes has been obtained and has been shown to be in good agreement with simulations. Moreover, other features of the BOA, e.g., the existence of an optimal target thickness for given laser and target density and the propagation of light and heavy ion species at comparable speed have been demonstrated in simulations and experiments.   

High Contrast Laser Interactions with Thin Targets: Juxtaposition of Material Composition, Thickness and Neutron Yield

Recent experiments on the 200TW Trident short-pulse laser (80J, 500fs) at high contrast ($>$10$^{-10})$ have shown an unexpected trend for proton beam energy and yield on target material with targets below 1 micron in thickness, which is completely opposite of that of thicker targets above 1 micron. Previous lower contrast experiments showing similar material dependence for thicker targets is also presented. In addition neutron production has a dependence on the target thickness, composition, and possibly shape. The highest neutron yield was found to lie near the target normal rear and front directions of the targets, with the majority of neutrons originating from an unintended but rather efficient neutron converter.   

Laser Proton Acceleration by Radiation Pressure and Its Scaling

The search for mono-energetic protons are always of great interests, both for science and for many applications, among the medical therapy of cancer and fast ignition of laser fusion. Radiation Pressure Acceleration (RPA) of quasi-monoenergetic protons by illuminating an ultra thin foil (thickness $<$ wavelength) with a short pulse, intense laser has been actively studied. RPA is the radiation pressure acceleration of the whole foil trapping protons in it or equivalently a ``light sail.'' In this paper, we present analytical modeling and PIC simulation of the scaling of proton energy by RPA of ultra thin foil. By defining the monoenergetic proton as having sufficiently small energy spread in PIC simulation, we studied the proton mono-energy profile as a function of the laser power and peak intensity, thickness of the thin foil and target density. We found that the Rayleigh-Taylor (R-T) instability plays significant role in increasing the energy spread of accelerated protons. The simulation results are able to provide experimentalists with optimal scaling for instability avoidance and optimal ion acceleration.   

Carbon Ion Energy Spectra from Hemispherical EP Targets

High-intensity laser acceleration of ions may be useful for several applications. Carbon hemishell targets have been shot at OMEGA-EP for ion focusing experiments (Flippo and Offermann, this meeting). This paper reports on carbon ion data collected with a Thomson parabola ion energy analyzer known as TPIE. With 1 kilojoule of laser energy on target, C$^{6+}$ has been detected at energies from $\sim $9 -- 33 MeV. Lower charge states down to C$^{3+}$ at energies from 6 -- 16 MeV are also detected. The dominant ion is C$^{5+}$. Representative carbon ion-energy distributions are shown for the hemishell targets, which were heated to drive off proton contaminants. Without the target heating, the carbon abundance is greatly reduced. The nominal EP irradiance for these experiments was $\sim $5 x 10$^{18}$ W/cm$^{2}$. TPIE has proven its worth for ion energy analysis for such experiments. EP focal-spot properties and contrast will be improved in the coming year, and this will be a significant benefit for ion experiments and applications.   

Laser-driven neutron production from bulk and pitcher-catcher targets

As an important step in the development of the highly directional compact neutron source from the reaction $^{7}$Li(d,xn) [1] we have studied the laser-driven fusion neutron production d(d,n)$^{3}$He from bulk deuterated plastic targets and compared it to a pitcher-catcher target method using the same laser and detector arrangement. For laser intensities of up to I = 3.10$^{19}$ Wcm$^{2}$ it was found that the bulk targets produced a high yield (5.10$^{4}$ neutrons/steradian) beamed preferentially in the laser propagation direction. The inhibition of the deuteron acceleration by a proton rich contamination layer is likely to significantly reduce the pitcher-catcher neutron production. Two-dimensional particle-in-cell simulations were performed to model the deuteron beam acceleration, the results of which were coupled to a Monte Carlo code to calculate the expected neutron beam properties. Numerical analysis suggests the pitcher-catcher targets would become more efficient at higher laser intensities. This work was supported by DTRA and the NRL. [1] J. Davis et al., PPCF 52, 045015 (2010).   

Laser Generated Neutron Source for Temperature Measurement

Neutron Resonance Spectroscopy (NRS) is a robust temperature measurement tool for shocked matter in the 0.1-3 eV range, with error bars as low as $\pm$100 Kelvin, using neutrons from a particle accelerator. Using NRS at high-energy laser facilities requires the development and optimization of a laser-generated neutron source. In this experiment, the Titan laser (150J/0.7ps) was used to accelerate protons from the rear of a Cu foil. The protons were incident on a LiF foil, which used the ${}^7$Li(p,n)${}^7$Be reaction to create up to $1.8\times10^9$ neutrons. Absolute measurements of neutron yields are used to validate simulations of the proton deceleration and reaction. This allows for optimization of neutron generation on Titan and for predicted yields at higher energy facilities. For example, $\sim2\times10^{10}$ neutrons are predicted at Omega EP. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Demonstration of a Laser-Driven, High Efficiency, Low Noise Argon Gas Jet X-Ray Source*

The electromagnetic pulse (EMP) from Ar gas jet plasmas has been measured to be 4x weaker and 20x shorter in time duration than the EMP from solid density plasmas of similar (3 keV) x-ray energy. We irradiated high density (10$^{20}$ cm$^{-3}$ atomic density) supersonic Ar gas jets and solid (6 x 10$^{22}$ cm$^{-3}$ atomic density) plastic C$_{2}$H$_{2}$Cl$_{2}$ targets with an ultra-high intensity (10$^{19}$ W/cm$^{2})$, petawatt-class 1053 nm laser. Electron spectroscopy shows that the electron distribution leaving the rear side of gas jet targets close to the laser axis is more than 4x higher in number and energy than for solid targets, in spite of the greatly reduced EMP. This suggests that target density is the more decisive factor. Monochromatic x-ray imaging and K-shell x-ray spectroscopy provide additional insight into the nature of the laser-target interaction. With competitive conversion efficiency from laser energy into x-rays, Ar gas jets are a bright and low-noise source of 3 keV x-rays for plasma diagnostics. *This work was supported by the DOE Plasma Physics Junior Faculty Award Program and was performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344. LLNL-ABS-442193   

Pulse shortening via Relativistic Transparency of Nanometer Foils

Intense lasers drive plasma electrons to velocities approaching light-speed. Increase of the electron mass causes optical transparency in otherwise classically over-dense plasma. Simulations indicate relativistic transparency can produce near-single-cycle rise time light pulses. It also lies enables a new mechanism for laser-based ion-acceleration yielding energy increases over earlier approaches. A direct signature of transparency is pulse-shortening thru over-dense plasmas in which relativistic intensity induces transmission. Using nm C foils (LMU) and the high-contrast Trident laser (LANL) we have made auto-correlation measurements showing $>$2x transmitted pulse duration reduction at intensities corresponding to $\sim $20-fold increase in electron mass. Spectral measurements agree with pulse shortening thru the target, and 1-D particle-in-cell simulations support the measurements.   

Laser pulse shaping due to self-induced relativistic transparency in laser -- nanofoil interactions

Shapes of the laser pulses transmitted through thin nanofoils due to self-induced relativistic transparency in intense, high contrast laser (80J, $\sim $600 fs, $>$10$^{20}$ W/cm$^{2}$ and 10$^{-10}$ contrast) -- nanofoil interactions were measured using a single shot second harmonic frequency resolved optical gating (FROG) system [Palaniyappan, et al., RSI, 81, 1 (2010)]. The FROG measurements show asymmetric pulse shapes, pulse shortening up to a factor of 2 and faster rise times on the leading edge than the falling edge. Part of the incident pulse is reflected/absorbed by the over dense plasma until it becomes relativistically under dense and transparent to the rest of the pulse. The measurements are qualitatively in agreement with a 1-D PIC simulation. Transmitted laser pulse shapes through 3 nm thick foil show large variations due to early target expansion.   

New results on the laser produced positrons using the TITAN and OMEGA EP lasers

We performed new experiments and simulations on generating positrons with intense lasers [1]. A cone shaped positron jet is produced by irradiating a gold target with an intense picosecond duration laser pulse. The jet has $\sim $20 degree angular divergence and a quasi-monochromatic energy distribution with energy 4 to 20 MeV. The conversion efficiency from laser energy to positrons in the jet is $\sim $ 2x10$^{-4}$. The positron angular and energy distributions are controlled by the laser and target conditions. The positron acceleration mechanism is identified experimentally as the sheath electric field on the rear surface of target. This talk will present the details of these new experimental and simulation results.\\[4pt] [1] Hui Chen, S. C. Wilks, D. D. Meyerhofer et al., PRL 105,015003 (2010)   

Spectroscopy of positron annihilation gamma rays from laser-exited media

Motivated by calculations for gamma ray yields and results of positron beam measurements from laser irradiated high Z targets [1], a Gamma-ray Crystal Spectrometer (GCS) was built by Artep Inc. and fielded at the Titan laser facility of LLNL. The spectrometer is equipped with heavy shielding around a cylindrically bent Ge crystal in a transmission geometry. The Bremsstrahlung continuum and the 511 keV annihilation gamma rays are dispersed by the Ge(440) crystal and detected by an image plate placed on the Rowland circle. The gamma rays originate inside the thick target material (1 to 3 mm Au disks) where positrons are produced in the intense field of the high energy (350 J) short pulse (10 ps) laser irradiation. In addition to the spectrometer, two different electronic detection systems also recorded the gamma ray spectra using the single hit per pixel technique. The first gamma ray spectra recorded with the crystal spectrometer and the electronic detectors will be reported. \\[4pt] [1] Hui Chen et al., PRL 105, 015003 (2010)