Session UI2: Particle Acceleration and Short Pulse Lasers

Electron-Positron Jets Created by Ultra-Intense Lasers

For decades, positrons could only be produced in the laboratory using either radioactive sources or electron accelerators. Recently, it has been shown that ultra-intense laser pulses (I$\lambda ^{2} >$ 10$^{19}$ W$\cdot \mu $m$^{2}$/cm$^{2})$ incident on solid targets can also provide copious positrons ($\sim $ 10$^{10})$ via pair production.\footnote{ H. Chen, S.C. Wilks, J.D. Bonlie, E.P. Liang, J. Myatt, D.F. Price, D.D. Meyerhofer, and P. Beiersdorfer, Phys. Rev. Lett., \textbf{102}, 105001(2009).} While time-integrated energy spectra of the positrons have been experimentally observed, the exact character of the relativistic positron-electron (e$^{+}$-e$^{-}$) beam is only now becoming clear. The detailed physics behind the generation and acceleration of the positrons will be presented. Particle-in-cell simulations using the hybrid code Lsp\footnote{D.R. Welch, et. al, Phys. Plasmas \textbf{13}, 063105 (2006).} of the entire process, from laser-generated electrons to positrons ejected from the solid target, compare favorably to observed energy distributions. Using this benchmark as a base, the actual spatial-temporal energy and density profiles of the ejected positron (and electron) beam are investigated, where space charge is found to be an important effect in determining the properties of this ``jet.'' In particular, this jet is found to consist of a leading, dense electron bunch that is immediately followed by a nearly charge-neutral e$^{+}$-e$^{-}$ beam. In addition to exploring the exciting possibility of using this source to create e$^{+}$-e$^{-}$ plasmas, injection of this jet into a low-density plasma behind the target reveals a strong plasma wakefield effect that dominates the beam-plasma interaction. This, in turn, suggests ways to use this jet as a source of positrons for small-scale laser wakefield accelerator research. This work was performed in collaboration with H. Chen, A.J. Link, and D.R. Welch under the auspices of the U.S. DOE under Contract DE-AC52-07NA27344 and LDRD 10-ERD-044.   

Ultra-relativistic laser-plasma interaction and beyond

Relativistic laser-plasma interaction (LPI) is of broad interest in modern physics, with applications ranging from particle acceleration, laboratory astrophysics, to fast ignition for inertial confinement fusion. LPI is a highly dynamic process, especially in the relativistic regime. The plasma conditions evolve rapidly upon intense laser irradiation, which modifies laser absorption and energy partition. This talk summarizes recent advances in understanding laser absorption and dynamics of ultra-relativistic LPI. It is found that the total absorption of laser pulses by solid targets is strongly enhanced in the ultra-relativistic regime, reaching a surprisingly high level of $\sim$90\% at intensities above $10^{20}W/cm^{2}$. Both presence of preplasma and hole boring contribute to the high absorption. The dynamics of hole boring is studied with a novel single-shot time-resolved diagnostic based on Frequency Resolved Optical Gating (FROG). Time history of the Doppler shift in the reflected light indicates that ponderomotive steepening occurs rapidly and majority of the laser pulse interacts with a sharpened density profile. Two-dimensional (2D) Particle-In-Cell (PIC) simulation results agree well with measurements for short pulses ($<$ 5 ps), however discrepancy showing up after 5ps for longer pulses, indicating 3D effect starts to play a role. In case of high-contrast laser pulses interacting with solid targets, the preplasma is minimal and the delicate competition between plasma creation and ponderomotive pushing results in a snake-like structure in the reflected spectrum. Finally, the talk will briefly cover potential schemes utilizing LPI as an amplification process of laser pulses for next-generation laser systems, which could enable ``vacuum boiling'' laser intensities for future experiments.   

Effect of Lattice Structure on Energetic Electron Transport in Solids Irradiated by Ultraintense Laser Pulses

The generation and transport of energetic (MeV) electrons in solids irradiated by ultraintense laser pulses is of fundamental importance to many topics in intense laser-solid interactions, including ion acceleration, warm dense matter studies and the fast ignition approach to inertial confinement fusion. An investigation into the effect of lattice structure on the transport of energetic electrons in solids irradiated by ultraintense laser pulses will be presented. The study involved the use of various forms (allotropes) of carbon. We observe smooth electron transport in diamond, whereas beam filamentation, arising from resistive instabilities, is observed with less ordered forms of carbon. The highly ordered lattice structure of diamond is shown to result in a transient state of warm dense carbon with metallic-like conductivity at temperatures of the order of 1-100 eV, leading to suppression of electron beam filamentation. First-principles models of the electrical conductivity of the various carbon allotropes under these highly non-equilibrium conditions were used in 3D simulations with the ZEPHYROS particle-based hybrid code. The experimental observations and the simulation results are in very good agreement. The lattice structure is shown to be important in defining the conductivity of the transient warm dense matter state induced by rapid heating of the solid and this defines the fast electron beam transport pattern. P. McKenna et al., Phys. Rev. Lett. 106, 185004 (2011)   

Towards Extreme Field Physics: Relativistic Optics and Particle Acceleration in the Transparent-Overdense Regime

A steady increase of on-target laser intensity with also increasing pulse contrast is leading to light-matter interactions of extreme laser fields with matter in new physics regimes which in turn enable a host of applications. A first example is the realization of interactions in the transperent-overdense regime (TOR), which is reached by interacting a highly relativistic (a0$>$10), ultra high contrast laser pulse [1] with a solid density target, turning it transparent to the laser by the relativistic mass increase of the electrons. Thus, the interactions becomes volumetric, increasing the energy coupling from laser to plasma, facilitating a range of effects, including relativistic optics and pulse shaping, mono-energetic electron acceleration [3], highly efficient ion acceleration in the break-out afterburner regime [4], and the generation of relativistic and forward directed surface harmonics. Experiments at the LANL 130TW Trident laser facility successfully reached the TOR, and show relativistic pulse shaping beyond the Fourier limit, the acceleration of mono-energetic $\sim $40 MeV electron bunches from solid targets, forward directed coherent relativistic high harmonic generation $>$1 keV Break-Out Afterburner (BOA) ion acceleration of Carbon to $>$1 GeV and Protons to $>$100 MeV. Carbon ions were accelerated with a conversion efficiency of $>$10{\%} for ions $>$20 MeV and monoenergetic carbon ions with an energy spread of $<$20{\%}, have been accelerated at up to $\sim $500 MeV, demonstrating 3 out of 4 for key requirements for ion fast ignition. The shown results now approach or exceed the limits set by many applications from ICF diagnostics over ion fast ignition to medical physics. Furthermore, TOR targets traverse a wide range of HEDP parameter space during the interaction ranging from WDM conditions (e.g. brown dwarfs) to energy densities of $\sim $10$^{11}$ J/cm$^3$ at peak, then dropping back to the underdense but extremely hot parameter range of gamma-ray bursts. Whereas today this regime can only be accessed on very few dedicated facilities, employing special targets and pulse cleaning technology, the next generation of laser facilities will operate in this regime by default, turning its understanding in a necessity rather than a curiosity.   

High Flux Spatially Coherent X-ray Generation from Laser Wakefield Accelerators

Nonlinear plasma waves driven by existing ultra-intense short-pulse lasers can trap large numbers of electrons from the plasma (as many as $5\times10^9$) and accelerate them to $\sim GeV$ energy over $\sim1~cm$. The details of the trapping process and plasma wave structure dictate that the trapped electrons undergo transverse oscillatory motion on the microscopic scale of the plasma structure, resulting in short wavelength betatron radiation. These x-ray beams are presumed to retain the short-pulse characteristic of the laser, resulting in high peak flux, making the source a candidate for ultrafast temporally resolved imaging applications. Presented here are experimental studies of the scalings of fluence upon laser power, gas jet length, and electron beam parameters. The spectrum was directly measured by single hit spectroscopy to be broad and smooth with peak photon energy exceeding $10~keV$. Additional measurements indicate that the beam source size can be as small as $1~\mu m$ and that the radiation exhibits spatial coherence. These two key characteristics allow advanced imaging capabilities including phase contrast imaging and tomography, as demonstrated by radiography studies of biological specimens. Collaborators: S.~Kneip (Imperial College London), T.~Matsuoka (Present affiliation: Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science), W.~Schumaker, V.~Chvykov, F.~Dollar, M.~Vargas, G.~Kalintchenko, V.~Yanovsky, A.~Maksimchuk, A.~G.~R.~Thomas, and K.~Krushelnick (University of Michigan)   

Generation, and applications of stable, 100-500-MeV, dark-current-free beams, from a laser-wakefield accelerator

This talk will report the production of high energy, quasi-monoenergetic electron bunches without the low-energy electron background that is typically detected from self-injected laser-wakefield accelerators. These electron bunches are produced when the accelerator is operated in the blowout regime, and the laser and plasma parameters are optimized. High-contrast, high power (30--60 TW) and ultra-short-duration (30 fs) laser pulses are focused onto He-gas-jet targets. The high energy (300-400 MeV) monoenergetic (energy spread $<$ 10{\%}) beams are characterized by 1--4-mrad divergence, pointing stability of 1--2 mrad, and a few-percent shot-to-shot fluctuation of peak energy. The results are scalable: the beam energy can be tuned by appropriate choice of acceleration length, laser power and plasma density. Three-dimensional particle-in-cell simulations show that these electron beams are generated when the accelerator is operated near the self-injection threshold, which suppresses dark current (continuous injection in the first bucket). Suppression of dark current is required to minimize noise, improve the quality of secondary radiation sources, and minimize shielding requirements for high repetition-rate operation. Also reported, is the application of this novel electron-beam source to radiography of dense objects with sub-millimeter spatial resolution. In this case, the energetic electron beam is incident on a 2''-thick steel target with embedded voids, which are detected with image plates. Current progress on the generation of GeV energy electron beams with petawatt peak power laser pulses, from the upgraded DIOCLES laser system, will also be discussed.