Session 15HE: Intense Laser-Matter Interactions

Blast waves in atomic cluster media using intense laser pulses.

We report on the progress of experimental and numerical investigations of the dynamics of strong ($>$Mach 50) blast waves driven by focusing sub-ps laser pulses into an extended medium of atomic clusters. A gas of atomic clusters is an extraordinarily efficient absorber of intense laser light and can be used to create high energy density plasmas with tabletop laser systems. These HED plasmas can launch shocks and strongly radiative blast waves with dimensionless parameters scalable to astrophysical objects such as supernova remnants, and have been used by us in a number of shock evolution and collision studies. To date such experiments have been conducted with modest laser energies of $<$1J. In order to study processes such as the Vishniac overstability and cooling instability in these systems significantly more input energy may be required due to the weak variation of blast wave velocity with deposited energy Vb $\propto $ E$^{1/4}$. We report on the scaling of cluster blast wave experiments to laser energies up 0.5kJ using the Vulcan laser at RAL. An extensive suite of diagnostics including multi-frame optical probe systems, streaked Schlieren imaging and keV imaging and spectroscopy was fielded in order to study the growth of spatial and temporal instabilities. To better match astrophysical scenarios with strong radiative pre-heat of material upstream of the shock an additional radiation field was also introduced using a secondary laser heated gold foil target and grazing incidence XUV guiding structure. This allowed us to compare blast wave propagation into cold versus hot ionized upstream gases. These experimental systems provide a useful test bed against which to benchmark numerical simulations, and have been compared to the 3D magnetoresistive hydrocode GORGON and radiation-hydrodynamics code NYM.   

Creating high energy density matter with intense laser driven proton beams

The interaction of a high-intensity short-pulse laser with a thin foil target can produce an intense highly directional beam of protons. The laser pulse produces a population of suprathermal electrons, which flood the target and set up an electrostatic sheath field at the rear surface. This highly transient field accelerates ions (predominantly protons) from a thin layer at the rear surface to multi-MeV energies on a timescale of just a few hundred femtoseconds. The properties of this proton beam make it an interesting candidate for application to the creation of high energy density matter. We describe experiments conducted on the 350J Titan Petawatt laser at the Lawrence Livermore National Laboratory and on the 500J Vulcan Petawatt laser at the Rutherford Appleton Laboratory to investigate the utility of laser driven proton beams for creating plasma conditions ranging from the warm dense matter regime of a few eV temperature at solid density, to the highly localized multi-keV hot spot temperatures necessary for proton fast ignition.   

Applications of the wave-kinetic approach: from wakefield accelerators to space plasmas

The theory of wave-kinetics is a new approach to the study of the propagation of broadband, incoherent waves in dispersive media. The wave-kinetic approach is centered around the propagation of individual wave modes, and both monochromatic and incoherent waves can be described easily through the wave mode distribution function. Where the classical approach to wave-driven processes restricts itself mainly to monochromatic waves coupling to other monochromatic waves, the wave-kinetic approach allows one to study the coupling of random and/or broadband pump waves coupling to monochromatic or coherent structures in the medium. The list of phenomena that can be studied through this approach is very long, reaching from laser-wakefield studies to lower-hybrid drift wave studies, turbulence in planetary atmospheres, and even neutrino-driven shock waves in collapsing stars. We have employed the wave-kinetic approach to study photon acceleration (changing the frequency of laser light using a spatially and temporally changing index of refraction) in ultrafast, ultra-intense laser-plasma interaction experiments. Photon acceleration can be used both as a means to prove the existence of laser-driven wakefields and as a tool to determine their properties. In addition, we have used the wave-kinetic approach to model drift wave turbulence interacting with zonal flows in a magnetized plasma. We have discovered a mechanism for spontaneous generation of soliton-like structures, which has applications ranging from turbulence at the magnetopause boundary layer to edge localized modes in tokamaks. For both laser-plasma interaction and drift mode turbulence, we will compare wave-kinetic simulations against the results of recent experiments.   

Weibel instability in colliding electron-positron-ion plasmas

The new regimes accessed in ultra intense laser plasma interactions and recent developments in relativistic astrophysics are giving rise to an increased interest in the Weibel instability. In fact, whenever colliding streams of plasmas (arbitrary mixtures of electrons-positrons-ions) are present, a fraction of the kinetic energy of the plasma flows can be converted to a sub-equipartition magnetic field. In this talk, and using a combination of particle-in-cell simulations and relativistic kinetic theory, I will first describe the recent theoretical advances in our understanding of the Weibel instability and the connection with the electromagnetic beam plasma instability. Emphasis will be given to the coupling with longitudinal modes, leading to the formation of tilted filaments, and to the effects of the collisions and the merging of the Weibel instability with the resistive filamentation instability. In light of these results, the relevance of Weibel instability to ultra intense laser matter interactions (e.g. fast ignition) and to astrophysical scenarios (e.g. in gamma ray bursters and for cluster magnetic fields) will be discussed. Finally, the role of the Weibel instability in the formation of relativistic shocks and in particle acceleration in these structures will also be addressed.   

Energy transport and isochoric heating of ultra-intense laser irradiated target

Irradiation of matter with ultra-intense short laser pulses generates MeV electrons and can create plasmas at solid density and temperatures of several hundred eV, i.e. several million degrees. Since hydrodynamic expansion of such micrometer-sized targets, driven apart by the Gigabar pressure of MeV hot electrons, limits their lifetime to a few picoseconds, energy must be deposited rapidly, i.e. on a sub-ps time scale, and deep in the target. Therefore, to realize various applications such as a compact neutron source, blight x-ray, and a good test bed of the high energy density physics, it is essential to understand the energy transport inside dense plasmas. We study ultra-fast heating of thin plastic foils by intense laser irradiation theoretically using collisional two-dimensional particle-in-cell simulations. We find that the laser-generated hot electrons are confined laterally by self-generated resistive magnetic fields, heating the laser focal area beyond keV electron temperatures isochorically in a few picoseconds. Also strong surface magnetic fields are excited deu to rapid lateral diffusion of MeV electrons. Using this confinement by the self-generated fields one can excite shock waves that compress the plasma beyond solid density and achieve keV thermal plasmas before the plasma disassembles. Such shocks can be launched at material interfaces inside the target where jumps in the average ionization state and thus electron density lead to Gigabar pressure. They propagate stably over picoseconds accompanied by multi-MegaGauss magnetic fields, and thus have a potential for various applications in high energy density physics.   

Transport of high intensity laser-generated hot electrons in cone coupled wire targets

In this talk, we present results from a series of experiments where cone-wire targets were employed both to assess hot electron coupling efficiency, and to reveal the source temperature of the hot electrons. Experiments were performed on the petawatt laser at the Rutherford Appleton Laboratory. A 500J, 1ps laser (I $\sim $ 4 x 10$^{20}$ W/cm$^{-2})$ was focused by an f/3 off-axis parabolic mirror into hollow aluminum cones joined at their tip to Cu wires of diameters from 10 to 40 $\mu $m. The three main diagnostics fielded were a copper K$_{alpha}$ Bragg crystal imager, a single hit CCD camera spectrometer and a Highly Oriented Pyrolytic Graphite (HOPG) spectrometer. The resulting data were cross-calibrated to obtain the absolute K$_{alpha}$ yield. Comparison of the axially diminishing absolute Cu K$_{\alpha}$ intensity with modeling shows that the penetration of the hot electrons is consistent with one dimensional ohmic potential limited transport (1/e length $\sim$ 100 $\mu$m). The laser coupling efficiency to electron energy within the wire is shown to be proportional to the cross sectional area of the wire, reaching 15{\%} for 40 $\mu $m wires. We find that the hot electron temperature within the wire was $\le $750 keV, significantly lower than that predicted by the ponderomotive scaling. A comparison of the experimental results with 2D hybrid PIC simulations using e-PLAS code will be presented and relevance to Fast Ignition will be discussed at the meeting. \newline *In collaboration with J.A. King, M.H. Key, K.U. Akli, R.R. Freeman, J. Green, S. P. Hatchett, D. Hey, P. Jaanimagi, J. Koch, K. L. Lancaster, T. Ma, A.J. MacKinnon, A. MacPhee, R. Mason, P.A. Norreys, P.K Patel, T. Phillips, R. Stephens, W. Theobald, R.P.J. Town, M. Wei, L. Van Woerkom, B. Zhang.