Bulletin of the American Physical Society
55th Annual Meeting of the APS Division of Plasma Physics
Volume 58, Number 16
Monday–Friday, November 11–15, 2013; Denver, Colorado
Session PO6: Short-Pulse Laser Plasma Interactions |
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Chair: Douglass Schumacher, Ohio State University Room: Governor's Square 11 |
Wednesday, November 13, 2013 2:00PM - 2:12PM |
PO6.00001: Characterization and scaling effect of the resistive magnetic field on guiding laser generated fast electrons in solid targets Yasuhiko Sentoku, Philippe Leblanc For applications such as fast ignition, laser generated fast electrons play an essential role in determining energy deposition mechanics. However, the physics behind the electron beam self-guiding in solid materials is poorly understood. Upon examination of experimental results and simulation data, it has been determined that understanding the resistive magnetic field is crucial in determining laser produced fast electron transport patterns in solid targets. The scaling of the resistive magnetic field and confinement conditions are derived and are compared with 2-dimensional collisional particle-in-cell simulations. We study the impact of the initial state of the material (Z dependence, conductor or insulator) on global electron transport patterns. The fast electron transport seen in the simulations are found to be consistent with our scaling rule. Previous experimental observations (e.g. Stephens PRE 2004 and Sentoku PRL 2011) that show confinement or divergence in various materials are explained by this empirical resistive scaling. Our scaling is a powerful tool to design applications of compact radiation source, where controlling fast electron transport is critical. [Preview Abstract] |
Wednesday, November 13, 2013 2:12PM - 2:24PM |
PO6.00002: Magnetic deflection of MeV electrons in 3D simulations of intense laser-solid interaction F. Perez, A. Kemp, L. Divol, C.D. Chen, P.K. Patel Using three-dimensional particle-in-cell (PIC) simulations of 10$^{19}$ - 10$^{20}$ W/cm$^{2}$ laser-solid interaction, we find that the reflected laser light, while only 15{\%} of the incident laser power, accelerates electrons (away from the target) at both a higher density and over a larger volume than the incident light. It results in a strong current that dominates the generation of magnetic fields. These fields have important consequences for the laser-accelerated relativistic electron beam, with a significant fraction of the multi-MeV electrons re-directed away from their initial direction along the laser axis. This effect, not described in previous literature, is strongly dependent on incident angle and can change the interpretation of divergence and directionality measurements in laser experiments. We have developed a reduced model reproducing the essential physics of this magnetic field generation and electron beam deflection. We investigate how this will affect experiments in Omega EP or future facilities which feature large spot sizes and longer pulses. [Preview Abstract] |
Wednesday, November 13, 2013 2:24PM - 2:36PM |
PO6.00003: On The Origin of Super-Hot Electrons from Intense Laser Interactions with Solid Targets having Moderate Scale Length Preformed Plasmas A.G. Krygier, D.W. Schumacher, R.R. Freeman The results of a numerical study investigating the acceleration mechanism for super-hot electrons by an intense laser interaction with moderate scale length preformed plasma are described. The particle-in-cell code LSP is used to model a $100J$, $175fs$, peak intensity $6 x 10^{20} W/cm^{2}$ laser in 2D Cartesian geometry. The laser interacts with a solid density Al target with a $L=3\mu m$ scale length preformed plasma. We find that a simple three-step mechanism that we call loop-injected direct acceleration (LIDA) is overwhelmingly dominant in the acceleration of the hottest electrons. LIDA involves only well-known physics and is numerically observed over a range of pre-plasma and laser conditions. In LIDA, the laser heats the plasma near the critical surface expelling electrons from the region. Some of the expelled electrons follow looping paths away from the target, guided by quasi-static magnetic fields, and are injected into the intense region of the laser pulse where they are laser-accelerated until they escape into the target with large energy. This work is supported by DOE contracts DE-FC02-04ER54789 and DE-FG02-05ER54834 and allocations of computing time from the Ohio Supercomputer Center. [Preview Abstract] |
Wednesday, November 13, 2013 2:36PM - 2:48PM |
PO6.00004: Shaping the Spectrum of Hot Electrons using Structured Targets Sheng Jiang, Andrew Krygier, Douglass Schumacher, Richard Freeman, Kramer Akli Hot electron generation is a crucial aspect of the intense laser solid interaction. Proper energy and angular distributions of the fast electrons greatly benefit subsequent processes such as X ray/gamma ray production and ion acceleration. Fast electrons generated using simple flat targets are large in charge, but usually have high divergence, low energy and broad spectrum, which limit the efficacy of their applications. We have used 3D LSP PIC simulations to develop a way to generate high energy, low divergence electrons using structures (spikes or fins) on the target front surface. When an intense, ultra-fast laser pulse interacts with these structures, electrons at the tip are accelerated via direct laser acceleration to energies much higher than the ponderomotive energy. The electric and magnetic fields from these super-hot electrons and the return current inside the structures guide the electrons, leading to a small divergence angle. Varying the structure shape can further tune the electron spectrum. [Preview Abstract] |
Wednesday, November 13, 2013 2:48PM - 3:00PM |
PO6.00005: Study of surface current confinement in high-intensity laser interactions with wire targets A. Maksimchuk, P. Belancourt, M.J.-E. Manuel, L. Willingale, A.G.R. Thomas, R.P. Drake, K. Krushelnick, A.V. Brantov, V. Yu. Bychenkov Understanding surface electron current generation and propagation is increasingly important for fast-ignition fusion research. From the interaction of a 400 fs, 20 TW laser pulse focused to intensity up to 4x10$^{19}$ W/cm$^{2}$on tungsten wire, we observed a highly collimated, hollow electron beam with a charge of several nC and electron energies greater than 1 MeV. The beam was confined and guided along the wires of different diameters to a distance of 40 cm. Simultaneous measurements of electron beams at both ends of the wire show symmetrical images for 0 degree angle of incidence for s- and p- laser polarizations. The measured electron spectrum extended to 4 MeV; the spatial structure for different components of the spectrum confirmed the production of a hollow electron beam and demonstrated a better confinement of the lower energy electrons. Electron beam radiography of a solid object was performed and showed spatial resolution better than several hundred microns. The experimental results were interpreted taking into account the generated electric and magnetic fields near the surface of the wire, which are the result of strong charge separation during the laser-plasma interaction. Test particle simulations shows that these fields provide the crucial conditions for collimating and confining the laser-produced hollow electron beams along the wire. [Preview Abstract] |
Wednesday, November 13, 2013 3:00PM - 3:12PM |
PO6.00006: Time-resolved measurements of fast electron-sourced sheath dynamics J.S. Green, C.D. Murphy, R.J. Gray, D.A. Maclellan, P. McKenna, R.J. Dance, C.P. Ridgers, A.P.L. Robinson, D. Rusby, L. Wilson Here we present unique fast electron generation and transport results from a recent experimental campaign on the Astra Gemini laser facility. Using complementary diagnostics a unique picture is presented of the evolution of electron sheath dynamics, and thus electron transport, in the interaction of an ultra-intense, short pulse (40 fs) laser with a solid target. Targets were irradiated at up to ($10^{21} Wcm^{-2}$), with a chirped optical probe used to spatially and temporally resolve the rear surface target reflectivity. Clear snapshots of rapid ionisation and fast electron transport at the target rear surface were observed for a range of target and laser parameters. As well as providing a valuable insight into fast electron transport at the rear surface the role of fast electron refluxing inside thin targets was also investigated through the use of thin foils and novel targetry. These studies aim to see how the very earliest stages of the fast electron dynamics directly affect both the sheath evolution and ion acceleration at the rear surface. [Preview Abstract] |
Wednesday, November 13, 2013 3:12PM - 3:24PM |
PO6.00007: Simulations of ps-laser channeling and fast electron generation in realistic density gradients Marija Vranic, Jorge Vieira, Hirotaka Nakamura, Ricardo Fonseca, Peter Norreys, Luis Silva Energetic electron beams are useful for many aplications such as radiation generation, laboratory astrophysics, energy transport and many more. Laser channel formation in plasmas is particularly interesting for the fast ignition scheme, where a long laser pulse is used to increase efficiency of the ignition laser energy delivery to the core. Electrons generated in the channel can be used to enhance this energy coupling. We have performed full-scale 2D PIC simulations of channeling of $\sim$ 15ps lasers through 2mm plasmas with realistic density gradients relevant for fast ignition scenario using OSIRIS 2.0. Full cavitation is not achieved - a fraction of electrons remains in the channel and is accelerated by the combination of laser and channel fields. We identify different regimes depending on the background plasma density by simulating the same plasma length with flat-density profiles. Evidence of hosing instability is observed at high densities. [Preview Abstract] |
Wednesday, November 13, 2013 3:24PM - 3:36PM |
PO6.00008: Investigation of fast electron-induced K$\alpha $ x-rays in laser-produced blow-off plasma Hiroshi Sawada, M.S. Wei, S. Chawla, A. Morace, K. Akli, T. Yabuuchi, N. Nakanii, M.H. Key, P.K. Patel, H.S. McLean, R.B. Stephens, F.N. Beg Interaction of a high-power, short-pulse laser exceeding the peak intensity of 10 18 W/cm$^2$ with solid targets is an efficient source of characteristic x-ray. We have quantitatively studied the K$\alpha $ x-ray production in laser-produced expanding plasma. Using two-beam TITAN laser at LLNL, a multilayered target was irradiated by the long pulse laser to create blow plasma and by the short pulse laser to generate fast electrons at the delay of 0, 1, 6 and 8 ns. The K$\alpha $ yields and monochromatic images were recorded with a Bragg crystal spectrometer and spherical crystal imager. The results show a decrease of the total K$\alpha $ yields by a factor of 8 from the refluxing to non-refluxing conditions. The size of the K$\alpha $ spot was unchanged at any delays. There is good agreement between the experimental data and modeling using hydrodynamic and hybrid-PIC codes in the K$\alpha $ yields. Results will be presented at the meeting. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and DE-FG-02-05ER54834 (ACE). [Preview Abstract] |
Wednesday, November 13, 2013 3:36PM - 3:48PM |
PO6.00009: Relativistic Transparency Experiments at the Trident Laser J.A. Cobble, S. Palaniyappan, D.C. Gautier, Y.H. Kim, D.D. Clark, R.P. Johnson, T. Shimada, J.C. Fernandez, H.W. Herrmann With near-diffraction-limited irradiance of 3 x 10$^{20}$ W/cm$^{2}$ on target and prelase contrast better than 10$^{-9}$, we have accessed the regime of relativistic transparency (RT) at the Trident Laser. The goal was to assess electron debris emitted from the target rear surface with phase-contrast imaging (PCI) and current density measurements (hence, the total electron current). Companion diagnostics show whether the experiments are in the target-normal-sheath-acceleration mode or in the RT regime. The superb laser contrast allows us to shoot targets as thin as 50 nm. PCI at 527 nm is temporally resolved to 600 fs. It has shown the evolution of electron behavior over tens of ps, including thermal electrons accompanying the ion jet, accelerated to many tens of MeV earlier in time. Faraday-cup measurements indicate the transfer of many uC of charge during the laser drive. As a ride-along experiment using a gas Cherenkov detector (GCD), we have detected gamma rays of energy \textgreater~5 MeV. This radiation has a prompt component and a lesser source, driven by accelerated ions, that is time resolved by the GCD. The ion time of flight is compared to Thomson parabola data. Electron energy spectra are also collected. [Preview Abstract] |
Wednesday, November 13, 2013 3:48PM - 4:00PM |
PO6.00010: Comparison of Measured and Simulated Properties of Laser-Driven Ion Beams Juan C. Fernandez, S. Palaniyappan, R. Shah, B.J. Albright, J. Cobble, D.C. Gautier, C. Hamilton, C. Huang, L. Yin, J. Williams, B.M. Hegelich, D. Jung This presentation expands on the results in the talk by Palaniyappan et al. Multiple laser-driven ion acceleration mechanisms have been studied in a series of experiments at the Trident laser facility, enabled by a variety of laser targets, ranging from nanofoil targets of different materials to foams that provide near-critical-density plasmas. These experiments have been extensively diagnosed with many instruments and techniques, including ion spectrometers, electron spectrometers, frequency-resolved optical gating of the reflected and transmitted laser beams, and a transmitted-laser-beam profiler. Ion acceleration has been observed in both the regimes where the laser plasma remains opaque and where it becomes transparent. In some cases a measure of ion-spectral control has been demonstrated, beyond the typical Maxwellian ion distribution. In this presentation, initial simulations of these experiments are compared with spectrally and angularly resolved ion-beam characterization measurements. [Preview Abstract] |
Wednesday, November 13, 2013 4:00PM - 4:12PM |
PO6.00011: Real-time diagnostics of laser acceleration of ions in overdense gas-jet plasma Chakra Maharjan, Oliver Tresca, Nathan Crook, Nicholas Dover, Mikhail Polyanskiy, Zulfikar Najmudin, Peter Shkolnikov, Igor Pogorelsky Laser acceleration of ions to MeV energies in overdense plasma at relativistic intensities shows promise for obtaining ion beams for important applications. Recently, gas jet targets were proposed for laser acceleration of ions, utilizing the fact that plasma overdense for CO$_{2}$ lasers ($\lambda$ $\sim$ 10 $\mu$m) is easily created in gas jets. Among advantages of gas targets are availability of optical plasma diagnostics, easy control of plasma density, non-destructiveness, and the ability to generate purely proton beams. With experiments underway, much of the physics of the process is still unknown. Several acceleration mechanisms are currently under consideration, with shock-wave acceleration been studied most extensively. We report our results on ion generation in overdense plasma of H and He gas jets. Laser-plasma processes have been studied using two probe pulses. Varying delays between these pulses and the CO$_{2}$ laser pulse, we observe and evaluate the evolution of the laser-driven shock which, we believe, is the process mainly responsible for ion acceleration in our parametric regime. We present our real-time study of the formation and evolution of plasma shock waves and their correlation with maximum ion energy. [Preview Abstract] |
Wednesday, November 13, 2013 4:12PM - 4:24PM |
PO6.00012: Absorption in Temporally Clean Ultra-Intense Laser Plasma Interactions Calvin Zulick, Franklin Dollar, Anthony Raymond, Louise Willingale, Vladimir Chvykov, Galina Kalintchenko, Anatoly Maksimchuk, Alexander Thomas, Victor Yanovsky, Karl Krushelnick Experiments at the HERCULES laser facility have been performed to measure the transmission and reflection of a temporally clean ultra-intense laser pulse interacting with a thin-film solid density target. The laser pulse had a nanosecond amplified spontaneous emission contrast of better than $10^{-15}$ which was achieved through a combination of cross polarized wave (XPW) pulse cleaning at the pre-amplification level as well as the use of dual plasma mirrors. Silicon based targets ranging in thickness from 10's of nanometers to millimeters demonstrated $\leq 1\%$ light transmission. Target reflectivity of up to 70\% was observed with S polarization, and up to 30\% with P polarization. An increase in target absorption was observed with thin-film targets which has been attributed to the deformation of the target critical surface. OSIRIS simulations have been performed to model the thin-film interactions, and have been compared to experimental results. [Preview Abstract] |
Wednesday, November 13, 2013 4:24PM - 4:36PM |
PO6.00013: Kinematic Constraints on Absorption of Ultraintense Laser Light Matthew Levy, Scott Wilks, Max Tabak, Matthew Baring A fundamental property of ultraintense laser interaction with matter is the trend towards increasing absorption with irradiance. Established through simulations and comprehensive experimental data over the years, the theoretical basis to date has remained predominantly heuristic. In this Letter, we propose an explanation for this universal scaling, which we show emerges naturally from a novel mathematical formulation of the laser plasma (LP) interaction. Using constrained minimization techniques, we demonstrate that the LP system is subject to severe restrictions, which become increasingly strict with intensity $I_L$. We show that the phase volume of states accessible to the LP system, $\Lambda$, scales in inverse proportion to the laser intensity and forbids states corresponding to low absorption. We further demonstrate that $\Lambda$ undergoes rapid contraction in the regime $10^{18} < I_L < 10^{20} W/cm^2$. These key properties suggest that the global trend towards increasing absorption with $I_L$ may be a reflection of the underlying phase volume contraction. This work was performed under the auspices of the U.S. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Wednesday, November 13, 2013 4:36PM - 4:48PM |
PO6.00014: Self-sustaining relativistic ionization wave launched by a sheath field Alexey Arefiev, Matt McCormick, Hernan Quevedo, Roger Bengtson, Todd Ditmire We present experimental evidence supported by particle-in-cell (PIC) simulations of a self-sustaining relativistic ionization wave launched into a surrounding gas by the sheath field of a high energy density plasma. We create a plasma filament with hot electrons by irradiating a supersonic clustering gas jet with a short pulse laser (115 fs) at an intensity of $5 \times 10^{17}$ W/cm$^2$. In contrast with a single atom, a cluster of atoms produces super-ponderomotive electrons in the field of the laser. These electrons generate a sheath field at the edge of the plasma filament strong enough to ionize the gas atoms in the sheath. We observe that a collisionless ionization wave is launched in this regime, propagating radially through the gas at up to 0.5 $c$ after the laser has passed. The expansion of the resulting plasma filament due to the ionization wave occurs in about 2 ps, more than doubling the initial radius of the filament. The remarkable longevity of the wave without continuous energy deposition into the electron population is explained by a moving field structure that traps the hot electrons near the boundary. 2D PIC simulations confirm that the trapped hot electrons maintain a sheath field required for the ionization despite the significant expansion of the filament. [Preview Abstract] |
Wednesday, November 13, 2013 4:48PM - 5:00PM |
PO6.00015: Radiation transport in ultrafast heated high Z solid targets Ioana Paraschiv, Yasuhiko Sentoku, Roberto Mancini, Tomoyuki Johzaki Ultra-intense laser-target interactions generate hot, dense, and radiating plasmas, especially in the case of high-Z target materials. In order to evaluate the effect of radiation and its transport on the laser-produced plasmas we have developed a radiation transport (RT) code and implemented it in a collisional particle-in-cell code, PICLS. The code uses a database of emissivities and opacities as functions of photon frequency, created for given densities and temperatures by the non-equilibrium, collisional-radiative atomic kinetics 0-D code FLYCHK together with its postprocessor FLYSPECTRA [1]. Using the two-dimensional RT-PICLS code we have studied the X-ray transport in an ultrafast heated copper target, the X-ray conversion efficiency, and the exchange of energy between the radiation field and the target. The details of these results obtained from the implementation of the radiation transport model into the PICLS calculations will be reported in this presentation. \\[4pt] [1] H.-K. Chung, M.H. Chen, W.L. Morgan, Y. Ralchenko, HEDP 1, 3 (2005) [Preview Abstract] |
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