Bulletin of the American Physical Society
2005 47th Annual Meeting of the Division of Plasma Physics
Monday–Friday, October 24–28, 2005; Denver, Colorado
Session CO2: High Intensity Laser Matter Interacting; Intense Coherent X-rays; Beams |
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Chair: Jorge Rocca, Colorado State University Room: Adam's Mark Hotel Governor's Square 11 |
Monday, October 24, 2005 2:00PM - 2:12PM |
CO2.00001: Short-Pulse Heavy Ion Production via Target Laser-Ablation-Cleaning and Proton Acceleration Suppression Kirk Flippo, B.M. Hegelich, M.J. Schmitt, E. Dodd, J.A. Cobble, D.C. Gautheir, R. Gibson, R. Johnson, S. Letzering, J.C. Fern\'{a}ndez, T. Lin, A. Maksimchuk, M. Rever*, D. Umstadter* In the last few years it has become apparent that the surface contamination on laser ion-acceleration targets is a major impediment to the acceleration of the actual target ions. To this end we have performed experiments at the Los Alamos Trident Laser facility using one arm of the Trident laser at 150 ps to ablatively clean the target's rear-surface. The front-surface is then irradiated by the Trident TW Short-pulse to accelerate the heavy-ions to high energies. This process was used on targets consisting of 15 microns of vanadium. Normally ions with the lightest charge to mass ratio (i.e. protons) would be accelerated preferentially from the surface at the expense of heavier ions. However, with the rear contamination layer removed, the TNSA mechanism is available to accelerate the bulk material ions to high energies. A lower energy proton component from the front surface is also present and has been observed experimentally and modeled recently with the TRISTAN PIC code, and mitigation is discussed. We report on the ion beam parameters achieved to date, including laser-beam conversion efficiency, ion energy, and beam divergence. Our ablation results are compared to the LASNEX code to validate and improve our predictive capabilities for future experiments. [Preview Abstract] |
Monday, October 24, 2005 2:12PM - 2:24PM |
CO2.00002: Target Optimization Stategies for Control Laser Driven Ion Acceleration Bjoern Manuel Hegelich, K.A. Flippo, Brian J. Albright, Lin Yin, J. Cobble, Cort Gautier, Samuel Letzring, Mark Paffet, Roland Schulze, Juan C. Fernandez Laser-induced ion acceleration, until recently, has been poorly controlled. In this paper, we describe the results from our latest experiments aimed at increasing the control over the ion species, its charge state and its energy spectrum. Focusing a $\sim $30 TW laser pulse on a thin metal foil target, different ions, from Z=1 up to Z=78 have been accelerated to multi-MeV/nucleon energies. Using different target materials, and a better controlled heating technique, we performed detailed studies of the effects of target conditions on the properties of the accelerated ions. Depending on material and temperature, the dominantly accelerated species, its charge state and its energy spectrum can be changed. Using 2 Thomson parabolas at different angles simultaneously enables us to recover information on the spatial ion distribution for given laser and target conditions. The data obtained in the experiments will than be used to expand our predictive capability, from that embodied in our current 1D hybrid model of the ion acceleration, to a 2D and later a full 3D model. [Preview Abstract] |
Monday, October 24, 2005 2:24PM - 2:36PM |
CO2.00003: Hot Electron and Positron Energy Distributions from Ultra-Intense Laser Solid Interactions Hui Chen, Scott Wilks, W. Kruer, S. Moon, N. Patel, P. Patel, R. Shepherd, R. Snavely We present experimental data of electron energy distributions from ultra-intense (greater than $10^{19}$ W/cm$^{2}$) laser-solid interactions using the Rutherford Appleton Laboratory Vulcan petawatt laser. These measurements were made using a magnetic spectrometer that used both CCD and image plates as detectors. We present details on the distinct effective temperatures that were obtained for a wide variety of targets as a function of laser intensity. In addition, we will also present the results of our first attempt at simultaneously measuring the positron energy distribution, along with the electrons. Positrons can be produced during these experiments if the number and effective temperature of hot electrons exceeds a threshold value, which we satisfied by using a gold target, and we report on possible signatures of positrons. This additional information is yet another point of information that allows for better determination of the effective electron temperature that exists in these targets. [Preview Abstract] |
Monday, October 24, 2005 2:36PM - 2:48PM |
CO2.00004: Numerical Calculations of Laser-Generated MeV Electrons and Characteristic X-Ray Production in Copper Foil Targets J. Myatt, J.A. Delettrez, W. Theobald, C. Stoeckl, M. Storm, A.V. Maximov, R.W. Short, R.P.J. Town, L.A. Cottrill We report on the numerical modeling of laser-produced hot-electron flow within solid foil targets. Using copper targets and parameters motivated by recent RAL experiments,\footnote{ C. Stoeckl\textit{ et al.}, Bull. Am. Phys. Soc \textbf{49}, 104 (2004).} calculations have been performed that include photon production due to collisions of hot electrons with target ions, i.e., both bremsstrahlung and characteristic x rays. The sensitivity of absolute K\textit{$\alpha $} yield to laser intensity and target geometry has been investigated. Comparison between the calculated and experimental yields over a range of laser intensities between 10$^{18}$ and 10$^{21}$ W/cm$^{2}$ are presented. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under the Cooperative Agreement No. DE-FC52-92SF19460. [Preview Abstract] |
Monday, October 24, 2005 2:48PM - 3:00PM |
CO2.00005: Stochastic Heating in High Intensity Laser-Plasma Interaction: Theory and PIC Code Simulations Alain Bourdier, David Patin, Erik Lefebvre The basic physical processes in laser-matter interaction, up to $10^{17}$ W/cm2, are now well understood, on the other hand, a large number of issues remain open in the study of the relativistic interaction regime above. Thus, the relativistic motion of a charged particle in a high intensity electromagnetic wave is studied in order to find conditions for stochastic heating to take place in laser-matter interaction. The Hamiltonian formalism is used, the Chrikov criterion allows predicting conditions for the onset of stochastic acceleration. Then, considering a high intensity wave and/or a low-density plasma, PIC code simulations results confirm, in the conditions previously defined, the occurrence of stochastic heating. Finally, it is shown that stochastic heating can help accelerating protons. [Preview Abstract] |
Monday, October 24, 2005 3:00PM - 3:12PM |
CO2.00006: Relativistic Oscillating Gyromagnetic Effects in Ultra-intense Laser-Matter Interaction Robin Marjoribanks, Jean-Paul Geindre, Patrick Audebert We demonstrate that in ultra-intense ultrafast laser-matter interaction, the interplay of laser-induced oscillating space-charge fields with laser E- and B-fields can strongly affect whether the interaction is relativistic or not: stronger laser fields may not in fact produce more-relativistic plasma interactions. We show that there exists a regime of interaction, in the relation of laser intensity and incident angle, for which the Brunel effect of electron acceleration is strongly suppressed by AC gyromagnetic fields, at a frequency different from the laser field. Analytically and with 1.5-D PIC code modelling, we show that from gyromagnetic effects, even in the absence of usual JxB second-harmonic contributions, there is strong impact on the harmonic emission and on the generation of attosecond pulses. [Preview Abstract] |
Monday, October 24, 2005 3:12PM - 3:24PM |
CO2.00007: Dielectric Response of a Laser-Exploded Cluster to a Perturbing Field J.P. Palastro, T.M. Antonsen, A. Gupta The optical properties of a gas of laser-pulse exploded clusters are determined by the time evolving polarizabilities of individual clusters. The polarizability depends on both the inner core electrons, which are largely shielded from the laser field, and an outer halo of hot electrons created by the laser pulse. We calculate the linear polarizability using the Vlasov equation. The equilibrium is calculated for a bi-maxwellian distribution that models both the hot and cold electrons. We then perturb the system to first order in field and integrate the response of individual electrons to the self consistent field following unperturbed orbits. By considering an ensemble of equilibrium orbits, the linear local and non-local responses have been accounted for. Using this method, we investigate the dependence of polarizability on frequency. Work supported by NSF and USDOE. [Preview Abstract] |
Monday, October 24, 2005 3:24PM - 3:36PM |
CO2.00008: Intense Laser-Cluster Interaction: Effect of hot electrons on propagation and studies with longer pulse widths A. Gupta, T. Antonsen, T. Taguchi, J. Palastro, H. Milchberg The non-linear interaction of intense short laser pulses with gases of atomic clusters has applications in areas such as x-ray and energetic particle generation, and nuclear fusion. We model the propagation of laser pulses through a medium of Argon clusters (14 to 54 nm diameter) and un-clustered Argon atoms. Our goal is to understand the competing effects of the non-linear dielectric response of the clusters and that of the free electrons on self-guiding. In our propagation model, the effective dielectric constant of the medium evolves in time due to the hydrodynamic expansion of clusters, ionization of un-clustered monomer gas atoms as well as due to the creation of `hot' electrons during the laser-cluster interaction. Hot electron generation is based on the PIC simulation of laser-cluster interaction[1]. Our simulation results show that for moderate intensities $\sim $6$\times $10$^{15}$ W/cm$^{2}$ the hot electrons lead to enhancement in self-guiding. We also investigate the dynamics of cluster explosion for a range of pulse lengths. Preliminary results suggest that early in the pulse cluster expansion is governed by hydrodynamic forces while kinetic effects dominate later in the pulse. [1] T. Taguchi et. al. PRL \textbf{92}, 205003 (2004). [Preview Abstract] |
Monday, October 24, 2005 3:36PM - 3:48PM |
CO2.00009: Energetic Ion Generation in Intense Femtosecond Laser Interaction with Clusters Katsunobu Nishihara, Vasilii Zhakhovskii, Toshiyuki Hirao, H. Amitani, Serugei Bulanov, Timur Esirkepov Interaction of an intense femtosecond laser pulse with clusters has given a new insight in ionization processes and collective phenomena such as strong radiation emission and generation of high energy ions. We have studied Coulomb explosion of large molecule and clusters driven by intense femtosecond laser interaction, with use of 3d-MD simulations including electron dynamics and 3d-PIC simulations, in which ionization processes are taking into account. We mainly discuss energy spectra of ions in Coulomb explosion of clusters following that laser light expels electrons. In addition to relatively uniform explosion of ions, it is also observed that in circularly polarized laser interaction of normalized laser field of eA/mc=50 with relatively large cluster of 800 nm in radius, energetic ions of up to 400 MeV are accelerated forward by electric field of 150 TV/m induced in a core of the cluster. This large electric field is induced due to density increase, approximately 40 times higher than its initial density. Both the ion energy and induced electric field intensity are about ten times greater than those previously known. The acceleration of ions forward is however not observed in linearly polarized laser interaction. [Preview Abstract] |
Monday, October 24, 2005 3:48PM - 4:00PM |
CO2.00010: High repetition rate tabletop soft x-ray lasers at wavelengths down to 11.9 nm in Nickel-like ions Bradley M. Luther, Yong Wang, Miguel A. Larotonda, David Alessi, Mark Berrill, Vyacheslav N. Shlyaptsev, Jorge J. Rocca There is significant interest in the development of high average power table-top soft x-ray lasers (SXL) for applications. The repetition rate of gain-saturated collisional SXL operating at wavelengths of less than 30nm has been limited to one shot every several minutes by the large laser pump energy required to heat the plasma. Recent experiments have demonstrated a large pump energy reduction by directing the heating pulse into the plasma at grazing incidence [1-3]. This pumping geometry takes advantage of the refraction of the pump beam in the plasma to deposit a large fraction of its energy into the gain region. Here we report 5Hz repetition rate operation of gain-saturated table-top lasers with 1-2$\mu $W average power in transitions of Ni-like ions (Mo, Ru, Pd, Ag and Cd) at wavelengths between 18.9nm and 13.2nm, using a 1J, 8 ps heating pulse from a Ti:Sa laser. Strong amplification was also observed at 11.9 nm in Ni-like Sn. 1. R. Keenan et al, Phys. Rev. Lett., 94, 103901, (2005). 2. B. M. Luther et al, Opt. Lett., 30, 165, (2005). 3. D. Alessi et al, Opt. Express, 13, 2093, (2005). [Preview Abstract] |
Monday, October 24, 2005 4:00PM - 4:12PM |
CO2.00011: Demonstration of a desk-top size high repetition rate soft x-ray laser based on a fast capillary discharge Scott Heinbuch, Michael Grisham, Dale Martz, Fong Deng, Elliot Bernstein, Jorge Rocca The demonstration of laser amplification in transitions of Ne-like ions in a capillary discharge plasma column opened the possibility to develop very compact short wavelength lasers for applications. We report a new type of high repetition rate 46.9 nm Ne-like Ar capillary discharge laser that fits on top of a small desk and that it does not require a Marx generator for its excitation [1]. The relatively low voltage required for its operation allows a reduction of nearly one order of magnitude in the size of the pulsed power unit relative to previous capillary discharge lasers. Laser pulses with an energy of $\sim $13 $\mu $J are generated at repetition rates up to 12 Hz. About (2-3) x 10 $^{4}$ laser shots can be generated with a single capillary. This new type of portable laser is an easily accessible source of intense short wavelength laser light for applications. Work supported by the NSF EUV ERC and DOE. \newline \newline [1] S. Heinbuch, M. Grisham, D. Martz and J.J. Rocca. Optics Express, \textbf{13}, 4050 (2005). [Preview Abstract] |
Monday, October 24, 2005 4:12PM - 4:24PM |
CO2.00012: Multiply Ionized Plasmas with index of refraction greater than one Jorge Filevich, Jonathan Grava, Mike Purvis, Mario C. Marconi, Jorge Rocca, James Dunn, Joe Nilsen, Jim Scofield, Stephen S. Moon, Raymond F. Smith, Roisin Keenan, Jim R. Hunter, V.N. Shlyaptsev We have obtained clear experimental evidence showing that the contribution of bound electrons can dominate the index of refraction in multiply ionized plasmas at soft x-ray wavelengths. We have conducted soft x-ray laser interferometry experiments at 14.7 nm with a Ni-like Pd laser and with a 46.9 nm Ne-like Ar laser that show anomalous fringe shifts in laser-created plasmas from different target materials. Comparison with code simulations shows that the observed anomalous fringe shifts are the result of the contribution of bound electrons of low charge ions to the index of refraction. The results have broad implications, as the bound electrons are shown to affect the index of refraction of many plasmas at soft x-ray wavelengths. Work sponsored by the NNSA-SSAA program through DOE Grant {\#} DE-FG03-02NA00062 and U.S. DOE by the U. of California LLNL through the ILSA, contract No. W-7405-Eng-48. [Preview Abstract] |
Monday, October 24, 2005 4:24PM - 4:36PM |
CO2.00013: Dynamic Study of an Intense Relativistic Electron Beam Transported in an Over-Dense Plasma Alain Piquemal We want to investigate the evolution of an intense relativistic electron beam when it is transported in an over-dense plasma. It is known that a collision-less beam which undergoes self-similar deforming, does not see emittance growth, even if it is submitted to non-linear electromagnetic forces. This property is still verified in an ionised gas in which the beam emittance growth is due only to collisions with the atoms and molecules of the medium. In this case, the beam goes progressively to a Bennett profile. But the problem is the understanding of the routes the beam follows, to go from a phase space limited distribution function (d.f.) at the generator output, to a Bennett-Maxwell d.f. when the beam parameters are degraded. We study some diagnostics taken from the chaos mechanics and propose some ideas about the internal structure of intense relativistic beams and associated mechanisms of thermalization. [Preview Abstract] |
Monday, October 24, 2005 4:36PM - 4:48PM |
CO2.00014: Equilibrium and stability of a high-intensity periodically twisted ellipse-shaped charged-particle beam Jing Zhou, Ronak Bhatt, Chiping Chen It is shown that there exists an exact paraxial cold-fluid equilibrium of a high-intensity, space-charge-dominated charged-particle beam with a periodically twisted elliptic cross section in a non-axisymmetric periodic magnetic field. Generalized envelope equations, which determine the beam envelopes, ellipse orientation, density, and internal flow velocity profiles, are derived. Effects of nonlinearities in the magnetic fields and instabilities at high vacuum phase advances are investigated. The parameter space for stable operation is identified. The beam equilibrium and stability properties are verified by two-dimensional self-consistent particle-in-cell (PIC) simulations using the MIT 2D Periodic Focused Beam (PFB2D) code. The beam equilibrium is further verified by 3D simulations using the commercial code OmniTrak. Applications in high-power microwave sources are discussed. [Preview Abstract] |
Monday, October 24, 2005 4:48PM - 5:00PM |
CO2.00015: Priming effects in a relativistic magnetron with a transparent cathode H.L. Bosman, S. Prasad, M.I. Fuks, E. Schamiloglu Theoretical and simulation studies have shown that the transparent cathode can cause shorter start-up times in relativistic magnetrons, compared with the traditional solid cathode (even when cathode priming or magnetic priming is applied). The transparent cathode consists of several individual cathode strips, arranged in a cylindrical geometry. This arrangement allows the RF electric fields to penetrate through the cathode strips, resulting in a larger E$_{\theta }$ field amplitude in the electron hub region which enhances electron capture into spokes. The transparent cathode also allows for priming of the magnetron, defined here as the bunching of electrons into the preferred spoke geometry before the Buneman-Hartree condition for the desired mode is satisfied. A detailed study was undertaken using the 3D electromagnetic particle-in-cell code MAGIC to investigate priming effects for the well-known A6 magnetron geometry with a transparent cathode. The simulations show a correlation between the number of cathode strips and the mode that is primed in the magnetron, and that the magnetron behavior is sensitive to the placement of the cathode strips relative to the anode vanes. However, once the Buneman-Hartree condition is satisfied, RF effects always dominate over priming effects in spoke formation. [Preview Abstract] |
Monday, October 24, 2005 5:00PM - 5:12PM |
CO2.00016: Development of the Indiana Rf Photocathode Source Simulator Mark Hess, Chong Shik Park, Daniel Bolton One of the challenging issues for simulating rf photocathode sources is how to incorporate fully electromagnetic effects which are generated by the bunched electron beam in the vicinity of complex conducting wall structures. This problem can be handled self-consistently by using a time-dependent Green's function formulation of the electromagnetic fields. We are currently developing a new simulation code called, IRPSS (Indiana Rf Photocathode Source Simulator), which utilizes the time-dependent Green's function method. We discuss the theory and current capabilities of IRPSS, and show the initial results of simulations performed on IRPSS using the experimental parameters of existing rf photocathode sources. [Preview Abstract] |
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