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 KO1: Plasma-Based Accelerators and Radiation Sources |
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Chair: Eric Esarey, Lawrence Berkeley National Laboratory Room: Adam's Mark Hotel Governor's Square 10 |
Wednesday, October 26, 2005 9:30AM - 9:42AM |
KO1.00001: Generation of 300 MeV Quasi-Monochromatic Electron Beams from Laser Wakefield and Initiation of Photonuclear Reactions A. Maksimchuk, S. Reed, N. Naumova, V. Chvykov, B. Hou, G. Kalintchenko, P. Rousseau, G. Mourou, V. Yanovsky, J.R. Beene, D.R. Schultz, D.W. Stracener, C.R. Vane In the interaction of 30 fs, 40 TW Ti:sapphire Hercules laser at the University of Michigan, which is focused to the intensity of 10$^{19}$ W/cm$^{2 }$onto a supersonic He gas jet with electron density close to the resonant density, we observed quasi-monoenergetic electron beams with energy up to 300 MeV and angular divergence of about 10 mrad. The results on characterization of relativistic electron beam in terms of energy spread, its charge, divergence and pointing stability will be presented. 2D PIC simulations performed for the parameters close to the experimental conditions show the evolution of the laser pulse in plasma, electron injection, and the specifics of electron acceleration observed in experiments. Resulted relativistic electron beams have been used to perform gamma-neutron activation of $^{12}$C and $^{63}$Cu and photo-fission of $^{238}$U. We demonstrated that approximately 10$^{6}$ reaction per shot has been produced in each case. This work was supported by the NSF through the Physics Frontier Center FOCUS. JRB, DRS, DWS, and CRV acknowledge support by the DOE under contract DE-AC05-00OR22725 with UT-Battelle, LLC. [Preview Abstract] |
Wednesday, October 26, 2005 9:42AM - 9:54AM |
KO1.00002: WITHDRAWN--Ponderomotive-based temperature sampling of relativistic electron beams Anthony Valenzuela, Anatoly Maksimchuk, Rahul Shah, Scott Sepke, Sudeep Banerjee, Donald Umstadter The physical process that produces an electron beam from self-modulated laser wakefield acceleration evolves rapidly. This is due to the growth of the plasma wave that creates the acceleration structure$^{1}$. Our previous work$^{2}$ has demonstrated the deflection of an ultrafast electron beam via the ponderomotive force of a second laser pulse at a counter-propagating angle. The strength of the deflection is related to the energy of the electron and provides a means to spectrally resolve energy components of a polychromatic electron beam. By varying the interaction time and observing the resulting deflected electron beam, we can relate the time delay to a temperature measurement of a section of the electron beam. This provides a means to more closely examine variations of electron beam temperature temporally, providing insight into the physical processes in acceleration. \\ 1. S. P. Leblanc, et al. Phys. Rev. Lett. 77, 5381 (1996) \\2. S. Banerjee, et al. Phys. Rev. Lett. 95, 035004 (2005) [Preview Abstract] |
Wednesday, October 26, 2005 9:54AM - 10:06AM |
KO1.00003: Progress on a 1 GeV laser accelerator at the LOASIS Facility of LBNL W.P. Leemans, B. Nagler, K. Nakamura, Cs. Toth, C.G.R. Geddes, J. van Tilborg, E. Esarey, C.B. Schroeder, A. Gonsalves, S. Hooker, C. Filip, E. Michel, T. Cowan, D. Bruhwiler, D. Dimitrov, J. Cary Progress towards a 1 GeV laser-driven plasma-based accelerator at the LOASIS facility will be presented. The design of the 1 GeV accelerator module consists of an all-optical electron injector and a plasma channel for laser guiding and electron acceleration to high energy via the laser wakefield acceleration (LWFA) mechanism. Experimentally we have previously demonstrated laser guiding at relativistic intensities in preformed plasmas and production of quasi-monochromatic electron beams with energy around 100 MeV [1]. Guiding experiments are underway using the 100 TW-class LOASIS laser with capillary discharges that provide multi-cm scale plasma channels in hydrogen gas at densities on the order of $10^{18} cm^{-3}$. Intensities in excess of $5\times 10^{17} W/cm^2$ have been guided in plasma densities that are sufficiently low to provide high phase velocity plasma wakes for GeV electron beams. Progress on acceleration of electrons will be presented. [1]C.G.R. Geddes et al., Nature 431, 538 (2004). [Preview Abstract] |
Wednesday, October 26, 2005 10:06AM - 10:18AM |
KO1.00004: Warm wavebreaking of laser-driven plasma waves C.B. Schroeder, E. Esarey, B.A. Shadwick, W.P. Leemans We present a calculation of the maximum amplitude (wavebreaking field) of a nonlinear electron plasma wave, valid for nonrelativistic plasma temperatures and arbitrary wave phase velocities, that can be applied to plasma waves driven by short-pulse laser-plasma interactions. This is in contrast to previous calculations of the wavebreaking limit, which are not valid in the regime of present short-pulse laser-plasma interaction experiments. We analyze the nonlinear electron plasma waves excited by intense short-pulse lasers propagating in underdense plasmas using a warm, relativistic fluid model of a nonequilibrium, collisionless plasma. Properties of the nonlinear plasma wave, such as the plasma temperature evolution and nonlinear wavelength, as well as the laser intensity required to reach wavebreaking, are presented. The influence of the presence of an intense laser field on the wavebreaking amplitude is examined. The relation between the wavebreaking amplitude and particle trapping is discussed. [Preview Abstract] |
Wednesday, October 26, 2005 10:18AM - 10:30AM |
KO1.00005: Improvement of electron beam quality in optical injection schemes using negative plasma density gradients Gwenael Fubiani, Eric Esarey, Carl Schroeder, Wim Leemans Enhanced electron trapping using plasma density down ramps as a method for improving the performance of laser injection schemes is proposed and analyzed. A decrease in density implies an increase in plasma wavelength, which can shift a relativistic electron from the defocusing to the focusing region of the accelerating wakefield, and a decrease in wake phase velocity, which lowers the trapping threshold. The specific method of two-pulse colliding pulse injector was examined using a three-dimensional test particle tracking code. A density down-ramp with a change of density on the order of tens of percent over a length greater than the plasma wavelength led to an enhancement of charge by two orders in magnitude or more, up to the limits imposed by beam loading. The accelerated bunches are ultrashort (fraction of the plasma wavelength), high charge ($>20$~pC at modest injection laser intensity $\sim 10^{17}~{\rm W/cm^2}$), with a good beam quality (relative energy spread of a few percent at a mean energy of 25 MeV, and a normalized RMS emittance on the order 0.5~mm.mrad). [Preview Abstract] |
Wednesday, October 26, 2005 10:30AM - 10:42AM |
KO1.00006: High-fidelity Simulations of Laser Pulses in long Plasma Channels D. Bruhwiler, P. Messmer, J. Cary, C. Nieter, D. Dimitrov, E. Esarey, W. Leemans, C. Geddes Capillary discharge plasma channels, with cm lengths and moderate densities, show promise for laser wakefield accelerators operating in a mildly nonlinear regime, with higher energy trapped electrons and fewer laser-plasma instabilities. High-fidelity simulations are required to verify that the laser pulse can sustain a stable plasma wake over such long distances. Absorbing boundary conditions are required to prevent reflections of scattered laser energy back into the domain. The 2nd-order electromagnetic field update used in particle-in-cell (PIC) codes may not propagate a laser pulse over many Rayleigh lengths with sufficient accuracy, so higher-order algorithms must be considered. A PIC model is necessary to see particle trapping and non-laminar flow in the wake, but the particle noise may degrade accuracy over long times, so fluid models are important. The ratio of time and space scales between a laser pulse and a capillary discharge plasma can be $\sim $100, requiring tremendous computational resources for explicit simulations, so the use of ponderomotive guiding center algorithms to average over the smallest time and space scales of the pulse are important. We present initial efforts to address these issues, using the VORPAL code. [Preview Abstract] |
Wednesday, October 26, 2005 10:42AM - 10:54AM |
KO1.00007: Beam dynamics in plasma-based accelerators including the effects of betatron radiation Pierre Michel, Carl B. Schroeder, Bradley A. Shadwick, Eric Esarey, Wim P. Leemans Spontaneous synchrotron radiation is emitted from relativistic electrons undergoing betatron motion in the focusing wakefields of a plasma-based accelerator. Significant beam energy can be radiated by this mechanism, typically in the hard x-ray domain with a broad spectrum. We present analytical and numerical results on the radiation spectrum and the dynamics of the strongly radiating beam undergoing betatron oscillations. Evolution of the electron beam phase space characteristics are investigated. Implications for the design of a laser-plasma- based accelerator are considered. [Preview Abstract] |
Wednesday, October 26, 2005 10:54AM - 11:06AM |
KO1.00008: Laser driven ion channel x-ray laser. C.S. Liu, Vipin Tripathi An intense short pulse laser, undergoing relativistic self-focusing in a plasma, pushes the low energy electrons radially outward to create an ion channel, while accelerates, directly by itself or by the wakefield driven plasma wave, the energetic ones axially to multi-MeV energies. When the energy spread of energetic electrons is contained, they form a beam that executes betatron oscillations in the channel and drives x-ray radiation unstable via betatron resonance, $\omega $-kv$_{z}=\omega _{b}$, producing coherent x-rays of frequency $\omega \mbox{ }=\mbox{ 2}\gamma _\mbox{0}^\mbox{2} \mbox{ }\omega _\mbox{b} \mbox{ ,}$ with optimum growth rate $\Gamma _\mbox{m} \mbox{ }\approx \mbox{ }\omega _\mbox{p} \mbox{ }\left( {\mbox{n}_{\mbox{ob}} \mbox{/ n}_\mbox{o} } \right)^{\mbox{1/3}}\mbox{ / 2 }\gamma _\mbox{0}^{\mbox{5/6}} \mbox{ ,}$ where $\mbox{k }=\mbox{ }\omega \mbox{/ c, }\omega _\mbox{b} \mbox{ }=\mbox{ }\omega _\mbox{p} \mbox{ / }\left( {\mbox{2 }\gamma _\mbox{0} } \right)^{\mbox{1/2}}\mbox{ }$ is the betatron frequency, $\omega _{p}$ is the plasma frequency, $\gamma _{0}$ is the Lorentz factor of the beam, v$_{z}$ is its axial velocity, and n$_{0b}$/ n$_{0}$ is the ratio of beam density to plasma density. The fractional energy spread in excess of 2 $\Gamma _{m}$/ 3 $\omega _{b}$ reduces the growth rate. [Preview Abstract] |
Wednesday, October 26, 2005 11:06AM - 11:18AM |
KO1.00009: Optical Probing of Magnetic Fields in a Self-Injected Laser Wakefield Accelerator M. Kaluza, S.P.D. Mangles, A.G.R. Thomas, C.D. Murphy, Z. Najmudin, A.E. Dangor, K. Krushelnick The use of lasers as particle accelerators has recently attracted new attention due to the generation of quasi- monoenergetic electron beams from a self-injected laser wakefield accelerator. A recent experiment was carried out at the ASTRA facility at Rutherford Appleton Laboratory (UK), where a 35~fs 400~mJ laser pulse was focused into a gas jet. While a magnetic spectrometer measured the electron energy distributions showing monoenergetic spikes, the Faraday rotation of a transverse probe pulse was used to measure the magnetic field distribution around the plasma channel with high spatial and temporal resolution. Taking into acount the electron density distribution in the plasma, which was measured with a Nomarski interferometer, clear evidence of Mega-Gauss magnetic fields could be observed in the wake of the laser pulse. These magnetic fields are directly related to the electron current in the plasma. The results are compared with $2-D$~PIC simulations. [Preview Abstract] |
Wednesday, October 26, 2005 11:18AM - 11:30AM |
KO1.00010: THz generation by ultra-short laser pulses propagating in nonuniform plasma channels T. Antonsen, J. Palastro, J. Cooley, A. York, S. Varma, H. Milchberg We consider the excitation of THz electromagnetic waves inn plasma by the ponderomotive force of an ultra-short laser pulse. For a uniform plasma such excitation is weak because electromagnetic waves have no density perturbation and do not couple to the ponderomotively driven plasma current. EM waves in a nonuniform plasma channel can be excited. Further, if the channel is axially modulated the EM waves can be slowed down and phase matched to the ponderomotive wave. We calculate the excitation of these waves by both fixed shape laser pulses and by parametric decay. Experimental techniques for generating modulated channels are also explored. [Preview Abstract] |
Wednesday, October 26, 2005 11:30AM - 11:42AM |
KO1.00011: Photon acceleration by co-propagating plasma waves Raoul Trines, C. Murphy, R. Bingham, J.T. Mendonca, L.O. Silva, A. Reitsma, P. Norreys We have studied photon acceleration of an intense laser pulse interacting with co-propagating plasma waves. This type of photon acceleration has several characteristics that distinguish it from blueshift induced by ionization fronts: an asymmetric redshift/ blueshift around the central frequency, a decrease in the intensity of the blueshifted light with increasing plasma density, and a split fundamental peak in the spectrum. Photon kinetic simulations reveal how these characteristics arise from the interplay between the laser pulse and the plasma wave. The results have important implications for laser-plasma accelerators. [Preview Abstract] |
Wednesday, October 26, 2005 11:42AM - 11:54AM |
KO1.00012: Acceleration of overdense plasmas using colliding laser pulses Edison Liang, Koichi Noguchi, Scott Wilks Most conventional laser-plasma acceleration schemes involve underdense plasmas. Using PIC simulations we demonstrate a radically different concept involving overdense plasmas. When a thin slab of overdense electron-positron plasma is irradiated with ultra-intense linearly polarized laser pulses from both sides, the slab is compressed to less than two relativistic skin-depths so that the laser pulses are transmitted. The transmitted pulses then capture and continuously accelerate a fraction of the particles via comoving Lorentz forces as the laser pulses are slowed by plasma loading. The maximum Lorentz factor grows as a power-law in time and the asymptotic momentum distribution forms a power law of slope close to --1. The highest energy particles are narrowly beamed, providing strong energy-angle selectivity. For 1 micron laser and 1.e21 Wcm$^{-2}$ intensity, the maximum energy exceeds GeV in a ps. We will also discuss applications of this concept to electron-ion plasma slabs. [Preview Abstract] |
Wednesday, October 26, 2005 11:54AM - 12:06PM |
KO1.00013: Collimated Multi-MeV Ion Beams in the Forward Direction from High-Intensity Laser Interactions With Underdense Plasma L. Willingale, S.P.D. Mangles, P.M. Nilson, Z. Najmudin, A.G.R. Thomas, M.C. Kaluza, M.S. Wei, C. Kamberides, S.R. Nagel, A.E. Dangor, K. Krushelnick, R.J. Clarke, K.L. Lancaster, S. Karsch, J. Schreiber, W. Mori Ions have been observed to be accelerated to multi-MeV energies in the forward direction from high-intensity (about $5 \times 10^{20} \rm{Wcm}^{-2}$) laser interactions with underdense plasma. The effect of the plasma density and laser parameters on the maximum ion energy and collimation is discussed. The acceleration method for these ions in the forward direction is the large electric field that is created by the fast electrons moving out of the back of the target in to the vacuum. The correlation between the longitudinal electron and ion spectra is considered as well as 2D-3V PIC code simulations to verify this acceleration method. [Preview Abstract] |
Wednesday, October 26, 2005 12:06PM - 12:18PM |
KO1.00014: Control of electron cloud in laser foil interaction and suppression of proton beam divergence Shigeo Kawata, Ryo Sonobe, Shuji Miyazaki, Masaki Nakamura, Takashi Kikuchi In recent years, a high energy ion is observed in experimental and numerical researches. We focus on a suppression of transverse proton beam divergence by a controlled electron cloud in laser-plasma interaction. When an intense laser pulse illuminates a thin foil plasma, first electrons are accelerated and form a strong electrostatic field at the target surface. Then protons are accelerated by the strong field. When a target has an appropriate hole at the opposite side of the laser illumination, an electron cloud is limited in transverse by the neutral plasma at the protuberant part [1]. By the protuberant part of the foil target the electron cloud shape is controlled transversely. The protons are accelerated and also controlled transversely by the electron cloud shaped, and consequently the transverse divergence of the proton beam is suppressed. In our 2.5-dimensional particle-in-cell simulations, the shape of the electron cloud is controlled well, and the transverse proton beam divergence is suppressed successfully without a significant energy loss. ~ [1]~R. Sonobe, et al., Phys. Plasmas, 12 (2005) 073104. [Preview Abstract] |
Wednesday, October 26, 2005 12:18PM - 12:30PM |
KO1.00015: Progress on neutralized drift compression experiment (NDCX-Ia) for high intensity ion beam P. Roy, S. Yu, E. Henestroza, W. Waldron, F. Bieniosek, S. Eylon, J. Coleman, A. Anders, M. Leitner, W. Greenway, D. Baca, D. Vanecek, G. Logan, D. Welch, D. Rose, C. Thoma, R. Davidson, P. Efthimion, I. Kaganovich, E. Gilson, A. Sefkow, W. Sharp Transverse and longitudinal beam compression, together, are a promising approach to the high intensities required for depositing the energy to create high energy density matter and fusion ignition conditions. We have experimentally demonstrated that beam space charge can be neutralized by passing the beam through a localized plasma ``plug.'' This makes it possible to focus a beam of several centimeters radius to a millimeter radius. We are also interested in longitudinal drift compression to a short pulse length of a few nanoseconds using neutralization. The NDCX-Ia facility at LBNL has been used to test these techniques. Here a 300 keV, 25-milliamp K$^{+}$ ion beam is given a head to tail energy variation using a tilt core induction cell. In the NDCX-Ia, simulations predicting a compression ratio of roughly 60, experimentally we have measured above 50 fold with $<$5 ns peak width at FWHM. (This work was supported by U.S. Department of Energy under Contract No. DE-AC02-05CH11231). [Preview Abstract] |
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