Bulletin of the American Physical Society
50th Annual Meeting of the Division of Plasma Physics
Volume 53, Number 14
Monday–Friday, November 17–21, 2008; Dallas, Texas
Session DI2: Plasma-Based Acceleration |
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Chair: Patric Muggli, University of Southern California Room: Landmark B |
Monday, November 17, 2008 3:00PM - 3:30PM |
DI2.00001: Laser plasma accelerator: Control of electron beam parameters in colliding laser pulses scheme Invited Speaker: Stable and high quality electron beams are produced when two laser pulses collide in underdense plasmas. In addition to the improvement of the stability of the electron beam, the use of a second laser pulse allows the control of the electron beam parameters (energy, relative energy spread, and charge). The experimental features are well explained by the use of PIC simulations which underline physics processes which were not predicted by fluid model. This control is obtained by changing laser pulse energy, laser pulses polarization or electron density. With a total of 1 J laser energy, a 10 pC electron beam at 200 MeV with relative energy spread smaller than 1\% has been measured for the first time. Using higher laser energy PIC simulations predicted that 3 GeV electron beam with 0.9\% should be produced in this scheme after 3.8 cm propagation length. In collaboration with J. Faure and C. Rechatin, Laboratoire d'Optique Appliqu\'ee, \'Ecole Nationale Sup\'erieure de Techniques Avanc\'ees, \'Ecole Polytechnique, CNRS, UMR 7639, 91761 Palaiseau, France; A. Ben-Ismail, Laboratoire d'Optique Appliqu\'ee, and LLR, \'Ecole polytechnique, CNRS-IN2P3, 91128 Palaiseau, France; J. Lim, Laboratoire d'Optique Appliqu\'ee; X. Davoine and E. Lefebvre, Commissariat \`a l'Energie Atomique, DIF, Bruy\`eres-le-Ch\^atel, France; and A. Specka and H. Videau, LLR, \'Ecole polytechnique. \\[2pt] This work has been partially supported by ANR-05-NT05-2-41699, by the European Community Research Infrastructure Activity under the FP6 Structuring the European Research Area program (CARE, contract number RII3-CT-2003-506395 and EU-ROLEAP, contract number 028514). [Preview Abstract] |
Monday, November 17, 2008 3:30PM - 4:00PM |
DI2.00002: Relativistic Electron Beams from Laser-Solid Interactions at 0.5 kHz Invited Speaker: Relativistic monoenergetic electron beams from laser-plasma sources can make accessible, in a laboratory set-up, both the science and applications that were once limited to large-scale accelerator facilities. Owing to their ultrafast nature -- typically sub-picosecond, comparable to the driving laser pulse duration -- these beams could be used to drive ultrafast radiation sources covering almost the entire spectrum, from $\gamma$-rays to infrared. Such sources are instrumental for the study of transient dynamics in solid-state physics, material science and bio-chemistry. Moreover, relativistic electrons beams are emerging as an alternative to x rays for cancer radiotherapy due to their superior penetration depth. All these applications would greatly benefit from high electron fluxes. However, to date, monoenergetic electron beams have been primarily obtained from wakefield accelerators produced by low repetition rate lasers in under-dense plasmas. Here we present recent findings on the production of electron beams from the interaction of a high repetition rate laser with an SiO$_2$ target around the relativistic-intensity threshold ($a_o \simeq 1$). In particular we investigate the effects of the plasma scale-length on the beam spatial and spectral characteristics. At the intermediate scale-length of $\lambda/2$, the electrons were emitted in collimated beams with a quasimonoenergetic distributions. Although the spectral peak occurs at moderately relativistic energies ($E_o \sim 0.8$ MeV) with a relatively large energy spread ($\Delta E/E_o \simeq 30\%$), these beams are potentially suitable as seeds to be injected in a two-stage accelerator scheme that could produce 50 MeV electron beams with better than $10^{-2}$ energy spread at kilohertz repetition rates. [Preview Abstract] |
Monday, November 17, 2008 4:00PM - 4:30PM |
DI2.00003: Nonlinear Depletion and Dephasing in Laser Wakefield Accelerators Invited Speaker: Pump depletion and electron dephasing lengths are conventional figures of merit used to assess laser-plasma accelerators. Scaling laws associated with these lengths can be of considerable utility in the design of multi-GeV acceleration stages. While approximate expressions for these lengths have been known for some time, such expressions are not particularly accurate. Through a comprehensive numerical study, we have obtained a detailed description of laser depletion and electron beam dephasing. The rate of energy deposition into the plasma by the laser and, correspondingly, the pump depletion length, is a non-linear process dependent on both the laser pulse amplitude and length and the ratio of the plasma density to the critical density. The combination of dephasing length and depletion length give a measure of the efficiency of the transfer of energy from the laser to the accelerated beam; any laser energy remaining after the beam has propagated a dephasing length is not available for acceleration. Our numerical studies have found that the evolution of a resonant laser pulse proceeds in two phases. In the first phase the pulse envelope is modified by group velocity dispersion resulting in pulse steepening and slight pulse shortening. The central wavenumber of the pulse is slowly reduced as energy is deposited in the plasma (the well-known redshifting). The second phase of evolution occurs after the pulse reaches it's minimum length at which point, the pulse length rapidly lengthens, loosing resonance with the plasma. A consequence of this lengthening is a significant reduction in accelerating gradient, effectively terminating the energy gain of the beam. We present a comprehensive examination of these effects over a wide range of laser and plasma parameters. We also present results concerning the rate at which laser energy is deposited in the plasma. We conclude with an examination of the laser energy remaining when the beam reaches the dephasing limit. [Preview Abstract] |
Monday, November 17, 2008 4:30PM - 5:00PM |
DI2.00004: Beam loading in the nonlinear regime of plasma-based acceleration Invited Speaker: An analytical theory for the interaction of a negatively charged bunch with a nonlinear plasma wave is developed to make it possible to design efficient laser- and/or beam-driven accelerators that generate truly monoenergetic electron beams. This theory allows us to choose the charge, the shape and the placing of the beam so that the efficiency is maximized and the beam quality optimized. For intense drivers the nonlinear wake is described by the trajectory of the blowout radius and beam loading arises when the radial space-charge force of the beam acts back on the trajectory. Starting from the nonlinear theory by W. Lu et al. [1], an equation for the wakefield in the presence of an electron bunch is derived. The shape of the ion channel in an unloaded wake is determined and the modification of the wake due to the presence of flat-top electron bunches is studied. It is shown that the energy spread of an externally injected flat-top (or Gaussian) electron bunch can be kept low by choosing the correct charge per unit length and the analytical results are confirmed with PIC simulations. The bunch profile that leads to zero energy spread is found to be trapezoidal. The conversion efficiency from the fields of the bubble to the accelerating electrons is determined, and it is shown that for optimal bunches it approaches 100{\%}. The differences between nonlinear and linear [2] theory are described and the advantages of operating in the nonlinear regime are discussed. [1] W. Lu et al., Phys. Rev. Lett. \textbf{96}, 165002 (2006); Phys. Plasmas \textbf{13}, 056709 (2006). [2] T. Katsouleas et al., Particle Accelerators, 1987, \textbf{22}, pp. 81-99. [Preview Abstract] |
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