UCLA, Los Angeles, California
Instituto Superior Tecnico, Lisbon
Funding: Fundacao Calouste Gulbenkian and Fundacao para a Ciencia e Tecnologia under grants SFRH/BD/35749/2007.
The possibility of using a CO2 laser (10 micron wavelength) to drive a plasma density compression and achieve effective ion acceleration in gaseous targets (density>~ 1019cm-3) is explored. A parameter scan is performed with a set of particle in cell simulations in OSIRIS*, both in 2D and 3D, for various laser intensities, linear/circular polarization pulses, and plasma densities. Results show that, to generate the shock compression, plasma density must be increased above the critical value to account for the relativistic motion of the electrons. Under these conditions, 2-5MeV ions are observed with moderate intensity (a0=3) laser pulses. Finally, configurations to generate a shock structure are suggested, that will more efficiently accelerate the particles. This scenario is also of particular relevance to fast-ignition, inertial confinement fusion, and implications to those regimes can be obtained from numerical simulations by using the appropriate density normalization.
*R. A. Fonseca et al, LNCS 2329, III-342, Springer-Verlag, (2002)
Instituto Superior Tecnico, Lisbon
UCLA, Los Angeles, California
Funding: F.C.Gulbenkian, F.C.T. [SFRH/BD/35749/2007, SFRH/BD/39523/2007, PTDC/FIS/66823/2006 (Portugal)], and European Community (project EuroLeap, contract #028514)
It is now accepted that self-trapped electrons in a laser wakefield accelerator operating in the "bubble" regime undergo strong periodic oscillations about the wakefield axis because of the focusing force provided by the ions. This betatron motion of the off-axis electrons results in the emission of x-ray radiation strongly peaked in the forward direction. Even though the x-rays are broadband with a synchrotron-like spectrum, their brightness can be quite high because of their short pulse duration and strong collimation. We employ particle tracking in particle in cell simulations with OSIRIS*, combined with a post-processing radiation diagnostic, to evaluate the features of the radiation mechanisms of accelerated electrons in LWFA experiments. We show and discuss results for a 1.5 GeV laser wakefield accelerator stage. A study of the angular dependence of the radiated power is also presented and compared with theoretical models. This analysis also allows for the direct calculation of the radiation losses of the self-injected bunch.
*R. A. Fonseca et al, LNCS 2329, III-342, Springer-Verlag, (2002)
UCLA, Los Angeles, California
Instituto Superior Tecnico, Lisbon
Funding: Work Supported by DOE Grant DEFG02-92ER40727
Controlling the trapping of electrons into accelerating wakefields is an important step to obtaining a stable reproducible electron beam from a laser wakefield accelerator (LWFA). Recent experiments at UCLA have focused on using the different ionization potentials of gases as a mechanism for controlling the trapping of electrons into an LWFA. The accelerating wakefield was produced using an ultra-intense (Io ~ 1019 W / cm2 ), ultra-short (τFWHM ~ 40 fs) laser pulses. The laser pulse was focused onto the edge of column of gas created by a gas jet. The gas was a mixture of helium and nitrogen. The rising edge of the laser pulse fully ionizes the helium and the first five bound electrons of the nitrogen. Only at the peak of the laser pulse is it intense enough to ionize the most tightly bound electrons of the nitrogen. Electrons which are ionized at the peak of laser pulse are born into a favorable phase space within the accelerating wakefield and are subsequently trapped and accelerated. The accelerated electrons were dispersed using a dipole magnet with a ~ 1 Tesla magnetic field onto a phosphor screen. Electron beam energy spectrum charge and divergence were measured.
Instituto Superior Tecnico, Lisbon
UCLA, Los Angeles, California
Funding: F.C.Gulbenkian, F.C.T. [SFRH/BD/35749/2007, PTDC/FIS/66823/2006 (Portugal)], and European Community - New and Emerging Science and Technology Activity, FP6 program (project EuroLeap, contract #028514)
We address full particle-in-cell simulations of the next generation of Laser Wakefield Accelerators with energy gains > 10 GeV. The distances involved in these numerical experiments are very demanding in terms of computational resources and are not yet possible to (easily) accomplish. Following the work on simulations of particle beam-plasma interaction scenarios in optimized Lorentz frames by J.-L. Vay*, the Lorentz transformation for a boosted frame was implemented in OSIRIS**, leading to a dramatic change in the computational resources required to model LWFA. The critical implementation details will be presented, and the main difficulties discussed. Quantitative comparisons between lab/boost frame results with OSIRIS, QuickPIC***, and experiment will be given. Finally, the results of a three-dimensional PIC simulation of a > 10 GeV accelerator stage will be presented, including a discussion on radiation emission.
* J.-L. Vay, PRL 98, 130405 (2007)
** R.A. Fonseca et al., LNCS 2329, III-342 (Springer-Verlag, 2002)
*** C. Huang, et al., JCP 217, Issue 2, 20 (2006)
USC, Los Angeles, California
UCLA, Los Angeles, California
Funding: Work supported by the US Department of Energy
Studies on propagation of electron beams in plasma have shown that in the blowout regime of the plasma wakefield accelerator (PWFA), the emittance of the incoming beam is preserved because of the linear focusing force exerted by a uniform ion column [1]. However, for positron beams the focusing force is nonlinear and they suffer emittance growth. We simulated the propagation of a positron beam in the uniform plasmas with different densities. We calculated the beam emittance from the simulation results and observed the beam size and emittance grow with increasing plasma density. Simulation results agree well with that of previous work.