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Pak, A. E.

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WEOBKI02 Evolution of Relativistic Plasma Wave-Front in LWFA 1919
  • C. E. Clayton, F. Fang, C. Joshi, K. A. Marsh, A. E. Pak, J. E. Ralph
    UCLA, Los Angeles, California
  • N. C. Lopes
    Instituto Superior Tecnico, Lisbon
  Funding: Work supported by DOE Grant Nos. DE-FG52-03NA00138 and DE-FG02-92ER40727, and Grant POCI/FIS/58776/2004 (FCT-Portugal)

In a laser wakefield accelerator experiment where the length of the pump laser pulse is several plasma period long, the leading edge of the laser pulse undergoes frequency downshifting as the laser energy is transferred to the wake. Therefore, after some propagation distance, the group velocity of the leading edge of of the pump pulse–and therefore of the driven electron plasma wave–will slow down. This can have implications for the dephasing length of the accelerated electrons and therefore needs to be understood experimentally. We have carried out an experimental investigation where we have measured the velocity vf of the 'wave-front' of the plasma wave driven by a nominally 50fs (FWHM), intense (a0~1), 0.8 micron laser pulse. To determine the speed of the wave front, time- and space-resolved shadowgraphy, interferometry, and Thomson scattering were used. Although low density data (ne ~ 1018 cm-3) showed no significant changes in vf over 1.5mm (and no accelerated electrons), high-density data shows accelerated electrons and an approximately 5% drop in vf after a propagation distance of about 800 microns.

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THPMS024 Experimental Investigation of Self-guiding using a Matched Laser Beam in a cm Scale Length Underdense Plasma 3052
  • J. E. Ralph, C. E. Clayton, F. Fang, C. Joshi, K. A. Marsh, A. E. Pak
    UCLA, Los Angeles, California
  Funding: This work was supported by NNSA Grant no. DE-FG52- 03NA00138, and DOE Grant no. DE-FG02-92ER40727.

High-intensity short-pulse laser guiding in plasma channels has extended the length over which acceleration occurs in laser wake field accelerators*. Recent multidimensional nonlinear plasma wave theory predicts a range of optimal characteristics for self-guiding of laser pulses in the blowout regime for pulses shorter than a plasma wavelength**. This theory predicts a robust, stable parameter space for self-guiding and wake production and has been verified through multidimensional particle-in-cell simulations. We experimentally explore the plasma dynamics and laser pulse propagation using a 50 fs multi-terawatt Ti:Sapphire laser in a helium plasma at plasma densities, laser powers, and spot sizes within this parameter space. Our parameters are in the range where the plasma is underdense and the laser power is much greater than the critical power for self focusing. The evolution of the laser pulse and plasma channel will be followed over several Rayleigh lengths.

* C. Geddes et. al., Nature (London) 431, 538 (2004)** W. Lu et. al., Phys. Plasmas 13, 056709 (2006)