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Physics of Plasmas / Volume 14 / Issue 4 / ARTICLES / Lasers, Particle Beams, Accelerators, Radiation Generation

Phys. Plasmas 14, 043105 (2007); doi:10.1063/1.2721068 (14 pages)

Coupling of laser energy into plasma channels

D. A. Dimitrov1, R. E. Giacone1, D. L. Bruhwiler1, R. Busby1, J. R. Cary2, C. G. R. Geddes3, E. Esarey3, and W. P. Leemans3

1Tech-X Corporation, 5621 Arapahoe Avenue, Suite A, Boulder, Colorado 80303
2Tech-X Corporation, 5621 Arapahoe Avenue, Suite A, Boulder, Colorado 80303 and University of Colorado, Boulder, Colorado 80309
3Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720

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(Received 28 July 2006; accepted 9 March 2007; published online 30 April 2007)

Diffractive spreading of a laser pulse imposes severe limitations on the acceleration length and maximum electron energy in the laser wake field accelerator (LWFA). Optical guiding of a laser pulse via plasma channels can extend the laser-plasma interaction distance over many Rayleigh lengths. Energy efficient coupling of laser pulses into and through plasma channels is very important for optimal LWFA performance. Results from simulation parameter studies on channel guiding using the particle-in-cell (PIC) code VORPAL [ C. Nieter and J. R. Cary, J. Comput. Phys. 196, 448 (2004) ] are presented and discussed. The effects that density ramp length and the position of the laser pulse focus have on coupling into channels are considered. Moreover, the effect of laser energy leakage out of the channel domain and the effects of tunneling ionization of a neutral gas on the guided laser pulse are also investigated. Power spectral diagnostics were developed and used to separate pump depletion from energy leakage. The results of these simulations show that increasing the density ramp length decreases the efficiency of coupling a laser pulse to a channel and increases the energy loss when the pulse is vacuum focused at the channel entrance. Then, large spot size oscillations result in increased energy leakage. To further analyze the coupling, a differential equation is derived for the laser spot size evolution in the plasma density ramp and channel profiles are simulated. From the numerical solution of this equation, the optimal spot size and location for coupling into a plasma channel with a density ramp are determined. This result is confirmed by the PIC simulations. They show that specifying a vacuum focus location of the pulse in front of the top of the density ramp leads to an actual focus at the top of the ramp due to plasma focusing, resulting in reduced spot size oscillations. In this case, the leakage is significantly reduced and is negligibly affected by ramp length, allowing for efficient use of channels with long ramps.

© 2007 American Institute of Physics

Article Outline

  1. INTRODUCTION
  2. COUPLING OF LASER PULSES INTO A PLASMA CHANNEL
    1. Preformed plasma channel profile and simulation parameters
    2. Optimal laser pulse parameters in the presence of a channel ramp
    3. Energy loss inside a plasma channel
  3. MODELING OF IONIZATION EFFECTS
  4. SUMMARY AND CONCLUSIONS

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KEYWORDS and PACS

Keywords

plasma light propagation, plasma simulation, plasma density, ionisation, plasma diagnostics, plasma oscillations, differential equations

PACS

  • 52.38.Kd

    Laser-plasma acceleration of electrons and ions

  • 52.38.Hb

    Self-focussing, channeling, and filamentation in plasmas

  • 52.65.Rr

    Particle-in-cell method

  • 52.25.Jm

    Ionization of plasmas

  • 52.70.Kz

    Optical (ultraviolet, visible, infrared) measurements

  • 52.35.Fp

    Electrostatic waves and oscillations (e.g., ion-acoustic waves)

ARTICLE DATA

Permalink
http://link.aip.org/link/doi/10.1063/1.2721068

PUBLICATION DATA

ISSN:

1070-664X (print)  
1089-7674 (online)

Publisher:

$abstract.society.value is a Member of CrossRef American Institute of Physics

For access to fully linked references, you need to log in.
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    C. G. R. Geddes, C. Toth, J. van Tilborg, E. Esarey, C. B. Schroeder, J. Cary, and W. P. Leemans, Phys. Rev. Lett. 95, 145002 (2005).

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