• W. Leemans, E. Esarey, C.G.R. Geddes, P. Michel, B. Nagler, K. Nakamura, C.B. Schroeder, C. Toth, J. Van Tilborg
  LBNL, Berkeley, California
• T.E. Cowan, C. Filip, E. Michel
  University of Nevada, Reno, Reno, Nevada
• A.J. Gonsalves, S.M. Hooker, D. J. Spence
  OXFORDphysics, Oxford, Oxon

Experimental progress towards the realization of a 1 GeV laser-driven plasma-based accelerator at the L’OASIS facility of LBNL will be discussed. The design of the 1 GeV accelerator module consists of two components: (1) an all-optical electron injector and (2) a plasma channel for laser guiding and electron acceleration to high energy via the laser wakefield acceleration (LWFA) mechanism. Experimental results on the injector development include the demonstration of laser guiding at relativistic intensities in preformed plasmas and production of quasi-monochromatic electron beams with energy around 100 MeV. Recently guiding experiments using the 100 TW-class laser upgrade of the L’OASIS facility have been started with capillary discharges. The capillary system provides multi-cm scale plasma channels in hydrogen gas at densities on the order of 10¹⁸ cm-3. Such densities are required to have sufficiently high phase velocity of the plasma wave to result in GeV electron beams.

• B. Nagler, E. Esarey, C. Filip, C.G.R. Geddes, W. Leemans, C. Toth
  LBNL, Berkeley, California
• T.E. Cowan
  University of Nevada, Reno, Reno, Nevada
• A.J. Gonsalves, S.M. Hooker, D. J. Spence
  OXFORDphysics, Oxford, Oxon

Experiments are underway at LBNL on guiding high peak power (up to 100 TW), ultra-short (<50 fs) laser pulses using a preformed plasma channel created by an electrical discharge in a capillary. The laser beam is produced by the multi-beam l'OASIS Ti:sapphire laser system and is focused onto the entrance of the capillary using a 2 meter focal length off-axis parabola. The capillary has been developed at Oxford University and creates a fully ionized plasma channel with a radial density profile that is suitable for guiding over a length ranging from 30 to 70 mm. The laser beam is monitored using a CCD camera based mode profile diagnostic, an optical spectrometer and a pulse length diagnostic. Experimental results will be presented on the plasma channel characteristics and on laser guiding and its dependenceo n laser and channel parameters.

• R. Giacone, D.L. Bruhwiler, J.R. Cary, D.A. Dimitrov, P. Messmer
  Tech-X, Boulder, Colorado
• E. Esarey, C.G.R. Geddes, W. Leemans
  LBNL, Berkeley, California

Optical guiding of the laser pulse in a laser wakefield accelerator (LWFA) via plasma channels can greatly increase the interaction length and, hence, the maximun energy of trapped electrons.* Energy efficient coupling of laser pulses from vacuum into plasma channels is very important for optimal LWFA performance. We present 2D particle-in-cell simulations of this problem using the VORPAL code.** Some of the mechanisms considered are enhanced leakage of laser energy transversely through the channel walls, enhanced refraction due to tunneling ionization of neutral gas on the periphery of the gas jet, ionization of neutral gas by transverse wings of the laser pulse and effect of the pulse being off axis of the channel. Using power spectral diagnostics,*** we are able to differentiate between pump depletion and leakage from the channel. The results from our simulations show that for short (≈λ_p) plasma ramp, very little leakage and pump depletion is seen. For narrow channel walls and long ramps, leakage increases significantly.

*C. G. R. Gedes et al., Nature 431 (2004), p. 538. **C. Nieter and J. R. Cary, J. Comp. Phys. 196 (2004), p. 448.***D. A. Dimitrov et al., Proc. Advanced Accel. Concepts Workshop (2004).

• P. Messmer, D.L. Bruhwiler, D.A. Dimitrov, P. Stoltz
  Tech-X, Boulder, Colorado
• E. Esarey, C.G.R. Geddes, W. Leemans
  LBNL, Berkeley, California

Capillary channels of cm-length and at plasma density low compared to gas jets are promising setups for low noise laser wakefield acceleration. Computationally, however, the large discrepancy of the length scales of the plasma and the laser are a big challenge. Methods are therefore sought that relax the need to concurrently resolve both length scales. Moving windows allow to reduce the size of the computational box to a few plasma wave-lengths, which can already be a big gain compared to the full length of the capillary. On the other hand, average methods allow to relax the constraint to resolve the laser wavelength. These methods split the laser induced current into a fast varying part and a slowly varying envelope. The average over the fast timescales is performed in a semi analytic way, leaving the evolution of the envelope to be modeled. Such an envelope model is currently being incorporated into the VORPAL code.* Preliminary results show considerable time savings compared to fully resolved simulations. The status of this ongoing work will be presented.

*C. Nieter and J. R. Cary, J. Comp. Phys. 196 (2004), p. 448.

• C.G.R. Geddes, E. Esarey, W. Leemans, C.B. Schroeder, C. Toth
  LBNL, Berkeley, California
• J.R. Cary, C. Nieter
  Tech-X, Boulder, Colorado
• J. Van Tilborg
  TUE, Eindhoven

A laser driven wakefield accelerator has been tuned to produce high energy electron bunches with low emittance and energy spread by extending the interaction length using a plasma channel. Wakefield accelerators support gradients thousands of times those achievable in RF accelerators, but short acceleration distance, limited by diffraction, has resulted in low energy beams with 100% electron energy spread. In the present experiments on the L’OASIS laser,* the relativistically intense drive pulse was guided over 10 diffraction ranges by a plasma channel. At a drive pulse power of 9 TW, electrons were trapped from the plasma and beams of percent energy spread containing >200pC charge above 80 MeV and with normalized emittance estimated at < 2 pi -mm-mrad were produced.** Data and simulations (VORPAL***) show the high quality bunch was formed when beam loading turned off injection after initial trapping, and when the particles were extracted as they dephased from the wake. Up to 4TW was guided without trapping, potentially providing a platform for controlled injection. The plasma channel technique forms the basis of a new class of accelerators, with high gradients and high beam quality.

*W.P. Leemans et al., Phys. Plasmas 5, 1615-23 (1998). **C.G.R. Geddes et al., Nature 431, 538-41 (2004). ***C. Nieter et al., J. Comp. Phys. 196, 448-73 (2004).

• J.R. Cary, D.L. Bruhwiler, J.R. Cary, R. Giacone, C. Nieter
  Tech-X, Boulder, Colorado
• E. Esarey, C.G.R. Geddes, W. Leemans
  LBNL, Berkeley, California

The accelerating gradient of laser-generated wake fields in plasmas can be orders of magnitude greater than the gradients obtainable in traditional, rf structures. One of the hurdles to overcome on the road to practical utilization of said plasma wake fields for production of high energy particles is the creation of quality beams having significant charge, low emittance, and narrow energy spread. To generate appropriate beams, various injection methods have been proposed. Injection by conventional means of beam prepartion using conventional technology is very difficult, as the accelerating buckets are only tens of microns long. Therefore, the field has turned to all-optical injection schemes, which include injection by colliding pulses, plasma ramps, wave breaking, and self-trapping through pulse evolution. This talk will review the various concepts proposed for injection, including plasma ramps, colliding pulses, and self trapping. The results of simulations and experiments will be discussed along with proposed mechanisms for improving the generated beams. Parameter studies to find optimal beam generation scenarios will be presented.