The experimental breakthrough, reported in the October 2006 issue of Nature Physics,¹ was spearheaded by the LOASIS (Laser Optical Accelerator Systems Integrated Studies) program at Berkeley Lab, headed by Wim Leemans. Defying the prediction that petawatt-class lasers would be needed to reach GeV energies, Leemans and his collaborators channeled a 40 terawatt peak-power laser pulse in a gas-filled capillary waveguide to produce a high-quality electron beam with 1 GeV energy.

Figure 1. A 3D visualization showing the density of the plasma wave driven by the laser (volume shading), and positions of particles accelerated by that wave (blue spheres). (Simulation by John Cary and Cameron Geddes. Visualization by Cristina Siegerist.) (Click on images to enlarge.)

The simulation project, led by Cameron Geddes of the LOASIS team, created the first high-resolution 3D models of these laser wakefield experiments, providing crucial understanding of the nonlinear laser–plasma interactions and particle distribution effects. The simulation activities are tightly tied to the experimental and theoretical efforts of the LOASIS program and collaborators at the University of Colorado and Tech-X Corporation.

Laser wakefield accelerators use laser-driven plasmas to accelerate particles in as little as a thousandth of the length required by conventional radiofrequency accelerators. The plasma-based accelerators are not subject to electrical breakdown that limits conventional accelerators and have demonstrated accelerating gradients thousands of times those obtained in conventional machines. Thus plasma-based accelerators offer a path to more compact, ultrafast particle and radiation sources for probing the subatomic world, for studying new materials and new technologies, and for medical applications.

Figure 2. A two-dimensional cut through a 3D simulation shows the plasma wave density (surface height) and reveals the particle momentum distribution versus position (spheres, height and color = momentum). (Simulation by Cameron Geddes. Visualization by Cameron Geddes and Peter Messmer.)

“INCITE advanced the understanding of particle beam formation and evolution in these devices through a very large simulation in three dimensions as well as a large series of two-dimensional cases to evaluate accelerator optimization,” said Geddes. “These simulations are computationally intensive because the laser wavelength (micron) must be resolved over the acceleration length of centimeters. Coupled with experiments, these simulations are developing the detailed understanding of laser acceleration needed to apply this technology to future higher energy particle physics experiments and to compact machines for medicine and laboratory science.”

In laser wakefield accelerators, plasma is formed by heating hydrogen gas enough to disintegrate its atoms into their constituent protons and electrons. A laser pulse traveling through this plasma creates a wake in which bunches of free electrons are trapped and ride along, much like surfers riding the wake of a big ship (Figures 1 and 2). After propagating for a distance known as the “dephasing length,” the electrons outrun the wake. This limits how far they can be accelerated and thus limits their energy. The LOASIS team’s method for increasing the acceleration length is to provide a guide channel for the drive-laser pulse that creates the plasma wakefield.

Particle-in-cell simulations are a crucial tool in interpreting these experiments and planning the next generation because they can resolve kinetics and particle trapping. These simulations have revealed why recent experiments succeeded in producing a narrow energy spread: the trapping of an initial bunch of electrons loads the wake, suppressing further injection and forming a bunch of electrons isolated in phase space; at the dephasing point, as the bunch begins to outrun the wake, the particles are then concentrated near a single energy, and a high quality bunch is obtained (Figure 3). Only a single wake period contributes to the high energy bunch, and hence the electron bunch length is near 10 femtoseconds, indicating that a compact ultrafast electron source with high beam quality has been developed.

Impressive as this is, “It’s the tip of the iceberg,” says Leemans. “We are already working on injection”—inserting an already energetic beam into an accelerating cavity—“and staging,” the handoff of an energetic beam from one capillary to the next and subsequently to others, until very high energy beams are achieved. “Brookhaven physicist Bill Weng has remarked that achieving staging in a laser wakefield accelerator would validate 25 years of DOE investment in this field.”

Figure 3. Particle-in-cell simulations show that a high-quality bunch is formed when beam loading and pulse evolution turn off injection after the loading of an initial bunch, resulting in an isolated bunch (A). These particles are then concentrated in energy at the dephasing length (B), forming a low-energy-spread bunch. The laser pulse envelope is shown at the bottom of pane (B), showing self modulation into sub-pulses at the plasma period.

Leemans’ group and their collaborators look forward to the challenge with confidence. “In DOE’s Office of Science, the High Energy Physics office has asked us to look into what it would take to go to 10 GeV. We believe we can do that with an accelerator less than a meter long—although we’ll probably need 30 meters’ worth of laser path.”

While it has often been said that laser wakefield acceleration promises high-energy accelerators on a tabletop, the real thing may not be quite that small. But laser wakefield acceleration does indeed promise electron accelerators potentially far more powerful than any existing machine—neatly tucked inside a small building.

This article written by: John Hules, Jon Bashor, Paul Preuss, and Ucilia Wang (Berkeley Lab)

1 W. P. Leemans, B. Nagler, A. J. Gonsalves, Cs. Tóth, K. Nakamura, C. G. R. Geddes, E. Esarey, C. B. Schroeder, and S. M. Hooker, “GeV electron beams from a centimetre-scale accelerator,” Nature Physics 2, 696 (2006). Funding: HEP, EPSRC.