| The long-term future of experimental high-energy physics research using accelerators can be strengthened by the successful development of novel ultra-high-gradient acceleration methods. New acceleration techniques using lasers and plasmas have already exhibited gradients and focusing forces more than a thousand times greater than conventional technologies. The challenge is first to control these high-gradient systems, and then to string them together. This technology could enable the development of ultra-compact accelerators, which could be used in industry and medicine as well as science. The potential impact of this advancement is truly staggering. In addition, plasma-based acceleration is just one aspect of a rapidly emerging field called high-energy density plasma physics. The on-focus energy density of the particle beam drivers and laser drivers used in this field approaches 1 Tbar of pressure. When such beams interact with plasma or un-ionized matter, the physics involved includes ultra-fast, relativistic, and nonlinear phenomena.
Three independent experimental groups have recently observed the creation of ~100 MeV monoenergetic beams carrying ~100 pC of charge, when a ~10 TW laser propagates through ~1 mm of plasma at densities of ~1019 cm–3. We would like to understand the nonlinear evolution of the laser, capture the self-injection of the electrons, and study how the electrons can be accelerated to form monoenergetic beams. For such a complex problem, simulations that follow the trajectories of individual particles (particle-in-cell methods) are required. Each of the published results mentioned also discusses supporting simulations; indeed, unraveling the underlying physics is not possible without fully nonlinear simulations of the particles involved. Two massively parallel particle-in-cell codes, OSIRIS and VORPAL, have been used to elucidate the key physics in these experiments. A sample result from a VORPAL simulation is shown in figure 4.
In addition, prior to these experiments, 3D OSIRIS simulations had already predicted that a modest 13 TW laser could indeed self-inject electrons.
As the laser evolves, a combination of frequency redshift and group velocity dispersion can produce a monoenergetic beam; the electrons in the forefront of the beam dephase into a decelerating region, while those in the back continue to be accelerated by the beam. The parameters of these simulations were not identical to those of the cited experiments, but they succeeded in predicting the essential physics. This physics is illustrated in figure 5, which shows the evolving plasma density in 2D slices through the 3D data. Within the next three years, laser power will increase from ~10 TW to 1 PW. Full-scale 3D OSIRIS simulations at these energies have already been carried out, and predict that a 200 TW laser should produce a 1.5 GeV monoenergetic beam with 0.5 nC of charge. This simulation followed 0.5×109 particles on a 4000×256×256 grid over 300,000 time steps. Figure 6 shows a 2D slice through the laser and plasma density from this simulation.
Contributor: Dr. Warren Mori, on behalf of SciDAC's Advanced Accelerator Group; work done under SciDAC AST project
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Figure 4.Shown above is plasma density, including the wake and trapped particle bunches (bright dots on black, on the center line), in a channel-guided wakefield accelerator from a SciDAC simulation using the VORPAL code. Particles are trapped, followed by loading of the wake by the trapped bunch, and concentration of the particles in energy when the higher energy particles outrun the accelerating structure.
Figure 5. A sequence of 2D cuts through the 3D data of the electron density. The laser is propagating from left to right. After 0.24 cm (left column) there are no selfinjected electrons. After 0.43 cm (middle column) the self-injected electrons are seen. The electron beam shape is different in the two planes (xz and yz), an effect which has been seen experimentally. After 0.64 cm the first bunch has completely outrun the plasma wave and a second bunch has been injected.
Figure 6. A 2D slice of a 3D simulation showing the laser envelope (in orange) and the plasma density (in blue). As the laser moves from right to left it blows out the electrons, which rush back to the axis once the laser has passed. There they feel a strong accelerating force and are self-injected in the laser's wakefield. They are accelerated until they outrun the wake.
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