There is the possibility of using gas targets as plasma based optical devices to manipulate laser pulses of ultra-high energy density. Some simulations and theory have offered tantalizing glimpses of the use of plasma as an optical device. If these effects could be controlled, then the propagation and applicability of the laser could be tailored by redirecting the laser energy, improving the focusing properties, compressing and guiding the pulse, broadening the pulse spectrum and eliminating pre- and post-pulses. In the course of this project, the PI and graduate students intend to perform experiments and numerical work to understand non-linear optical effects at the ultra-high intensities available on the Hercules laser system at the University of Michigan. Following demonstrations of optical control of the laser pulse, the modified, tailored pulses could then be used for laser-driven particle acceleration, both solid target proton acceleration by radiation pressure acceleration and laser wakefield acceleration of electrons in a gas jet.
This research may lead to new practical techniques for manipulating laser pulses for ultra-intense applications. By implementing the techniques learnt during the course of the research, improved stability for particle acceleration, and particle beams of higher quality should result
This proposal was submitted to the NSF-DoE Partnership in Plasma Science and Engineering joint solicitation 08-589. This award is being funded by the Plasma Physics Program in the Division of Physics.
The plasma in a gas target can be preformed, and manipulated through known hydrodynamic evolution or the presence of other laser beams. It should be possible to use plasma to make optical devices to manipulate laser pulses of ultra-high energy density. High-intensity laser plasma interactions are not well understood, for example in the context of self-guiding and compression, the effect of the spatially varying phase and amplitude of a real pulse on propagation, laser hosing, photon acceleration, laser multi-filamentation and inhomogeneous densities. The propagation of the laser can be modified by redirecting the laser energy, improving the focusing properties, compressing and guiding the pulse, broadening the pulse spectrum and eliminating pre- and post-pulses. We set up a new vacuum chamber for laser-gas jet experiments using the lambda-cubed laser system and ran full 3D calculations of the experiments, trying to understand the electron injection process under these conditions. Experimental measurements have demonstrated repeatable electron beams with small divergence, but relatively low energies. The spatial profile of the laser, modified by a deformable mirror, can affect the angular distribution of these electrons. A magnetic field was also added, and the polarization of the laser was rotated and also changed to circular polarization, all of which allowed us to control the angular distribution of the electrons. The wakefield of this accelerator has demonstrated efficient (>90% transmission with a relatively good pulse near field profile) compression of the pulse duration from 35 fs down to 16 fs, measured on a FROG device. We are currently trying to understand exactly the mechanism that enables the pulse compression, but it is likely to be phase modulation due to a combination of effects including ionization and wakefield generation. The pulse spectrum is broadened, predominantly towards shorter wavelengths, and has been independently verified on a spectrometer. Undergraduate students have set up a gas target characterization laboratory, and used it to develop a periodic gas target using a supersonic gas jet nozzle and a fine adjustable wire array. Interferometry of the target shows a sinusoidal density structure at a distance of 1 mm from the orifice. The periodic nozzle was subsequently further developed by Spencer Jolly to be manufactured in a different way using a rapid prototyping machine. This uses stereolithography, which is a process in which plastic is cured in ascending layers to produce a three-dimensional final product. Stimulated Raman side-scattering was imaged on a transverse spectrometer, which indicated that the laser propagated over the full 10 mm length, despite the variation in density. We also modified the source code of a plasma code to include the ``real world'' effects of an aberrant pulse and study its propagation in a laser wakefield accelerator. We demonstrated that comatic optical aberrations in a nominally Gaussian laser pulse are self-corrected by the plasma response, leading to stable propagation and therefore little variation in peak energy, energy spread, or peak current of the accelerated bunch, even for serious aberrations. However, the comatic aberration does lead to enhanced transverse beam emittance in the direction of the coma. Although this may be deleterious to the performance of an accelerator, one useful outcome is that the increased oscillation amplitude of electrons in the wake structure may lead to increased synchrotron radiation emission, which would be partially polarized in the direction of coma.
