Downward density transitions have been shown to be a promising injection mechanism for laser plasma accelerators (LPAs). We combined a gas jet and movable blade assembly as the LPA source with a transverse probe laser for density measurements to correlate the electron beam parameters to density profile. The wavefront-based density diagnostic, which senses both neutral gas and plasma, was used to inform a model explaining the observation of tunable 100-MeV-level beams.
Electron injection at a downward density transition created by a shock front has been shown by several groups to provide low energy spread electron beams. The produced beams are very attractive as injectors for multi-stage high-energy systems, as well as light-source applications. We report experiments where the gas jet was installed on a three-dimensional translation stage while the shock-producing blade was positioned independently allowing adjustment of shock parameters. We have precisely characterized the injection parameters using a wavefront sensor on a transverse probe laser. We measured the density profiles and electron beam performance for a range of shock configurations. This method has produced a tunable injector with electron beams of < 10% energy spread. Scaling laws have been developed which parameterize observed performance using the measured density profiles as input.
We performed the experiment at the BELLA Center using the TREX Ti:Sapphire laser providing 800nm, 47fs, 1.8J on target pulses at a0 = 1.5. We produced a shock by inserting a razor blade into a supersonic gas jet, and the shock density profile was examined and parametrized in-situ by a probe beam on a wavefront sensor. The electron energy spectrum was measured using a magnetic spectrometer. A Lanex scintillating screen imaged onto a CCD camera was used to quantify the electron beam profile and pointing. By changing the blade position along the laser axis (thus covering more, or less, of the jet nozzle), we controlled the sharp and smooth parts of the plasma profile. For example, control of the down-ramp plasma profile allowed for tuning of the energy spectrum, with central energy ranging from 20 MeV to 200 MeV.
Work supported by the US DOE under Contract No. DE-AC02-05CH11231.
|Are you a student?||Yes|