Tech-X, Boulder, Colorado
University of Nevada, Reno, Reno, Nevada
LBNL, Berkeley, California
Laser wakefield accelerators produce accelerating gradients up to hundreds of GeV/m and narrow energy spread, and have recently demonstrated energies up to GeV and improved stability [*,**] using electrons self trapped from the plasma. Controlled injection and staging can further improve beam quality by circumventing tradeoffs between energy, stability, and energy spread/emittance. We present experiments demonstrating production of a stable electron beam near 1 MeV with 100 keV level energy spread and central energy stability by using the plasma density profile to control self injection, and supporting simulations. A 10 TW laser pulse was focused near the downstream edge of a mm-long hydrogen gas jet. The plasma density near focus is decreasing in the laser propagation direction, which changes the wake phase velocity and reduces the trapping threshold. This allows stable self trapping and low absolute energy spread. Simulations indicate that such beams can be post accelerated to form high energy, high quality, stable beams, and experiments are under investigation.
* Geddes et al, Nature v431 no7008, 538 (2004).** Leemans et al, Nature Physics v2 no10, p696 (2006)
UCLA, Los Angeles, California
Instituto Superior Tecnico, Lisbon
In a laser wakefield accelerator experiment where the length of the pump laser pulse is several plasma period long, the leading edge of the laser pulse undergoes frequency downshifting as the laser energy is transferred to the wake. Therefore, after some propagation distance, the group velocity of the leading edge of of the pump pulse–and therefore of the driven electron plasma wave–will slow down. This can have implications for the dephasing length of the accelerated electrons and therefore needs to be understood experimentally. We have carried out an experimental investigation where we have measured the velocity vf of the 'wave-front' of the plasma wave driven by a nominally 50fs (FWHM), intense (a0~1), 0.8 micron laser pulse. To determine the speed of the wave front, time- and space-resolved shadowgraphy, interferometry, and Thomson scattering were used. Although low density data (ne ~ 1018 cm-3) showed no significant changes in vf over 1.5mm (and no accelerated electrons), high-density data shows accelerated electrons and an approximately 5% drop in vf after a propagation distance of about 800 microns.