We report observations of self-modulated laser wakefields (SM-LWF) generated using the 10-micron wavelength CO2 Terawatt laser system at the Accelerator Test Facility (ATF) in Brookhaven National Lab. The wakefields were produced in a 1-mm diameter hydrogen gas jet and probed by forward collective Thomson scattering (CTS) of a co-propagating Nd:YAG pulse. Anti-Stokes sidebands were observed at densities down to 5*10^17cm^-3, limited by spectral detection range.
The very favorable wavelength scaling of the interaction strength between plasma and the driving laser pulses when moving from the common 0.8-micron to 10-micron laser drivers allows lowering of laser intensity and plasma density with a corresponding increase in spatial dimensions of the wakes. Consequently, spatial and temporal synchronization with external injection and probing beams is simplified and greater charge can be accelerated, leading to the possibility of production of more controllable beams with greater brightness. The larger wake dimensions at lower densities also open the way to more detailed probing of the wakes.
The experiment used the ATF CO2 laser (10 um, ~1 TW, 3-4 ps) focused onto a 1-mm hydrogen gas jet. A 14-ps, 1.064-micron probe co-propagated with the pump pulse through the interaction region. The 10-micron pump was blocked by a vacuum chamber window after the interaction, and the probe fundamental light was suppressed 6 orders of magnitude by a notch filter and further suppressed by the analyzing spectrometer. The plasma density range was limited at the upper end by the critical density for the drive pulse, and at the low density end by the spectral edge of the notch filter.
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