Field mapping of CO2-laser-driven LWFA at low density using electron beam probing*

Laser wakefield accelerators (LWFAs) have been experimentally shown to produce sustained gradients of tens of GeV/m over tens of centimetres. While the strength of these fields has been demonstrated, a direct measurement of the field configurations inside an LWFA represents an emerging research area. 

A recently published study(1) suggests that the unique combination of parameters used during the experiment, where the laser pulse length spans only a few plasma wake periods, places laser plasma interaction in a novel regime, as an intermediary stage between self modulated and blowout regime. OSIRIS simulations support the presence of an elongated cylindrical cavity with an almost flat accelerating field along the length of the ion channel, making this regime a potential candidate for laser plasma lens. 

Here we report on the transverse probing of the fields inside an LWFA at such a regime in some of the lowest densities ever observed, i.e. in the range of 1015 — 1017 cm-3. The LWFA is driven by BNL Accelerator Test Facility’s long-wave-infrared CO2 laser (9.2 μm) pulse, which currently generates 2 ps long pulses at 2-3 TW. The linac-produced electron beam has an energy of 50-60 MeV and about a 200 fs long bunch length. Electrons traversing the wakefield are detected using YAG:Ce scintillator screens, with one being stationary at 50 cm and the other placed on a translation stage imaging the electron beam density profile at distances of up to 10 cm from the plasma. Particle-In-Cell Simulations using OSIRIS are used to corroborate the results of the experiment.