High-Flux X-Ray Emission due to Injection into a Laser-Wakefield Accelerator beyond Its Depletion Length

Laser wakefield accelerators are bright, compact sources of synchrotronlike x-rays. By driving the wakefield using a 110 TW laser in a variable length gas cell, we are able to map the evolution of the electron beam and resulting x-ray emission. We find that using a laser pulse initially focused to larger than the matched spot size and extending the plasma length beyond the depletion length, dramatically increases the x-ray flux. Self-injected electrons are initially accelerated to >2  GeV, emitting x-rays with critical energy E_{c}>20  keV. But, as these electrons dephase, a second high-charge electron bunch is produced and accelerated to ≈ 0.7  GeV. The x-rays produced by this bunch dominate the total emission with >5×10^{10} photons per shot with E_{c}=12-17  keV and a flux of 7×10^{4}  photons/mrad^{2}/0.1%BW. Combining electron and x-ray measurements reveals that the increased emission results from the larger betatron amplitude of the secondary bunch. Simulations attribute this to rapid bubble expansion driven by laser compression over the depletion length, which leads to injection of a high-charge electron bunch with increased transverse momentum and thus stronger betatron radiation.