Quasi-mono-energetic and Tunable Compton X-rays from a Laser Wakefield Accelerator

A quasi-mono-energetic x-ray source, produced by laser Compton scattering of laser wakefield accelerated electrons, is demonstrated for the first time [1]. The peak spectral brightness of the source compares with or exceeds that of conventional, radio-frequency (RF)-based sources, but at significantly lower cost and smaller footprint. The beams are expected to have among the shortest temporal duration of any high energy x-ray source produced to date. The source was composed of a laser wakefield accelerator (LWFA) and Compton scattering undulator. The LWFA uses a laser to drive a plasma wave. Spatial and temporal properties of the drive beam were matched to the plasma resonance conditions. A near-diffraction-limited, near-transform limited and low contrast intense laser was produced using spatial and temporal phase correction and temporal gating. The high-quality laser parameters resulted in effective self-guiding over several Rayleigh lengths and stable acceleration. A dual-staged gas jet target was designed to independently control e-beam characteristics [2]. The first stage used a He/N2 gas mixture to inject electrons, by ionization, into the accelerator. Increasing the density and nitrogen content of this jet increased the e-beam charge. The second stage used pure helium to further accelerate the injected electrons. The e-beam energy was increased, while maintaining the energy spread by increasing the density and length of the second jet. The e-beams were characterized by relatively high charge (∼20 pC), low divergence (∼10 mrad), small source size (∼1 µm), and low energy spread (\u3c25%). The Compton undulator used a second laser pulse, split from the drive beam, to scatter from the e-beams and generate x-rays. The beam focus was spatio-temporally overlapped with the e-beam. Since the drive and scattering beams are produced by the same laser, timing jitter is reduced. Quasi-mono-energetic e-beams were used to generate x-rays with correspondingly quasi-mono-energetic spectral distributions. The x-ray beam energy was tuned from 40 keV–1 MeV. A Ross filter was designed to characterize the spectrum. Tunability of the x-ray central energy was accomplished by tuning the e-beam energy. The central energy was measured using transmission filters. The stability and source size of the x-rays were also studied. [1] N. D. Powers, I. Ghebregziabher, G. Golovin, C. Liu, S. Chen, S. Banerjee, J. Zhang, and D. P. Umstadter, Nat. Photonics 8, 28 (2014) URL: http://dx.doi.org/10.1038/nphoton.2013.314