Laser wakefield accelerators (LWFA) hold great potential to produce high-quality high-energy electron beams (e beams), and wiggling of these LWFA e beams either in the Conventional period magnetic field structure (undulator radiations), strong focusing laser wakefield (betatron radiation), or intense laser fields (Compton scattering) can emit high-energy x-ray photons. By experimentally generating the high-quality LWFA e beams with a good stability and repeatability, we have recently produced tunable quasi-monochromatic ultrahigh brilliance MeV γ-ray via the self-synchronized all-optical Compton scattering scheme and realized a scheme to enhance betatron radiation by manipulating transverse oscillation of electrons in a deflected wakefield with a tilted shock front. The concurrent generation of high-quality e beams and bright x-rays in a compact LWFA may provide practical applications in ultrafast pump-probe study and x-ray radiology fields.
One of the major goals of developing laser wakefiled accelerators (LWFAs) is to produce compact high-energy electron beam (e-beam) sources, which are expected to be applied in developing compact x-ray free-electron lasers and monoenergetic gamma-ray sources. Although LWFAs have been demonstrated to generate multi-GeV e-beams, to date they are still failed to produce high quality e beams with several essential properties (narrow energy spread, small transverse emittance and high beam charge) achieved simultaneously.
Here we report on the demonstration of a high-quality cascaded LWFA experimentally via manipulating electron injection, seeding in different periods of the wakefield, as well as controlling energy chirp for the compression of energy spread. The cascaded LWFA was powered by a 1-Hz 200-TW femtosecond laser facility at SIOM. High-brightness e beams with peak energies in the range of 200-600 MeV, 0.4-1.2% rms energy spread, 10-80 pC charge, and ~0.2 mrad rms divergence are experimentally obtained. Unprecedentedly high 6-dimensional (6-D) brightness B6D,n in units of A/m2/0.1% was estimated at the level of 1015-16, which is very close to the typical brightness of e beams from state-of-the-art linac drivers and several-fold higher than those of previously reported LWFAs.
Furthermore, we propose a scheme to minimize the energy spread of an e beam in a cascaded LWFA to the one-thousandth-level by inserting a stage to compress its longitudinal spatial distribution via velocity bunching. In this scheme, three-segment plasma stages are designed for electron injection, e-beam length compression, and e-beam acceleration, respectively. A one-dimensional theory and two-dimensional particle-in-cell simulations have demonstrated this scheme and an e beam with 0.2% rms energy spread and low transverse emittance could be generated without loss of charge.
Based on the high-quality e beams generated in the LWFA, we have experimentally realized a new scheme to enhance the betatron radiation via manipulating the e-beam transverse oscillation in the wakefield. Very brilliant quasi-monochromatic betatron x-rays in tens of keV with significant enhancement both in photon yield and peak energy have been generated. Besides, by employing a self-synchronized all-optical Compton scattering scheme, in which the electron beam collided with the intense driving laser pulse via the reflection of a plasma mirror, we produced tunable quasi-monochromatic MeV γ-rays ( 33% full-width at half-maximum) with a peak brilliance of ~3.1×1022 photons s-1 mm-2 mrad-2 0.1% BW at 1 MeV, which is one order of magnitude higher than ever reported value in MeV regime to the best of our knowledge.
1. J. S. Liu, et al., Phys. Rev. Lett. 107, 035001 (2011).
2. X. Wang, et al., Nat. Commun. 4, 1988 (2013).
3. W. P. Leemans, et al., Phys. Rev. Lett. 113, 245002 (2014)
4. W. T. Wang et al., Phys. Rev. Lett. 117, 124801 (2016).
5. Z. J. Zhang et al., Phys. Plasmas 23, 053106 (2016).
6. C. H. Yu et al., Sci. Rep. 6, 29518 (2016).
Previous research on achieving the alignment of compression gratings has mainly focused on the parallelism of the gratings. We propose a promising method to achieve parallelism and especially accurately adjust the grating to its optimum working angle to achieve dispersion compensation. A spectrometer and a precisely adjustable mirror pair are cooperatively used to measure the wavelength of the light diffracted by the grating, satisfying the Littrow condition. Meanwhile, the tiny slit of the spectrometer can decrease the grating-tip and in-plane rotation error during the alignment procedure. Using this technique, the residual phase of the compressed pulse is optimized and the compressed pulse duration is 25.4 fs, which is 1.06 times that of the Fourier-transform-limited compressed pulse.
We report a generation of 10.6% conversion efficiency near 1053 nm first order Stokes pulse in stimulated Raman
scattering pumped using 800 nm Ti:sapphire based femtosecond pulses that are stretched to 460 ps, obtained by use of a
single pass ethonal Raman shifter. The Stokes pulse almost maintains the bandwidth of the pump and is compressed to
~10 ps using a mismatched grating-pair. The spectral characteristic of the Raman pulse is calculated and the results
explain the observed transient features.