Beam quality degradation during the transition from a laser wakefield accelerator to the vacuum is one of the reasons that cause the beam transport distortion, which hinders the way to compact free-electron-lasers. Here,we performed transition simulation to initialize the beam parameters for beam optics transport. This initialization was crucial in matching the experimental results and the designed evolution of the beamline. We experimentally characterized properties of high-quality laser-wakefield-accelerated electron beams, such as transverse beam profile, divergence, and directionality after long-distance transport. By installing magnetic quadrupole lenses with tailored strength gradients, we successfully collimated the electron beams with tunable energies from 200 to 600 MeV.
In this paper,we report the recent progress on the 1 PW/0.1 Hz laser beamline of Shanghai Superintense Ultrafast Laser Facility(SULF).The SULF-1 PW laser beamline is based on the double chirped pulse amplification(CPA)scheme,which can generate laser pulses of 50.8 J at 0.1 Hz after the final amplifier;the shot-to-shot energy fluctuation of the amplified pulse is as low as 1.2%(std).After compression,the pulse duration of 29.6 fs is achieved,which can support a maximal peak power of 1 PW.The contrast ratio at-80 ps before main pulse is measured to be 2.5×10^-11.The focused peak intensity is improved by optimizing the angular dispersion in the grating compressor.The maximal focused peak intensity can reach 2.7×10^19W/cm2 even with an f/26.5 off-axis parabolic mirror.The horizontal and vertical angular pointing fluctuations in 1 h are measured to be 1.89 and 2.45μrad,respectively.The moderate repetition rate and the good stability are desirable characteristics for lasermatter interactions.The SULF-1 PW laser beamline is now in the phase of commissioning,and preliminary experiments of particle acceleration and secondary radiation under 300–400 TW/0.1 Hz laser condition have been implemented.The progress on the experiments and the daily stable operation of the laser demonstrate the availability of the SULF-1 PW beamline.
We experimentally investigate the high-order harmonic generation in argon gas cell driven by a multi-cycle broadband infrared laser pulse from a tunable optical-parametric-amplifier （OPA） source. The generation of high-order harmonic continuum with the cut-off photon energy up to 110 eV is observed by tuning the chirp of the 800-nm laser pulse which pumps OPA source. The generation of harmonic continuum is understood in terms of the two-hump structure of the OPA output spectrum and the optimal relative phase of the two humps. The demonstrated scheme is of importance for the generation of extreme ultraviolet （XUV） continuum at higher photon energy region.
Laser wakefield accelerators（LWFAs） are compact accelerators which can produce femtosecond high-energy electron beams on a much smaller scale than the conventional radiofrequency accelerators. It is attributed to their high acceleration gradient which is about 3 orders of magnitude larger than the traditional ones. The past decade has witnessed the major breakthroughs and progress in developing the laser wakfield accelerators. To achieve the LWFAs suitable for applications,more and more attention has been paid to optimize the LWFAs for high-quality electron beams. A single-staged LWFA does not favor generating controllable electron beams beyond 1 Ge V since electron injection and acceleration are coupled and cannot be independently controlled. Staged LWFAs provide a promising route to overcome this disadvantage by decoupling injection from acceleration and thus the electron-beam quality as well as the stability can be greatly improved.This paper provides an overview of the physical conceptions of the LWFA, as well as the major breakthroughs and progress in developing LWFAs from single-stage to two-stage LWFAs.
Real-time single-shot measurement of the femtosecond electron beam duration in laser wakefield accelerators is discussed for both experimental design and theoretical analysis that combines polarimetry and interferometry.The probe pulse polarization is rotated by the azimuthal magnetic field of the electron beam and then introduced into a Michelson-type interferometer for self-interference. The electron beam duration is obtained from the region size of the interference fringes, which is independent of the pulse width of the probe laser. Using a larger magnification system or incident angle, the measurement resolution can be less than 1 fs.
A mode-locked（ML）picosecond ytterbium-doped thin disk laser using a monolayer Mo S2as the saturable absorber（SA）is demonstrated.The monolayer MoS2 is fabricated through the method of low-pressure chemical vapor deposition.The laser directly produces stable ML picosecond pulses at a slope efficiency of 9.71%.The maximum output power is approximately 890 mW,while the corresponding repetition,pulse energy,and pulse duration are 48.6 MHz,18.3 nJ,and 13.1 ps,respectively.Results suggest that the monolayer MoS2 is a promising SA for ultrafast lasers system.
We present a velocity-gauge model for the generation of even-order high harmonics, and reveal that the even-order harmonics originate from the multiple-step transitions among the energy bands in momentum space, while the odd-order harmonics are mainly from direct transitions. The lower valence band is found vital for the generation of even harmonics. Relative intensity of even-order harmonics versus the odd orders is calculated and shows a growing trend as the laser field amplitude increases.
The underlying mechanism of the spectral cleaning effect of the cross-polarized wave（XPW） generation process was theoretically investigated. This study shows that the spectral noise of an input spectrum can be removed in the XPW generation process and that the spectral cleaning effect depends on the characteristics of the input pulses, such as the chirp and Fourier-transform-limited duration of the initial pulse, and the modulation amplitude and frequency of the spectral noise. Though these factors codetermine the output spectrum of the XPW generation process, the spectral cleaning effect is mainly affected by the initial pulse chirp. The smoothing of the spectrum in the XPW generation process leads to a significant enhancement of the coherent contrast.
In this study, we investigate a new simple scheme using a planar undulator(PU) together with a properly dispersed electron beam(e beam) with a large energy spread(～1%) to enhance the free-electron laser(FEL) gain. For a dispersed e beam in a PU, the resonant condition is satisfied for the center electrons, while the frequency detuning increases for the off-center electrons, inhibiting the growth of the radiation. The PU can act as a filter for selecting the electrons near the beam center to achieve the radiation. Although only the center electrons contribute, the radiation can be enhanced significantly owing to the high-peak current of the beam. Theoretical analysis and simulation results indicate that this method can be used for the improvement of the radiation performance, which has great significance for short-wavelength FEL applications.
Ke FengChanghai YuJiansheng LiuWentao WangZhijun ZhangRong QiMing FangJiaqi LiuZhiyong QinYing WuYu ChenLintong KeCheng WangRuxin Li
A new polarization gating is demonstrated by our principle-of-proof experiment, which is theoretically proposed to generate the isolated or double attosecond pulses with the multi-cycle driving laser pulse in the previous work [Optics Express. 20, 5196 （2012）]. In the experiment, the polarization gating is formed by two orthogonally-polarized linearly chirped multi-cycle laser pulses, and the spectral bandwidths of tile harmonics are broadened by more than two times, which agree with the previous theoretical work.
