Controlling the Self-Injection Threshold in Laser Wakefield Accelerators

Review of Quality Optimization of Electron Beam Based on Laser Wakefield Acceleration

Compared with state-of-the-art radio frequency accelerators, the gradient of laser wakefield accelerators is 3–4 orders of magnitude higher. This is of great significance in the development of miniaturized particle accelerators and radiation sources. Higher requirements have been proposed for the quality of electron beams, owing to the increasing application requirements of tabletop radiation sources, specifically with the rapid development of free-electron laser devices. This review briefly examines the electron beam quality optimization scheme based on laser wakefield acceleration and presents some representative studies. In addition, manipulation of the electron beam phase space by means of injection, plasma profile distribution, and laser evolution is described. This review of studies is beneficial for further promoting the application of laser wakefield accelerators.

Generation of ultrahigh-brightness pre-bunched beams from a plasma cathode for X-ray free-electron lasers

The longitudinal coherence of X-ray free-electron lasers (XFELs) in the self-amplified spontaneous emission regime could be substantially improved if the high brightness electron beam could be pre-bunched on the radiated wavelength-scale. Here, we show that it is indeed possible to realize such current modulated electron beam at angstrom scale by exciting a nonlinear wake across a periodically modulated plasma-density downramp/plasma cathode. The density modulation turns on and off the injection of electrons in the wake while downramp provides a unique longitudinal mapping between the electrons’ initial injection positions and their final trapped positions inside the wake. The combined use of a downramp and periodic modulation of micrometers is shown to be able to produces a train of high peak current (17 kA) electron bunches with a modulation wavelength of 10’s of angstroms - orders of magnitude shorter than the plasma density modulation. The peak brightness of the nano-bunched beam can be O(10²¹A/m²/rad²) orders of magnitude higher than current XFEL beams. Such prebunched, high brightness electron beams hold the promise for compact and lower cost XEFLs that can produce nanometer radiation with hundreds of GW power in a 10s of centimeter long undulator.

Laser-produced plasma can be used for acceleration and tuning of particle beams. Here the authors discuss the generation of a bunched electron beam using simulations and its application to X-ray free-electron laser.

Development of a density-tapered capillary gas cell for laser wakefield acceleration

A capillary gas cell for laser wakefield acceleration was developed with the aid of three-dimensional computational fluid dynamics simulations. The gas cell was specially designed to provide upward density tapering in the longitudinal direction, which is expected to suppress the dephasing problem in laser wakefield acceleration by keeping the accelerated electrons in the acceleration phase of the wake wave. The density-tapered capillary gas cell was fabricated by sapphire plates, and its performance characteristics were tested. The capillary gas cell was filled with a few hundred millibars of hydrogen gas, and a Ti:sapphire laser pulse with a peak power of 3.8 TW and a pulse duration of 40 fs (full width at half maximum) was sent through the capillary hole, which has a length of 7 mm and a square cross section of 350 × 350 µm². The laser-produced hydrogen plasma in the capillary hole was then diagnosed two-dimensionally by using a transverse Mach-Zehnder interferometer. The capillary gas cell was found to provide an upward plasma density tapering in the range of 10¹⁸ cm^-3-10¹⁹ cm^-3, which has a potential to enhance the electron beam energy in laser wakefield acceleration experiments.

Polarization-Dependent Self-Injection by Above Threshold Ionization Heating in a Laser Wakefield Accelerator

We report on the experimental observation of a decreased self-injection threshold by using laser pulses with circular polarization in laser wakefield acceleration experiments in a nonpreformed plasma, compared to the usually employed linear polarization. A significantly higher electron beam charge was also observed for circular polarization compared to linear polarization over a wide range of parameters. Theoretical analysis and quasi-3D particle-in-cell simulations reveal that the self-injection and hence the laser wakefield acceleration is polarization dependent and indicate a different injection mechanism for circularly polarized laser pulses, originating from larger momentum gain by electrons during above threshold ionization. This enables electrons to meet the trapping condition more easily, and the resulting higher plasma temperature was confirmed via spectroscopy of the XUV plasma emission.