Modular supersonic nozzle for the stable laser-driven electron acceleration.

The sharp density down-ramp injection (shock injection) mechanism produces the quasi-monoenergetic electron beam with a bunch duration of tens of femtoseconds via laser wakefield acceleration. The stability of the accelerated electron beam strongly depends on the stability of the laser beam and the shock structure produced by the supersonic gas nozzle. In this paper, we report the study of a newly designed modular supersonic nozzle with a flexible stilling chamber and a converging-diverging structure. The performance of the nozzle is studied both numerically and experimentally with the computational fluid dynamics simulation and the Mach-Zehnder interferometry method. The simulation results and the experimental measurements are well consistent, and both prove the effectiveness of the stilling chamber in stabilizing the gas flow.

Notable improvements on LWFA through precise laser wavefront tuning.

Laser wakefield acceleration (LWFA) continues to grow and awaken interest worldwide, especially as in various applications it approaches performance comparable to classical accelerators. However, numerous challenges still exist until this can be a reality. The complex non-linear nature of the process of interaction between the laser and the induced plasma remains an obstacle to the widespread LWFA use as stable and reliable particle sources. It is commonly accepted that the best wavefront is a perfect Gaussian distribution. However, experimentally, this is not correct and more complicated ones can potentially give better results. in this work, the effects of tuning the laser wavefront via the controlled introduction of aberrations are explored for an LWFA accelerator using the shock injection configuration. Our experiments show the clear unique correlation between the generated beam transverse characteristics and the different input wavefronts. The electron beams stability, acceleration and injection are also significantly different. We found that in our case, the best beams were generated with a specific complex wavefront. A greater understanding of electron generation as function of the laser input is achieved thanks to this method and hopes towards a higher level of control on the electrons beams by LWFA is foreseen.

High-spatiotemporal-quality petawatt-class laser system.

We have developed a femtosecond high-intensity laser system that combines both Ti:sapphire chirped-pulse amplification (CPA) and optical parametric CPA (OPCPA) techniques and produces more than 30 J broadband output energy, indicating the potential for achieving peak powers in excess of 500 TW. With a cleaned high-energy seeded OPCPA preamplifier as a front end in the system, for the compressed pulse without pumping the final amplifier, we found that the temporal contrast in this system exceeds 10(10) on the subnanosecond time scales, and is near 10(12) on the nanosecond time scale prior to the peak of the main femtosecond pulse. Using diffractive optical elements for beam homogenization of a 100 J level high-energy Nd:glass green pump laser in a Ti:sapphire final amplifier, we have successfully generated broadband high-energy output with a near-perfect top-hat-like intensity distribution.