Recently, A 200TW/1Hz laser at Shanghai Institute of Optics and Fine Mechanics (SIOM) has been upgraded, in order to better drive laser wakefield acceleration (LWFA) of electrons and make an advance on X-ray free electron lasers (XFEL) project. Based on this upgraded driving laser, high-quality electron beams can be reproduced in consecutive shots. This work offers the technical guidance for the construction of high peak power femtosecond lasers, and provides the principle verification for the realization of XFEL. The relevant result has recently been published on Optics and Laser Technology (F. Wu, et al. Performance improvement of a 200TW/1Hz Ti:sapphire laser for laser wakefield electron accelerator, 131, 2020).

The XFEL project was formally started in 2015, the aim of which is to achieve high-brilliance X-ray radiation by LWFA. Although significant progress on LWFA has been achieved in 2016, the researchers are still suffered great difficulty in the XFEL project, which is mainly limited by the stability of electron beams. In addition, the stability and quality of produced electron beams are still essential issues for the practical applications, like the compact mono-energetic Gamma-ray source and the candidate for healthcare.

LWFA is based on the interaction between an intense laser and a plasma, which is a high nonlinear process occurring in relativistic regime and very sensitive to the property of driving laser.

Thereby, in order to obtain high-quality and high-stable electron beams, the research team in SIOM comprehensively upgraded the 200TW/1Hz driven laser. In terms of laser stability: the energy fluctuation (std) of amplified pulse is 0.55%, and the beam pointing fluctuation on target (std) is 1.5μrad, which are simultaneously measured in 90 minutes for consecutive 5400 pulses. To the best of our knowledge, it is the best beam pointing stability of high peak power lasers for such a long time measurement. In terms of laser spatial-temporal quality: the duration of compressed pulse is 23.7fs, and the focal spot obtained by an f/30 off-axis parabolic mirror is 54μm×52μm at 1/e2, while the Fourier-transform limit duration and the diffraction-limited focal spot of this laser are 21fs and 51.2μm, respectively. Consequently, a peak intensity of 6.2×1018W/cm2 is obtained, which is about 2.6 times of the previous 200TW/1Hz laser. It is worth noting that, the improvement on laser spatial-temporal quality is realized without utilizing any active control optics , which greatly reduces the cost and complexity of laser.

Benefiting from this upgraded 200TW/1Hz driving laser, quasi-mono-energetic electron beams are reproduced for successive shots in LWFA experiments. For consecutive 300 shots, the reproducibility of electron beams approached 100%. The peak energy of these 300 electron beams averages about 667MeV, with a fluctuation (std) of only 3.4%. The progresses on both driving laser and LWFA make the realization of XFEL possible.

In the following work, a detailed research of the correlation between laser pointing stability and electron beams stability will be carried out, and a further upgrade based on double-CPA will also be implemented to enhance pulse contrast and make this laser can be used not only for electron acceleration but also for proton acceleration.

The engineering drawing of upgraded 200TW/1Hz laser. (Image by SIOM)

Article website:
https://www.sciencedirect.com/science/article/pii/S0030399220310860

Contact:
Mr. Wu Xiufeng
General Administrative Office
Shanghai Institute of Optics and Fine Mechanics, CAS
Email: xfwu@siom.ac.cn