Optical control of transverse motion of ionization injected electrons in a laser plasma accelerator

In recent years, the plasma wakefield acceleration driven by ultra-short and ultra-intense laser pulses has become increasingly mature, which can produce electron beams with ultra-high beam density and femtosecond beam duration; By using this electron beam, a new table-top radiation light source with collimation, ultrafast and high brightness can be produced. Compared with large synchrotron radiation light source, the radiation light source based on laser wakefield acceleration has the characteristics of femtosecond pulse duration, small source size, and high photon energy. In particular, the natural synchronization between X-ray pulse and driving laser pulse has incomparable advantages in the research of ultra-fast pump detection.

 

Moreover, if the light source has tunable polarization and orbital angular momentum, it can further be used to analyze complex structures or diagnose the magnetic moment and magnetic field distribution in magnetic materials. However, at present, the research on polarization generation and regulation of Betatron radiation is still relatively lacking, which is mainly due to the tiny wakefield structure (tens of micrometers) and its ultra-high transverse confinement field. It is difficult to control the transverse motion of the accelerated electron beam through the method of external electromagnetic field, and achieve the regulation of the polarization, angular momentum and other characteristics of radiation.

 

The related research results are published on High Power Laser Science and Engineering, Vol. 9, Issue 1, and is selected as cover paper (Jie Feng, Yifei Li, Jinguang Wang, et al. Optical control of transverse motion of ionization injected electrons in a laser plasma accelerator[J]. High Power Laser Science and Engineering, 2021, 9(1): 03000e05). They proposed a simple method of controlling the transverse motion of an electron beam in a plasma bubble. Laser pulses with a power of 100 TW drive a plasma wakefield and accelerate an electron beam in the regime of ionization injection. The transverse motion of the accelerated electron beam can be controlled by changing the intensity distribution of the laser focus by adjusting the posture of the off-axis-parabolic mirror. When the shape of the laser focus changes from circular to slanted elliptical, the geometrical symmetry of the transverse force in the plasma bubble is changed, resulting in the motion of the electron beam changing from undulating to helical. Moreover, the profile of the electron beam also changes with the laser focal spot's shape. The experimental results were verified by 3D-PIC simulations. This method is expected to conveniently control the transverse motion of an electron beam in a plasma wakefield and to generate synchrotron radiation with orbit angular momentum.

 

Figure. 1. The results of electron beam transversal motion controlled by laser focal spot. (a) The first three columns correspond to the case of asymmetrical focus, the last column corresponds to the case of symmetrical focus; (b) Deflected electron distributions and charges for the case of asymmetrical focus and the case of symmetrical focus, respectively; (c) Trajectories of the electrons correspond to the case of circular laser focus and the case of elliptical laser focus, respectively.