A method of using intense Laguerre-Gaussian (LG) laser pulse is proposed to generate ultrarelativistic (multi-GeV) electron beams with controllable helical structures based on a hybrid electron acceleration regime in underdense plasmas, where both the longitudinal charge-separation electric field and transverse laser electric field play the role of accelerating the electrons. By directly interacting with the LG laser pulse, the topological structure of the accelerated electron beam is manipulated and it is spatially separated into multi-slice helical bunches. These results are clearly demonstrated by our three-dimensional particle-in-cell simulations and explained by a theoretical model based on electron phase-space dynamics. This novel regime offers a new degree of freedom for manipulating ultrashort and ultrarelativistic electrons, and it provides an efficient way for generating high-energy highangular-momentum helical electron beams, which may find applications in wide-ranging areas.

Energetic electron acceleration processes in a plasma hollow tube irradiated by an ultraintense laser pulse are investigated. It is found that the longitudinal component of the laser field is much enhanced when a linear polarized Gaussian laser pulse propagates through the plasma tube. This longitudinal field is of π/2 phase shift relative to the transverse electric field and has a π phase interval between its upper and lower parts. The electrons in the plasma tube are first pulled out by the transverse electric field and then trapped by the longitudinal electric field. The trapped electrons can further be accelerated to higher energy in the presence of the longitudinal electric field. This acceleration mechanism is clearly illustrated by both particle-in-cell simulations and single particle modelings.

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The dynamics of electron injection in the direct laser acceleration (DLA) regime was investigated by means of three-dimensional particle-in-cell simulations and theoretical analysis. It is shown that when an ultra-intense laser pulse propagates into a near-critical density or relativistically transparent plasma, the longitudinal charge-separation electric field excites an ion wave. The ion wave modulates the local electric field and acts as a set of potential wells to guide the electrons, located on the edge of the plasma channel, to the central region, where the DLA takes place later on. In addition, it is pointed out that the self-generated azimuthal magnetic fields tend to suppress the injection process of electrons by deflecting them away from the laser field region. Understanding these physical processes paves the way for further optimizing the properties of direct-laser accelerated electron beams and the associated X/gamma-ray sources.High-energy electron beams are attractive for many applications ranging from fast ignition of inertial confinement fusion [1], radiography [2], and novel light sources [3][4][5], to neutron sources [6]. With the ability of supporting huge accelerating electric fields (above 100 GV/m), the laser-based plasma accelerators, which are promising to revolutionize the conventional accelerator technologies, recently have attracted much research attention. Based on laser-plasma interactions, two distinct regimes have been proposed to produce high-energy electron beams, which are known as the laser wake-field acceleration (LWFA) and the direct laser acceleration (DLA). In LWFA regime, so far quasi-monoenergetic electron beams with energies up to several GeV have been obtained [7][8][9][10][11]. However, the rather low-density plasma (typically less than 10 20 cm −3 ) used in this scheme generally limits the charges of accelerated electrons to about 100 pC, which makes this regime infeasible for many applications that requires high-charge electron beams.The DLA regime becomes dominant for electron acceleration in relatively high density plasmas, and in this regime high-charge (higher than 10 nC) electron beams with energies up to hundreds of MeV have been obtained [12]. This regime occurs when the betatron oscillation frequency of the electrons in the laser-produced plasma channel is equal to the Doppler-shifted laser frequency [13]. Recently, several different schemes have been proposed to enhance the electron energy in this regime [14][15][16]. So far little attention has been paid to the injection process of the electrons in this regime [17,18], which plays a significant role on the subsequent electron acceleration process and determines the qualities of the accelerated electrons. In particular, the DLA electron beams usually display as broad energy spectra. Understanding the injection mechanism is an important step to optimize and improve the electron beam qualities in this regime. The injection process of electrons in the plasma channel seems complicated and chaotic [12,19], and onl...