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electrons to MeV energies by a high-intensity subpicosecond laser pulse (10 19 W/cm 2 , 300 fs). Geddes et al. [2] observed a high-quality electron beam from a laser wake field accelerator using a plasma channel guiding. They proposed peak energy in simulation of about 200 MeV which is close to experimental results. An electron in vacuum may be accelerated by the laser field directly. A planarlaser pulse cannot be used for electron acceleration; since it overtakes an electron, the latter is ponderomotively driven forwards in the raising part, but then backwards in trailing part, resulting in no net gain by the electron [6]. An electron can gain and retain significant energy if a magnetic field of suitable magnitude and period is externally applied. The magnetic field strengthens the cyclotron oscillations due to � v × � B force and hence contributes significantly to forward drift and rate of gain of electron energy. The electron energy gain of 1.2 GeV was obtained [7] with an intense laser pulse peak intensity of 8 × 10 21 W/cm 2 . The characteristics variations in polarizations of laser pulse were studied to investigate the betterment in interaction of laser pulse with electron for high energy gain [8-10]. Sohbatzadeh and Aku [8] proposed that the circularly polarized (CP) laser pulse is more efficient in electron bunch acceleration in comparison with elliptical and linear polarizations. The pulse parameters of interaction with electrons are time-averaged with circular polarization which increases the interaction of laser pulse with electrons. Gupta et al.[10] observed the electron energy gain of 1.5 GeV with a radially polarized laser pulse in the presence of magnetic field of about 1 MG. After attaining the maximum energy gain, the electron gets decelerated, losses energy, and tends to get out of phase with the field even with high-intensity laser pulse. A pre-accelerated electron was employed to enforce a confined trajectory for longer duration [9] with a CP laser pulse. It is easier to inject a pre-accelerated Abstract Electron acceleration by a frequency-chirped circularly polarized (CP) laser pulse in vacuum in the presence of azimuthal magnetic field has been studied. A laser pulse propagating along +z-axis interacts with a pre-accelerated electron injected at a small angle in the direction of propagation of laser pulse in vacuum. The electron is accelerated with high energy in the presence of azimuthal magnetic field till the saturation of betatron resonance. A linear frequency chirp increases the duration of interaction of laser pulse with electron and hence enforces the resonance for longer duration. The presence of azimuthal magnetic field further improves the electron acceleration by keeping the electron motion parallel to the direction of propagation for longer distances. Thus, resonant enhancement appears due to the combined effect of chirped CP laser pulse and azimuthal magnetic field. An electron with few MeV of initial energy gains high energy of the order of GeV. Higher energy gain is obtained ...

We present a scheme of electron injection for enhanced electron energy gain by using a radially polarized (RP) laser pulse in vacuum under the influence of magnetic wiggler. The inherent symmetry of an RP laser pulse enforces the trapping and acceleration of electrons in the direction of propagation of laser pulse during laser electron interaction. A magnetic wiggler encircles the trajectory of accelerated electron and improves the strength of v→×B→ force which supports the retaining of betatron resonance for longer duration and leads to enhance electron acceleration. Four times higher electron energy is observed with a RP laser pulse of peak intensity 8.5×1020  W/cm2 in the presence of magnetic wiggler of 10.69 kG than that in the absence of magnetic wiggler. We have also analyzed the electron injection for enhanced energy gain and observe that the electron energy gain is relatively higher with a sideway injection than that of axial injection of electron. Injection angle δ is optimized and found that at δ=10° to the direction of propagation of laser pulse, maximum energy is obtained.

Hermite–Gaussian (HG) laser beam with transverse electromagnetic (TEM) mode indices (m, n) of distinct values (0, 1), (0, 2), (0, 3), and (0, 4) has been analyzed theoretically for direct laser acceleration (DLA) of electron under the influence of an externally applied axial magnetic field. The propagation characteristics of a TEM HG beam in vacuum control the dynamics of electron during laser–electron interaction. The applied magnetic field strengthens the $\vec v \times \vec B$ force component of the fields acting on electron for the occurrence of strong betatron resonance. An axially confined enhanced acceleration is observed due to axial magnetic field. The electron energy gain is sensitive not only to mode indices of TEM HG laser beam but also to applied magnetic field. Higher energy gain in GeV range is seen with higher mode indices in the presence of applied magnetic field. The obtained results with distinct TEM modes would be helpful in the development of better table top accelerators of diverse needs.

Electron injected in the path of a circularly polarized Gaussian laser beam under the influence of an external axial magnetic field is shown to be accelerated with a several GeV of energy in vacuum. A small angle of injection δ with 0∘<δ<20∘ for a sideway injection of electron about the axis of propagation of laser pulse is suggested for better trapping of electron in laser field and stronger betatron resonance under the influence of axial magnetic field. Such an optimized electron injection with axial magnetic field maximizes the acceleration gradient and electron energy gain with low electron scattering.