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Register a new userGeneration of Super-Ponderomotive Electrons due to a non-Wakefield interaction between a Laser Pulse and a Longitudinal Electric Field
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It is shown that electrons with momenta exceeding the `free electron limit of $m_eca_0^2/2$ can be produced when a laser pulse and a longitudinal electric field interact with an electron via a non-wakefield mechanism. The mechanism consists of two stages: the reduction of the electron dephasing rate $gamma-p_x/m_ec$ by an accelerating region of electric field and electron acceleration by the laser via the Lorentz force. This mechanism can, in principle, produce electrons that have longtudinal momenta that is a significant multiple of $m_eca_0^2/2$. 2D PIC simulations of a relatively simple laser-plasma interaction indicate that the generation of super-ponderomotive electrons is strongly affected by this `anti-dephasing mechanism.
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We use 3D simulations to demonstrate that high-quality ultra-relativistic electron bunches can be generated upon reflection of a twisted laser beam off a plasma mirror. The unique topology of the beam with a twist index $|l| = 1$ creates an accelerating structure dominated by longitudinal laser electric and magnetic fields in the near-axis region. We show that the magnetic field is essential for creating a train of dense mono-energetic bunches. For a 6.8~PW laser, the energy reaches 1.6~GeV with a spread of 5.5%. The bunch duration is 320 as, its charge is 60~pC and density is $sim 10^{27}$~m$^{-3}$. The results are confirmed by an analytical model for the electron energy gain. These results enable development of novel laser-driven accelerators at multi-PW laser facilities.
Since it is possible to form laser pulses with a frequency much larger than the frequency of visible light, Prof. T.Tajima proposed using such pulse to accelerate the particles at its injection into the crystal. Here, the wakefield excitation in the metallic-density plasma and the electron acceleration by laser pulse are simulated. The accelerating gradient has been ob-tained approximately 3TV/m. It is shown that, as in ordinary plasma, with time beam-plasma wakefield acceleration is added to laser wakefield acceleration.
We investigate the generation of twin $gamma$ ray beams in collision of an ultrahigh intensity laser pulse with a laser wakefield accelerated electron beam by using particle-in-cell simulation. We consider the composed target of a homogeneous underdense preplasma in front of an ultrathin solid foil. The electrons in the preplasma are trapped and accelerated by the wakefield. When the laser pulse is reflected by the thin solid foil, the wakefield accelerated electrons continue to move forward and passing through the foil almost without the influence of the reflected laser pulse and the foil. Consequently, two groups of $gamma$ ray flashes, with tunable time delay and energy, are generated by the wakefield accelerated electron beam interacting with the reflected laser pulse from the foil as well as another counter propagating petawatt laser pulse in the behind the foil. The dependence of the $gamma$ photon emission on the preplasma densities, driving laser polarization and the foil are studied.
The effect of an external transverse magnetic field on ionization injection of electrons in a laser wakefield accelerator (LWFA) is investigated by theoretical analysis and particle-in-cell simulations. On application of a few tens of Tesla magnetic field, both the electron trapping condition and the wakefield structure changes significantly such that injection occurs over a shorter distance and at an enhanced rate. Furthermore, beam loading is compensated for, as a result of the intrinsic trapezoidal-shaped longitudinal charge density profile of injected electrons. The nonlinear ionization injection and consequent compensation of beam loading lead to a reduction in the energy spread and an enhancement of both the charge and final peak energy of the electron beam from a LWFA immersed in the magnetic field.
The dependence of the mean kinetic energy of laser-accelerated electrons on the laser intensity, so-called ponderomotive scaling, was derived theoretically with consideration of the motion of a single electron in oscillating laser fields. This scaling explains well the experimental results obtained with high-intensity pulses and durations shorter than a picosecond; however, this scaling is no longer applicable to the multi-picosecond (multi-ps) facility experiments. Here, we experimentally clarified the generation of the super-ponderomotive-relativistic electrons (SP-REs) through multi-ps relativistic laser-plasma interactions using prepulse-free LFEX laser pulses that were realized using a plasma mirror (PM). The SP-REs are produced with direct laser acceleration assisted by the self-generated quasi-static electric field and with loop-injected direct acceleration by the self- generated quasi-static magnetic field, which grow in a blowout plasma heated by a multi-ps laser pulse. Finally, we theoretically derive the threshold pulse duration to boost the acceleration of REs, which provides an important insight into the determination of laser pulse duration at kilojoule- petawatt laser facilities.
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