Enhancement of laser-driven electron acceleration in an ion channel

A long laser beam propagating through an underdense plasma produces a positively charged ion channel by expelling plasma electrons in the transverse direction. We consider the dynamics of a test electron in a resulting two-dimensional channel under the action of the laser field and the transverse electric field of the channel. A considerable enhancement of the axial momentum can be achieved in this case via amplification of betatron oscillations. It is shown that the oscillations can be parametrically amplified when the betatron frequency, which increases with the wave amplitude, becomes comparable to the frequency of its modulations. The modulations are caused by non-inertial (accelerated/decelerated) relativistic axial motion induced by the wave regardless of the angle between the laser electric field and the field of the channel. We have performed a parameter scan for a wide range of wave amplitudes and ion densities and we have found that, for a given density, there is a well pronounced wave amplitude threshold above which the maximum electron energy is considerably enhanced. We have also calculated a time-integrated electron spectrum produced by an ensemble of electrons with a spread in the initial transverse momentum. The numerical results show that the considerable energy enhancement is accompanied by spectrum broadening. The presented mechanism of energy enhancement is robust with respect to an axial increase of ion density, because it relies on a threshold phenomenon rather than on a narrow linear resonance

Dynamic positive column in long-gap barrier discharges

A simple analytical model of the barrier discharge in a long gap between opposing plane electrodes is developed. It is shown that the plasma density becomes uniform over large part of the gap in the course of the discharge development, so that one can speak of a formation of a dynamic positive column. The column completely controls the dynamics of the barrier discharge and determines such characteristics as the discharge current, discharge duration, light output, etc. Using the proposed model, all discharge parameters can be easily evaluatedComment: 7 pages, 8 figures; submitted to the Journal of Applied Physic

Parametric Amplification Of Laser-Driven Acceleration In A Plasma Channel

Two-dimensional particle-in-cell simulations are presented for a laser-irradiated solid-density target with and without an underdense preplasma. It is shown that an underdense preplasma can generate an energetic electron tail in addition to the warm electrons generated at the critical surface. Preplasma electrons are accelerated in a quasi-static positively charged channel formed by the laser. At ultra-relativistic laser intensities (a(0) = 10), the acceleration mechanism is not sensitive to the laser polarization. An energetic tail with energies significantly exceeding the energy expected for a single electron in a vacuum is present in simulations with s and p-polarized beams. This suggests that the mechanism of parametric amplification of laser-driven electron acceleration is a likely explanation for the observed phenomenon.Institute for Fusion Studie

Electron avalanche sliding along a dielectric surface

Abstract We consider the dynamics of an electron avalanche propagating along a dielectric surface in a channel wherein the Townsend breakdown condition does not hold. We show that when the angle θ between the electric field and the channel wall is less than some critical value θ cr , the electron avalanche grows. The function θ cr = θ cr (E) is found by kinetic Monte-Carlo simulations for a mixture of Xe and Ne gasses

Spontaneous emergence of non-planar electron orbits during direct laser acceleration by a linearly polarized laser pulse

An electron irradiated by a linearly polarized relativistic intensity laser pulse in a cylindrical plasma channel can gain significant energy from the pulse. The laser electric and magnetic fields drive electron oscillations in a plane making it natural to expect the electron trajectory to be flat. We show that strong modulations of the relativistic gamma-factor associated with the energy enhancement cause the free oscillations perpendicular to the plane of the driven motion to become unstable. As a consequence, out of plane displacements grow to become comparable to the amplitude of the driven oscillations and the electron trajectory becomes essentially three-dimensional, even if at an early stage of the acceleration it was flat. The development of the instability profoundly affects the x-ray emission, causing considerable divergence of the radiation perpendicular to the plane of the driven oscillations, while also reducing the overall emitted energy. (C) 2016 AIP Publishing LLC