7 Search Results

We present an analytical theory that reveals the importance of the longitudinal laser electric field in the course of the resonant acceleration of relativistic electrons by a tightly confined laser beam. It is shown that this laser field component always counteracts the transverse one and effectively decreases the final energy gain of electrons via the direct laser acceleration (DLA) mechanism. This effect is demonstrated by carrying out particle-in-cell simulations of the DLA of the electrons injected into the accelerating phase of the plasma wake. It is shown that the electron energy gain from the wakefield is substantially compensated by themore » quasiresonant energy loss to the longitudinal laser field component. The analytically obtained scalings and estimates are in good agreement with the results of the numerical simulations.« less

In this paper, we predict that electrons in an ion channel can gain ultra-relativistic energies by simultaneously interacting with a laser pulse and, counter-intuitively, with a decelerating electric field. The crucial role of the decelerating field is to maintain high-amplitude betatron oscillations, thereby enabling constant rate energy flow to the electrons via the inverse ion channel laser mechanism. Multiple harmonics of the betatron motion can be employed. Injecting electrons into a decelerating phase of a laser wakefield accelerator is one practical implementation of the scheme.

We advance here a theory of quasistatic approximation and investigate the excitation of nonlinear plasma waves by the driving beam of ultrarelativistic electrons using a novel electrostatic-like particle-in-cell code. Assuming that the beam occupies an infinitesimally small volume, we find the radius and the length of the plasma bubble formed in the wake of the driver for varying values of the beam charge. The mechanism of bubble formation is explained by developing simple models of the bubble at large charges. Plasma electrons expelled by the driver charge excite secondary plasma waves, which complicate the plasma electron flow near the bubblemore » boundary.« less

Collisionless shocks generated by colliding relativistic plasmas are studied using particle-in-cell (PIC) simulations. The shock is produced due to the Weibel instabilities that generate current and density filaments and small-scale magnetic fields that are amplified from initial fluctuations. Localized regions of the strong magnetic field in the form of magnetic dipole vortices upstream of the shock are observed in the simulation developed during the nonlinear evolution of the electron and ion filaments. The vortices developing from the merger and subsequent pinching of the small-scale filaments are shown to be moving in the direction opposite to that of the shock. Lastly,more » we also found an analytical estimate of the drift velocity of the vortices that are confirmed by the PIC simulations.« less

Here, we analytically investigate the acceleration of electrons undergoing betatron oscillations in an ion channel, driven by a laser beam propagating with superluminal (or luminal) phase velocity. The universal scalings for the maximum attainable electron energy are found for arbitrary laser and plasma parameters by deriving a set of dimensionless equations for paraxial ultra-relativistic electron motion. One of our analytic predictions is the emergence of forbidden zones in the electrons' phase space. For an individual electron, these give rise to a threshold-type dependence of the final energy gain on the laser intensity. The universal scalings are also generalized to themore » resonant laser interaction with the third harmonic of betatron motion and to the case when the laser beam is circularly polarized.« less

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 γ-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,more » even if at an early stage of the acceleration it was flat. In conclusion, 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.« less