Gennady Shvets

Stable Laser-Driven Proton Beam Acceleration from a Two-Ion-Species Ultrathin Foil

By using multidimensional particle-in-cell simulations, we present a new regime of stable proton beam acceleration which takes place when a two-ion-species shaped foil is illuminated by a circularly polarized laser pulse. In the simulations, the lighter protons are nearly instantaneously separated from the heavier carbon ions due to the charge-to-mass ratio difference. The heavy ion layer expands in space and acts to buffer the proton layer from the Rayleigh-Taylor-like (RT) instability that would have otherwise degraded the proton beam acceleration. A simple three-interface model is formulated to explain qualitatively the stable acceleration of the light ions. In the absence of the RT instability, the high quality monoenergetic proton bunch persists even after the laser-foil interaction ends.

Cherenkov radiation in a surface wave accelerator based on silicon carbide (SWABSiC)

We report on theoretical investigations of Cherenkov-type emission of surface phonon polaritons (SPhPs) by relativistic electron bunches. The polaritons are confined by a planar waveguide comprised of two SiC slabs separated by a vacuum gap. The SPhPs are generated in the reststrahlen band, where the dielectric permittivity of SiC is negative. Two surface modes are analyzed: the accelerating (symmetric) and the deflecting (anti-symmetric) wakes. Both form Cherenkov cones that exhibit rapid spatial oscillations and beats behind the moving charge. Moreover, the accelerating mode forms a reversed Cherenkov radiation cone due the negative group velocity for sufficiently small gaps. The wakefield acceleration of electron bunches inside the structure is also discussed, as well as our recent experimental progress in propagating the electron beam through the structure at the Advanced Test Facility (ATF) that resulted in > 12% beam transmission.

Direct laser acceleration of electrons in plasma bubbles or ion channels with and without a longitudinal wakefield

We investigate the motion of electrons in a plasma bubble (or an ion channel) under combined action of an oscillating laser field, quasistatic transverse wakefield, and longitudinal electric field. The longitudinal field E∥ significantly influences the broadband resonance between betatron oscillations of electrons and oscillations of the laser wave, which results in the profoundly different electron dynamics at different signs and magnitudes of the longitudinal force −eE∥. Specifically, we make a contrast between three representative cases: when this force is absent (−eE∥ = 0), when it accelerates electrons (−eE∥ > 0), and when it decelerates them (−eE∥ < 0). We estimate the electron energy gain at given laser-plasma parameters.

Single-shot visualization of evolving plasma wakefields

We measure the evolution history of terawatt-laser driven plasma wakefields in the nonlinear bubble regime using an all-optical frequency-domain streak camera (FDSC) technique. The longitudinal profiles of the plasma “bubble” at different propagation distances within a 3u2005mm long ionized helium gas target are imaged in single shots, illustrating formation, propagation and coalescence of the bubble. 3D particle-in-cell (PIC) simulations validate the observed density-dependent bubble evolution, and correlate it with generation of a quasi-monoenergetic ∼ 100u2005MeV electron beam. To visualize petawatt-laser- or e-beam driven plasma wakefields, FDSC is extended to multi-object-plane imaging (MOPI) to measure evolution of wakefield transverse profiles over acceleration distance up to ∼ 1u2005m in a single shot.

Excitation of accelerating plasma waves by counter-propagating laser beams

Generation of accelerating plasma waves using two counter-propagating laser beams is considered. Colliding-beam accelerator requires two laser pulses: the long pump and the short timing beam. We emphasize the similarities and differences between the conventional laser wakefield accelerator and the colliding-beam accelerator (CBA). The highly-nonlinear nature of the wake excitation is explained using both non-linear optics and plasma physics concepts. Two regimes of CBA are considered: (i) the short-pulse regime, where the timing beam is shorter than the plasma period, and (ii) parametric excitation regime, where the timing beam is longer than the plasma period. Possible future experiments are also outlined.

Excitation of Accelerating Plasma Waves by Counter-propagating Laser Beams

Generation of accelerating plasma waves using two counter-propagating laser beams is considered. Colliding-beam accelerator requires two laser pulses: the long pump and the short timing beam. We emphasize the similarities and differences between the conventional laser wakefield accelerator and the colliding-beam accelerator (CBA). The highly nonlinear nature of the wake excitation is explained using both nonlinear optics and plasma physics concepts. Two regimes of CBA are considered: (i) the short-pulse regime, where the timing beam is shorter than the plasma period, and (ii) the parametric excitation regime, where the timing beam is longer than the plasma period. Possible future experiments are also outlined.