Ponderomotive and resonant effects in the acceleration of particles by electromagnetic modes

Abstract

In the present analysis we study the dynamics of charged particles under the action of slowly modulated electromagnetic carrier waves. With the use of a high-frequency laser mode along with a modulated static magnetic wiggler, we show that the ensuing total field effectively acts as a slowly modulated high-frequency beat-wave field typical of inverse free-electron laser schemes. This effective resulting field is capable of accelerating particles in much the same way as space-charge wake fields do in plasma accelerators, with the advantage of being more stable than plasma related methods. Acceleration occurs as particles transition from ponderomotive to resonant regimes, so we develop the ponderomotive formalism needed to examine this problem. The ponderomotive formalism includes terms that, although not discussed in the usual applications of the approximation, are nevertheless of crucial importance in the vicinity of resonant capture. The role of these terms is also briefly discussed in the context of generic laser-plasma interactions.

Non-resonant acceleration of charged particles driven by the associated effects of the radiation reaction

“…We reiterate that even though our relativistic model is purely electrostatic, as far as the particle dynamics parallel to the wavevector axis is concerned, the underlying physics is similar to that of a particle submitted to the combined action of collinear electromagnetic and wiggler fields, as discussed in Almansa et al. (2019). We therefore expect that the present simplified model can be of help in understanding the most relevant properties of the acceleration mechanism resulting from RR forces.…”

“…2010; Ruiz & Dodin 2017; Almansa et al. 2019). When the carrier's phase velocity is lower than the speed of light, the full mechanism involves an initial ponderomotive step that accelerates low-velocity particles up to the carrier's phase velocity, at which point these particles are resonantly trapped in the wave's troughs and resonantly accelerated to much larger energies than the initial energy (Tajima & Dawson 1979; Shukla et al.…”

“…Wave–particle resonance usually needs some adjustment or manipulation of the carrier phase velocity so that it can be made slower than the speed of light and, therefore, reachable by the particles (Almansa et al. 2019). To avoid technical difficulties with the speed reduction, it is thus of relevance to investigate the acceleration when one works with as few as possible processing procedures applied on the wave mode.…”

“…Acceleration of charged particles by localized packets of high-frequency electromagnetic carrier waves has been the subject of investigation in a number of recent works (Mulser & Bauer 2010;Smorenburg et al 2010;Ruiz & Dodin 2017;Almansa et al 2019). When the carrier's phase velocity is lower than the speed of light, the full mechanism involves an initial ponderomotive step that accelerates low-velocity particles up to the carrier's phase velocity, at which point these particles are resonantly trapped in the wave's troughs and resonantly accelerated to much larger energies than the initial energy (Tajima & Dawson 1979;Shukla et al 1986;Bingham 2003;Esarey, Schroeder & Leemans 2009;Zhang, Khudik & Shvets 2015).…”

“…In the present work, we examine the case of purely electrical waves moving at the speed of light. The model is simple enough to illustrate the method of analysis and generic enough to represent not only acceleration by electrostatic waves themselves, but also acceleration by the ponderomotive beat wave formed by collinear laser modes for instance (Almansa et al 2019).…”