Beyond the ponderomotive limit: Direct laser acceleration of relativistic electrons in sub-critical plasmas

Abstract

We examine a regime in which a linearly-polarized laser pulse with relativistic intensity irradiates a subcritical plasma for much longer than the characteristic electron response time. A steady-state channel is formed in the plasma in this case with quasi-static transverse and longitudinal electric fields. These relatively weak fields significantly alter the electron dynamics. The longitudinal electric field reduces the longitudinal dephasing between the electron and the wave, leading to an enhancement of the electron energy gain from the pulse. The energy gain in this regime is ultimately limited by the superluminosity of the wave fronts induced by the plasma in the channel. The transverse electric field alters the oscillations of the transverse electron velocity, allowing it to remain anti-parallel to laser electric field and leading to a significant energy gain. The energy enhancement is accompanied by development of significant oscillations perpendicular to the plane of the driven motion, making trajectories of energetic electrons three-dimensional. Proper electron injection into the laser beam can further boost the electron energy gain.

Investigation of laser pulse length and pre-plasma scale length impact on hot electron generation on OMEGA-EP

“…Another shortcoming found in the first experiment was that the BMXS has a very limited range in examining hot electrons. To examine super-ponderomotive electrons such as those seen in many previous simulations [11][12][13][14][15][16] the EPPS diagnostic was introduced. The electrons measured by the EPPS are those that have enough energy to escape the target, which supplements the BMXS perfectly.…”

“…The source of these electrons was investigated via 1 and 2D PIC simulations, which showed that an electrostatic potential well develops in the pre-plasma which traps electrons. Simulations performed by Robinson et al [11], Sorokovikova et al [12], Krygier [13] and Arefiev et al [14] show these electrons gain large amounts of energy from direct laser acceleration after being trapped and pre-accelerated inside the potential. Simulations performed by Paradkar et al [15] show that stochastic heating of trapped electrons in the potential well is also a source of these electrons.…”

“…These experiments showed that the electron spectrum can significantly change due to an underdense pre-plasma. The reported spectra have a pronounced two-component structure; the low energy component has a slope predicted by ponderomotive scaling, while additionally there is a hot electron tail with energies that can significantly exceed ( )  = +a 0.511 1 2 max 0 2 , the maximum energy of a free electron accelerated by a plane wave in a vacuum [14]. It is convenient to differentiate these two components of energetic electrons.…”

Radiation from laser-microplasma-waveguide interactions in the ultra-intense regime

“…Under such circumstance, the ponderomotive force arises due to the spatial variation of intensity of the EM wave that in general pushes the charged particles out the regime of high intensity and exert pressure on the target [33]. The electron acceleration due to ponderomotive force has been broadly referred to as the "direct laser acceleration" [34][35][36][37][38] [in the following we use the term "ponderomotive laser acceleration (PLA)" to avoid possible confusing]. In our case, the electron acceleration by the laser pulses inside the MPW takes place in presence of both of TM modes and ponderomotive force, apparently it benefits from both mechanisms.…”

Two-color hybrid laser wakefield and direct laser accelerator

“…Direct laser acceleration (DLA) [2][3][4][5] accelerates electrons through the direct interaction between laser electromagnetic field and electrons inside a plasma channel [2,6], in a nonlinear plasma wave [3], or inside a plasma bubble [5,7] created as a result of a complete ponderomotive blow-out of the plasma electrons from the laser's path [8,9]. The merger of the LWFA and DLA, referred henceforth as the laser wakefield and direct acceleration (LWDA) mechanism, is attracting considerable attention because of the possibilities for producing high energy electron beams for high energy physics, as well as copious high-energy X-rays [3,5,7,[10][11][12][13] for a variety of applications that require high-brightness photon beams.…”