Efficient bright γ-ray vortex emission from a laser-illuminated light-fan-in-channel target

Abstract

X/γ-rays have many potential applications in laboratory astrophysics and particle physics. Although several methods have been proposed for generating electron, positron, and X/γ-photon beams with angular momentum (AM), the generation of ultra-intense brilliant γ-rays is still challenging. Here, we present an all-optical scheme to generate a high-energy γ-photon beam with large beam angular momentum (BAM), small divergence, and high brilliance. In the first stage, a circularly polarized laser pulse with intensity of 1022 W/cm2 irradiates a micro-channel target, drags out electrons from the channel wall, and accelerates them to high energies via the longitudinal electric fields. During the process, the laser transfers its spin angular momentum (SAM) to the electrons’ orbital angular momentum (OAM). In the second stage, the drive pulse is reflected by the attached fan-foil and a vortex laser pulse is thus formed. In the third stage, the energetic electrons collide head-on with the reflected vortex pulse and transfer their AM to the γ-photons via nonlinear Compton scattering. Three-dimensional particle-in-cell simulations show that the peak brilliance of the γ-ray beam is photons·s–1·mm–2·mrad–2 per 0.1% bandwidth at 1 MeV with a peak instantaneous power of 25 TW and averaged BAM of /photon. The AM conversion efficiency from laser to the γ-photons is unprecedentedly 0.67%.

Promises and challenges of high-energy vortex states collisions

“…An ingenious, all-optical scheme based on non-linear Compton backscattering was recently proposed in [182]. An extremely intense circularly polarized laser pulse irradiates a micro-channel target, drags out electrons from the channel wall, accelerates them to high energies (hundreds of MeV) and, at the same time, transfers its spin angular momentum to this electron beam.…”

“…Upon reflection, this intense vortex light pulse collides with the energetic electron pulse and gets upscattered into the MeV energy range, taking a very large OAM from the electrons. The very detailed numerical study reported in [182] was based on the 3D particle-in-cell simulations, with QED effects computed via a Monte Carlo algorithm, which included radiation reaction and the feedback between the plasma and photon emission. The parameters of the pulse were chosen to ensure a moderately non-linear regime for the Compton scattering, with ξ = 0.55.…”

“…Although the resulting gamma beam, as a whole, possesses a very large OAM which can exert significant torque on a target, it is not clear whether individual photons can be considered in a vortex state. The authors of [182] carefully label the resulting AM as the beam angular momentum and do not discuss the issue of coherence or the properties of individual gamma photons.…”

“…In Section 4.3 we also described schemes [179,180,182] based on non-linear Compton backscattering to generate gamma beams carrying large OAM, with the average value of the OAM up to 10 6 per photon [182]. However since these simulations are intrinsically semiclassical, it is not clear whether individual photons with such a large value of OAM can exhibit a phase vortex.…”

“…Although specific processes for generation of vortex states via scattering have been proposed -such as Compton scattering on electrons [32,33,170,176,177,179,180,182] and on relativistic partially stripped heavy ions [164,165,167],…”