Realising single-shot measurements of quantum radiation reaction in high-intensity lasers

C D Baird,C D Murphy,M Marklund,C P Ridgers,S P D Mangles,A Ilderton,T G Blackburn

doi:10.1088/1367-2630/ab1baf

Abstract

Modern laser technology is now sufficiently advanced that collisions between high-intensity laser pulses and laser-wakefield-accelerated (LWFA) electron beams can reach the strong-field regime, so that it is possible to measure the transition between the classical and quantum regimes of light–matter interactions. However, the energy spectrum of LWFA electron beams can fluctuate significantly from shot to shot, making it difficult to clearly discern quantum effects in radiation reaction (RR), for example. Here we show how this can be accomplished in only a single laser shot. A millimetre-scale pre-collision drift allows the electron beam to expand to a size larger than the laser focal spot and develop a correlation between transverse position and angular divergence. In contrast to previous studies, this means that a measurement of the beam’s energy-divergence spectrum automatically distinguishes components of the beam that hit or miss the laser focal spot and therefore do and do not experience RR.

Highlights

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It can be seen that the central region of the electron bunch, i.e. where the bunch overlaps with the laser pulse, has experienced radiation reaction, resulting in a long tail of low energy electrons
We found that the optimum drift distance was dependent on the angular divergence of the electron beam and that a drift of 5-10 mm was optimal; if the beam drifts too far it becomes too large such that the counter-propagating laser interacts with only a small fraction of the electrons and the signal-to-noise at the detector becomes too low

Summary

QED-PIC Simulations ce

We model quantum effects, including RR, using the now-standard approach based on the ‘locally constant field approximation’. If the drift length is long enough, this correlation dominates over the initial spread of position This drift was incorporated into the simulations by first initialising, and redistributing the electrons by extrapolating their starting positions based on the divergence angle (neglecting space charge effects), i.e. xf = xi + d(px /pz ), where d is drift. With these initial conditions, we reach ψ ' 1, corresponding to the radiation dominated regime [49]. It can be seen that the central region of the electron bunch, i.e. where the bunch overlaps with the laser pulse, has experienced radiation reaction, resulting in a long tail of low energy electrons. We will extend this work by exploring the possibility of direct experimental measurement of this region

Experimental Constraints

Optimum drift distance ce

Findings

Discussion ce

Conclusions