Experimental Evidence of Radiation Reaction in the Collision of a High-Intensity Laser Pulse with a Laser-Wakefield Accelerated Electron Beam

The dynamics of energetic particles in strong electromagnetic fields can be heavily influenced by the energy loss arising from the emission of radiation during acceleration, known as radiation reaction. When interacting with a high-energy electron beam, today’s lasers are sufficiently intense to explore the transition between the classical and quantum radiation reaction regimes. We present evidence of radiation reaction in the collision of an ultrarelativistic electron beam generated by laser-wakefield acceleration ($ϵ>500\mathrm{MeV}$) with an intense laser pulse ($a_0>10$). We measure an energy loss in the postcollision electron spectrum that is correlated with the detected signal of hard photons ($\gamma$ rays), consistent with a quantum description of radiation reaction. The generated $\gamma$ rays have the highest energies yet reported from an all-optical inverse Compton scattering scheme, with critical energy ${ϵ}_{\mathrm{crit}}>30\mathrm{MeV}$.

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Popular Summary

Whenever charged particles accelerate, they radiate photons and, therefore, must lose some energy as they move. This effective decelerating force, called radiation reaction, is normally very weak but can dominate the dynamics in extreme astrophysical environments and in the laser-plasma physics that will be explored at next-generation laser facilities. Though several models exist to describe the particle motion and photon emission in this regime, radiation reaction has not yet been directly observed in an intense electromagnetic wave. We have performed an experiment designed to amplify the effects of radiation reaction to a measurable level, and we conclude that radiation reaction is required to explain the results we observe.

The strongest electric fields produced in the laboratory are found at the focus of intense laser pulses, where the field strength can reach $10^{14}\mathrm{V}{\mathrm{m}}^{-1}$. By colliding an energetic electron beam with an intense laser pulse, the radiation reaction force on the electrons becomes sufficiently strong that they lose a significant fraction of their energy. This energy is radiated as a bright, collimated gamma-ray beam. Because the most intense laser pulses are very short, we use the laser-wakefield acceleration technique (which accelerates electrons using plasma waves driven by laser pulses) to generate a high-energy, short, synchronized electron beam. By measuring the electron energy after the collision and the spectrum of the emitted gamma rays, we find that these two measurements are only consistent if the electron beam has experienced significant radiation reaction, losing approximately 15% of its energy. This very thin $10\text{−}\mu \mathrm{m}$ sheet of laser light is as effective at slowing down the electron beam as a sheet of lead that is 100 times as thick.

In the future, we will be able to extend this technique to constrain different models of radiation reaction in extreme fields with much greater precision.

Figure 1

Schematic of the experimental setup. All components are inside a vacuum chamber except for the CsI array.

Figure 2

(a) Electron spectrometer screen image, transformed onto a linear energy axis. (b) Angularly integrated electron spectrum. (c) Raw image of CsI crystal stack detector. (d) Integrated CsI signal as a function of penetration depth into the stack.

Figure 3

The total CsI signal as a function of the integrated squared energy of the electron spectrum, and the linear fit to the “beam-off” shots. The shaded area represents the 68% confidence interval (CI) for the linear fit.

Figure 4

Consecutive electron spectra. Each spectrum has been normalized to its own maximum.

Figure 5

The background-subtracted CsI signal as a function of the energy of the edge in the electron spectrum. The histogram on the top shows the distribution of electron beam energies recorded during a separate sequence of shots where no radiation reaction signal could occur. Overlaid is a kernel density estimate (KDE) to the distribution of electron beam energies.

Figure 6

CsI spectrometer measurements recorded for a successful collision and “beam-off” shots, and associated best-fit response functions from the Monte Carlo model of the detector. The maximum likelihood estimate of the fit parameter ${ϵ}_{\mathrm{crit}}$ is displayed next to each fit.

Figure 7

The spectrum of ICS photons radiated by a single electron as simulated for various ${ϵ}_{\text{initial}}$ and $a_0$ for the three radiation reaction models described in the text.

Figure 8

Simulated retrievals of ${ϵ}_{\mathrm{crit}}$ assuming the collision of a plane wave of given peak $a_0$ with the experimentally measured electron spectra. The shaded regions represent $\pm 1\sigma$ variations arising from the measured electron beam spectral fluctuations.

Figure 9

Experimentally measured ${ϵ}_{\mathrm{crit}}$ as a function of ${ϵ}_{\text{final}}$ measured at the electron spectral feature (points). The shaded areas correspond to the results a hypothetical ensemble of identical experiments would measure 68% of the time under different assumed radiation reaction models for a uniform distribution of $a_0$ between 4 and 20.