Abstract
Two signatures of quantum effects on radiation reaction in the collision of a ${\sim}$GeV electron beam with a high intensity (${>}3\times 10^{20}~\text{W}~\text{cm}^{-2}$) laser pulse have been considered. We show that the decrease in the average energy of the electron beam may be used to measure the Gaunt factor $g$ for synchrotron emission. We derive an equation for the evolution of the variance in the energy of the electron beam in the quantum regime, i.e. quantum efficiency parameter $\unicode[STIX]{x1D702}\not \ll 1$. We show that the evolution of the variance may be used as a direct measure of the quantum stochasticity of the radiation reaction and determine the parameter regime where this is observable. For example, stochastic emission results in a 25 % increase in the standard deviation of the energy spectrum of a GeV electron beam, 1 fs after it collides with a laser pulse of intensity $10^{21}~\text{W}~\text{cm}^{-2}$. This effect should therefore be measurable using current high-intensity laser systems.
Highlights
Radiation reaction is the effective recoil force on an accelerating charged particle caused by the particle emitting electromagnetic radiation
This effect will play an important role in laser–matter interactions at the intensities set to be reached by generation high-intensity laser facilities ( 1023 W cm−2), where radiation reaction can lead to almost complete absorption of the laser pulse: Bashinov & Kim (2013) and Zhang, Ridgers & Thomas (2015), have shown that radiation reaction gives an imaginary part in the
Radiation reaction models we describe the radiation reaction models considered here: (i) classical – using the ultra-relativistic form of the Landau and Lifshitz prescription; (ii) modified classical – as the classical model but including a function describing the reduction in the power radiated due to quantum effects, the Gaunt factor g
Summary
Radiation reaction is the effective recoil force on an accelerating charged particle caused by the particle emitting electromagnetic radiation. The weak-field approximation allows us to assume that the rate of photon emission (and the energy spectrum of the emitted photons) is well described by the well-known rate in an equivalent set of constant fields as given in Ritus (1985) (for constant crossed electric and magnetic fields) and Erber (1966) (for a constant magnetic field) The accuracy of this quasi-classical approach has recently been demonstrated by comparison to full QED calculations for the electron energies and laser intensities considered here by Dinu et al (2016). Using this quasi-classical model (making the quasi-static and weak-field approximations), it is possible to include the quantum radiation reaction force in a kinetic equation describing the evolution of the electron distribution, as given by. This is reasonable in the moderately quantum regime described by Di Piazza et al (2010), i.e. where η ∼ 0.1
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