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We r e p o r t on measurements of quantum electrodynamic processes in an intense electromagnetic wave, where nonlinear e ects (both multiphoton and vacuum polarization) are prominent. Nonlinear Compton scattering and electronpositron pair production have been observed in collisions of 46.6 GeV and 49.1 GeV electrons of the Final Focus Test Beam at SLAC with terawatt pulses of 1053 nm and 527 nm wavelengths from a Nd:glass laser. Peak laser intensities of 0:5 10 18 W/cm 2 have been achieved, corresponding to a value of 0:4 for the parameter = eErms=m!0c, and to a value of 0:25 for the parameter e = E ? rms =Ecrit = eE ? rms h=m 2 c 3 , where E ? rms is the rms electric eld strength of the laser in the electron rest frame. We p r e s e n t data on the scattered electron spectra arising from nonlinear Compton scattering with up to four photons absorbed from the eld. A convolved spectrum of the forward high energy photons is also given. The observed positron production rate depends on the fth power of the laser intensity, as expected for a process where ve photons are absorbed from the eld. The positrons are interpreted as arising from the collision of a high-energy Compton scattered photon with the laser beam. The results are found to be in agreement with theoretical predictions.

A free electron can temporarily gain a very significant amount of energy if it is overrun by an intense electromagnetic wave. In principle, this process would permit large enhancements in the center-of-mass energy of electron-electron, electron-positron, and electron-photon interactions if these take place in the presence of an intense laser beam. Practical considerations severely limit the utility of this concept for contemporary lasers incident on relativistic electrons. A more accessible laboratory phenomenon is electron-positron production via an intense laser beam incident on a gas. Intense electromagnetic pulses of astrophysical origin can lead to very energetic photons via bremsstrahlung of temporarily accelerated electrons.PACS numbers: 03.65. Sq, 41.75.Fr, 52.40.Mj The prospect of acceleration of charged particles by intense plane electromagnetic waves has excited interest since the suggestion by Menzel and Salisbury [1] that this mechanism might provide an explanation for the origin of cosmic rays. However, it has generally been recognized that if a wave overtakes a free electron the latter gains energy from the wave only so long as the electron is still in the wave, and reverts to its initial energy once the wave has passed [2][3][4][5]. There is some controversy as to the case of a "short" pulse of radiation, for which modest net energy transfer between a wave and electron appears possible [6][7][8][9][10]. Acceleration via radiation pressure is negligible [11]. It has been remarked that even in the case of a "long" pulse, some of the energy transferred from the wave to the electron can be extracted if the electron undergoes a scattering process while still inside the wave [3,5]. This paper is an elaboration of that idea. We do not discuss here the observed phenomenon that an electron ionized from an atom in a strong wave can emerge from the wave with significant energy [12].We consider a plane electromagnetic wave (often called the background wave) with dimensionless, invariant field strengthHere the wave has laboratory frequency v 0 , reduced wavelength l -0 , root-mean-square electric field E rms , and 4-vector potential A m ; e and m are the charge and mass of the electron, and c is the speed of light. A practical realization of such a wave is a laser beam. Laser beams with parameter h close to one have been used in recent plasma physics experiments [9] and in highenergy physics experiments [13,14].When such a wave overtakes a free electron, the latter undergoes transverse oscillation (quiver motion), with relativistic velocities for h * 1 [2][3][4][5]15,16]. The v 3 B force then couples the transverse oscillation to a longitudinal drift in the direction of the wave. In the nonrelativistic limit, this effect is often said to be due to the "ponderomotive potential" associated with the envelope of the electromagnetic pulse [3]. The resulting temporary energy transfer to the longitudinal motion of the electron can in principle be arbitrarily large.A semiclassical description of this process exist...

We report on the collision of 1.5 ps (FWHM) laser pulses traversing at 17° a short 7 ps (FWHM) 46.6 GeV electron bunch. The phase-locked system used to maintain the correct timing of the laser pulses and the appropriate diagnostics are described. The jitter between the laser and electron pulses is determined from the stability of the observed rate of Compton scatters and can be described by a Gaussian distribution with ~j N 2.2 ps.