Electron acceleration via collisions with ions in plasma under the action of a relativistically strong laser field
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Electron Acceleration via Collisions with Ions in Plasma under the Action of a Relativistically Strong Laser Field A. A. Balakin and G. M. Fraiman Institute of Applied Physics, Russian Academy of Sciences, Nizhni Novgorod, 603950 Russia e-mail: [email protected] Received April 12, 2006
Abstract—Electron–ion collisions in plasma in a strong electromagnetic field are considered in the ultrarelativistic limit (in which the vector potential A is such that a = eA/mc2 1). Expressions relating the electron drift coordinates and momentum to those in the laboratory frame are obtained using exact canonical transformations with allowance for adiabatic effects. The appearance of ultrafast particles with a maximum energy proportional to the third power of the laser pulse vector potential is predicted. Expressions for the energy (and number) distribution function of such high-energy (hot) electrons appearing as a result of electron–ion collisions are obtained. These distribution functions obey a power law, which agrees with the results recently obtained by Mangles et al. [1] in experiments with a petawatt laser. PACS numbers: 45.20.Jj, 52.20.Dq, 52.27.Ny, 52.35.Mw DOI: 10.1134/S1063776106090068
1. INTRODUCTION As is known, electron–ion collisions with ions in strong electromagnetic fields on a nonrelativistic intensity level give rise to a number of interesting phenomena such as inverse bremsstrahlung heating [2, 3], harmonic generation [4], and high-energy particle production [5]. The passage to relativistic intensities of the electromagnetic field leads to rather unexpected modification of these phenomena. In particular, as will be shown below, electron–ion collisions in ultrarelativistic fields can lead to the appearance of ultrafast particles 3 (hot electrons) with energies up to p osc /m2c, where posc = –eA/c is the oscillatory momentum of a particle and A is the vector potential of the electromagnetic wave. It should be noted that the maximum energy of such hot particles grows even faster than that in the nons relativistic case, where it is limited by the 2 p osc /m value. Evidence for the appearance of ultrafast particles is probably provided by experiments [1, 6], where electrons with energies up to several gigaelectronvolts have been detected in plasmas interacting with laser radiation at intensities at a level of 1020 W/cm2. Traditionally, the appearance of hot electrons in such experiments is explained within the framework of collisionless models, where the acceleration is provided by excited plasma waves [1, 6, 7]. The aim of this study was to evaluate the efficiency of hot electron generation in transparent plasma due to electron–ion collisions in the field of a relativistically strong laser pulse. The analysis will be performed within the framework of the classical equations of
motion with neglect of radiative losses. The paper consists of two parts. In the first part, the problem of instantaneous (collision-induced) electron injection into the field of a smooth wave packet is considered
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