Measurement of the Gaponov-Miller force produced in vacuum by tightly focused intense femtosecond laser radiation
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Measurement of the Gaponov–Miller Force Produced in Vacuum by Tightly Focused Intense Femtosecond Laser Radiation S. A. Aseyev*, B. N. Mironov, V. G. Minogin, S. V. Chekalin Institute of Spectroscopy, Russian Academy of Sciences, Troitsk, Moscow oblast, 142190 Russia *email: [email protected] Received November 16, 2010
Abstract—A method for measuring the Gaponov–Miller force (GMF) is demonstrated based on the deflec tion of a picosecond photoelectron beam exposed to tightly focused intense femtosecond laser radiation. It is shown experimentally that the action of this force produced by femtosecond laser pulses linearly depends on their intensity. The method can be used to verify the correctness of measuring the duration of an ultrashort electron bunch based on the GMF. DOI: 10.1134/S1063776111040133
1. INTRODUCTION The possibility of acting on the translational degrees of freedom of charged particles, in particular, free electrons, by means of a spatially inhomogeneous electromagnetic field was theoretically predicted by Gaponov and Miller back in the mid1950s [1]. This mechanism leads to the ejection of charge particles from the strong field region. When very intense laser radiation is used, the Gaponov–Miller force (GMF), or the pondermotive (gradient) force [2], which is defined as the spatial gradient of the pondermotive potential Up, can achieve quite high values, which can be used to efficiently control electron beams in vac uum. For example, for an intensity of tightly focused 0.8μm laser radiation at the center of a 2μm focal spot of 1015 W/cm2 (Up ≈ 60 eV), the GMF [F(grad) ≈ 10–11 N] will be approximately equal to the repulsion force between two electrons spaced 5 nm apart. Ultrashort laser radiation opens up fundamentally new possibilities for control and can be used to act on electrons propagating in vacuum on the femtosecond time scale [3]. This makes it possible to produce ultrashort photoelectron bunches [4] and measure their duration [5–7]. The temporal parameters of a pulsed electron beam are determined by scanning the delay time between laser radiation producing a photo electron bunch and a tightly focused laser beam changing the velocity distribution of photoelectrons under the GMF action [5–8]. This method surpasses a standard streak camera in its limiting characteristics and can be used for femtosecond electron beams [6]. Studies of the action of a spatially inhomogeneous electromagnetic field on free electrons were started soon after the article by Gaponov and Miller and were mainly devoted to the possibility of constructing traps by using microwave techniques [9]. Special attention was paid to investigations of the propagation of elec
trons with a certain kinetic energy through a ponder motive potential, which could be used, for example, to determine the value of Up [9, 10]. In [11], the scatter ing of photoelectrons by the pondermotive potential produced by an intense subnanosecond laser pulse was demonstrated for the first time. A pulsed photoelec tron b
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