Low-frequency transition radiation from a short laser pulse at the plasma boundary
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Low-Frequency Transition Radiation from a Short Laser Pulse at the Plasma Boundary L. M. Gorbunova and A. A. Frolovb a Lebedev
Institute of Physics, Russian Academy of Sciences, Leninskii pr. 53, Moscow, 119991 Russia b Institute for High Energy Densities, Russian Academy of Sciences, ul. Izhorskaya 13/19, Moscow, 127412 Russia e-mail: [email protected] Received January 27, 2006
Abstract—The transition radiation that appears when a short laser pulse crosses the vacuum–plasma boundary and that is attributable to an vortex electric current produced at the plasma boundary by averaged ponderomotive forces has been considered for the first time. The spectral, angular, and spatiotemporal characteristics of the transition radiation in a vacuum are analyzed. It is shown that under typical conditions of present-day laser plasma experiments, the frequency of the transition radiation lies in the terahertz range and its power can reach several megawatts. PACS numbers: 52.25.Os, 52.38.–r DOI: 10.1134/S1063776106060033
1. INTRODUCTION As was first shown in [1], when a uniformly and rectilinearly moving charged particle crosses the boundary between two material media, electromagnetic radiation called transition radiation appears (see [2] for more detail). It was pointed out in [2] and [3] that, apart from charged particles, the transition radiation can be generated by electric and magnetic dipoles, light pulses, charged particle bunches, and other sources capable of polarizing the medium. The transition radiation that appears when an intense short laser pulse is focused on a jet of neutral gas has recently been detected and theoretically interpreted in [4–6]. The physical mechanism behind the generation of transition radiation under the experimental conditions [4, 6] lies in the fact that a short intense laser pulse ionizes the gas and excites a plasma wake in the forming plasma that traps and accelerates plasma electrons and forms compact bunches from them. When these electron bunches exit the plasma, the transition radiation is generated at the plasma–vacuum boundary. In this paper, we consider another generation mechanism of low-frequency transition radiation when a short laser pulse crosses the plasma boundary. In contrast to [4–6], where the transition radiation was produced by a charged particle bunch, in our case, the generation of electromagnetic waves is attributable to an vortex electric current excited at the plasma boundary by the ponderomotive action of a laser pulse. We analyze the spectral, angular, and spatiotemporal characteristics of the transition radiation and show that they depend significantly on the laser pulse duration. When a laser pulse with a duration shorter than or on the order
of the plasma oscillation period interacts with a semibounded plasma, the transition radiation has a frequency close to the plasma frequency and lasts for a time considerably longer than the pulse–plasma boundary interaction time. If the laser pulse duration exceeds the plasma oscillation period, then the tr
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