High-Frequency Quasi-Potential Waves in the Plasma Formed under Tunnel Ionization of Atoms
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MA DYNAMICS
High-Frequency Quasi-Potential Waves in the Plasma Formed under Tunnel Ionization of Atoms K. Yu. Vagina, *, T. V. Mamontovaa, b, and S. A. Uryupina, b, ** a
Lebedev Physical Institute, Russian Academy of Sciences, Moscow, 119991 Russia b National Research Nuclear University MEPhI, Moscow, 115409 Russia *e-mail: [email protected] **e-mail: [email protected] Received November 9, 2017; in final form, January 25, 2018
Abstract—The dispersion law and collisionless damping rate of quasi-potential waves in the plasma formed upon tunnel ionization of gas atoms in the field of a short pulse of circularly or linearly polarized radiation are found. It is shown how the frequency and damping rate of quasi-potential waves depend on the wave propagation direction relative to the symmetry axis of the photoelectron distribution. It is established that, in plasma with a toroidal photoelectron velocity distribution, weakly damped waves with a linear dispersion law and frequency above the electron plasma frequency can propagate in a wide range of angles. In the case of a bi-Maxwellian photoelectron distribution, the frequency of weakly damped waves is comparable with the electron plasma frequency and the anisotropy of electron motion manifests itself in relatively small corrections to the dispersion law. DOI: 10.1134/S1063780X18080093
1. INTRODUCTION A specific feature of plasmas formed upon ionization of gas atoms by a laser pulse is that the photoelectron velocity distribution differs strongly from Maxwellian (see, e.g., [1–5]). Therefore, the properties of photoionized plasmas, which depend on the shape of the electron distribution function, differ substantially from those previously established for thermodynamically equilibrium plasmas. In particular, due to the anisotropy of the photoelectron velocity distribution, photoionized plasma is unstable against the development of aperiodic instability [6–8], which makes it possible to amplify low-frequency probing pulses [9– 11]. The nonequilibrium photoelectron distribution leads to a change in the dispersion law and collisionless damping rate of surface waves [12]. It is clear that such changes in the properties of electron waves should also take place for bulk waves, as was described in the brief communication [13], devoted to the dispersion law and collisionless damping of high-frequency longitudinal waves propagating across the anisotropy axis of the toroidal photoelectron velocity distribution in plasma formed upon tunnel ionization of gas atoms by circularly polarized radiation. In [14], the dispersion relation for high-frequency bulk waves propagating in a similar plasma at an arbitrary angle to the anisotropy axis was found using a model description disregarding the spread in photoelectron velocities.
In the present work, which continues our previous studies [13, 14], we consider specific features of the dispersion law and collisionless damping of high-frequency quasi-longitudinal plasma waves propagating at an arbitrary angle to the anisotropy axis of t
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