Generation of low-frequency nonlinear currents in plasma by an ultrashort pulse of high-frequency radiation
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Generation of Low-Frequency Nonlinear Currents in Plasma by an Ultrashort Pulse of High-Frequency Radiation V. E. Grishkov and S. A. Uryupin Lebedev Physical Institute, Russian Academy of Sciences, Leninskii pr. 53, Moscow, 119991 Russia National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Kashirskoe sh. 31, Moscow, 115409 Russia e-mail: [email protected] Received November 5, 2014; in final form, January 19, 2015
Abstract―A kinetic theory of low-frequency currents induced in plasma by an ultrashort high-frequency radiation pulse is developed. General expressions for the currents flowing along the propagation direction of the pulse and along the gradient of the field energy density are analyzed both analytically and numerically for pulse durations longer or shorter than or comparable with the electron collision time in plasma. It is demonstrated that the nonlinear current flowing along the gradient of the field energy density can be described correctly only when the modification of the isotropic part of the electron distribution function is taken into account. DOI: 10.1134/S1063780X15070028
1. INTRODUCTION The interaction of high-frequency electromagnetic radiation with conductive media is accompanied by the generation of nonlinear currents at both higher frequencies (multiple of the fundamental frequency of the incident radiation) and lower frequencies. Interest in studying low-frequency nonlinear currents is caused by the fact that such currents are sources of a quasi-stationary magnetic field and low-frequency radiation. The kinetic theory of generation of low-frequency nonlinear currents was developed in [1–3]. In those works, it was assumed that the amplitude of the high-frequency field varied on time scales much longer than the electron collision time in plasma. Such a theory fails to describe low-frequency nonlinear currents on time scales shorter than the electron collision time. At the same time, rapid progress in the development of methods of generation of ultrashort laser pulses with durations shorter than the electron collision time makes the problem of analyzing nonlinear currents on such short time scales quite challenging. Earlier, low-frequency nonlinear currents in plasma were analyzed using equations of collisionless hydrodynamics (see, e.g., [4, 5]). However, as will be shown below, such an approach not only leads to quantitative errors, but also does not allow one to reveal some important qualitative effects. A simplified approach based on the equation for the average electron velocity and taking into account electron collisions by introducing a given (but unknown) effective collision frequency [6, 7] also cannot be considered satisfactory.
The first attempt to construct the kinetic theory of nonlinear currents driven by an ultrashort laser pulse was made in [8], where a vortical nonlinear current acting as a source of a quasi-stationary magnetic field was analyzed. In the present work, using the approach proposed in [8], we construct a theory of g
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