S. A. Uryupin

Electron distribution functions in laser fields

A short review of the properties of electron distribution functions in a fully ionized plasma in the presence of high-frequency laser radiation is given. Weak and strong field situations are considered. In a weak field, when the amplitude of the electron quiver velocity in the field is smaller than the electron thermal velocity, the distributions of both the thermal and the under thermal electrons are considered. The conditions are shown when it is necessary to take into account the deviation of the electron distribution function from a Maxwellian. In a strong field the kinetics of the electron is strongly determined by the field intensity and the distribution function in the coordinate system oscillating with the radiation frequency is anisotropic under broad physical conditions, and may be approximated by a bi-Maxwellian distribution with two different transverse and longitudinal temperatures. Among the most peculiar features of the laser modified electron distribution functions, it is worth quoting the pathway of the anisotropy evolution. Depending on the laser field parameters, the distribution function in the initial stages of the laser-plasma interaction is either elongated or squeezed parallel to the field polarization. In the later stages it evolves towards isotropization, which is always approached from the elongated shape. Thus, an initially squeezed distribution function first evolves towards an elongated shape and subsequently towards isotropization. The physical origin and the consequences of the reported features are discussed. In addition, a number of physical processes for which the theory demands the use of the described non-equilibrium distribution functions are briefly addressed.

Harmonic generation and wave mixing in a plasma in the presence of two linearly polarized laser fields

A theoretical treatment of the high-order harmonic generation by electrons undergoing collisions with ions in a plasma in the presence of two single-mode linearly polarized fields is presented. Except for polarization, the two radiation fields may have arbitrary parameters. The treatment is based on an approximate solution of the Fokker–Planck equation for the plasma electron velocity distribution function. A set of representative numerical calculations is carried out. It is found that the generation of odd harmonics of each field is followed by the generation as well of satellites at mixed frequencies. For a given odd harmonic of one field, the satellites are shifted from it by multiple numbers of twice the frequency of the other field. When there are no coincident frequencies among the generated ones, the intensity of the odd harmonics of one field is found to decrease owing to the action of the other field. When, instead, the satellite frequencies of low-order harmonics coincide with the frequencies of high-order harmonics, a considerable enhancement of the latter is predicted. It is shown also that the harmonic intensity and the polarization direction depend significantly on the angle between the polarization directions of the two mixing fields.

Inverse bremsstrahlung in a plasma with electron temperature anisotropy

Inverse bremsstrahlung absorption of electromagnetic radiation in plasma with anisotropic two-temperature bi-Maxwellian electron distribution function over velocities is investigated. In the case of a weak field, absorption is more effective if the radiation field is polarized in the plane in which the plasma electrons have the smaller of the two temperatures. In the case when the distribution function is highly anisotropic, absorption changes strongly when the field polarization changes its direction with respect to the temperature anisotropy axis. In the intermediate domain, when the field is strong in directions not very close to that of the larger temperature, both absorption efficiency and degree of its anisotropy decrease. The conditions when the absorbed power practically does not depend on the field are established. Finally, in the case of a strong field, absorption decreases further while the degree of anisotropy is a weakly changing logarithmic function of effective electron temperatures.

Even harmonics generation of high frequency radiation in current-carrying plasmas

Generation of high frequency radiation harmonics in a current-carrying plasma is studied. The physical mechanism responsible for harmonics generation is provided by electron-ion collisions. The current in the plasma is sustained by a constant electric field. It is shown that the electron distribution function anisotropy due to the static field yields generation of even harmonics. As a result, the radiation spectrum emitted by the current-carrying plasma contains both even and odd harmonics, the latter being attributed to currentless plasma. For a broad range of plasma and high frequency radiation parameters, a detailed analysis of the even harmonics properties is reported.

Radiation absorption and reflection by a plasma with cold and hot electrons

Reflection and collisionless absorption of a test wave by a plasma, with a sharp boundary and containing a small amount of hot electrons, besides the bulk of more cold electrons, are investigated. It is established that, in the high-frequency skin-effect regime, the conditions are possible, when the absorption coefficient is basically determined by the hot electrons. On the contrary, in the transition to the anomalous skin-effect, the absorption by the cold electrons becomes dominant. In both the cases of high-frequency and anomalous skin-effect it is found that the reflected wave phase shift is determined by the cold electron bulk.

Fast plasma heating in the high-frequency skin-effect regime

Dense plasma heating by an ultrashort laser pulse is investigated in the regime of the high-frequency skin effect. The electron temperature evolution on the plasma surface and the properties of the classical heat transfer into the plasma interior are studied. Three possible plasma heating mechanisms are considered. At relatively not large temperatures the plasma heating, due to radiation absorption, is controlled by the electron–ion collisions in the skin layer. The others two pathways are related to high temperatures and take place in the regime of collisionless radiation absorption when the electrons are reflected by the plasma surface either specularly or diffusely. It is shown that, compared with the specular reflection, the diffuse one yields a larger increase of the electron temperature on the plasma surface and a faster propagation of the heat flux in the plasma interior, even when the fraction of diffusely reflected electrons is not high.

Magnetic field generation in a plasma in the presence of an ultrashort laser pulse

The description of the Weibel instability linear stage for a plasma interacting with an ultrashort laser pulse is given. Pulse durations both smaller and larger than the inverse collision frequency of the thermal electrons are considered. In the latter case the pulse duration is smaller than the thermal electron heating time. The growth rate of the instability is derived and the possibility of a considerable quasistationary magnetic field amplification demonstrated.

Electron-ion collision-induced harmonics generation in a plasma with an isotropic bi-Maxwellian distribution

A treatment is given of harmonics generation resulting from nonlinear inverse bremsstrahlung in a plasma with an anisotropic bi-Maxwellian electron velocity distribution function. A complete characterization of the process is reported. In particular, analytically and numerically we established how the efficiency of the odd harmonics generation and their polarization depend on such process parameters as: (1) the degree of effective temperature anisotropy, (2) the frequency and the intensity of the fundamental wave, and (3) the angle between the fundamental wave field direction and the symmetry axis of the electron distribution function.

Thresholds and spectra of SBS in plasmas with two species of ions

Abstract The thresholds of the SBS instability and the non-linear influence of the driving field on the SBS spectrum are studied for plasmas with two species of ions. The spectrum of those quasimodes, which define the frequency shift of the scattered radiation in the process of SBS, is studied.

Amplification of spontaneous magnetic fields due to inverse bremsstrahlung absorption of high frequency radiation

The theory of the linear stage of the Weibel instability growth due to anisotropic heating of electrons undergoing inverse bremsstrahlung absorption of high frequency electromagnetic radiation is reported. The possibility is shown of a significant amplification of spontaneous magnetic fields for both the cases of circularly and linearly polarized high frequency radiation. Numerical calculations and comparisons are reported for a wide range of the process parameters, covering the cases of weak and strong high-frequency radiation fields. Among others, if the linearly polarized field is strong, the magnetic field amplification for typical conditions is found to be less significant as compared to the case of strong circularly polarized field. The reverse result is found for the case of weak linearly and circularly polarized fields.