Igor A. Kotelnikov

On the role of photoionization in generation of terahertz radiation in the plasma of optical breakdown

The effect of multiphoton and tunnel ionization on the generation of terahertz radiation for optical breakdown in the focus of femtosecond bichromatic laser pulses is discussed.

Diffraction of a surface wave on a conducting rectangular wedge

Diffraction of a surface wave on a rectangular wedge with impedance faces is studied using the Sommerfeld-Malyuzhinets technique. An analog of Landaus bypass rule in the theory of plasma waves is introduced for selection of a correct branch of the Sommerfeld integral, and the exact solution is given in terms of imaginary error function. The formula derived is valid both in the near-field and far-wave zones. It is shown that a diffracted surface wave is completely scattered into freely propagating electromagnetic waves and neither reflected nor transmitted surface waves are generated in case of bare metals which have positive real part of surface impedance. The scattered waves propagate predominantly at a grazing angle along the direction of propagation of the incident surface wave and mainly in the upper hemisphere regarding the wedge face. The profile of radiated intensity is nonmonotonic and does not resemble the surface wave profile which exponentially evanesces with the distance from the wedge face. Comparison with experiments carried out in the terahertz spectral range at Novosibirsk free electron laser has shown a good agreement of the theory and the experiments.

l=1 diocotron instability of single charged plasmas

The linear stability analysis of the l=1 diocotron perturbations in a low density single charged plasma confined in a cylindrical Penning trap is critically revisited. Particular attention is devoted to the instability due to the presence of one or more stationary points in the radial profile of the azimuthal rotation frequency. The asymptotic analysis of Smith and Rosenbluth for the case of a single-bounded plasma column (algebraic instability proportional to t1/2) is generalized in a few respects. In particular, the existence of unperturbed density profiles that give rise to l=1 algebraic instabilities growing with time proportionally to t1−1/m, m≥ 3 being the order of a stationary point in the rotation frequency profile, and even proportionally to t, is pointed out. It is also shown that smoothing the density jumps of a multistep density profile can convert algebraically growing perturbations into exponentially decaying modes. The relevant damping rates are computed. The asymptotic analysis (t → ∞) of the fundamental diocotron perturbations is then generalized to the case of a cylindrical Penning trap with an additional coaxial inner conductor. It is shown that the algebraic instability found in the case of a single-bounded plasma column becomes exponential at longer times. The relevant linear growth rate is computed by a suitable inverse Laplace transform (contour integral in the complex plane). In the particular case of an uncharged inner conductor of radius a, the growth rate is shown to scale as a4/3 for a → 0. The theoretical results are compared with the numerical solution of the linearized two-dimensional drift Poisson equations.

Thermodynamic equilibrium of hollow non-neutral plasmas

New annular confinement configurations of one component plasmas, corresponding to global thermal equilibria in a cylindrical Penning–Malmberg trap with an axial conductor, are investigated both numerically and analytically. In the case of infinite length plasma, analytical solutions are calculated explicitly in the limit of small Debye length. In the case of finite length plasma, the self-consistent solution of the thermal equilibrium Poisson’s equation is obtained numerically, and the dependence of the density distribution on the different parameters of the system is illustrated.

Impact of the dipole contribution on the terahertz emission of air-based plasma induced by tightly focused femtosecond laser pulses

The present paper studies the generation mechanism of terahertz (THz) radiation from tightly focused femtosecond laser pulses in a gas medium. We measured the angular radiation pattern under different focusing conditions and observed that, with the deepening of focus, the angular radiation pattern changes and optical-to-THz conversion efficiency increases. The analysis of the observed phenomena led to the assumption that the dipole radiation prevails in most cases despite the existing conception regarding the dominating role of the quadrupole mechanism of radiation. Based on these assumptions, the transient photocurrent theory of the phenomenon presented in this paper was developed by us and used for the numerical fit of the experimental data.

Interaction of High-Intensity Femtosecond Radiation With Gas Cluster Beam: Effect of Pulse Duration on Joint Terahertz and X-Ray Emission

This paper studies the phenomenon of joint generation of terahertz (THz) and X-ray radiation in the argon nanocluster jet under the action of high-power femtosecond laser pulse in both the single-color and dual-color regimes. It was discovered that in a gas cluster beam the pulse duration affects the properties of THz and X-ray emission differently. For the same given total energy of optical pulse in the dual-color excitation regime of cluster medium, more than a five times increase of THz radiation power was observed in comparison with the single-color regime, while the conversion efficiency to the argon X-ray K-line reached 7 × 10-6 and remained unchanged. The possibility of separation of contributions of different beam components into the THz signal was demonstrated experimentally, using contributions from clusters and nonclustered gas as an example. We suggest an interpretation of experimental results based on a theoretical model of cluster ionization that self-consistently predicts the level and dynamics of ionization and electron temperature in the clusters.

Formation of an ion beam in an elementary cell with inhomogeneous emission current density.

A well-known Pierce solution that allows focusing a beam of charged particles using properly shaped electrodes outside the beam aperture is generalized to the case of an accelerating system with inhomogeneous emission current density. It is shown that the defocusing effect of the space charge can, in principle, be evenly compensated over the entire cross section of the beam. In contrast to the beam with a uniform emission current density, both the electric potential and the transverse electric field must be controlled along the beam boundary in order to eliminate the angular divergence. However, eliminating the angular spread evenly across the beam constitutes a mathematically ill-posed problem which needs to be solved with the use of one or another method of regularization. An alternative way of diminishing beam emittance is proposed for the beam where the emission current is uniform across the entire aperture except for a narrow beam edge layer and a simple formula for the Pierce electrodes is derived. Numerical simulation has proved the reasonable accuracy of our analytical theory.

Thermodynamic equilibrium of pure electron plasmas in a Malmberg-Penning trap

A new class of annular confinement configurations of a single-charged plasma corresponding to global thermodynamic equilibria in a cylindrical Malmberg-Penning trap with an axial conductor is investigated both numerically and analytically. In the case of an infinite plasma length, the density turns out to be essentially constant inside a surface of revolution and to fall off abruptly outside of it. Analytical limiting cases are calculated explicitly in the limit of small Debye lengths. In the case of a finite plasma length, the self-consistent solution to Poissons equation describing thermodynamic equilibrium is obtained numerically and the dependence of the plasma density distribution on the various parameters of the system is investigated.

On the density limit in the helicon plasma sources

Existence of the density limit in the helicon plasma sources is critically revisited. The low- and high-frequency regimes of a helicon plasma source operation are distinguished. In the low-frequency regime with

Diocotron instability in non-neutral plasmas with a stationary point in the rotation frequency profile

\omega \sqrt{\omega_{ci}\omega_{ce}}