M. V. Chegotov

Charge-exchange-induced formation of hollow atoms in high-intensity laser-produced plasmas

For the first time registration of high-resolution soft x-ray emission and atomic data calculations of hollow-atom dielectronic satellite spectra of highly charged nitrogen have been performed. Double-electron charge-exchange processes from excited states are proposed for the formation of autoionizing levels in high-intensity laser-produced plasmas, when field-ionized ions penetrate into the residual gas. Good agreement is found between theory and experiment. Plasma spectroscopy with hollow ions is proposed and a temperature diagnostic for laser-produced plasmas in the long-lasting recombining regime is developed.

Generation of a wakefield during gas ionization

One-dimensional equations are derived that describe the hydrodynamic and electrodynamic properties of a plasma created through gas ionization by a short intense laser pulse. Different approaches (in particular, the particle-in-cell method) are used to show that, with ionization processes included, the excitation of a wakefield by an intense laser pulse can be described by the method of slowly varying amplitudes. It is shown that ionization processes enhance the wakefield excited by a moderate-intensity laser by about 10% in the case of a linearly polarized laser and by about 50% in the case of a circularly polarized laser. Ionization processes in light gases irradiated with high-intensity laser pulses have essentially no effect on the wakefield during the resonant excitation of a plasma wave by the ponderomotive force and play a governing role far from the resonance.

Wakefield generation by elliptically polarized femtosecond laser pulse in ionizing gases

Wakefield generation by a femtosecond laser pulse is described in the frame of the slowly varying amplitudes approximation. The amplitude of the wakefield A, is studied as a function of laser pulse and background gas parameters, and is compared with well-known results for preformed, completely ionized plasma A/sub p,i/. It is found that the ionization processes can increase A/sub p/ as compared to A/sub p,i/ at comparatively high laser peak intensities. It is shown that the increase of the wakefield amplitude due to gas ionization is more pronounced for circularly polarized laser pulses than for linearly polarized laser pulses. The strongest enhancement of A/sub p/ in comparison with A/sub p,i/ takes place for longer laser pulses with a duration in excess of the plasma wave period when the resonant conditions for ponderomotive excitation of the wakefield are not matched. Thus, ionization processes can expand the region of parameters for efficient generation of the laser wakefields.

Propagation of intense laser pulses through inhomogeneous ionizing gas profiles

By simultaneously and self-consistently solving Maxwells equations, the Ammosov-Delone-Krainov (ADK) field ionization equation, and the relativistic cold plasma equations, we have investigated the propagation of intense, ultrashort laser pulses through spatially inhomogeneous longitudinal gas gradients. Along with highly accurate calculations of the spatial and temporal beam profiles of the pulse at the end of various gradients, we have also determined simple scaling rules for the location of the vacuum-gas interface in order to minimize the pulse distortion at the focus. We show the benefits of using either preionized or low-Z gases, and we discuss the implications of this work for plasma-channel laser wakefield acceleration.

Wakefield Generation as the Mechanism behind Spectral Shift of a Short Laser Pulse

An equation describing the dynamics of plasma wave generation by a short intense laser pulse is analyzed to find a relation between the difference in mean-square pulse frequency before and after laser-matter interaction and the electric field amplitude in the wakefield plasma wave generated by the laser pulse. This relation can be effectively used in systems for wakefield diagnostics. The relation is applied to several geometries of interaction between a pulse and an ionizing gas or preformed plasma.

Ionization focusing of a short intense laser pulse and generation of wake plasma waves

The propagation of a short intense laser pulse is studied in a gas taking into account the ionization of gas atoms by the high-frequency electromagnetic field of the pulse. The conditions are found under which the ionization structures produced by the laser pulse cause the pulse focusing accompanied by a substantial increase in its intensity. It is shown that the leading edge of the pulse is subjected to ionization refraction at the ionization front, the temporal profile of the pulse becoming steeper. This results in the efficient generation of a wake wave at the ionization front, which is amplified during the development of self-modulation instability. The amplitude of the wake plasma wave achieves a substantial value already at small paths of the pulse in matter (smaller than the diffraction length of the pulse).

Residual Electron Momentum and Energy in a Gas Ionized by a Short High-Power Laser Pulse

abstractA study is made of the nonadiabatic dynamics of photoelectrons produced during interaction of an elliptically polarized, high-power laser pulse with a gas. Expressions for the so-called residual momentum and energy of the electrons (i.e., the mean electron momentum and energy after the passage of the pulse through the gas) are derived. The residual electron momentum and energy are investigated analytically as functions of the gas and laser parameters. A relationship is established between the residual energy and the electron temperature tensor.

Third-harmonic generation in an ionized gas and its relation to the residual energy of electrons

The generation of low-order harmonics by a short ionizing laser pulse passing through a gas is investigated in regard to the space-time phase-synchronism conditions. This investigation is based on the results of hydrodynamic calculations for the model of [1] supplemented by taking into account the ionization current [2–5] and on the numerical solution of a one-dimensional time-dependent Schrödinger equation. As applied to the description of the third-harmonic spectrum, the hydrodynamic model with ionization current is shown to be in good agreement with a quantum-mechanical model. In this case, the amplitude of the third harmonic is determined by the intensity of the laser field at the moment of maximal ionization rate; this fact allows one to relate the amplitude of the third harmonic to the residual energy of electrons [5–8] and may provide grounds for the diagnosis of the residual energy by the spectrum of the third harmonic, which is important for the development of X-ray lasers based on ionization nonequilibrium plasma.

Laser plasma source for highly charged ions

A production of large amount of ions with low temperature (less than 100 eV), low momentum spread and narrowed charge state distribution by petawatt-class short-pulse laser is under consideration. Residual ion energy as the least unavoidable ion energy after ionization of gases by a short intense laser pulse is calculated as a function of laser pulse parameters. Electron thermal energy coupling to the ions is estimated taking into account a multi-group structure of free electrons produced by optical field ionization.

Thomson scattering in a plasma created by a short intense laser pulse

Thomson scattering spectra from a plasma created through ionization of a gas consisting of multielectron atoms by a laser pulse with an intensity of about 1016 W/cm2 or higher and with a duration τimp≤100 fs are studied theoretically with allowance for electron groups with different temperatures.