Natural sources of intense ultra-high-frequency radiation in high-voltage atmospheric discharges.

We study the sources of intense ultra-high-frequency (UHF) radiation (in the frequency range 1-6 GHz) arising during the development of high-voltage atmospheric discharges. The discharges were initiated in a long discharge gap by applying an approximately 1-MV pulse with positive or negative polarity. By employing a radio registration system based on ultrawideband antennas, we managed to localize the UHF radiation sources in the discharge with centimeter accuracy and investigate their temporal and spatial correlation with the discharge structures. The vast majority of the localized sources turned out to be concentrated in the near-electrode regions. It is found that the generation mechanism of intense UHF radiation in a laboratory discharge cannot be unambiguously associated with such basic processes as the head-on collision of opposite-polarity streamers or the interaction of single streamers with the near-electrode plasma at the surface of metal electrodes. We discovered that the observed UHF emission appears basically as a precursor of the intense plasma development in a certain discharge region, whereinto a bright counterstreamer comes a bit later. The findings were confirmed by the statistical observations and results of imaging the dynamics of the discharge structures with a nanosecond temporal resolution.

Electromagnetic emissions in the MHz and GHz frequency ranges driven by the streamer formation processes.

We provide comprehensive data on the spectral and temporal characteristics of low-frequency (LF) (MHz) and high-frequency (HF) (GHz) radio emissions and investigate their correlation with the streamer formation. We show that the propagation of streamers from the cathode is accompanied only by the LF radio emission (10-150 MHz). In contrast, the HF radio emission (1-4 GHz) arises during the travel of counterstreamers from the anode, which is also indicated by radio interferometric measurements. The power of the LF radio emission sharply increases almost synchronously with that of the HF radio emission. We find that the HF radio emission has a complex spectral and temporal structure and appears as multiple short (less than 1 ns) bursts characterized by various frequency components, existing in subnanosecond time intervals.

Streamer formation processes trigger intense x-ray and high-frequency radio emissions in a high-voltage discharge.

For a laboratory discharge initiated in a long air gap by a microsecond megavolt pulse, we simultaneously register wideband high-frequency microwave and hard-x-ray emissions and thoroughly analyze the temporal relationship of the emissions depending on the discharge evolution. The temporal structure of microwave radiation is found to consist of numerous short intense bursts with high-frequency components. We directly show that x-ray and microwave emissions can appear almost synchronously in the discharge but only when a complex net of countless plasma channels forms and spans the entire discharge gap. The channel formation is closely related to the intense development of multiple streamers.

Dataset illustrating the construction of dislocations and running dislocation dynamics using the periodic array of dislocation quadrupoles in FCC Al.

The data illustrates the process of creation of dislocation quadrupoles in FCC Al. The density of dislocations in our data is 3.8 × 1013 m-2 which is the lowest published to date and is comparable to the density of dislocations in real crystals. The data set can be found at the following URL:https://data.mendeley.com/datasets/mvmychgk4j/1. The detailed research article is "A topologically correct method of dislocations construction for atomistic modeling" (K. Yu. Khromov et al., 2019).

Correlation of radio and gamma emissions in lightning initiation.

The results of simultaneous radio and gamma emission measurements during thunderstorms are presented. A gamma detector situated at the height 3840 m and two radio detectors of Tien-Shan Mountain Scientific Station (altitude 3340 m) registered intensive gamma flashes and radio pulses during the time of lightning initiation. The radio-gamma correlation grows abruptly at the initial moment (a few hundred microseconds), and the correlation coefficient reaches 0.9-0.95. The gamma-energy spectrum measured during lightning initiation is close to the characteristic spectrum of runaway breakdown. Radio pulses observed at the same time have highest amplitudes. Combined observation of gamma and radio emissions confirm the conception of lightning initiation due to multiple simultaneous electric discharges at hydrometeors stimulated and synchronized by low-energy electrons generated in the runaway breakdown process.

Observation of neutron bursts produced by laboratory high-voltage atmospheric discharge.

For the first time the emission of neutron bursts in the process of high-voltage discharge in air was observed. Experiments were carried out at an average electric field strength of ∼1 MV·m(-1) and discharge current of ∼10 kA. Two independent methods (CR-39 track detectors and plastic scintillation detectors) registered neutrons within the range from thermal energies up to energies above 10 MeV and with an average flux density of ≳10(6) cm(-2) per shot inside the discharge zone. Neutron generation occurs at the initial phase of the discharge and correlates with x-ray generation. The data obtained allow us to assume that during the discharge fast neutrons are mainly produced.

Strong flux of low-energy neutrons produced by thunderstorms.

We report here for the first time about the registration of an extraordinary high flux of low-energy neutrons generated during thunderstorms. The measured neutron count rate enhancements are directly connected with thunderstorm discharges. The low-energy neutron flux value obtained in our work is a challenge for the photonuclear channel of neutron generation in thunderstorm: the estimated value of the needed high-energy γ-ray flux is about 3 orders of magnitude higher than that one observed.

Dynamics on a torus.

The dynamics of atoms on the surface of a torus is considered. The simple illustration of motion with regard to rotating and fixed space gives a model of a four-dimensional (4D) torus. Two different schemes including rotation and shear in angular frame are used to take into account shears of the surface. In general, a variable-cell-shape molecular dynamics method analogous to the Parrinello-Raman one is developed. The six dynamical variables, the three radiuses and the three angles, specifying the deformations of the surface describe the cell dynamics. The new equations of motion contain no vectors of translations of the cell making its shape irrelevant for the structural and thermodynamical description of the system. The new method was tested on two problems concerning structure transformations of two-dimensional lattices.

Constant pressure molecular dynamics on a hypercylinder.

A Lagrangian formalism for variable-cell-shape molecular dynamics is derived from first principles. It is based on consideration of a crystal as arranged on the surface of hypercylinder in an extended coordinate frame. The artificial curvature along the additional degrees of freedom upsets the balance of forces acting on every atom in a periodically repeating cell. Since the distance between atoms is not the metric tensor the proposed method provides an essential simplification of the equations of motion compared to those of Parinello and Rahman approach. The Lagrangian of the system eliminates the cell orientation from the dynamics, thus avoiding symmetry-breaking effects and physically irrelevant cell rotation. The change from a fully flexible cell to an isotropically flexible cell is realized within the same computational framework. Simulations for the transformation of a model He lattice under isotropic applied pressure are used to illustrate the application of this method.