Igor S. Samoylov

Molecular dynamics simulation of the electrochemical interface between a graphite surface and the ionic liquid [BMIM][PF6].

The structure of the electrical double layer in the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) near a basal plane of graphite was investigated by molecular dynamics simulation. The calculations were performed both for an uncharged graphite surface and for positively and negatively charged ones. It is found that near an uncharged surface the ionic liquid structure differs from its bulk structure and represents a well-ordered region, extending over approximately 20 A from the surface. Three dense layers of ca 5 A thick are clearly observed at the interface, composed of negative ions and positively charged rings. It is established that in the first adsorption layer the imidazolium ring in the [BMIM]+ cation tends to be arranged in parallel to the graphite surface at a distance of 3.5 A. The [PF6]- anion is oriented in such a way that the phosphorus atom is at a distance of 4.1 A from the surface and triplets of fluorine atoms form two planes parallel to the graphite surface. Ions adsorbed at the uncharged surface are arranged in a highly defective 2D hexagonal lattice and the corresponding lattice spacing is approximately four times larger than that of the graphene substrate. The influence of the electrode potential on the distribution of electrolyte ions and their orientation has also been investigated. Increase in the electrode potential induces broadening of the angle distribution of adsorbed rings and a shift of the most probable tilt angle towards bigger values. It was shown that there are no adsorbed anions on the negatively charged surface (sigma = -8.2 microC cm(-2)), but the surface concentration of adsorbed cations on the positively charged surface (sigma = +8.2 microC cm(-2)) has a nonzero value. In addition, the influence of the surface charge (+/- sigma) on the volume charge density and electric potential profiles in an electrolyte was studied. The differences in the cation and anion structure result in the fact that the integral capacitance of the electrical double layer depends on the electrode polarity and equals C = 4.6 microF cm(-2) at sigma = -8.2 microC cm(-2) and C = 3.7 microF cm(-2) at sigma = +8.2 microC cm(-2).

On the Nature of Copper Cathode Erosion in Negative Corona Discharge

Erosion of the copper cathode in Trichel-pulse negative corona discharge was investigated in the atmospheric pressure air in point-to-plane electrode configuration. Nanometer-size craters were found on the cathode surface as a result of discharge treatment. Therefore, the current density on the cathode surface was estimated to be around 108 A/cm2. The value of the integral of specific current action of Trichel pulse argues in favor of electroexplosive mechanism of cathode erosion in Trichel-pulse negative corona.

Diffuse Vacuum Arc on the Nonthermionic Lead Cathode

Experimental study of the diffuse vacuum arc discharge on the nonthermionic lead cathode is presented. At the working cathode temperatures of 1.2-1.6-kK current-voltage characteristic of the discharge, cathode heat operation regime and its erosion rate were measured. Using probe methods, electron temperature (0.3-1.2 eV) and heavy particles (atoms and ions) mean charge (0.17-0.28 e) were determined in after-anode plasma. Evaluated current densities on the cathode are in agreement with a hypothesis on the ion mechanism of charge transfer on its surface. Parameters of the obtained discharge have been compared with the characteristics of the earlier investigated diffuse vacuum arcs on nonthermionic chromium and on thermionic gadolinium cathodes. The obtained data might be useful when creating plasma sources for verifying the plasma separation method of the spent nuclear fuel using the nonradioactive substances.

Melt formation on the graphite electrode surface in a diffuse arc discharge

We present the results of investigation of a discharge with a current of 15–200 A between graphite electrodes at argon pressure of 10–90 kPa after long-term specimen heating by the DC at a temperature of about 3 kK. The change in the electrode shape is evidence of the existence of a melt film on the electrode surfaces at the temperature of 3.3–3.5 kK. The anode vaporization velocity is in agreement with the data on the pressure of the saturated vapor of the atomic carbon.

Study of the feasibility of distributed cathodic arc as a plasma source for development of the technology for plasma separation of SNF and radioactive wastes

One of the key problems in the development of plasma separation technology is designing a plasma source which uses condensed spent nuclear fuel (SNF) or nuclear wastes as a raw material. This paper covers the experimental study of the evaporation and ionization of model materials (gadolinium, niobium oxide, and titanium oxide). For these purposes, a vacuum arc with a heated cathode on the studied material was initiated and its parameters in different regimes were studied. During the experiment, the cathode temperature, arc current, arc voltage, and plasma radiation spectra were measured, and also probe measurements were carried out. It was found that the increase in the cathode heating power leads to the decrease in the arc voltage (to 3 V). This fact makes it possible to reduce the electron energy and achieve singly ionized plasma with a high degree of ionization to fulfill one of the requirements for plasma separation of SNF. This finding is supported by the analysis of the plasma radiation spectrum and the results of the probe diagnostics.

High-voltage discharge in supersonic jet of plumbum vapor

During study of vacuum discharge in plumbum evaporating from molybdenum crucible in identical geometry of discharge gap and the same crucible temperature existence of two different discharge forms were observed. These two forms are vacuum arc with current above 10 A and voltage about 15 V and high-voltage discharge with current about 10 mA and voltage of 340 V. Plumbum was placed in heat-isolated crucible (cathode). Electron-beam heater was situated under the crucible. At the temperature of 1.25 kK that corresponds to plumbum saturated vapor pressure about 0.1 kPa voltage from power source (380 V, 200 A) was applied to anode and high-voltage discharge initiated with characteristics mentioned above. After a few seconds this discharge could turn into arc or could exist hundreds of seconds until total plumbum evaporation. Glow of discharge could take the form of a cone, harness or plasma bunch that hanged at the appreciable distance from the electrodes. The estimations of plasma parameters are presented.

Diffuse vacuum arc on cerium dioxide hot cathode

A diffuse (spotless) vacuum arc was investigated on a hot cathode made from cerium dioxide. The discharge is obtained in the following range of current, voltage, and cathode temperature of I = 15–150 A, Va = 9–14 V, and Tc = 2.1–2.4 kK. The main characteristics of the plasma flow in space behind the anode with a hole were determined: it was found that the electron temperature at the working parameters lies in the range of 0.4–1 eV, the ions are predominantly singly charged, the average charge of the outgoing heavy particles reaches 0.9 e (elementary charge), and the most probable kinetic energy of the ions does not exceed 9 eV. Potentially found regimes of vacuum arc operation are promising for use in the work on implementation of the plasma method for spent nuclear fuel and/or radioactive waste reprocessing.

Liquid Phase Formation on Graphite Electrode Surface in Arc Discharge

The graphite electrode surface is studied after impact by an electric arc burning in an argon atmosphere at a pressure of 50 kPa. The arc occurred as a result of the local destruction of a graphite rod heated by electric current and preliminarily kept for 2 × 103 s at a temperature of about 3 kK. After the arc discharge with a current of about 100 A with a duration within 1 s, we found drop-like particles, 0.1–0.3 mm in size, on the graphite electrode surface, which is evidence of the local occurrence of liquid carbon phase at a temperature of about 3.3 kK. With longer arc burning, the melting zone propagated over the entire working surface of the electrodes; the surface became smoothed.

Arc ignition at heating of graphite by fixed current

Arc ignition after the destruction of graphite samples under prolonged heating by electric current was described. Evidences of liquid film formation on the graphite surface at a temperature of 3.3 kK were presented.

Diffuse vacuum arc with cerium oxide hot cathode

Diffuse vacuum arc with hot cathode is one of the perspective plasma sources for the development of spent nuclear fuel plasma reprocessing technology. Experimental data is known for such type of discharges on metal cathodes. In this work discharge with cerium dioxide hot cathode was studied. Cerium dioxide properties are similar to uranium dioxide. Its feature as dielectric is that it becomes conductive in oxygen-free atmosphere. Vacuum arc was studied at following parameters: cathode temperatures were between 2.0 and 2.2 kK, discharge currents was between 30 and 65 A and voltages was in range from 15 to 25 V. Power flows from plasma to cathode were estimated in achieved regimes. Analysis of generated plasma component composition was made by radiation spectrum diagnostics. These results were compared with calculations of equilibrium gaseous phase above solid sample of cerium dioxide in close to experimental conditions. Cerium dioxide vacuum evaporation rate and evaporation rate in arc were measured.