N. P. Kyrie

Current sheets in magnetic configurations with singular X-lines

The possibility of the formation of current sheets in 3D magnetic configurations with singular X-lines was studied experimentally. It is shown that a sheet can be formed in the presence of the longitudinal magnetic-field component directed along the X-line, in which case the longitudinal component can exceed the transverse component everywhere inside the plasma. Characteristic of the CS formation in 3D magnetic configurations with X-lines are an increase in the longitudinal magnetic-field component inside the sheet and a decrease in the plasma compression ratio as compared to 2D configurations with null-lines. If the longitudinal component exceeds a certain critical value, a sheet cannot be formed: instead of a sheet, there appear two sheaths separated by a cavity with a local minimum in the electron density.

Spectroscopic Measurements of the Electron and Ion Temperatures and Effective Ion Charge in Current Sheets Formed in Two- and Three-Dimensional Magnetic Configurations

The spatial distributions of the electron temperature and density, the effective and average ion charges, and the thermal and directed ion velocities in current sheets formed in two-dimensional magnetic fields and three-dimensional magnetic configurations with an X line were studied using spectroscopic and interference holographic methods. The main attention was paid to studying the time evolution of the intensities of spectral lines of the working-gas (argon) and impurity ions under different conditions. Using these data, the electron temperature was calculated with the help of an original mathematical code based on a collisional-radiative plasma model incorporating the processes of ionization and excitation, as well as MHD plasma flows generated in the stage of the current-sheet formation. It is shown that the electron temperature depends on the longitudinal magnetic field, whereas the ion temperature is independent of it. The effective ion charge of the current-sheet plasma was determined for the first time.

Plasma dynamics in laboratory-produced current sheets

Evolution of currents and Ampere forces in current sheets are analyzed on the basis of magnetic measurements. Two new effects are observed in the current distributions at the later stage of the sheet evolution: first, a broadening of the current area at the side edges of the current sheet; second, a generation of reverse currents followed by their propagation from the edges to the center of the sheet. Super-thermal plasma flows moving across the width of the current sheet are observed by spectroscopic methods. The energies of plasma jets are consistent with the spatial structure and time dependences of the Ampere forces in the current sheets. The assumption is advanced that plasma acceleration may be more effective at the regions with lower plasma density, which are located at some distances from the sheet mid-plane. Generation of reverse currents provides an additional confirmation of transfer of energetic plasma jets toward the sheet edges.

Suprathermal plasma flows in current sheets formed in two- and three-dimensional magnetic configurations

Dynamics of the thermal and directed motions of argon plasma ions in current sheets formed in various magnetic configurations was investigated experimentally Measurements in three-dimensional magnetic configurations with an X line were carried out for the first time. The results of these measurements were compared with the data obtained in experiments with two-dimensional magnetic configurations. The ion temperature and the energies and velocities of directed plasma flows within the current sheet were determined by analyzing the shapes of argon ion spectral lines broadened due to the Doppler effect. It is found that, under the given experimental conditions, the axial magnetic field does not affect the ion temperature and plasma acceleration in the sheet.

Generation of superthermal plasma flows in current sheets

The profiles of the He II 3203 Å and He II 4686 Å spectral lines of helium ions have been detected and analyzed. Using these data, the processes of acceleration and heating of a plasma in current sheets that are formed in magnetic configurations with X-type singular lines have been analyzed. The generation of plasma flows with energies of 400–1300 eV, which are much higher than the thermal energy of ions, has been revealed. The acceleration of the plasma induced by Ampere forces is likely spatially inhomogeneous, which should lead to shear plasma flows in a current sheet.

Plasma acceleration in current sheets formed in helium in two- and three-dimensional magnetic configurations

The processes of heating and acceleration of plasma in current sheets formed in 2D and 3D magnetic configurations with an X-line in helium plasma have been investigated using spectroscopic methods. It is found that, in 2D magnetic configurations, plasma flows with energies of 400–1000 eV, which are substantially higher than the ion thermal energy, are generated and propagate along the width (the larger transverse dimension) of the sheet. In 3D configurations, the influence of the longitudinal (directed along the X-line) component of the magnetic field on the plasma parameters in the current sheet has been studied. It is shown that plasma acceleration caused by the Ampère force can be spatially inhomogeneous in the direction perpendicular to the sheet surface, which should lead to sheared plasma flows in the sheet.

Spectroscopic study of anomalous electric fields in peripheral regions of a current sheet plasma

We observed an unusual/anomalous asymmetry of the HeI 667.8 nm spectral line profiles emitted from the peripheral regions of a current sheet plasma, which is characterized by high electron density gradients. This asymmetry can be explained only by assuming that, in the plane perpendicular to the current in the sheet, there is a strong low-frequency oscillatory electric field, which considerably exceeds the ion electric microfield. Our calculations show that this field seems to correspond to either a circularly or nearly-circularly polarized wave in the current sheet plasma.

Plasma compression into a planar sheet under high-pressure He-filling

The results of experimental research are presented on the fundamental possibility and conditions of the current sheet formation under high-pressure He-filling in the 2D magnetic fields with a null-line. It was revealed that the formation of the current sheet and efficient plasma compression into the planar sheet could take place under high initial pressure, provided that the gradient of the magnetic field was large enough. The maximal electron density in the sheet peaked at ∼ 10 17 cm -3 and was an order of magnitude higher than the initial electron density and the atomic density of the gas. Both the electron and ion temperatures were lower than 5 eV resulting in suppression of the electron thermal conductivity along the sheet surface. In these conditions the other loss channels played a dominant part, specifically the losses due to plasma radiation.

Distributions of the ion temperature, ion pressure, and electron density over the current sheet surface

The distributions of the ion temperature, ion pressure, and electron density over the width (the major transverse dimension) of the current sheet have been studied for the first time. The current sheets were formed in discharges in argon and helium in 2D and 3D magnetic configurations. It is found that the temperature of argon ions in both 2D and 3D magnetic configurations is almost uniform over the sheet width and that argon ions are accelerated by the Ampère force. In contrast, the distributions of the electron density and the temperature of helium ions are found to be substantially nonuniform. As a result, in the 2D magnetic configuration, the ion pressure gradient across the sheet width makes a significant contribution (comparable with the Ampère force) to the acceleration of helium ions, whereas in the 3D magnetic configuration, the Ampère force is counterbalanced by the pressure gradient.

Diagnostics of a current-sheet plasma with the use of helium spectral lines with forbidden components

Results are presented from the first measurements of the profiles of the HeI 447.1-nm and HeI 492.2-nm neutral helium spectral lines emitted by the plasma of a current sheet formed in the CS-3D experimental device. A theoretical analysis of these profiles is performed with the model microfield method. A comparison of the theoretical and experimental profiles shows that the electron density in the peripheral regions of the current sheets amounts to (1.0–2.0)×1015 cm−3.