V. I. Demidov

Probe measurements of electron-energy distributions in plasmas: what can we measure and how can we achieve reliable results?

An electric-probe method for the diagnostics of electron-distribution functions (EDFs) in plasmas is reviewed with emphasis on receiving reliable results while taking into account appropriate probe construction, various measurement errors and the limitations of theories. The starting point is a discussion of the Druyvesteyn method for measurements in weakly ionized, low-pressure and isotropic plasma. This section includes a description of correct probe design, the influence of circuit resistance, ion current and plasma oscillations and probe-surface effects on measurements. At present, the Druyvesteyn method is the most developed, consistent and routine way to measure the EDF. The following section of the review describes an extension of the classical EDF measurements into higher pressures, magnetic fields and anisotropic plasmas. To date, these methods have been used by a very limited number of researchers. Therefore, their verification has not yet been fully completed, and their reliable implementation still requires additional research. Nevertheless, the described methods are complemented by appropriate examples of measurements demonstrating their potential value.

Nonlocal effects in a bounded low-temperature plasma with fast electrons

Effects associated with nonlocality of the electron energy distribution function (EEDF) in a bounded, low-temperature plasma containing fast electrons, can lead to a significant increase in the near-wall potential drop, leading to self-trapping of fast electrons in the plasma volume, even if the density of this group is only a small fraction (∼0.001%) of the total electron density. If self-trapping occurs, the fast electrons can substantially increase the rate of stepwise excitation, supply additional heating to slow electrons, and reduce their rate of diffusion cooling. Altering the source terms of these fast electrons will, therefore, alter the near-wall sheath and, through modification of the EEDF, a number of plasma parameters. Self-trapping of fast electrons is important in a variety of plasmas, including hollow-cathode discharges and capacitive rf discharges, and is especially pronounced in an afterglow plasma, which is a key phase of any pulse-modulated discharge. In the afterglow, the electron tem...

Influence of the transverse dimension on the structure and properties of dc glow discharges

Two–dimensional (2D) simulations of a dc glow discharge with a cold cathode in argon have been performed for various radii of the discharge tube. It is shown that the loss of the charged particles to the walls can significantly affect plasma parameters as well as properties of the cathode sheath. The longitude dimensions of the negative glow and Faraday dark space depend on the transverse loss of the charge particles and are not consistently predicted with a 1D model. The common assumption that the cathode sheath can be analyzed independently of the plasma also may not be valid. The transverse inhomogeneity of the plasma leads to a change in the current density distribution over the cathode surface. The thickness of the cathode sheath can vary with radial distance from the discharge axis, even for the case of negligible radial loss of the charge particles. The 2D model results provide an analysis of the conditions of applicability of the 1D model.

Nonlocal effects in a bounded afterglow plasma with fast electrons

Effects connected with nonlocality of the electron energy distribution function (EEDF) in a bounded, afterglow plasma with fast electrons can lead to a significant (many times of Te/e) increase in the near-wall potential drop, even if the density of this fast group is only a small fraction of the total electron density. This can substantially change the near-wall sheath thickness and electric field. Nonlocal fast electrons which are partially trapped in the plasma volume can increase the rate of stepwise excitation, supply additional heating to slow electrons and reduce their diffusion cooling rate. Altering the source terms of these fast electrons, to change their production rate will, therefore, alter the near-wall sheath and, through modification of the EEDF, a number of plasma parameters. Another possibility of modifying the EEDF is by application of a negative potential to a portion of the plasma boundary. This can allow modification of the fast part of the EEDF. The above effects and methods can be used in various research and technical applications

Active electron energy distribution function control in direct current discharge using an auxiliary electrode

The electron energy distribution functions are studied in the low voltage dc discharge with a constriction, which is a diaphragm with an opening. The dc discharge glows in helium and is sustained by the electron current emitted from a heated cathode. We performed kinetic simulations of dc discharge characteristics and electron energy distribution functions for different gas pressures (0.8 Torr-4 Torr) and discharge current of 0.1 A. The results of these simulations indicate the ability to control the shape of the electron energy distribution functions by variation of the diaphragm opening radius.

Non-local collisionless and collisional electron transport in low-temperature plasma

This paper reviews recent advances in non-local electron kinetics in low-pressure discharges. Non-local electron kinetics, non-local electrodynamics with collisionless electron heating and non-linear processes in the sheaths are typical for such discharges. Progress in understanding the non-local interaction of electric fields with real, bounded plasma created by the fields has been one of the major achievements of the past few decades.

Baffled probe for real-time measurement of space potential in magnetized plasma

A probe for measurements of space potential in magnetized plasma is tested in a fully ionized, barium, Q-machine plasma. The probe consists of a tungsten wire tip, situated perpendicular to the magnetic field, that is partially shielded by ceramic baffles (masks). The probe works under the condition that the electron Larmor radius is much smaller than the probe radius, and that the ion Larmor radius is comparable to or larger than the probe radius. By rotating the baffle configuration around the probe tip, the ratio between the electron and ion probe current can be adjusted. The probe uses the same principles as Katsumata and plug probes [V. I. Demidov et al., Rev. Sci. Instrum. 73, 3409 (2002)], but has the advantage of convenient control of the ratio between the electron and ion current, and is not sensitive to uncertainties in the orientation of the probe tip relative to the direction of the magnetic field. Measurements of potential are made while the probe floats electrostatically. When saturated electron and ion currents have comparable magnitudes, accurate, real-time measurements of space potential can be acquired.

Is the negative glow plasma of a direct current glow discharge negatively charged

A classic problem in gas discharge physics is discussed: what is the sign of charge density in the negative glow region of a glow discharge? It is shown that traditional interpretations in text-books on gas discharge physics that states a negative charge of the negative glow plasma are based on analogies with a simple one-dimensional model of discharge. Because the real glow discharges with a positive column are always two-dimensional, the transversal (radial) term in divergence with the electric field can provide a non-monotonic axial profile of charge density in the plasma, while maintaining a positive sign. The numerical calculation of glow discharge is presented, showing a positive space charge in the negative glow under conditions, where a one-dimensional model of the discharge would predict a negative space charge.

Modeling a short dc discharge with thermionic cathode and auxiliary anode

A short dc discharge with a thermionic cathode can be used as a current and voltage stabilizer, but is subject to current oscillation. If instead of one anode two anodes are used, the current oscillations can be reduced. We have developed a kinetic model of such a discharge with two anodes, where the primary anode has a small opening for passing a fraction of the discharge current to an auxiliary anode. The model demonstrates that the current-voltage relationship of the discharge with two anodes is characterized everywhere by positive slope, i.e., positive differential resistance. Therefore, the discharge with two anodes is expected to be stable to the spontaneous oscillation in current that is induced by negative differential resistance. As a result, such a discharge can be used in an engineering application that requires stable plasma, such as a current and voltage stabilizer.

Metastable atom and electron density diagnostic in the initial stage of a pulsed discharge in Ar and other rare gases by emission spectroscopy

Temporal measurements of the emission intensities of the Ar 419.8 and 420.1 nm spectral lines combined with Ar plasma modeling were used to examine the metastable atom and electron density behavior in the initial stage of a pulsed dc discharge. The emission intensity measurements of these spectral lines near the start of a pulsed dc discharge in Ar demonstrated a sharp growth of metastable atom and electron densities which was dependent on the applied reduced electric fields. For lower electric fields, the sharp growth of metastable atom density started earlier than the sharp electron density growth. The reverse situation was observed for larger electric fields. This presents the possibility for controlling plasma properties which may be useful for technological applications. Similar measurements with spectral lines of corresponding transitions in other rare gases are examined.