In this work, diagnostics and comparative analysis of microhollow cathode discharges with an open and covered external surface of the cathode in helium at atmospheric pressure were carried out. It is shown that by covering the external surface of the cathode, it is possible to achieve the effect when the current–voltage characteristics of the discharge grows in a wider range of discharge currents. Obviously, in this case, the negative glow (NG) plasma of the hollow cathode occupies a significant part of the discharge cell, as evidenced by the measurements carried out with the additional electrode. The electron temperature values determined from the I–U characteristic of the additional electrode are 0.3–0.4 eV for a microhollow cathode discharge with a covered external surface, which indicates that the plasma of such discharge is similar to NG plasma of glow discharge. The fast parts of the second derivatives of the I–U characteristic of the additional measuring electrode demonstrate the detection of the spectra of fast electrons produced as a result of Penning ionization reactions. Moreover, by covering the external surface of the cathode, conditions are created under which the entire discharge volume is occupied by a NG plasma, which makes it possible to achieve a better resolution for recording the spectra of fast electrons.
It was shown that the spectrum of Penning electrons in a nonlocal equipotential plasma is associated with the second derivative of their current to a wall electrode (sensor). Analytical calculations are confirmed by experimental studies in a negative glow plasma of short DC glow discharge at moderate and high pressures (up to 150 Torr).
The temperature and density of electrons are measured in a high pressure dc He microdischarge plasma. An additional electrode sensor, placed at the boundary of the discharge volume, is used to measure the sensor current-voltage characteristics and their first and second derivatives. The electron temperature is determined from the transition region of the sensor current-voltage characteristics, in particular from the slow part of the Electron Energy Distribution Function. The electron density is determined from the ion saturation current. The results from the measurements show that the temperature of the slow group of electrons is low about 0.2–0.3 eV and the electron density is about 1012–1014 cm−3.
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