It has been shown (Demidov et al 2005 Phys. Rev. Lett. 95 215002) that even a small number of nonlocal fast electrons, which do not significantly affect the overall mean electron energy, can dramatically change both the plasma and near-wall sheath properties. In this work, Langmuir probe measurements of the electron energy distribution function (EEDF) show the presence of fast electrons created due to collisions of pairs of metastable atoms and collisions of the second kind between metastable atoms and bulk electrons. These measurements were made in the afterglow of a 100% modulated radio-frequency inductive-coupled-plasma discharge in argon, neon and helium. It is shown that this fast component of the EEDF can be controlled independently of the slow electrons, which is a direct consequence of the EEDF nonlocality. Both EEDF and plasma emission spectroscopy measurements are presented for the helium afterglow. These data allow us to determine the absolute density of metastable atoms and their temporal decay during the afterglow. It is shown that under the experimental conditions, stepwise excitation of helium metastable atoms is the primary process for populating excited states and, therefore, decay of the excited atoms is governed by the decay of metastable states. The presence of a trace amount of nitrogen in the system, which does not significantly change the properties of the helium discharge, allowed us to independently measure the decay of helium metastables by monitoring the N+2(B–X) emission resulting from Penning ionization of N2(X) and confirmed the above conclusions regarding the presence and importance of metastable atoms.

Characteristics of electron near-wall transport under two-dimensional dynamic sheath in a Hall effect thruster Phys. Plasmas 17, 074505 (2010);

A new approach leading to the development of gas analytical detectors is reported. The approach is based on measurements in the near-cathode plasma of fine structures associated with atomic and molecular plasma processes of the high energy portion of the electron energy distribution function (EEDF). A short (without positive column) dc discharge with cold cathode and conducting walls was used. The EEDF measurements in a dc discharge are technically simpler and have dramatically better sensitivity than in the afterglow since temporal resolution is not required. Additional increased probe sensitivity is achieved by using a large-area, larger-radius-of-curvature conducting wall as the probe instead of the more common thin cylindrical Langmuir probe. The wall probe, being almost flat, also greatly reduces the ion current contribution to the measurements. The new approach allows for the development of micro-analytical, dc plasma electron spectroscopy (PLES) gas detectors that are operational up to atmospheric pressure.

Plasma emission from the afterglow of a low-pressure, 100% modulated, radio frequency (rf) excited discharge can originate from collisions between metastable atoms and fast electrons. The fast electrons are generated by collisions between pairs of metastables (Penning ionization of one metastable by another) and collisions of metastables with slower electrons (superelastic collisions). Using time-resolved Langmuir probe data, measurements were made of the electron energy distribution function (EEDF) containing these fast electrons in a helium afterglow. The EEDF data were used, along with measured optical cross sections out of the 2s3S and 2s1S metastable levels, to predict intensities of various He emission lines seen in the afterglow. A comparison between the measured and predicted emission is presented.