Plasma electron spectroscopy in microhollow discharge with integrated wallprobe

The plasma characteristics of a novel microhollow electrode discharge with an integrated wall probe have been analyzed in noble gases. The microplasma device was fabricated with three layers of thin molybdenum metal sheets separated by two thin sheets of mica insulation. A 200 micrometer diameter hole formed a microhollow electrode cylindrical cavity that passed through the entire five layers. The outer layers of molybdenum acted as a hollow anode and hollow cathode, while the inner molybdenum layer formed an effective wall probe. The working gas could be introduced as a flow through the microcavity into open air or the entire microplasma device could be operated in a closed vacuum chamber with a selected gas composition and pressure. Under certain microdischarge conditions, wall probe I-V measurements showed peaks in the electron energy spectra revealing an excess of fast electrons at specific energies. Analysis of these preliminary experiments using plasma electron spectroscopy (PLES) techniques produced promising results, revealing evidence in the spectra of energy exchange with specific neutral and excited gas species. This demonstrates, for the first time, that a wall probe can be useful for interpretation of plasma properties and the plasma-boundary interaction in a microplasma with a nonlocal electron energy distribution at atmospheric pressure. The condition of nonlocality has been analyzed in relation to the dimension of this microhollow electrode discharge and the operating conditions. In noble gases with elastic collisions of electrons, the condition of nonlocality is typically met when p L <10 Torr-cm, where p is the gas pressure and L is a characteristic plasma dimension. Further development of wall probes in microplasmas would enable a thorough understanding of non-Maxwellian electron energy distributions as well as the possible use of a device as an analytical sensor employing PLES analysis.