The positive column argon glow discharge plasma : an excited state gas studied by fast flow glow discharge mass spectrometry

• Main Author: Mitchell, D. J.
• Published: Swansea University 2003
• Subjects: 530.44
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.638206

This thesis reports studies into the chemical and electrical behaviour of the direct current, fast flowing, argon Glow Discharge plasma using mass spectrometry. Evidence presented suggests that an Excited State model, rather than the conventional Ion/Electron perception describes the Positive Column plasma of the glow discharge. It is proposed that contrary to common belief this cold plasma is a population of stabilised highly excited and potentially long lived, argon Rydberg atoms rather than an ionised medium. Studies involving the titration of small amounts of H₂, Ar, He and CC1₄ into the discharge, and the variation of the macroscopic properties of the plasma, were undertaken to examine the chemical and electrical and properties of the plasma. These results, in addition to those obtained by double probe studies, were inconsistent with processes pertaining to the Ion/Electron model of the plasma. The macroscopic properties of the discharge varied included gas glow rate, pressure, residence time, discharge voltage and current, sampling cone bias (positive and negative voltages). The effects of changing these parameters on the voltage distribution across the plasma was also examined. The results indicated that contrary to conventional 'free' ion/electron theory the discharge is sustained by the action of highly excited neutral argon states existing as a continuum and chemically distinct resonances. Detected ions were concluded to originate from outside the plasma by chemi-, auto-, or field-ionisation. Electric current flow across the plasma is proposed to be a facet of discharge gas excitation rather than ionisation with charge transfer between excited atoms providing the means of electron conduction rather than diffusive random motion. A simple model of the electrical properties of the discharge is presented which derives theoretically the experimental observation of a direct, linear, proportionality between the resistance of the discharge and reciprocal discharge current.