Experimental and Theoretical Studies of the Argon Pre-Breakdown Discharge

Abstract

This work concerns the use of an optical absorption technique to measure the concentration of metastable excited argon atoms within a 'chopped' prebreakdown or Townsend discharge. A diode laser tuned to the argon 1s5 - 2p9 transition at 811.5 nm was used as the light source. The absorption was measured with a spatial resolution of less than 2 mm and a time resolution of as low as 0.1 ms. The reduced diffusion coefficient NDm of the 1s5 state was determined by measurements of the decay of the discharge current in the afterglow. A value of (1.57 ± 0.05) x 10⁻¹⁸cm⁻¹s⁻¹ was returned, which is consistent with other published values. An approximate value of the efficiency with which the 1s5 atoms eject secondary cathodic electrons was determined from spatially-resolved measurements of the quasi-steady-state concentration of these atoms. The resulting value of 85% is broadly consistent with published values. The primary ionisation coefficient ai/N was determined at 454 Td of reduced electric field by spatially and time-resolved measurements of the 1s5 concentration in the first millisecond after the discharge was switched on. The measured value of 6 x 10⁻¹⁷cm² is about 25% lower than the previously accepted value. Measurements were also made from 36 to 82 Td, using just the current passed by the discharge: these values were about 10% lower than the canonical values. The 1s5 excitation coefficient αj/N was measured at the same pressure and field using both the quasi-static technique and the time-resolved technique, returning values of 8.0 and 7.3 x 10⁻¹⁸cm² respectively. The only previously published data report a value of 27 x 10⁻¹⁸cm² for this quantity.