Ionization in oxygen being a thesis on the measurement of ionization and attachment coefficients in low pressure oxygen discharges

Abstract

Ionization processes in oxygen have been studied using a Townsend type apparatus 'using electron swarms of mean energy of about a few electron volts. From a measurement
of currents and gap distances the ionization and attachment coefficients have been calculated using the steady state current growth equation, while breakdown voltage measurements have enabled the evaluation of the secondary coefficient.

As it is well known that the ionization coefficients depend very much on the gas used and that even traces of another gas can lead to quite erroneous values of the coefficients, an ultrahigh vacuum system was designed and constructed in glass. The background pressures were less than lO-u torr in the experimental tube while the experimental gas pressures were of the order of a few torr. The oxygen was initially produced from the electrolysis of Ba(OH)2 in distilled water, but since the silver diffusion tube meant to be used in conjunction with it to purify the oxygen was found to be almost inoperable, spectroscopically pure oxygen supplied by BOC Ltd. was used for most of the work after further purification in freezing traps cooled by liquid nitrogen.

The experimental tubes were all made of glass while the electrodes were evaporated gold films on glass substrates in the first four tubes and solid platinum in the final tube. The gap distance could be altered magnetically from the outside by means of iron slugs fixed to the electrode assembly, and a guard ring around the cathode ensured uniform field conditions.

Vibron and Keithly electrometers were used to measure the currents of the order of 10- 11 h, while the pressure was measured on an oil manometer and the gap distance could be measured accurately by means of a travelling microscope . A ban~ of 120 V dry batteries supplied the
stable voltage necessary and an accurate potentiometer was used in conjunction with a resistance chain to measure it to o.1 V.

An analysis was formulated to determine the ionization and attachment coefficients, * and *, from current growth equations and an I BM 1620 computer was used to simplify the calculations.

a/p values have been obtained for 50 <E/p <250 V/cm torr and compared with other available values; the agreement is reasonably good being approximately 10% lower than the previous values. */p is found to reduce to practically zero at about 80 V/cm torr which could indicate the presence of a photodetachment process. The breakdown voltages and the corresponding secondary coefficients have also be en measured in the range 0·1 < pd <5 cm torr.