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Title: Ionization and breakdown in argon
Author: Abdulla, R. R.
Awarding Body: University College of Swansea
Current Institution: Swansea University
Date of Award: 1981
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Abstract:
This thesis describes a study of the fundamental collision processes which contribute to the spatial growth of pre-breakdown ionization current in argon in a uniform electric field, E, over a pressure range of 0.5 1 < p < 750 torr. Measurements were carried out on the growth of small photoelectric currents, Io, in purified argon over the P range of 5 <, P <, 1200 volt/cm.torr. Measurements of sparking potential as a function of pd and p were also made for argon. Following the analysis of Gosseries, measurements were carried out in order to establish the limiting value of E/p above which the electron swarm in the discharge space was no longer in a steady-state. In all the measurements of the pre-breakdown ionization current measured at constant E/p and constant p with varying d, was found to be in agreement with the Townsend growth equation, 1 0 exp(ad)1 - m/a [exp (cad) -1]. Using the above equation, accurate evaluations were made of the primary, a, and secondary, m/a, ionization coefficients. It was known from previous work that the values of ionization coefficients for rare gases were critically dependent on the state of purity of the gas, so that the ionization chamber was baked and pumped to an ultra-high vacuum base pressure ( -10-9 torr) before the measurements were carried out. The argon gas used was a standard grade argon of initial purity of 99.99%, further purified by passing the gas through a B.O.C. rare gas purifier. A mass-spectroscopic analysis of the sample used in the present work showed that impurity levels were of the order of 2-3 ppm. Analysis of the experimental data showed that over the range investigated the primary ionization coefficient is solely dependent on E/p, while the secondary ionization coefficient depends on p as well as E/P. A review of previous experimental and theoretical values of Cc/ p has been made and compared with the results obtained in the present work. The present values of a/p were lower than all the previous experimental results by at least 4% over the whole range investigated, and is attributed to the purer gas sample employed, compared with other workers. The values of the secondary ionization coefficient obtained show a minimum in their dependence on E/p. The decrease of m/a with increasing E/p at low E/p ranges up to 20 volt/cm.torr., indicated that the predominant secondary ionization process in argon was the emission of electrons at the cathode by the action of non-resonance photons produced in the gas body by three-body collision involving a metastable state and two ground-state atoms. The values of m/a in this range were found to be in good agreement, within the experimental error ( 30%), with theoretically calculated values with the assumption that the effect of non-resonance photons at the cathode was the dominant process. w/a values remained constant and independent of E/p over the range 20-200 volt/cm.torr., indicating that the predominant secondary ionization process was the action of positive ions. For E/p > 200 volt/cm.torr, W, increased as E/p increased, and this variation is attributed to the combination of the impact of positive ions and metastable states at the cathode. Sparking potential measurements made in purified argon show a deviation from Paschen's Law. At a particular value of pd, the values of Vs were higher at the lower potential. Finally, Gosseries plots were linear except for a slight upward deviation for E/p > 700 volt/cm.torr. at smaller electrode separations. This could be caused by a slight distortion due to the holes in the anode. This linearity indicates that the ionization coefficients are independent of d, and that the electron swarms are in equilibrium with the electric field over the whole discharge region.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.635832  DOI: Not available
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