Embargoed until 2020-09-03
- Doctoral Thesis
A good insulation gas comprises, among others more practical considerations, these physical properties: high ionization energy thresholds, good electron moderation resulting in a low average electron energy, and high probabilities for the formation of strong anions with free electrons. This thesis deals with the evaluation of measurements in the Pulsed Townsend experiment of gases in which electron detachment from anions occurs. Due to the acceleration by the electric field, anions may gain enough internal energy to lose the surplus electron in a collision. This mechanism can have a negative impact on the electric strength of the insulation gas. The time-resolved Pulsed Townsend experiment allows the evaluation of rates and probabilities of collision-induced electron detachment. Based on a suitable model, simulations are fitted to the measured signals, which are obtained for different pressures, electric field strength and electrode separation distances. The simulation code is implemented in C++ and CUDA C and its structure is presented. Further, measurements of several different gases and gas mixtures are analyzed with this method. In oxygen and oxygen-nitrogen mixtures electron detachment rate coefficients as well as anion conversion rate coefficients are obtained, based on ion-kinetic models that are given in literature. In case of C4F7N, a suitable model is derived based solely on the present Pulsed Townsend measurements, and the effect of electron detachment on the electric strength is estimated. This is then compared to breakdown experiments. It is well known that electron detachment occurs in discharges in SF6 at a low rate. The results are in partial agreement with previous findings, and suggests rather high electron detachment rate coefficients. In N2O, evidence for two to three different electron detachment processes are found. The results indicate that discharge models that were suggested are incomplete. In measurements in CF3OCF=CF2, i-C4F10 and c-C4F8O2, electron detachment is found to occur, and the corresponding rate coefficients are evaluated. Models for these gases are proposed and discussed. Show more
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ContributorsExaminer: Franck, Christian M.
Examiner: Ebert, Ute
Organisational unit03869 - Franck, Christian / Franck, Christian
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