Roland Friedl »  Martin Wolf
Roland Friedl
Experimental investigations on the caesium dynamics in H2/D2 low temperature plasmas
Supervisor: Prof. Dr.-Ing. Ursel Fantz [Experimental plasma physics]
Date of oral examination: 12/17/2013
256 pages, english ,
The fusion experiment ITER requires powerful neutral beam injection (NBI) systems for heating and current drive. The neutral beam with a power of 16.5 MW at an energy of 1 MeV is generated via accelerating negative hydrogen ions and subsequent neutralization in a gas target. A key component of the NBI system is the ion source which has to provide accelerated current densities of 200 A/m^2 D^- and 300 A/m^2 H^-. Such ion sources are currently under development and are based on the surface conversion mechanism: atoms and positive ions from a low temperature hydrogen plasma are converted into negative ions at a low work function surface, which is therefore coated with the alkali metal caesium. For that purpose Cs is introduced into the ion source via evaporation from a reservoir. Due to its high chemical reactivity, the adsorbed Cs layer is susceptible to impurities from the residual gas, which degrades the work function of the converter surface. Consequently, the stability and reliability of a high negative ion current density significantly depends on the Cs dynamics in the hydrogen plasma and in the vacuum phases between the pulses. In order to investigate the Cs dynamics on a fundamental level including plasma and surface chemistry, dedicated studies are performed in a flexible laboratory experiment under ion source relevant conditions. The inductively coupled plasma (ICP) setup is equipped with a comprehensive set of diagnostics to measure Cs fluxes, the Cs density, local and global plasma parameters, the work function of a sample surface as well as the impurity content. Owing to the strong Cs dynamics the diagnostics can moreover be applied simultaneously to relate parameters from different diagnostics to the same experimental condition. Furthermore, a reliable Cs source is developed, capable of stable Cs evaporation with well adjustable evaporation rates. The fundamental investigations have emphasized that the Cs dynamics in vacuum as well as in hydrogen and deuterium plasmas are dominated by the surface chemistry of caesium compounds. Thus, gettering of residual gases upon evaporation of Cs is observed and the increased work function of Cs layers exposed to background pressures of 10^-6 mbar is quantified to 2.75 eV compared to 2.14 eV for pure Cs layers. In hydrogen and deuterium plasmas electron density and temperature in the volume are not affected by Cs amounts relevant to ion sources for ITER. Coating of the vessel surfaces with Cs, however, leads to a considerable gettering of hydrogen atoms from the plasma volume, which explains the observed decrease of the performance of negative ion sources upon excessive caesium evaporation. Detailed measurements above a sample surface moreover revealed a decrease of the electron density close to the surface due to the formation of negative ions. This is the first experimental demonstration of the explanation for the typically measured decrease of the co-extracted electron current upon Cs evaporation in ion sources. Furthermore, the long-term behavior of Cs adsorption and redistribution processes leads to a hysteresis of the observed effects, which explains the loss of correlation between the Cs density and its impact on the extracted current densities observed in ion sources.