Characteristics and dynamics of the boundary layer in RF-driven sources for negative hydrogen ions

The design of the neutral beam injection system of the upcoming ITER fusion device is based on the IPP (Max-Planck-Institut fuer Plasmaphysik, Garching) prototype source for negative hydrogen ions. The latter consists of a driver, in which hydrogen (or deuterium) molecules are dissociated in a large degree in a hydrogen plasma; the plasma expands then towards the plasma grid, on which negative hydrogen ions are formed by conversion of atoms or positive ions by the surface process and are extracted in the following accompanied by the co-extraction of electrons via a three grid system. Electrons are removed out of the extracted beam prior full acceleration using deflection magnets, bending them onto the second grid. The thermal load limits the tolerable amount of co-extracted electrons. A magnetic filter field in the expansion chamber reduces the electron temperature and density, on the one hand in order to minimize the destruction process of negative hydrogen ions by electron collisions and on the other hand in order to reduce the co-extracted electron current density. Caesium is evaporated into the source for an effective production of negative hydrogen ions, lowering the work function of the plasma grid. Due to the high chemical reactivity of caesium, the high vacuum condition in the source and the plasma-wall interaction, complex redistribution processes of Cs take place in the ion source. The boundary layer is the plasma volume between the magnetic filter field and the plasma grid, in which the most important physics of the negative ion source takes place: the production of negative hydrogen ions at the plasma grid, their transport through the plasma and the following extraction. A deeper understanding of the plasma and Cs dynamics in the boundary layer is desirable in order to achieve a stable long-pulse operation as well as to identify possible future improvements. For this reason, the boundary layer of the prototype source has been characterized in this work using multiple diagnostics at the same time: the Cs density is measured using a laser absorption spectroscopy, the density of negative hydrogen ions is determined using Cavity Ring-Down Spectroscopy and the plasma potential, plasma density and electron temperature is measured by Langmuir probes. Determination at the same time is required since in particular the Cs dynamics can differ in different experimental campaigns. Relevant processes have been identified by comparison of the plasma parameters with the extracted current densities of negative hydrogen ions and electrons. A strongly differing Cs dynamics has been detected between vacuum and plasma phases: whereas the Cs dynamics is mainly determined by the high chemical reactivity during vacuum phases, a large and almost homogeneous redistribution of Cs takes place in plasma phases. A comparison of hydrogen and deuterium operation showed a similar plasma dynamics with respect to negative ions (similar amount of extracted ion current density and an increased density of negative ions in the volume due to the mass difference in deuterium), as well as a differing dynamics with respect to electrons (increased co-extracted electron current density due to an increased electron density as well as a differing electron transport in deuterium).