Mainz 2022 – wissenschaftliches Programm

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P: Fachverband Plasmaphysik

P 9: Poster I

P 9.22: Poster

Dienstag, 29. März 2022, 16:00–17:30, P

Work Function Measurements of the Cesiated Surface in a Negative Hydrogen Ion Source Using LEDs — •Jacob Maria Berner, Christian Wimmer, and Ursel Fantz — Max-Planck-Institut für Plasmaphysik, 85748 Garching, Germany

The work function of a surface can be determined by measuring the nA-photocurrents arising from the irradiation by light of different wavelengths. When the chosen photon energies are close to the work function, this energy threshold can be calculated using the Fowler method. As a novelty for such a measurement, fiber coupled LEDs are used as a light source of sufficient power.

State of the art negative hydrogen ion sources, as used for the ITER neutral beam injectors, rely on the surface conversion process of H and H_x⁺ from a low temperature plasma at cesiated surfaces with low work function. In order to reach longer extraction durations at high extraction currents, a stable work function, which is governed by cesium dynamics, impurities, and the interaction with the plasma, is crucial. Taking on the challenges of measuring small nA-photocurrents in a corresponding environment, the method is developed to determine the work function for the first time ever directly inside a high-performance negative ion source, namely BATMAN Upgrade. Monitoring the temporal stability of the work function is essential, as variations can worsen the extraction performance of H_x⁻ and countermeasures can be taken accordingly. The results from the feasibility study are expected to enable the establishment of this diagnostic tool for in-situ monitoring of the work function.