Caesium Capture by POCO CZR-2 Graphite and Characterisation Experiments on VESPA

摘要

Experiments on the Vessel for Extraction and Source Plasma Analyses (VESPA) to investigate the effect of heated graphite on caesium diagnostics are presented. Current operational caesiated Penning-type H- sources on ISIS provide 35 mA, 200 μs beam pulses at 50Hz, using a 90° dipole bend and cold box to separate H- from co-extracted electrons and caesium. Previous experiments on VESPA have shown that up to 40% of the extracted beam is lost through the dipole. Ongoing development on a 2X source to guide future upgrades to the accelerator facility and use on the Front End Test Stand would benefit from a new caesium capture mechanism with better beam transport. Chemical capture is preferred to minimise the need for cooling. Graphite grade POCO CZR-2 was chosen in this first set of experiments due to its routine use capturing caesium vapour in atomic clocks. A system to mount and heat graphite was designed in ANSYS to permit in-situ baking and heating to 350 °C. Quartz Crystal Microbalances (QCMs) and optical emission spectrometry were used to study caesium dynamics as graphite was held at room temperature, 160 °C, and 190 °C. A decrease in caesium accumulation on QCMs in the vessel was observed at 160 °C compared to other temperatures. Additional characterisation experiments were performed in order to distinguish changes in caesium dynamics due to changes to operational settings from any effect of the graphite. Both sets of experiments revealed a need for improved time resolution in both diagnostics to reveal caesium behaviour on the time scale of a single pulse. Plans to obtain and develop this equipment are described.