It is assumed that persistent cesiation in the SNS RF SPS is related to deposition of carbon film on the collar converter. The work function dependence for graphite with alkali deposition has no minimum typical for metals and semiconductors and the final work function is higher. For this reason, the probability of H- secondary emission from cesiated metal and semiconductors can be higher than from cesiated carbon films but the carbon film maintains cesiation longer and can operate with low cesium consumption.
@article{arxiv.1808.06003,
title = {Carbon Film in Radio Frequency Surface Plasma Source with Cesiation},
author = {Vadim Dudnikov and B. Han and S. Murray and T. Pennisi and C. Stinson and M. Stockli and R. Welton and A. Dudnikov},
journal= {arXiv preprint arXiv:1808.06003},
year = {2018}
}