A model for real time, in situ estimation of cesium coverage on metal substrate using infrared imaging under vacuum

The present work is to develop an infra-red (IR) camera based in situ diagnostic tool for the determination of cesium (Cs) coverage suitable for ion source applications. Cs seeding is done to reduce the surface work function that enhances the surface assisted negative hydrogen ion production. The temporal Cs deposition on a metal surface (for, e.g., tungsten or molybdenum) follows Langmuir adsorption isotherm (LAI) kind of behavior. The surface temperature varies while the Cs deposition is reflected in the IR camera temperature measurements for a constant surface emissivity value. In this paper, a model on the relationship between Cs coverage in correlation with surface emissivity and temperature variation based on the theory of LAI is presented. A surface ionization probe (SIP) in the form of a cathode-anode assembly together with an IR camera viewing arrangement is designed to measure the Cs flux and the surface temperature simultaneously to test our model. In the present experiment, the Cs flux measurement using SIP is validated with a standard quartz crystal microbalance (QCM). The proposed model would be useful to correlate Cs coverage on plasma grid-like surface conditions under negative ion source relevant vacuum conditions.