Development of a Collisional Radiative model of hydrogencaesium plasmas and its application to the measurement of caesium density in SPIDER

SPIDER (Source for the Production of Ions of Deuterium Extracted from a Radio frequency plasma) is the full scale prototype for the ITER Heating Neutral Beam (HNB) source. The production of negative ions in the source mostly relies on the conversion of H/D atoms and positive ions on a converter surface, which is covered with caesium in order to reduce its work function and maximise the conversion efficiency. Caesium is transported to the converter during both the vacuum and the plasma phases, and caesium coverage shall be properly balanced. Therefore the study of caesium density and transport in the source plays a crucial part in understanding and monitoring the ion production. During plasma operation, the measurement of caesium density and population distribution in the source can be obtained by Optical Emission Spectroscopy (OES). This is done by tracking the intensity of certain emission lines and comparing these measurements with models mimicking the dynamics of caesium in the plasma. Caesium balance at the surface of plasma grid and bias plate can be estimated from the neutral caesium density in the proximity of the surfaces. On the other hand, this can be estimated via a collisional radiative (CR) model for low temperature hydrogen-caesium plasmas, recently developed in order to perform such measurements in SPIDER. It includes the caesium ground-state and 14 excited states evolving in a background made up of singly charged caesium positive ions, electrons, hydrogen atoms and negative ions. This contribution describes the model and its use in conjunction with OES to carry out caesium density measurements during the first operation with caesium in SPIDER. The views and opinions expressed herein do not necessarily reflect those of the ITER Organization. Part of this work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme. The views and opinions expressed herein do not necessarily reflect those of the European Commission.