Cs Evaporation in a Negative Ion Source and Cs Cleaning Tests by Plasma Sputtering

The compact radio frequency negative ion source negative ion optimization phase 1 (NIO1) has been designed, built, and operated by Consorzio RFX and INFN-LNL in order to study and optimize the production and acceleration of
$\text{H}^{-}$
ions in continuous operation. In 2020, Cs was evaporated in the source to increase the total extracted ion current. After an initial reduction of extracted electron to ion ratio and subsequently an increase of extracted negative ion current, the source performances progressively worsened, because of the excessive amount of Cs evaporated in the source; the extracted electron to ion ratio increased from below 1 to more than 10, while ion current density reduced from max. 67 A/m2 ion current to not more than 30 A/m2. This article presents the experimental observations collected during Cs evaporation (reduction of plasma light, Cs emission,
$H_{\beta } / H_{\gamma }$
ratio, and so on) that can help stopping the process before an excessive amount of Cs is introduced in the source. This article also reports the cleaning techniques tested to remove the Cs excess by the action of hydrogen or argon plasmas, while argon was predictably more effective in surface sputtering, and a 3-h Ar plasma treatment was not sufficient to recover from overcesiation.