Cs evaporation in a negative ion source and Cs cleaning tests by plasma sputtering

TL;DR

This study investigates cesium evaporation effects in a negative ion source and evaluates plasma sputtering methods for Cs cleaning, highlighting the impact on source performance and potential recovery techniques.

Contribution

It provides experimental insights into Cs evaporation impacts and tests plasma sputtering cleaning methods to mitigate Cs overaccumulation in negative ion sources.

Findings

Excessive Cs evaporation worsens source performance and increases electron to ion ratio.

Hydrogen and argon plasma treatments can partially remove Cs, with argon being more surface-effective.

A 3-hour argon plasma treatment was insufficient to fully recover source performance.

Abstract

The compact radio frequency negative ion source NIO1 (Negative Ion Optimization phase 1) has been designed, built and operated by Consorzio RFX and INFN-LNL in order to study and optimize the production and acceleration of 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). The paper presents the experimental observations collected during Cs evaporation (reduction of plasma light, Cs emission and H$\beta$/H$\gamma$ ratio, etc.) that can help stopping the process before an excessive amount of Cs is introduced in the source. The paper 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, a 3 h Ar plasma treatment was not sufficient to recover from overcesiation.