Summary of caesium evaporation and deposition during SPIDER first campaign

Abstract

SPIDER (Source for the Production of Ions of Deuterium Extracted from a Radio frequency plasma) is currently in a major shutdown period dedicated to the upgrade of several components in order to enhance its performances and guarantee their agreement with the ITER requirements. During this phase, an in-depth inspection of the beam source components is fundamental to understand SPIDER behaviour during the previous experimental campaigns, and in particular caesium evaporation inside the source by means of Cs ovens. Caesium evaporation and its deposition on the plasma grid (PG) is fundamental to minimize the work function of PG surface causing an increased generation of negative ions and a reduction of the electron density in front of the PG, which means less co-extracted electrons. This work presents the analysis of the caesium ovens performances and of their status after removal. Inductively Coupled Plasma Mass Spectrometry (ICP-MS) analyses were carried out on samples of plasma source walls, and the experimental data was compared to the results obtained from other investigations: numerical models for the caesium evaporation and deposition, chemical surface analyses (performed in strategical locations of the source walls and extraction grid), and Laser Absorption Spectroscopy (LAS) measurements. The non-uniformity of Cs distribution has been taken into account since it has repercussions on negative ion extraction efficiency and uniformity.