Correlation of H- beam properties to Cs-coverage

Abstract

A caesiated RF driven source delivers H- ions that, after stripping at the end of the 160 MeV H- linear injector, provides protons to CERN's accelerator complex including LHC, where the protons reached a record energy of 6.8 TeV. In Caesiated RF sources, H- ions are produced via dissociative attachment of electrons onto roto-vibrationally excited H2-molecules (volume) and re-emission as negative ions of protons or hydrogen atoms colliding on a low work function caesiated molybdenum plasma electrode (surface). During initial caesiation, the production mechanism evolves from the initial Cs-free volume production to a predominant surface production mode; the observed stunning reduction of co-extracted electrons is concomitant to an increase of the H- ion current to RF-power yield. This paper describes the evolution of the beam-profile at today's operational beam intensities of 35 mA for various ratios of volume and surface ion-origin. The presence of surface produced ions occurring on a conical plane is characterized by the electron to ion ratio and by measurement of the Cs-coverage of the molybdenum plasma electrode down to a fraction of a monolayer. Angular distributions are extracted from beam profile and Beam Emission Spectroscopy (BES) measurements. These experimental results provide an initial comparison to beam formation simulation that, at a later stage, could be coupled to beam transport software packages. The paper focuses on the caaesiation transient to present experimental evidence for 3D beam formation studies, it provides insight into the mixing of volume and surface production modes, reduction of co-extracted electrons and Cs-coverage. The paper also establishes magnetic field induced asymmetries in the beam's current density.