Hydrogen negative-ion surface production in low-pressure plasmas

Low-pressure negative-ion sources have a wide range of applications, including particle injection in linear accelerators and cyclotrons, surface analysis via Secondary Ion Mass Spectrometry (SIMS), and neutral beam injection for magnetically confined fusion. These sources operate based on the process of negative-ion surface production, whereby positive ions or atoms scattered from a plasma-facing surface capture one or two electrons from the material, resulting in the formation of negative ions.

This talk focuses on investigating this process within low-pressure hydrogen plasmas. Electron capture at the surface critically depends on the electronic structure of the material, which is often substantially altered by plasma exposure. To explore this phenomenon, we have developed an in situ photoemission diagnostic capable of monitoring changes in the electronic structure following plasma treatment and correlating these modifications with variations in the negative-ion surface production yield.

During the talk, we will present the experimental tools designed for this study, including a magnetized retarding field energy analyzer and a mass spectrometer for detecting surface-emitted negative ions, alongside in situ photoemission yield spectroscopy to characterize surface properties. We will illustrate negative-ion surface production in plasma on selected materials such as diamond, low work-function metals, and conductive ceramics.