Recent improvements to the design, diagnostics and modelling of the SNS H^- ion source

Recent improvements to the design, diagnostics and modelling of the SNS H^- ion source and electrostatic Low Energy Beam Transport (LEBT) are summarized. Design improvements have dramatically boosted the available H^- beam current to >125 mA at 6% duty-factor providing further margin for SNS upgrade plans. Specifically, experimental efforts included testing larger outlet apertures for increased beam current; testing the continuous application of 27 MHz RF for improved plasma ignition and measurements of the plasma heat flux to the Cs collar to inform future collar designs. Diagnostic improvements include implementation of a thermal camera monitoring LEBT electrode temperatures to circumvent arcing and the first time-resolved plasma spectroscopy measurements (microsecond resolution) to help understand Cs dynamics during the plasma pulse. We also acquired a turn-key Langmuir probe system (n_e , T_e , ϕ_p , EEDF) and a photoemission laser spectrometer for surface work function measurements which will be used, for the first time, to quantify these basic plasma and surface parameters in the SNS source. Modelling efforts include developing a simple COMSOL model of the ion source RF/plasma as well as basic Cs transport within the source. This paper will first summarize these developments and then describe in more detail a basic picture of Cs transport within the source which informed the design of a more efficient Cs system that is also presented.