Time-resolved plasma spectroscopy of the SNS RF-driven H^- ion source

The SNS RF-driven H^- ion source can produce beam currents of more than 100 mA in 1 ms pulses at 60 Hz. The H^- are surface-produced on a caesiated molybdenum converter adjacent to the extraction aperture. The SNS ion source is equipped with a viewport for monitoring the time-averaged hydrogen and caesium line intensities during the caesiation procedure and source operation by optical emission spectroscopy. A time-resolved spectroscopy setup based on bandpass filters and photodiodes has been recently implemented to measure the transient behaviour of the hydrogen and caesium emissions with µs resolution. The sensitivity of the diagnostics system allows detecting the weak Cs emission lines and recording the intensities of several atomic emission lines or molecular bands within each high-power plasma pulse. This information can be used for example to study the Cs dynamics and temporal evolution of the molecular dissociation rate during the plasma pulse. In this paper we describe the optical diagnostics system and report the results of an experimental campaign where the ion source control parameters, i.e. the 2 MHz RF power, H₂ flow rate, converter temperature and the RF duty factor were systematically varied and the emission intensities of atomic and molecular hydrogen (Balmer-alpha, Balmer-beta and Fulcher band) as well as neutral Cs (852 nm and 894 nm) were measured. It was observed that there is a strong correlation between the H^- production and Balmer-series emission implying that the atomic hydrogen density is critical for H^- production, commensurate with the surface production model. A transient peak of H₂ Fulcher-band and neutral Cs emissions was observed at the beginning of the high-power RF pulse. It is argued that the peak is caused by transition from capacitive to inductive coupling mode, i.e. so-called E-H mode transition.