First hydrogen operation of NIO1: characterization of the source plasma by means of an optical emission spectroscopy diagnostic

TL;DR

This paper reports the first hydrogen operation of the NIO1 ion source, using optical emission spectroscopy to characterize plasma behavior and identify transition signatures to H mode, advancing understanding of source performance in hydrogen.

Contribution

It introduces a spectroscopic diagnostic setup for NIO1 and presents the first plasma measurements during hydrogen operation, including transition signatures to H mode.

Findings

Identification of plasma emission signatures during hydrogen operation

Observation of a transition to H mode with increased light emission

Development of a collisional-radiative model for plasma analysis

Abstract

NIO1 is a compact and flexible radiofrequency H- ion source, developed by Consorzio RFX and INFN-LNL. Aim of the experimentation on NIO1 is the optimization of both the production of negative ions and their extraction and beam optics. In the initial phase of its commissioning, NIO1 was operated with nitrogen, but now the source is regularly operated also with hydrogen. To evaluate the source performances an optical emission spectroscopy diagnostic was installed. The system includes a low resolution spectrometer in the spectral range of 300-850 nm and a high resolution (50 pm) one, to study respectively the atomic and the molecular emissions in the visible range. The spectroscopic data have been interpreted also by means of a collisional-radiative model developed at IPP Garching. Besides the diagnostic hardware and the data analysis methods, the paper presents the first plasma measurements across a transition to the full H mode, in a hydrogen discharge. The characteristic signatures of this transition in the plasma parameters are described, in particular the sudden increase of the light emitted from the plasma above a certain power threshold.