Plasma emission monitored via optical emission spectroscopy during the Cs conditioning at SPIDER

Abstract

The SPIDER test facility is the full-size ITER neutral beam injector (NBI) ion source, required to provide 355 A/m2 extracted negative ion current density in hydrogen (285 A/m2 in deuterium) with an electron-to-ion ratio lower than 0.5 (one in deuterium). The negative ion source is attached to a three-grids extraction and acceleration system. The operational conditions for the cases presented in this work involve short pulses (up to about 30 s length) repeated every five to six minutes. The duty cycle can be adjusted. In order to fulfil the requirement on the extracted negative ion current with reduced amount of co-extracted electrons, the evaporation of caesium into the ion source through Cs ovens (three in SPIDER) and the optimisation of Cs conditioning techniques are mandatory. At SPIDER, the plasma is monitored via optical emission spectroscopy techniques measuring the plasma emission in a line-of-sight (LOS) integrated manner in several positions inside the ion source. In particular, close to the extraction region, two sets (centred at 5 mm and at 35 mm distance from the plasma grid) of four horizontal LOSs are used to retrieve the vertical profile of the plasma emission. During the Cs conditioning campaign performed at SPIDER, although the extraction capabilities were reduced due to technical problems, the RF power coupled to the plasma reached 400 kW with all four RF generators working simultaneously. The negative ions produced by surface emission affects the plasma radiation, and the ion source performances in terms of extracted negative ions and co-extracted electrons. The aim of this work is to study the evolution of the plasma emission over the initial phase of the Cs conditioning campaign. The effect of Cs conditioning on the extracted negative ions and electrons is also presented and discussed.