Stabilizing long pulse D− extraction for NBI using a Cs shower

Abstract

The Neutral Beam Injection (NBI) system of ITER is based on sources for negative hydrogen ions (H${}^{-}$ & D${}^{-}$). Negative ions are produced by surface conversion of plasma particles on low work function materials. Cs is steadily evaporated from an oven into the ion source for the reduction of the surface work function. The extraction of negative ions from the source plasma is accompanied by the co-extraction of electrons. In long pulses, the electron current is steadily increasing and thus limiting the source performance in particular in deuterium operation. Simulations and measurements give hints that the flux of Cs onto the plasma grid – the first grid of the extraction system and the most important conversion surface – decreases in long pulses.

In order to increase the flux of neutral Cs, a new version of a Cs shower is tested at the BATMAN Upgrade test facility. The Cs shower consists of a small tube containing a multitude of small orifices for directed evaporation of Cs very close to the convertor surface. This allowed for the first time a steady state with extremely stable performance in long deuterium pulses (1000 s, beam time limited by radiation restrictions). However, the evaporation from the Cs shower (measured by Tunable Diode Laser Absorption Spectroscopy) shows a strong vertical asymmetry. Measures for homogenizing the evaporation need to be adapted in a next step.