Influence of the magnetic field topology on the performance of the large area negative hydrogen ion source test facility ELISE

In negative hydrogen ion sources a too high amount or a too rapid increase in co-extracted electrons can prevent achieving the required negative ion current density and restrict the pulse duration. One important measure for reducing and stabilizing the co-extracted electron current is a magnetic filter field. The half-ITER-size NNBI test facility ELISE—in which the filter field is created by a current flowing through the extraction system—is used for performing experiments on the reaction of the source performance on modifying the filter field topology; external magnet bars are attached to the source in different polarities and extensive parameter variations have been done in volume and surface operation. A significant correlation of the extracted ion and electron currents and their temporal stability with the field topology is seen: with the external magnets strengthening the standard filter field a strong reduction of the co-extracted electrons and additionally a reduced increase in these electrons during the pulses is observed. In this configuration it was possible to perform the very first 1 h deuterium pulse in ELISE—an important step toward fulfilling the requirements to the ion source for ITER NBI.