An H− source for ESS

A new version of the high efficiency source is being developed for the European Spallation Source. The goal of these experiments is to produce a nearly pure H− ion beam with a low electron to H− ratio and high current (70 mA). To achieve this, the discharge chamber is divided into two parts of different plasma parameters by means of a filter magnetic field. Furthermore, the plasma chamber is aligned unsymmetrically with respect to the z axis of the extractor. We obtained a current density of 10 mA/cm2 for an arc power as small as 1 kW. In preliminary experiments, a beam of 10 mA was extracted at an extraction voltage of 8 kV (1.8% impurities). Measurements of the beam current density, electron to H− ratio, and the ion composition for a wide range of discharge conditions will be presented in this article.