Comparison of measured and modelled negative hydrogen ion densities at the ECR-discharge HOMER

As the negative hydrogen ion density nH− is a key parameter for the investigation of negative ion sources, its diagnostic quantification is essential in source development and operation as well as for fundamental research. By utilizing the photodetachment process of negative ions, generally two different diagnostic methods can be applied: via laser photodetachment, the density of negative ions is measured locally, but only relatively to the electron density. To obtain absolute densities, the electron density has to be measured additionally, which induces further uncertainties. Via cavity ring-down spectroscopy (CRDS), the absolute density of H− is measured directly, however LOS-averaged over the plasma length. At the ECR-discharge HOMER, where H− is produced in the plasma volume, laser photodetachment is applied as the standard method to measure nH−. The additional application of CRDS provides the possibility to directly obtain absolute values of nH−, thereby successfully bench-marking the laser photodetachment system as both diagnostics are in good agreement. In the investigated pressure range from 0.3 to 3 Pa, the measured negative hydrogen ion density shows a maximum at 1 to 1.5 Pa and an approximately linear response to increasing input microwave powers from 200 up to 500 W. Additionally, the volume production of negative ions is 0-dimensionally modelled by balancing H− production and destruction processes. The modelled densities are adapted to the absolute measurements of nH− via CRDS, allowing to identify collisions of H− with hydrogen atoms (associative and non-associative detachment) to be the dominant loss process of H− in the plasma volume at HOMER. Furthermore, the characteristic peak of nH− observed at 1 to 1.5 Pa is identified to be caused by a comparable behaviour of the electron density with varying pressure, as ne determines the volume production rate via dissociative electron attachment to vibrationally excited hydrogen molecules.