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

Efficiency of Cs-free materials for negative ion production in H2 and D2 plasmas

“…The measured relative negative ion density n H − /n e is translated to absolute values via the ion density from Langmuir probe measurements. The absolute density n H − has been benchmarked against cavity ring-down spectroscopy [16]. Furthermore, the Langmuir probe is used to determine the ion density, the electron energy distribution function and the plasma and floating potential.…”

“…Thus, the negative hydrogen ion density produced in the volume is sensitive to the atomic hydrogen density dominantly determining the destruction rate on the one hand, and to the electron density and the vibrational population determining the production rate on the other hand. The presented H − pressure dependence (figure 2), however, is mainly linked to the H − production side only: the behavior of n e (maximum at 1 Pa) together with the decreasing evolution of T vib,high determines the observed maximum in n H − around 0.5 Pa. For more details on this analysis see [16].…”

“…The electron density, however, is peaked at 1 Pa with a maximum of about 3.2 × 10 16 m −3 . In order to explain the observed pressure dependence and furthermore to determine the dominant destruction processes for negative ions within the plasma volume 0-dimensional modeling of the H − density is performed using the solver YACORA [16,18]. H − production via dissociative attachment (DA) is balanced by the well-known destruction mechanisms: electron detachment (ED), mutual neutralization (MN) with positive hydrogen ions (H, H + 2 , H + 3 ), associative and non-associative detachment (AD, nAD) by collisions with hydrogen atoms and collisional detachment (CD) by hydrogen molecules.…”

“…Further input parameters like electron density and temperature are taken from measurements directly in the downstream region. Modeled H − densities are then fitted to measured ones by variation of T vib,high [16]. In figure 2 (b) the pressure dependent vibrational temperatures T vib,high required to fit the measurements are shown additionally to n e .…”

“…The negative ion density is measured via laser photodetachment, while a Langmuir probe and optical emission spectroscopy (OES) are applied to characterize and monitor the plasma. Besides measurements also modeling of the H − volume production is performed [16] in order to deduce the influence of different material surfaces on the plasma itself. Like this, possible H − surface effects can be distinguished from the influence on the background plasma.…”