Combined Langmuir probe and collisional-radiative optical diagnostics of discharge conditions relevant to H - production in CeWNIS

The performance of surface-production negative-ion sources depends on the balance between H - production and competing loss processes. This balance depends on strongly coupled plasma parameters, including the electron density (n e ), electron temperature (T e ), electron energy probability function (EEPF), and hydrogen dissociation. The Cesiated Tungsten Dust-driven Negative Hydrogen Ion Source (CeWNIS) is a laboratory-scale facility under development for surface-assisted H - production. In this pre-beam commissioning study, the discharge characteristics of CeWNIS were investigated using combined Langmuir probe and optical emission spectroscopy (OES) diagnostics. The probe-measured non-Maxwellian EEPF was used as input to metastable-resolved collisional-radiative actinometry model. This connected the probe derived electron parameters to excitation sensitive optical line ratios. OES also provided the gas temperature and degree of hydrogen dissociation. The axial plasma density profile was used to select a practical operation separation of 80 mm between the multicusp magnetic cage and the dust coating unit (DCU). The selected configuration was then used to investigate the effect of working pressure. The degree of hydrogen dissociation decreased from 6.1±0.9% to 1.9±0.3% with increasing pressure. The measured plasma parameters were subsequently used as input to a particle balance model. Among the investigated pressures, the model predicted the highest steady-state H - density of (7.46±1.32)×10 15 m -3 near 1.5×10 -3 mbar. The study identifies a model based discharge condition window for subsequent H - production in CeWNIS.