Isotope effect and electron-temperature dependence in volumeH−andD−ion sources

We present the results of a numerical study of isotope effects in volume negative-ion sources and compare them with experimental results. We have studied both hydrogen and deuterium discharges over a range of discharge current. For basic plasma parameters, the model reproduces the behavior measured in the discharge region of a tandem multicusp source. The theory predicts a local maximum in negative-ion production at an optimal electron temperature ${\mathit{T}}_{\mathit{e}}^{\mathrm{opt}}$≊0.6–0.8 eV. This prediction is supported by our analysis of ${\mathrm{D}}^{\mathrm{-}}$ and ${\mathrm{H}}^{\mathrm{-}}$ densities measured in a hybrid multicusp source and in the extraction region of a tandem source. We show how the local maximum is related to the temperature dependence of the dissociative attachment rate coefficients. The most significant isotope effect suggested by this modeling effort is a stronger cooling of the vibrational distribution in deuterium due to vibration-translation reactions between molecules and atoms. This vibrational cooling reduces negative-ion production in high-power deuterium discharges below that in equivalent hydrogen discharges.