Benchmarking and validation of global model code for negative hydrogen ion sources

Abstract

Benchmarking and validation are prerequisites for using simulation codes as predictive tools. In this work, we have developed a Global Model for Negative Hydrogen Ion Source (GMNHIS) and performed benchmarking of the GMNHIS against another independently developed code, Global Enhanced Vibrational Kinetic Model (GEVKM). This is the first study to present a quite comprehensive benchmarking test of this kind for models of negative hydrogen ion sources (NHIS), and excellent agreements have been achieved for collisional energy loss per electron-ion pair created, electron number density, electron temperature, densities of H3+ and H2+ ions, and densities of H(n = 1–3) atoms. Very small discrepancies in number densities of H− ions and H+ ions, as well as the vibrational distribution function of hydrogen molecules, can be attributed to the differences in the chemical reactions datasets. The GEVKM includes additional chemical reactions that are more important at high pressures. In addition, we validated the GMNHIS against experimental data obtained in an electron cyclotron resonance discharge used for H− production. The model qualitatively (and even quantitatively for certain conditions) reproduces the experimental H− number density. The H− number density as a function of pressure first increases at pressures below 1.6 Pa and then saturates for higher pressures. This dependence was analyzed by evaluating contributions from different reaction pathways to the creation and loss of the H− ions. The developed codes can be used for predicting the H− production, improving the performance of NHIS, and ultimately optimizing the parameters of negative ion beams for fusion reactors.