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

Abstract

Benchmarking and validation are prerequisite 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 GMNHIS against another independently developed code, Global Enhanced Vibrational Kinetic Model (GEVKM). This is the first study to present quite comprehensive benchmarking test of this kind for models of negative hydrogen ion sources (NHIS), and very good agreement has been achieved for electron temperature, vibrational distribution function (VDF) of hydrogen molecules, and H e nn − ratio. The small discrepancies in number densities of negative hydrogen ions, positive ions, as well as hydrogen atoms can be attributed to the differences in the predicted electron number density for given discharge power. Higher electron number density obtained with GMNHIS is possibly due to fewer dissociation channels accounted for in GMNHIS, leading to smaller energy loss. In addition, we validated GMNHIS against experimental data obtained in an electron cyclotron resonance (ECR) 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 12 mTorr, 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 ITER.

Direct measurements of electronic ground state ro-vibrationally excited D2 molecules produced on ECR plasma-facing materials by means of VUV-FT absorption spectroscopy

“…In Fig. 11a, a break is observed but there is no clear plateau beyond v = 3: higher vibrational levels (up to v = 10 for Ta) follow a Gaussian-like distribution with no clear plateau observed like those obtained by Yang et al 36 when only E-V mechanism is considered in similar plasmas. In our case, firstly, the D-atoms density, estimated during these experimental series, is low (1%).…”

“…The cross section for the excitation of this first state is higher than for the other states. This makes this B state as the main contributor to the vibrational excitation 36 . Contributions of other singlet states (C, EF, ...) to the E-V excitation are estimated to be about 25% 37 .…”

“…It is worth mentioning that, the efficiency of the ECR coupling allows the production of electrons with high energy, E. When E >15.43 eV, there is the possibility that some of the D 2 molecules are directly ionized. According to Yang et al 36 , several reaction paths are leading either to D, D + or to D * 2 , then contributing for the two first to process (i) or to process (ii) for the latter.…”

Semiempirical zero-dimensional approach for the estimation of beam features from plasma parameters in proton sources

“…In past decades, numerical simulations have already become a valuable tool for improving the understanding of discharge physics and can provide theoretical predictions, especially in cases where diagnostic devices are expensive or difficult to include in an experimental setup. Zero-dimensional models applied to hydrogen discharges have been already reported [14][15][16][17][18][19][20]. Four fundamental conservation laws are usually used in the formulation of a zero-dimensional mode: (i) mass conservation, described via a particle balance for each species except for the H 2 molecules; (ii) charge conservation, reduced to the quasineutrality condition; (iii) neutral density conservation, determined by the equation of state for the total particle number density; and (iv) the energy conservation law, expressed through power balance.…”

“…Four fundamental conservation laws are usually used in the formulation of a zero-dimensional mode: (i) mass conservation, described via a particle balance for each species except for the H 2 molecules; (ii) charge conservation, reduced to the quasineutrality condition; (iii) neutral density conservation, determined by the equation of state for the total particle number density; and (iv) the energy conservation law, expressed through power balance. The resulting balance conditions consist of a system of 24 coupled nonlinear ordinary differential equations which can be numerically solved to obtain relevant plasma parameters [14].…”