A Global Enhanced Vibrational Kinetic Model for Radio-Frequency Hydrogen Discharges and Application to the Simulation of a High Current Negative Hydrogen Ion Source

TL;DR

This paper introduces a comprehensive kinetic model for high-current hydrogen ion sources, capturing vibrational, chemical, and plasma dynamics to optimize ion production in RF discharges.

Contribution

The development and validation of a detailed global kinetic model for hydrogen RF discharges, including vibrational states and surface processes, tailored for high-current negative hydrogen ion sources.

Findings

Model accurately predicts plasma species densities and temperatures.

Validated against experimental pressure measurements.

Provides insights into optimizing ion source performance.

Abstract

A Global Enhanced Vibrational Kinetic (GEVKM) model is presented for a new High Current Negative Hydrogen Ion Source (HCNHIS) developed by Busek Co. Inc. and Worcester Polytechnic Institute. The HCNHIS consists of a high-pressure radio-frequency discharge (RFD) chamber where high-lying vibrational states of the hydrogen molecules are produced, a bypass system, a nozzle, and a low-pressure negative hydrogen ion production region where $\text{H}^-$ are generated by the dissociative attachment of low energy electrons to rovibrationally excited hydrogen molecules. The GEVKM is developed from moment equations for multi-temperature chemically reacting plasmas derived from the Wang Chang-Uhlenbeck equations for a cylindrical geometry of an inductively coupled RFD chamber. The species included into the model are $\text{H}(n), n=0-3, \text{H}_2(v), v=0-14, \text{H}^+, \text{H}_2^+, \text{H}_3^+, \text{H}^-$, and $e$. The volume-averaged steady-state continuity equations coupled with the electron energy equation, the total energy equation and heat transfer to the chamber walls are solved simultaneously to obtain the volume-averaged number densities of the plasma components, the electron, heavy-particle and wall temperatures. The GEVKM is supplemented by a comprehensive set of surface and volumetric chemical processes governing vibrational and ionization kinetics of hydrogen plasmas. Analytic boundary conditions developed for the flow in the bypass tubes and in the nozzle take into account rarefaction effects and are validated by comparing pressures predictions in the RFD chamber with the pressure measurements. The GEVKM is verified and validated in the wide range of pressures and absorbed powers of different plasma reactors and is used in a parametric investigation of the RFD chamber of the HCNHIS with hydrogen inlet flow rates 5-1000 sccm and absorbed powers 200-1000 W.