Low-pressure hydrogen plasmas explored using a global model

Abstract

Low-pressure hydrogen plasmas have found applications in a variety of technology areas including fusion, neutral beam injection and material processing applications. To better understand these discharges, a global model is developed to predict the behaviour of electrons, ground-state atomic and molecular hydrogen, three positive ion species (H + , H 2 + , and H 3 + ), a single negative ion species (H − ), and fourteen vibrationally excited states of molecular hydrogen (H 1 2 (υ = -14)). The model is validated by comparison with experimental results from a planar inductively coupled GEC reference cell and subsequently applied to the MAGPIE linear helicon reactor.The MAGPIE reactor is investigated for a range of pressures from 1 to 100 mTorr and powers up to 5 kW. With increasing power between 50 W and 5 kW at 10 mTorr the density of all charged species increases as well as the dissociative fraction while the electron temperature remains almost constant at around 3 eV. For gas pressures from 1-100 mTorr at an input power of 1 kW, the electron density remains almost constant, the electron temperature and dissociative fraction decreases, while H 3 + density increases in density and also dominates amongst ion species. Across these power and pressure scans, electronegativity remains approximately constant at around 2.5%. The power and pressure determines the dominant ion species in the plasma with H 3 + observed to dominate at high pressures and low powers whereas H + tends to be dominant at low pressures and high powers.A sensitivity analysis is used to demonstrate how experimental parameters (power, pressure, reactor wall material, geometry etc) influence individual species' density as well as the electron temperature. Physical reactor changes including the length, radius and wall recombination coefficient are found to have the largest influence on outputs obtained from the model.

Concepts, Capabilities, and Limitations of Global Models: A Review

“…For investigations of general properties, a simple planar or cylindrical geometry is usually sufficient . However, if a particular device is to be modeled for the purposes of comparison with experiment, then more complicated geometries are often required, as well as considerations of gas flow and mechanical pumping systems …”

“…Neutral particles are often taken to have a Maxwellian EDF with a particular temperature, although it is possible to model this temperature self consistently . Ionic species are not necessarily in thermal equilibrium, and so may deviate from a Maxwellian EDF; despite this they are often modeled as such due to the complications of including such a deviation . Electrons can be described by a Maxwellian EDF only under certain specific conditions, and so it is often more appropriate to make a different assumption.…”

“…In general, excited states play an important role in many systems investigated by global models . Generally, three different types of excitation exist: electronic, rotational, and vibrational.…”

“…Additionally, vibrationally excited molecules often have larger dissociative attachment cross sections, and as such their presence can enhance the production of negative ions. An important example is the formation of H − ions in plasmas containing H 2 , such as those used in negative ion sources for fusion applications …”

“…A number of different expressions exist for this factor, often termed , that relate to a particular set of conditions. They are typically derived from analytic assumptions or through the finding of empirical relations …”