Hydrogen plasmas and their interaction with fusion-relevant materials

Abstract

Numerical modelling and experimental measurements are used to investigate hydrogen plasmas in the MAGPIE reactor, a low pressure radio frequency helicon device designed for plasma-material interaction experiments. The interaction between these plasmas and fusion-relevant surface materials are explored. A global model was developed and is implemented to describe the behaviour of electrons, ground-state atomic and molecular hydrogen, three positive ion species, a single negative ion species, and fourteen vibrationally excited states of molecular hydrogen in a low pressure discharge. The interactions between these species are defined through 33 processes, comprising 352 individual reactions. The model's operation is validated using experimental measurements of the electron density, electron temperature, and atomic hydrogen density. Subsequently, the model is applied to the MAGPIE reactor where it is employed to predict neutral and charged species densities as well as the electron temperature as a function of power, pressure, atomic loss coefficient, quenching coefficient, gas temperature, gas flow rate, and radial diffusion strength. The plasma chemistry contributing to each particle species' densities is evaluated and the dominant generation and loss mechanisms ascertained. Model simulations show that the atomic loss coefficient is observed to drive significant changes in plasma chemistry without changes in electron density or electron temperature. For a 1000 W, 10 mTorr MAGPIE plasma, it is demonstrated that at low loss coefficient values, neutral hydrogen atom density exceeds the molecular density and H+ becomes the dominant ion. For loss coefficient values approaching one, the dissociative fraction approaches zero and H3+ becomes dominant. The simulations demonstrate the large degree to which the choice of reactor wall material can impact plasma conditions. Experimental measurements of loss coefficient for stainless steel, copper, nickel, tungsten, and graphite samples are made using pulsed induced fluorescence (PIF) in MAGPIE. The loss coefficient is shown to be enhanced by plasma exposure. It is suggested that ion bombardment causes the production of additional adsorption and absorption sites on the material surface. These measurements are made for a range of powers between 100 and 800 W and at pressures between 5 and 100 mTorr. The loss coefficient is observed to change as a function of pressure, power, and sample material type. To further investigate the behaviour of neutral species, Doppler broadening of the Balmer-alpha emission line and optical emission spectroscopy of ro-vibrationally excited H2 is used to measure the atomic and molecular gas temperatures in the MAGPIE reactor. In low power operation (< 5 kW), the molecular hydrogen temperature is observed to be linearly proportional to the pressure while the atomic hydrogen temperature is inversely proportional. The energy imparted to atomic fragments of the exothermic dissociation process is shown to be a significant source of neutral atom heating. A maximum temperature of approximately 1200 K is measured during high power operation (20 kW) with localised cooling observed at sample surfaces. Show more