Investigation of the Paschen Curve for Various Electrode Geometries in IEC Fusion Devices through Monte Carlo Simulations

Abstract

The creation of plasma is key for achieving fusion in Inertial Electrostatic Confinement (IEC) fusion devices, and the conditions for such electrical breakdown are modelled by Paschen's law, which predicts the breakdown voltage of a system as a function of the product of pressure and gap distance (pd) between electrodes. However, the Paschen curve only models parallel plate configurations of electrodes and is rarely explored in the more complex electrode geometries often seen in IEC fusion devices, including Farnsworth-Hirsch fusors. To bridge this gap, we study the Paschen curve for various electrode configurations by use of Monte Carlo (MC) simulations in Mathematica and Java. We compute alpha - the rate of ionizations per unit length - in constant E-field systems to explore differences between predicted alpha values from the scientific literature and those from our MC simulations. We explore a Markov chain model for 1D alphas which outperforms the literature alpha prediction in modelling our 1D MC-derived alphas and more closely matches parallel plate MC breakdown results at the minimum. 3D simulations and breakdown plots are created for concentric sphere and sphere-in-cylinder geometries. We observe a widened breakdown curve for concentric spheres and sphere-in-cylinder geometries and note the reduced growth of breakdown voltage at small pd values for the sphere-in-cylinder case. These findings help explain the data collected from our own experimental fusor setup and suggest further work in the simulation of more complex electrode configurations and the incorporation of experimental data to verify results.