Electron density and velocity functions in a low beta Polywell

Abstract

Langmuir probe data from a low beta Polywell device at the University of Sydney are analysed to compare models describing its electron velocity distribution function (EVDF). Three models are compared using the Bayesian evidence across datasets measured at different spatial points in the device at varying coil voltage biases and coil currents. The best performing model of the EVDF across all datasets is found to be a mixture of Maxwellian and Gaussian distributions. This outperforms a model with a pure Maxwellian distribution and another model with an advanced non-thermalised distribution, indicating that this device partially thermalises its confined electron population. From the same analysis, the Bayesian posterior is used to give statistical distributions of plasma parameters, such as the plasma potential and density. Compared across the different datasets, these parameters indicate the successful formation of an electric potential well, crucial to the operation of the Polywell, and the conditions in which it appears. They also indicate that the electron density forms a dense shell near the centre of the device, which contracts with increasing bias voltage and coil current.