Modelling of electron cyclotron energy gain in the tokamak pre-ionization phase

This work presents a model for predicting electron energy gain from electron cyclotron (EC) wave–particle interactions during the breakdown phase of tokamak pre-ionization. Investigation of this phase to infer the EC requirements inevitably demands high-fidelity simulation study covering gas breakdown physics, plasma dynamics, and EC absorption. Under the significantly low density and temperature, electrons interact nonlinearly with EC wave and gain energy of which picture is distinct from the linear or quasilinear EC heating. For room temperature electrons, analytic estimates of energy gain have been derived using adiabatic nonlinear theory (Farina 2018 Nucl. Fusion 58 066012). However, electrons with large incident perpendicular energy can be generated from the ionization reactions and even be present in the Maxwellian tail during the breakdown. In this work, we refined the theory into a semi-analytic form that predicts nonlinear energy gain of arbitrary perpendicular energy. The semi-analytic model is demonstrated to be accurate and efficient in calculating macroscopic electron energy absorption: the expected total absorbed energies agree well with the numerical counterparts for key characteristic electron distributions, and computation time is significantly reduced by several orders of magnitude. This work is envisaged to offer a practical tool for electron avalanche dynamics study, contributing to development of predictive capability on EC pre-ionization phase of future tokamaks such as ITER and K-DEMO.