A new method for the calculation of the energy confinement time in General Fusion’s plasmas

General Fusion (GF) is developing a magnetized target fusion concept to compress a spherical tokamak plasma to fusion conditions. The Plasma Injector 3 (PI3) device is the largest testbed at GF to study thermal energy confinement in pre-compression plasma targets. PI3 plasmas are formed via coaxial helicity injection into an aluminum flux conserver periodically coated with lithium and are ohmically heated. Knowledge of the energy confinement time is essential to determine the plasma compression time appropriately.

In this work, we calculate the energy confinement time for PI3 plasmas using radial profiles of thermal energy and ohmic power density. We compute the ohmic heating from profiles of plasma current density obtained from the equilibrium reconstruction algorithm developed at GF. We show how PI3 mode of operation affects the plasma inductance and subsequently the ohmic power distribution within the plasma. When the vacuum toroidal flux in the flux conserver drops rapidly during the plasma lifetime, current is driven at the plasma edge, leading to hollow ohmic heating density profiles. If, instead, the toroidal flux is sustained for 8-10 ms after plasma formation, the current profile assumes a peaked configuration. Energy confinement times up to ~10 ms are obtained with high and sustained toroidal flux, when the current distribution is broad but not hollow. We explore the effect of the toroidal magnetic flux and of lithium coating on energy confinement time for a large set of PI3 shots.