Plugging of multi-mirror machines by a traveling rotating magnetic field

Abstract: Axial plugging is a critical challenge for fusion in open-ended magnetic confinement systems. Multi-mirror systems, consisting of a series of axially aligned magnetic mirrors, aim to enhance axial confinement by increasing the effective diffusion coefficient; however, additional plugging is required to meet the Lawson criterion. In [T. Miller et al., Phys. Plasmas 30, 072510 (2023)], it was found that applying a traveling and rotating electric field in multi-mirror machines can significantly suppress axial loss due to a selectivity effect induced by the Doppler shift of the ion cyclotron resonance. However, this method is energetically expensive and vulnerable to plasma screening effects. Here, we propose using a traveling, rotating magnetic field that can achieve comparable plugging effectiveness while offering better penetration and lower energy costs. Two limiting scenarios, with and without an induced electric field, were considered. The confinement enhancement is calculated using a semi-kinetic rate equation model, in which the flux coefficients are determined from single-particle simulations. While both scenarios exhibit significant confinement enhancement, the scenario without an induced electric field is much more energetically efficient, as it relies on phase-space mixing rather than on energy deposition in the escaping particles. The decoupling of confinement from plasma collisionality enables fusion conditions in the central cell while allowing affordable and efficient confinement enhancement in the multi-mirror sections.