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

Axial plugging is a critical challenge for fusion in open-ended magnetic confinement systems. Unlike simple magnetic mirrors, which suffer from direct axial flow, multi-mirror systems utilise a series of aligned magnetic cells to suppress plasma loss; however, the resulting confinement still requires additional plugging to reach Lawson criterion levels. In Miller et al. (Phys. Plasmas, 2023, vol. 30, p. 072510), it was found that applying a travelling 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 show that using a travelling, rotating magnetic field 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 rate 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.