  One of the main challenges of fusion reactors based on magnetic mirrors is
the axial particle loss through the loss cones. In multi-mirror (MM) systems,
the particle loss is addressed by adding mirror cells on each end of the
central fusion cell. Coulomb collisions in the MM sections serve as the
retrapping mechanism for the escaping particles. Unfortunately, the confinement
time in this system only scales linearly with the number of cells in the MM
sections and requires an unreasonably large number of cells to satisfy the
Lawson criterion. Here, it is suggested to reduce the outflow by applying a
traveling RF electric field that mainly targets the particles in the outgoing
loss cone. The Doppler shift compensates for the detuning of the RF frequency
from the ion cyclotron resonance mainly for the escaping particles resulting in
a selectivity effect. The transition rates between the different phase space
populations are quantified via single-particle calculations and then
incorporated into a semi-kinetic rate equations model for the MM system,
including the RF effect. It is found that for optimized parameters, the
confinement time can scale exponentially with the number of MM cells, orders of
magnitude better than a similar MM system of the same length but without the RF
plugging, and can satisfy the Lawson criterion for a reasonable system size.
