  The plasma equilibrium in a linear trap at $\beta\approx 1$ (or above the
mirror-instability threshold) under the topology-conservation constraint
evolves into a kind of diamagnetic "bubble". This can take two forms: either
the plasma body greatly expands in radius while containing the same magnetic
flux, or, if the plasma radius is limited, the plasma distribution across
flux-tubes changes, so that the same cross-section contains a greatly reduced
flux. If the magnetic field of the trap is quasi-uniform around its minimum,
the bubble can be made roughly cylindrical, with radius much larger than the
radius of the corresponding vacuum flux-tube, and with non-paraxial ends. Then
the effective mirror ratio of the diamagnetic trap becomes very large, but the
cross-field transport increases. The confinement time can be found from
solution of the system of equilibrium and transport equations and is shown to
be $\tau_E\approx\sqrt{\tau_\parallel\tau_\perp}$. If the cross-field
confinement is not too degraded by turbulence, this estimate in principle
allows construction of relatively compact fusion reactors with lengths in the
range of a few tens of meters. In many ways the described here diamagnetic
confinement and the corresponding reactor parameters are similar to those
claimed by the FRCs.
