  The problem of calculating the ac losses in a superconductor strip with a
transport current placed inside superconducting environments is studied
analytically in the frame of the critical state model. Exact results obtained
by the method of images for the commonly employed flat ground plates are used
to derive power losses and, consequently, the nonlinear resistance depending on
the ac frequency, current amplitude and the distance to the ground plates. The
resistance is strongly reduced when the distance between the strip and the
shields becomes small.
