or more skin depths at the frequency of operation this means that the ferromagnetic layer 12 is shielded from the rf or mw field by the conductive layer 14 also the upper surface of the conductive layer 14 is perfectly smooth such that the power losses encountered when using artificially roughened surfaces is avoided thus the invention allows to avoid multipacting without significant power loss which makes this invention especially attractive in cases where power supply is limited such as in satellites or other space applications the spatially or locally varying magnetic field 16 is generated by passive means namely by a static magnetization of ferromagnetic layer 12 with a rapidly varying magnetization pattern however instead of locally magnetizing ferromagnetic layers such as layer 12 on fig1 in an alternative embodiment it would also be possible to provide a locally varying distribution of ferromagnetic material which is exposed to a macroscopic or external magnetic field namely if a macroscopic magnetic field is applied for example by a bending magnet in an accelerator structure the ferromagnetic material will locally enhance the magnetic field and thus lead to a locally varying magnetic field as well in a preferred embodiment not shown the distribution of ferromagnetic material is inhomogenous on a microscopic length scale but homogenous on a macroscopic length scale such as to allow for a spatially rapidly varying magnetic field close to the surface of the conducting layer which becomes more and more uniform as one moves away from the surface this way the net field.