The design of high attenuation periodic microwave structures and their application during the past few years as internal collinear terminating loads for high power linear accelerator systems is described. An extension of these concepts for the development of small high vacuum loads which were located external to the actual accelerator waveguide and capable of absorbing high levels of r.f. power without adversely affecting the matching characteristics of the system is also discussed. Avoidance of the pulse shortening or beam blow-up phenomena enables beam currents considerably greater than the maximum conversion efficiency value to be attained and this results in the regeneration of r.f. power within the accelerator waveguide. Several examples of this condition and an analysis of the various beam induced power conditions in passive and "positron" phased non-uniform impedance circuits are presented. It is shown that in certain circumstances high current multisection machines, utilizing waveguide designs of differing conversion efficiency, can encounter beam induced electric field strengths sufficient to cause voltage breakdown. Exploitation of the stored energy mode of operation and demonstration of short pulses of beam current an order of magnitude greater than the steady state maximum conversion efficiency value is reported together with the associated capability of generating short pulses of very high r.f. power by beam induction.