This invention relates to radiofrequency (rf) cavities and couplers that comprise metallic or dielectric rods to provide specified concentration of field patterns for the operating modes in the interaction region, for applications in particle accelerators, pulsed rf power sources, amplifiers, mode converters and power couplers.
 This invention was made with government support under Grant No. DE-FG03-02ER83400 and Grant No. DE-FG02-03ER83845 awarded by the U.S. Energy Department. The government may have certain rights in the invention.