Optimum Generator Characteristics of RF Amplifiers for Heavily Beam-Loaded Acceterators

Abstract

As the beam loading is increased in a proton linear accelerator, it becomes increasingly more difficult to maintain the design gradient. To avoid excessive beam spill, this gradient must be controlled to a tolerance of ~ ± 1% during the initial transient of beam turn-on and during any subsequent fluctuation in beam loading due to uncontrolled or programmed variations in the amount of beam injected. Since the beam obtains its energy from the field, more power must be supplied by the rf generator when the beam is on. High-power microwave triodes, klystrons, and crossed-field amplifiers are being evaluated as generators to drive the accelerator structure of the LAMPF proton linac. The delivery of power from these tubes to the time-varying load impedance presented by the linac is a complicated interplay among such factors as the internal design of the tube's active region and output coupling section, the length of line between the generator and the load, and the coupling to the load. An optimum system design can enable the generator to provide a degree of self-compensation providing a programmed amount of power to match the changing load impedance. This eases the problem of providing fine control to the 1% tolerance by a closed-loop system. Experimental studies have been conducted on each tube type.