Heavy beam loading measurements of an rf accelerating cavity under amplitude and phase control

Abstract

A technique for measuring both resistive and reactive particle-beam-loading effects on an rf accelerating cavity is described. A high-power, six-port reflectometer is used to measure the complex cavity impedance, both with and without beam, as a function of cavity resonant frequency. Resistive and reactive beam loading appears as a decrease in the cavity quality factor Q and as a change in the cavity resonant frequency, respectively. An equivalent circuit model for the drive system, cavity, and beam is used to quantify these effects as a function of cavity voltage and phase, beam current, and cavity detuning. Measurements on a heavily beam-loaded two-cell drift-tube linac are compared with predictions from the equivalent circuit model and good agreement is found. The measurements are performed while maintaining constant cavity-field amplitude and phase. Thus, this technique is a nonperturbative measurement.