Waveguide stub-line tuning of RF cavities with heavy beam loading

A method is proposed for using adjustable, waveguide stub-line tuners to match the load impedance of the cavity plus beam to the power source, without the need to detune the cavity. Adjustable stub-line tuners are shown to be able to completely match the resistive and reactive parts of the load impedance to a generator under all conditions of beam loading, ensuring optimum power transfer between generator and cavity. This technique may have advantages in high-current storage rings such as the SLC damping rings and the new PEP II storage rings. The coupled-bunch instability driven by the fundamental mode of the cavity is re-appraised in this coupling scheme in which the cavity is no longer detuned. The consequences of this matching scheme are also considered for the beam loading stability limit.