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 reappraised 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