RF Distribution optimization in the main linacs of the ILC

The nominal design gradient in the main linacs of the International Linear Collider (ILC) is 31.5 MV/m for a beam current of 9.0 mA. However, the superconducting cavities built to date have demonstrated a range in sustainable gradient extending well below this goal, being limited by Q drop-off and quenching. For the current distribution of gradient limits in the RF cavities of the ILC linacs we have optimized the overall gradient of a 26 cavity RF unit, assuming the availability of various combinations of circulators and variable power tap-offs. One simple way of running such a unit is to set RF power, beam arrival time, and all loaded Q values so that the power is matched and the gradient in all cavities equals the gradient limit in the poorest performing cavity. One improvement strategy is to adjust the cavity couplings individually (possible, since circulators are assumed in the baseline ILC design) or in pairs (when circulators are not needed) using the movable antennae of the fundamental mode couplers. Another strategy is to use variable power tap-offs (VTOs) by which the RF power to succeeding pairs of cavities can be made to differ.