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. Thus, an economically feasible cavity acceptance rate will include a certain percentage of sub-performing cavities. An important question that needs to be addressed is, For a string of cavities rated to various levels of gradient and powered from a common source how can we optimize the overall gradient? Along with adjustable cavity coupling--or loaded Q factor--we assume adjustable RF power so that gradient can be leveled in nonnominal cavities, to avoid quench-inducing overshoots. In the ILC an RF unit comprises three cryomodules containing a total of 26 nine-cell cavities, which are fed by one klystron that nominally feeds equal power to all cavities. One simple way of running such a unit is to set RF power, beam arrival time, and all loaded Q's so that the power is matched and the gradient in all cavities equals the gradient limit in the poorest performing cavity. This conservative strategy, however, sacrifices gradient and can be improved upon. 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 (VTO's)[1] by which the RF power to succeeding pairs of cavities can be made to differ. These solutions will not be matched, resulting in power inefficiency and a gradient variation along the beam that needs to be limited. In this report we study the effect on overall gradient for various combinations of these ideas. Deciding which strategy is best can then be done by balancing gradient with the cost and complexity of hardware such as circulators and variable power tap-offs