Optimum frequency and gradient for the CLIC main linac accelerating structure

A novel procedure for the optimization of CLIC main linac parameters including operating frequency and the accelerating gradient is presented. The optimization procedure takes into account both beam dynamics and high power rf constraints. Beam dynamics constraints are given by emittance growth due to short- and long-range transverse wakefields. RF constraints are given by rf breakdown and pulsed surface heating limitations of the accelerating structure. Interpolation of beam and structure parameters in a wide range allows hundreds of millions of accelerating structures to be analyzed to find the structure with the highest ratio of luminosity to main linac input power, which is used as the figure of merit. The frequency and gradient have been varied in the ranges 12-30 GHz and 90-150 MV/m respectively. It is shown that the optimum frequency lies in the range from 16 to 20 GHz depending on the accelerating gradient and that the optimum gradient is below 100 MV/m. Based on our current understanding of the constraints, changing the frequency and gradient from current values of 30 GHz and 150 MV/m to the optimum ones doubles the luminosity for the same main linac input power. Nevertheless, overall extension of the collider and investment cost considerations are not taken into account and impose gradient larger than 100 M/m to 120 MV/m.