Optimum choice of RF frequency for two beam linear colliders

Recent experimental results on normal conducting RF structures indicate that the scaling of the gradient limit with frequency is less favourable than was believed. We therefore reconsider the optimum choice of RF frequency and iris aperture for a normal conducting, two-beam linear collider with E/sub CMS/=3 TeV, a loaded accelerating gradient of 150 MV/m and a luminosity of 8/spl middot/10/sup 34/cm/sup -2/s/sup -1/. The optimisation criterion is minimizing RF costs for investment and operation with constraints put on peak surface electric fields and pulsed heating of accelerating structures. Analytical models are employed where applicable, while interpolation of simulation program results is used for the calculation of luminosity and RF structure properties.