Scaling linear colliders to 5 TeV and above

Detailed designs exist at present for linear colliders in the 0.5-1.0 TeV center-of- mass energy range. For linear colliders driven by discrete rf sources (klystrons), the rf operating frequencies range from 1.3 GHz to 14 GHz, and the unloaded accelerating gradients from 21 MV/m to 100 MV/m. Except for the collider design at 1.3 GHz (TESLA) which uses superconducting accelerating structures, the accelerating gradients vary roughly linearly with the rf frequency. This correlation between gradient and frequency follows from the necessity to keep the ac "wall plug" power within reasonable bounds. For linear colliders at energies of 5 TeV and above, even higher accelerating gradients and rf operating frequencies will be required if both the total machine length and ac power are to be kept within reasonable limits. An rf system for a 5 TeV collider operating at 34 GHz is outlined, and it is shown that there are reasonable candidates for microwave tube sources which, together with rf pulse compression, are capable of supplying the required rf power. Some possibilities for a 15 TeV collider at 91GHz are briefly discussed. Detailed design parameters have been developed for linear colliders in the 0.5- 1.0 TeV center-of-mass energy range (see, for example, the report of the International Linear Collider Technical Review Committee (1)). Some basic rf- related parameters for these machines at the 1 TeV design energy are given in Table 1. The designs listed are the SBLC (S-band Linear Collider) at DESY; the JLC (Japan Linear Collider) C-band and X-band designs developed at KEK, Japan; the NLC (Next Linear Collider) at SLAC; and the VLEPP (Russian acronym for colliding linear electron positron beams) at the Branch Institute for Nuclear Physics, Protvino. Parameters for the first (and as yet the only) operating linear collider, the 0.1 TeV SLC machine at SLAC, are shown for comparison. All the machines shown in the table use copper accelerating structures. Not shown is the TESLA linear collider at DESY, which is based on superconducting rf technology. Operating at a frequency of 1.3 GHz and a gradient of 25 MV/m, the TESLA technology is difficult to scale to a higher gradients and energy. Also not shown in Table I are two proposed linear colliders which are based on the two-beam accelerator approach: CLIC at CERN and the Two-Beam NLC proposed by LBNL, Berkeley, and LLNL, Livermore. Although the drive beam of a two-beam accelerator is capable of producing copious amounts of rf power at good efficiency, for the purpose of scaling linear colliders with frequency we consider here only colliders powered by discrete rf sources.