Collider constraints in the choices for wavelength and gradient scaling

During the last few years many suggestions have been made for novel accelerating schemes, with the aim of making centre of mass energies of 1 TeV and above available for e+e-collisions at a socially acceptable cost. A wavelength range from microns to centimetres, and field gradients from tens to thousands of MeV per metre have been considered. It is now evident that the need for high luminosity with acceptable power cost in the face of destructive collider constraints imposes severe restrictions on the choices available. An attempt is made to examine these in a general way, but with emphasis on wavelength and gradient scaling. Non-harmonic schemes, where an equivalent wavelength must be assigned, will be included in the survey, and the use of multiple bunches considered. The need for essentially new inventive suggestions will be emphasized. (author)