Physical processes controlling dark current emission and resulting breakdown in Linacs.

Higher accelerating gradients are required for future high-power devices such as the TeV electron linear collider and muon-muon collider. A limit in the maximum field gradient is imposed by the rf dark current and the resulting electrical breakdown. Two critical questions need to be answered: What causes high dark current and breakdown? Can the breakdown threshold be increased? These problems have been the focus of significant experimental and theoretical investigations for over a century. The purpose of this report is to shed some light on the possible mechanisms that limit the field gradient in linacs with and without guiding magnetic fields. The idea of enhanced field emission due to separation of grains from a material surface by the electric field tension acting normally to the surface is discussed. The electric force can be high enough to overcome the binding energy of grains in the material. Thus, small-size separated grains result in enhanced field emission. The stopping power of relativistic and ultra-relativistic energies of ejected electrons at high electric field results in more production of secondary electrons. At high energy, E>>mc{sup 2}, the stopping power of relativistic and ultra-relativistic electrons becomes similar to the stopping power of ions with the same energy but much less velocity. It results in more production of the secondary electrons. The strong influence of a guide magnetic field results in more electrons reaching the opposite electrode due to straightening of electron trajectory and, consequently, more production of the secondary electrons. For a more accurate understanding of these complicated issues, detailed experimental and theoretical investigations are required