Effects of external magnetic fields on the operation of high-gradient accelerating structures

Field emission in an rf cavity in the presence of external magnetic fields is examined. We show that emitted electrons from a sharp protrusion are focused by the magnetic field into small spots at another location in the cavity where they heat its surface. Scaling laws are established for the beam's induced heat in terms of macroscopic quantities such as magnetic field, accelerating gradient and spot dimensions. We find that when the magnetic field is of the order of a Tesla, the induced thermal stresses by the pulsed electron flux exceed the elastic limit and the surface becomes prone to cycling fatigue. The implication of these findings on the observed surface damage and magnetic-field-depended breakdown of an 805 MHz cavity is addressed.