A Plasma Model for RF Breakdown in Accelerator Structures

A plasma model is presented for the formation of \cath-ode spots " and subsequent crater development near eld emission sites on a copper surface in the presence of a strong dc electric eld. Adding to previously published models, we propose that the two-stream plasma instabil-ity relates the plasma density to its dimensions. Argu-ments are presented that suggest that the formation and dynamics of such plasma spots are essentially the same phenomenon for both dc and rf elds. A consequence for accelerating structures is that, when such plasma spots are present, they erode the copper surface at a rate on the order of 300 micrograms per Coulomb of ion bom-bardment current. This erosion rate can have a measur-able eect on the rf properties of an accelerating structure after processing at high gradients for a few hundred hours.