On stabilization of linac cavities

For a number of years long Alvarez linac cavities, operating in the usual (β = 0) Alvarez accelerating mode, have been stabilized by means of a lower frequency passband associated with the excitation of drift tube stems and/or special coupling posts. This lower passband can be coupled to the usual Alvarez mode yielding a nonzero group velocity and a larger mode spacing at the working point. In this paper a field theoretical approach to the stabilizaton problem is given. Some general ideas about crossing dispersion curves are presented and illustrated, and it is shown that at the crossing point the two modes are present in equal amounts, which leads to a formula for the crossing group velocity in terms of the fields of the modes which are mixed. Some simple examples for stemmed structures are worked out. (author)