Technical Paper Abstract

Rf-Focused Drift Tube Linac Structure

Donald A. Swenson,
Linac Systems, L.L.C

A new rf-focused drift tube linac structure will be presented. It resembles a drift tube linac with rf focusing incorporated into each drift tube. As in conventional drift tube linacs, these drift tubes are supported by single stems along the axis of a cylindrical cavity that is excited in the TM₀₁₀ rf cavity mode. These "drift tubes" comprise two separate electrodes, operating at different electrical potentials as determined by the rf fields in the cavity, and supporting a total of four fingers that produce an rf quadrupole field distribution along the axis.

The fundamental periodicity of this structure is equal to the particle wavelength. Particles, traveling along the axis, traverse two distinct regions, namely the gaps between drift tubes, where the acceleration takes place, and the regions inside the drift tubes, where the rf quadrupole focusing takes place. This structure uses both phases of the rf fields to effect the beam; one for accelerating the beam and the other for focusing the beam. In this case, the "reverse phase" does not decelerate the beam because the fields inside the drift tubes are distorted into transverse focusing fields with little longitudinal component. The orientation of the fingers in the focusing regions alternate so as to create an alternating focusing and defocusing action on the beam in each transverse plane.

This new linac structure does not replace or compete with the conventional RFQ structure, but rather provides a graceful way to accelerate the small diameter, tightly bunched beams that come from RFQ linacs to higher energies. It offers additional degrees of freedom for the designer to exploit in the pursuit of enhanced performance and lends itself to miniaturization more readily than its magnetically focused counterpart.