A high‐voltage, fixed frequency re‐entrant cavity of moderate power requirements designed for use in the Cornell 1.5‐Bev synchrotron is described. Design considerations and details of the cavity gap structure are presented. The problem of multipactoring is described. Advantages of coaxial gap construction using an internal insulator are given. In particular a dc bias to prevent multipactoring can easily be applied. A simplified analysis of the operation of the cavity in terms of equivalent circuits is also given.