This paper reports on progress on a high‐power mm‐wave source concept being pursued at Los Alamos. The concept is based on passing a high‐brightness, sheet electron beam through a slow‐wave structure created from a vane‐loaded waveguide. Component development was conducted with at 10‐kV experiment, and design work and fabrication work has been conducted for a demonstration experiment with a 120‐kV, 20‐A sheet electron beam operating at 95 GHz. © 2003 American Institute of Physics