Electron Beam Excitation of a Surface Wave in mm-Wave Open Accelerating Structures

As part of research on the physics of rf breakdowns we performed experiments with high gradient traveling-wave mm-wave accelerating structures. The accelerating structures are open, composed of two identical halves separated by an adjustable gap. The electromagnetic fields are excited by an ultra-relativistic electron beam. We observed that a confined travelling-wave mode exists in half of the accelerating structure. The experiments were conducted at FACET facility at SLAC National Accelerator Laboratory. Depending on the gap width, the accelerating structure had beam-synchronous frequencies that vary from 90 to 140 GHz. When we opened the gap by more than half wavelength the synchronous wave remains trapped. Its behavior is consistent with the so called “surface wave”. We characterized this beam-wave interaction by several methods: measurement of the radiated rf energy with the pyrodetector, measurement of the spectrum with an interferometer, measurement of the beam deflection by using the beam position monitors and profile monitor. INTRODUCTION We studied physics of rf breakdown in open mm-wave accelerating structures [1]. The picture of one side of the accelerating structure is shown in Fig. 1 The fields were excited by an ultra-relativistic electron beam. We changed the interaction with the beam by changing the gap width. By opening the gap, the number of trapped modes is reduced. When the gap is opened beyond half wavelength the synchronous wave remains trapped. With larger gaps the parameters of the wave such as frequency and group velocity are less dependent on gap width. By increasing the gap, the rf power is guided by the corrugations with no radiation, behavior consistent with the so called “surface wave”. Surface waves were studied for applications to communications by [2], showing that guided waves do not necessesarly need to be confined within physical boundaries. G.Goubau [3, 4, 5] presented single conductor surface wave transmission lines. W. Rotman studied a single surface corrugated waveguide [6]. A review of surface waves is presented by G. John [7]. H. M. Barlow [8] discussed *Work supported by the US DOE under contract DE-AC0276SF00515. ** dalforno@slac.stanford.edu Output waveguide Output horn Output horn Coupler iris Electron beam Coupler cell