Wave Excitation in Waveguides below Cut-Off

A 400 kV, 2kA, electron beam has been used to grow space charge waves in a 5.0 cm diameter drift tube. The waves are excited by propagating the pencil beam through a series of annular metallic rings with alternate rings grounded and the others capacitively coupled to the beam. Measurements have been made and will be presented of the wave characteristics downstream of the exciting structure. The wave excitation may result from a parametric excitation of eigenmodes of the beam in the guide.