Diffusion of a test electron beam in a discrete spectrum of waves propagating along a traveling wave tube

The velocity diffusion of particles in a field of randomly phased waves is experimentally investigated. An arbitrary waveform generator is used to launch a prescribed discrete spectrum of waves along the helix of a traveling wave tube. A cold test electron beam propagates along the axis of the tube and interacts with the waves without self-consistently perturbing their amplitudes. A trochoidal energy analyzer records the beam energy distribution at the output of the tube. The energy spread of the beam is measured as the position of the emitter probe is varied. A velocity diffusion coefficient can thus be measured. Two different situations are compared: one with a large overlap parameter between neighboring modes where standard quasilinear diffusion theory is valid; the other one with an intermediate overlap parameter where numerical simulations have shown that the diffusion coefficient exceeds the quasilinear value by a factor of about 2.3.