Experimental test of the quasilinear theory of the interaction between a weak warm electron beam and a spectrum of waves

Recent theoretical work has suggested that the quasilinear description of the weak-warm-beam--plasma instability is incomplete. It has been argued that beam-mediated mode-coupling effects neglected in standard quasilinear theory play an important role, and that their average effect is a zeroth-order increase in the growth rate. We have experimentally observed strong mode-coupling effects when a weak warm beam interacts with waves on a slow wave structure. However, when we average the effects of the mode coupling we do not observe the predicted zeroth-order increase in the growth rate.