The use of the random phase approximation in quasilinear theory has been controversial for some time. For the bump‐on‐tail instability this approximation leads to the neglect of mode coupling effects mediated by the resonant particles. Recently, it has been argued theoretically and numerically that resonant particle mediated mode coupling effects actually play an important role, and that the statistically averaged effect of this mode coupling is a zeroth‐order increase in the growth rate. The quasilinear theory of the interaction between a warm beam and a slow wave structure is formally identical to the quasilinear theory of the interaction between a warm beam and a plasma in the weak beam limit. Strong mode coupling effects have been experimentally observed when a weak warm beam interacts with waves on a slow wave structure. When a statistical average is done over the mode coupling, however, the predicted zeroth‐order increase in the growth rate is not observed.

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