A large amplitude upper hybrid wave has potential to be employed as a wiggler for the generation of high frequency coherent radiation via free electron laser (FEL) instability. At a density fluctuation level of a few percent, due to the upper hybrid wave, the growth time of FEL instability, with electron beam current of a few kilo ampere, turns out to be of the order of a nanosecond. The growth rate of FEL instability depends sensitively on wi and this dependence comes through v osc , the beam

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... scillatory velocity. v OBC acquires large values at k Oz ~ [wo -(wc/7 §)]/f6 =s w c [l -(l/ 7o°) ]/«; 6 . At this value of k Oz , wi ~ 2TO 2 W C [2 -(I/70)]. However, this scheme of FEL operation suffers from a severe limitation due to the parametric instability of the upper hybrid wave. The process of parametric decay into lower hybrid and upper hybrid waves may have growth rate comparable to that of the FEL instability.