Self-induced distributed feedback in plasma-filled radiation sources

We show that the efficiency enhancement in plasma filled radiation sources and its dependence on the density of the background plasma can be accounted for by a mechanism of self-induced distributed feedback. Two counter propagating waves modulate the plasma density by the ponderomotive potential of their multiplication causing thus a grating in the refractive index of the plasma. The two waves are coupled by Brag reflections. The distributed feedback enhances the interaction between the resonant wave and the beam, and reduces the threshold gain for oscillations. Both effects enhance the efficiency of the oscillator.