The power of electromagnetic emission near the plasma frequency during collective electron beamplasma interaction is found to be significantly increased in a plasma with preformed large-amplitude density perturbations. Laboratory experiments at the GOL-PET facility show that injection of a kiloampere relativistic electron beam into a magnetized plasma with strong radial density gradients is accompanied with an order of magnitude more intense generation of sub-millimeter waves than in the case of smooth density profile. As a possible mechanism for the enhanced generation of the longitudinal radiation flux observed in these experiments, we discuss the direct beam pumping of longitudinally propagating electromagnetic plasma modes due to their coupling with the Doppler shifted beam branch in the presence of oblique modulation of plasma density.

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