The problem of the linear conversion of a Langmuir wave to a transverse electromagnetic wave in the presence of a density gradient has been solved numerically with appropriate boundary conditions. A reciprocity principle was found, allowing the deduction of solutions of this problem from those obtained from the transverse-Langmuir conversion. This model has been applied to study the spectrum emitted from an inhomogeneous plasma, including the effect of the antenna radiation pattern. Experiments have been performed in a large unmagnetized dc discharge plasma (ne∼5×1010 cm−3, Te = 1.3 eV, gradient scale length L = 100−1000 cm). The shape of the spectrum observed with a horn antenna agrees with the theoretical one, but the deduced level of Langmuir fluctuations is much higher than the thermal level. This enhancement is due to the presence of primary energetic (E = 60 eV) electrons.

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