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Observations of electromagnetic radiation from plasma in presence of a double layer

In earlier laboratory investigations of a double layer a plasma region with an intense, spontaneous HF-generation was observed by means of internal HF-probes. The region was located 50-200 Debyelengths from the double layer, on the high potential side. It was concluded that the HF-field is generated by beam-plasma interaction, and can best be classified as electrostatic Langmuir waves. Such a wave field should in first approximation not radiate electromagnetic waves. The present paper reports on electromagnetic radiation in the near-field region outside the plasma, measured by means of small, electrostatically shielded loop-probes. At the electron gyro frequency a broad peak is seen in the spectrum, with spatial maximum outside the region of internal HF-generation. Unexpectedly the azimuthal magnetic HF-component is stronger than the axial component. At the plasma frequency (higher than the gyro frequency, and less well defined) a broad range of HF emission is seen, extending to the upper hybrid frequency. The spatial maximum is outside the region of intense interaction, and the axial component dominates, suggesting oscillations at the upper hybrid frequency. Groups of intensity peaks are also seen near the upper hybrid frequency.