Interplay among various cavity modes in a microwave plasma system with well-defined cavity geometry

In an experimental microwave ion source plasma system with a well-defined cavity geometry, multiple cavity resonant mode excitations have been observed. The interactions among these modes can influence microwave coupling to the plasma, enhance plasma uniformity, and affect plasma oscillations. The superposition of closely spaced cavity resonant modes leads to a temporal modulation of the plasma due to the beating effect between pairs of modes. As a result, a new range of plasma oscillations is recorded at the same modulation frequency. This modulation is confirmed by the experimentally measured frequency emission spectra and the accumulation of hot electrons within the plasma-filled cavity. The plasma's resonance with the modulated wave contributes to an increase in the hot electron population. Additionally, the phenomenon of parametric decay (PD) can help explain the rise in hot electron populations in over-dense plasma. The frequency emission spectra show evidence of ion acoustic waves, whose daughter electrostatic waves are resulting from the PD. These appear as two sideband frequency peaks around each excited cavity mode frequency, adhering to the frequency and k-vector selection rules. The observed daughter wave peaks are identified as ion acoustic waves. All experimental findings have been further supported by analytical calculations and microwave plasma simulations conducted using the Finite Element Method in COMSOL Multiphysics software. This paper highlights the temporal phase modulation and the PD phenomena induced by the excitation of different closely spaced cavity modes around the broadly launched microwave frequency of approximately 200 MHz at 2.45 GHz and their interactions.