breathing oscillation driven by non-linearly coupled spokes in partially magnetized E×B discharge

The underlying mechanism for the intermittent appearance of breathing oscillations below 20 kHz in E×B sources has been a long-standing puzzle. Recently, alongside these oscillations, two rotating spokes with frequencies of 30 and 60 kHz (mode numbers 2 and 3) have been observed in the Penning E×B source. The sporadic nature of the spokes' appearance led to the hypothesis that their interaction or three-wave coupling may cause the breathing oscillations. To test this, the Penning discharge with two spokes was analyzed using a high-speed camera and a two-tip probe. During breathing oscillations, changes in internal characteristics were measured: decreased internal neutral pressure, increased electron density for one spoke, and reduced global visible light emission. The question arose about the source of energy for additional ionization. Non-linearly coupled relations for the three waves were confirmed via bi-coherence analysis, showing increased intensity in other frequency domains. The non-linear electric field energy transfer rate indicated significant energy transfer from the spokes to the breathing oscillations. Additionally, a new third wave at 90 kHz was confirmed, indicating mode coupling instability. We conclude that mode coupling of the sporadic spokes results in the non-linear transfer of electric field energy to other frequencies, causing additional ionization from neutral pressure. In the lab frame, the reduced light emission due to decreased neutral pressure appears as 'breathing'.