Experimental study of flute instability by even/odd-parity quadrupole fields in Keda Mirror with AXisymmetricity (KMAX)

Abstract This study investigates the stabilization of flute mode instabilities in the Keda Mirror with AXisymmetry device, specifically focusing on the m = 1 mode that propagates in the direction of electron diamagnetism due to the E × B force. To mitigate this instability, quadrupole field (QPF) antennas were designed with either even or odd parity, depending on the direction of the induced current. Experimental results reveal that even parity significantly enhances stability, reducing fluctuation values at the plasma boundary by approximately 50%, while odd parity achieves only a 10% to 20% reduction. Radial particle flux measurements indicate a decrease to one-tenth of the original value when the even-parity QPF is applied during the wash-gun injection phase, with no correlation observed between ion saturation current and angular electric field fluctuations. The stability enhancement is attributed to the positive magnetic pressure gradient generated by the QPF. Further analysis incorporating magnetic field curvature reveals that the beneficial effects are more pronounced with even parity. However, the density measured by the interferometer decreases significantly with increasing even-parity QPF current, while the density with odd parity remains largely unchanged.