Unipolar arc studies of the graphite Langmuir probe and biased electrode in KTX plasmas

Abstract Unipolar arcs are commonly observed in magnetically confined plasmas and play a significant role in plasma-wall interactions, particularly when impurities are introduced. Moreover, unipolar arcs can negatively affect the accuracy of Langmuir probe measurements as well as the effectiveness of biased electrodes in improving confinement. Consequently, understanding the initiation mechanisms of unipolar arcs is essential for developing effective mitigation strategies. This work investigates the arc phenomena of graphite Langmuir probes and biased electrodes in the Keda Torus eXperiment (KTX) reversed field pinch device. Initially, under high bias voltage, the probe reached an arc-free state, indicating that the probe surface was sufficiently clean and no longer prone to arcing. However, in the subsequent continuous multi-day probe experiments, the phenomenon of unipolar arcs intermittently disappearing and then reappearing was observed. Similar phenomena also existed in electrode arcs. This behavior cannot be fully explained by surface conditions from prior studies, suggesting that the probes and electrode regained arc-inducing medium after the arc disappeared. This arc-induced medium is likely gas retained by graphite. Accordingly, calculations were performed to assess changes in the sheath electric field of the probe tip, while simulations were conducted to model the temperature shift caused by ion bombardment. The analysis revealed that the initiation of unipolar arcs is associated with the enhancement of the sheath electric field.