Negative consequences of dynamo Wall mode locking
The tearing modes that are responsible for the dynamo process tend to lock in phase, due to their non-linear interaction, thus forming a localized magnetic perturbation. In RFX, where this perturbation extends toroidally for about 40 degrees, the phenomenon of wall locking is also present.
The perturbation hooks to magnetic field errors caused by the presence of gaps and portholes in the shell, and remains stationary throughout the discharge.
Since the localized magnetic perturbation is associated to a local helical distortion of the plasma column, a consequence of the wall locking is that a portion of the graphite first wall is exposed to severe heat loads, of the order of 100 MW per square meter.
The strong heat load can cause carbon blooming, and this has prevented for some time the achievement of plasma currents higher than 800 kA. More recently a technique has been devised to induce a rotation of the locked modes, so as to distribute uniformly the heat load over the whole first wall and to avoid this limitation.
