Resistive-Wall-Mode Control
Stabilizing slow, wall-limited magnetohydrodynamic modes with feedback coils to allow operation above the no-wall pressure limit.
What an RWM is
At high plasma pressure, an ideal external kink mode would grow too fast to control - but a nearby conducting wall slows it to the wall's resistive timescale, turning it into a resistive-wall mode (RWM). This slowing is what makes the mode controllable: it grows on milliseconds rather than microseconds, within reach of feedback.
Why we care
The RWM sets a pressure ceiling. Without wall stabilization, plasma pressure is capped at the no-wall beta limit; a perfectly conducting wall would raise it to the ideal-wall limit. A real, resistive wall gives neither for free - the RWM grows slowly and must be actively stabilized to operate in the gap between the two limits, where performance is higher.
Two stabilization routes
- Plasma rotation: sufficient toroidal rotation can stabilize the RWM through dissipative coupling
- Active feedback: coils sense the mode's field and apply an opposing field to hold it down
- In practice: a combination, with feedback covering regimes where rotation is insufficient
The feedback loop
RWM feedback measures the non-axisymmetric field of the growing mode with saddle loops and drives correction coils to cancel it. Because the mode is slow, the loop bandwidth is modest, but it must reject error fields and sensor pickup from the correction coils themselves. Loop design centers on distinguishing the real mode from its own control signal.
Relation to error fields
Static error fields and the RWM are closely linked: a resonant error field can drive the mode, and RWM feedback coils are often the same hardware used for error-field correction. The two functions are usually designed and operated together.