Feedback control stabilises plasmas
Principle of Real Time Control of Magnetic Fields at JET. Note: Large Coils are wound around the Machine Axis
In the previously mentioned example of computer mouse control, the mouse cursor position is sensed by your eyes, the target area is the reference, your brain is the controller and your arm the actuator. Instead of the plasma environment there is a mouse, computer and monitor between the actuator (arm) and the sensor (eyes), with a much more predictable behaviour.
Actuators must be designed so that they have enough power to change the quantities measured by sensors, but possibly without modifying other characteristics of the system. The controller, on the other hand, should be designed so that it can respond to errors within an appropriate time, usually referred to as a deadline. In today’s plasma physics, the controller commonly consists of a Proportional-Integral-Derivative (PID) element as used in many industrial process controllers, e.g. in chemical plants. However, more sophisticated controllers based on multiple-input multiple-output models, state-space models, and neural networks are being developed.
In magnetic confinement fusion research, the earliest examples of real time control were in the sensing and control of the magnetic fields used to keep the very hot plasma away from the vessel walls. Feedback stabilises the confining magnetic fields, counteracting plasma forces that randomly disturb the configuration. The magnetic field is monitored by tiny magnetic probes and from their data the plasma boundary position is calculated. The distance of the plasma boundary from points within the vacuum vessel produces a Sensor measurement. The controller takes the errors in these distances and drives large poloidal coils (the actuator) to correct the magnetic fields. Scientists realised in the early sixties that without this feedback control, high temperature plasmas would never survive for more than a few tens of milliseconds. Of course, the feedback at that time was completely hard-wired and analogue. It was based on resistive, inductive and capacitive elements and amplifiers, not on digital processors.
