Current-Profile and q-Profile Control

Controlling the current profile controls the safety-factor profile, which sets both stability and confinement in advanced scenarios.

Why the current profile is central

The radial distribution of plasma current determines the safety factor profile, usually called the q-profile. The q-profile sets where rational surfaces sit, whether the plasma is stable to tearing and kink modes, and whether transport barriers can form. Controlling it is the foundation of advanced, high-performance operation.

The slow actuator problem

The current profile evolves on the resistive diffusion timescale, which can be seconds long, and it responds to the integral of past current drive rather than the instant command. This makes it a slow, memory-laden system to control. Feedforward planning does most of the work; feedback makes gentle corrections around the plan.

Actuators

Current is driven non-inductively by neutral beams and by radio-frequency waves, each with a controllable deposition radius. The bootstrap current, a self-generated current tied to the pressure profile, is also part of the picture and is influenced indirectly through pressure control. A profile controller combines these to sculpt the total current.

Keeping q above one

A common goal is to keep the central safety factor above one to avoid sawteeth, or to hold a broad low-shear region to support a transport barrier. Real-time estimates of the q-profile, from equilibrium reconstruction constrained by internal measurements, tell the controller where to add or remove current drive.

In the Kronos program

The Hyperion breeder targets a design current of 9.86 megaamperes, and its current-profile control aims for a q-profile that avoids seeding low-order tearing modes while supporting good confinement. Because the profile responds slowly, the ramp-up scenario is designed to establish the right shape early, and feedback holds it during flat-top. This is exercised in simulation before operation.