To get a uniform load of the walls, impurities may be injected in a controlled way which transform the loss of energy by fast particles into radiation ("radiating boundary layer").

At ASDEX Upgrade, one succeeded in combining a favourable discharge regime with such a 'smooth' mode of energy removal for inevitable loss processes. The figure shows plasma discharge #1871 which reveals this milestone.

It is just the boundary layer where ASDEX Upgrade reaches conditions like a future fusion reactor.