Plasma discharge
In a stellarator discharge first the modular magnetic field is switched on, its confinement properties already being present without plasma.
As in the tokamak, hydrogen gas is admitted to the empty vessel just before the discharge. The plasma, however, is produced not by induction of a peripheral voltage and the ensuing plasma current, but by beaming in high-frequency electromagnetic waves or by neutral particle injection. The high-frequency waves accelerate and heat the electrons in the hydrogen gas or in the evolving plasma, where they then completely ionise the gas through collisions.
As the slow and controlled current build-up occurring in the tokamak is absent, the initial phase of the discharge is governed solely by the density build-up, so that the flat-top phase so crucial for plasma experiments is quickly reached. It is the heating time alone that determines the end of the discharge, thus making steady-state operation possible in principle.
Plasma discharge in Wendelstein 7-X
The plasma is observed by a video camera and measuring instruments. During the 30 second discharge they record the heating power and the power radiated by the plasma, the plasma density and the gas refill, the temperature of the plasma electrons, the energy content of the plasma and the temperature of the divertor (measurement curves from top down).