Present-day tokamaks – intended for basic research in plasma physics – mostly work with normal-conducting copper magnet coils. They therefore produce the plasma in short discharges lasting from 1 to 20 seconds. The ITER experimental reactor is fitted with superconducting magnet coils and will attain discharge times of up to quarter of an hour. In a future tokamak power plant pulse lengths of about an hour are conceivable.
Prior to the discharge there is a high vacuum of 10-8 millibar in the vacuum vessel. First the external magnetic field is built up and the current started up in the transformer coil. Shortly before the discharge is initiated, hydrogen gas is admitted to the vessel, whereupon the pressure rises to a few 10-5 millibar. The transformer is then discharged; i.e. the current is slowly reduced. In this way a peripheral voltage of about 10 volts is induced, which triggers the discharge:
The hydrogen gas is ionised – it transforms into a plasma built up by feedback-controlled adjustment of the plasma position, plasma cross-section, and plasma current. In keeping with the current rise, the plasma density is brought to the required value by gas intake. Once plasma heating has commenced, the actual experiments then proceed in the state with constant plasma current that has now set in.
The end of the discharge is governed by the transformer, which drives the plasma current. Once the current minimum has been attained, no further flux change is possible: the current in the plasma slowly decreases.
Plasma discharge in IPP's ASDEX Upgrade tokamak