Bootstrap current measurements
Banana orbits and the pressure gradient in the plasma create trapped particles, which collide with passing particles and result in an intrinsic toroidal current, the bootstrap current.
At the starting of the plasma a counter current is induced in the plasma that completely masks the bootstrap current. This shielding current is ohmically dampened and decays exponentially. The total toroidal current, the sum of the bootstrap and the shielding current, therefore starts at zero and rises towards the bootstrap current.
One of the optimisation criteria in the design of W7-X was to keep the bootstrap current low. This limits the movement of the strikelines and keeps them on the divertor during the time it takes the toroidal current to fully develop. It furthermore helps to avoid the occurrence of low order rationals in the iota profile to avoid internal island chains.
To experimentally validate this optimisation and to benchmark the models employed in the optimisation and data analysis, the bootstrap current was measured at several magnetic configurations, densities and heating powers.
Since the bootstrap current is initially screened by the shielding current, long programs are needed for the bootstrap current and its effects to become measurable. Rogowski coils measure the toroidal current. A phase in which all other plasma parameters are constant over a time that is comparable with the decay time of the shielding current is employed to fit an exponentially decaying shielding current to the measured toroidal current and thereby infer the bootstrap current.
Electron cyclotron current drive is minimised in these measurements as it would falsify the results. To achieve this the gyrotrons necessary for heating are used in symmetric pairs.