Continuous and segmented Rogowski coils
Toroidally driven currents, e.g. bootstrap currents, generate poloidal magnetic flux changes, which are measured by a continuous Rogowski coil poloidally encircling the plasma. Additional arrangements of poloidally and toroidally spaced Rogoswki coil segments detect current distributions. Both continuous Rogowski coils and coil segments are installed inside and outside the plasma vessel at the same toroidal position to study the effect of currents induced in the plasma vessel itself.
The saddle coils are flux loops on the plasma vessel with sections in toroidal direction close to the torus in board and outboard midplane and connecting sections in poloidal direction, resembling the outline of a saddle. In contrast to the diamagnetic loops, which collect magnetic flux due to poloidal currents, the saddle loops detect poloidal and radial magnetic field components due to toroidal currents. Hence the saddle loops are sensitive to Pfirsch-Schlüter currents. For a fixed magnetic configuration, the Pfirsch-Schlüter currents are proportional to the plasma energy. The integrated signals of saddle coils can therefore also be used to determine the plasma energy.
The diamagnetic loops are flux loops enclosing the plasma poloidally. They are sensitive to changes in the toroidal magnetic flux, which is dominated by the main magnetic field generated by the non-planar and planar superconducting field coils of W7-X. If a plasma is generated, the diamagnetic current balancing the pressure gradient of the plasma will reduce the magnetic field in the plasma region, thus changing the magnetic flux through the diamagnetic loop. This flux change can be expressed by a term, which is linear in the diamagnetic plasma energy. The diamagnetic loop in the triangular plane is surrounded by four extra coils not encircling the plasma to compensate against unwanted influence of current fluctuations in the superconducting main field coils or the plasma vessel.
Calibrated temperature sensors (75 PT1000 sensors) are located at the magnetic diagnostics.