W7-X magnetic diagnostics: Rogowski coil performance for very long pulses

Stellerators can be operated without the need of significant plasma current for generation of poloidal magnetic field components. Wendelstein 7-X (W7-X) is optimized for stiff magnetic configurations by minimizing pressure driven currents, such as bootstrap and Pfirsch-Schluter currents. In Addition to the largely reduced plasma currents, the HELIAS-type device is a low magnetic shear configuration required for magnetic boundary islands. However, the diverting magnetic field structure is already influenced by small net plasma currents. In particular, the W7-AS divertor results revealed a significant sensitivity of the X-point location with respect to favorable edge plasma states. Thus, a precise X-point control for W7-X is envisaged and, although expected net plasma currents range in the order of 10-100 kA only, stabilization by adaptive controllers is foreseen to an accuracy of about 1 kA. This stabilization should be reliable even after 2000 s of plasma operation. A precondition for achieving this is a reliable current detection. For this purpose, W7-X will be equipped with Rogowski of which the first has now been assembled onto one of the plasma vessel sectors. The integral performance and sensitivity inhomogeneities have been tested by employing the digital integrator of W7-X [A. Werner, Rev. Sci. Instrum. 77, 10E307 (2006)] and a cable energized with 200 A, representing the plasma current.