Diamagnetic energy measurement during the first operational phase at the Wendelstein 7-X stellarator

The magnetic diagnostic system at the Wendelstein 7-X stellarator includes three diamagnetic loops to measure magnetic flux changes in the plasma. Their signals are directly related to the plasma energy. The diagnostic design with respect to materials, component cooling and data acquisition is built to be fully steady-state capable within the harsh environment of a fusion plasma device. During the first operational phase, two diamagnetic loops have been put into operation, each of them close to one of the up-down symmetric main planes of the plasma column with a bean-shaped and triangular-shaped cross-section, respectively. Both loops measured reliable energies in accordance to theoretical expectations. The triangular-shaped diamagnetic loop is equipped with four compensation coils. They are used to compensate errors during the energy measurement due to small fluctuations of externally driven currents in the main superconducting magnetic field coils and eddy currents in the adjacent vacuum vessel and thereby increase the time-resolution allowing to measure fast changes of the plasma energy. The diamagnetic flux measurements agree well with corresponding estimations of diamagnetic signals using three-dimensional Biot–Savart calculations. A consistency check for the diamagnetic energy is performed by a reconstruction of the associated Pfirsch–Schlüter current distribution and a comparison of predicted signals with measurements of an arrangement of eight plasma encircling Rogowski coil segments. Additionally, the measured diamagnetic energy is compared to kinetic energy calculations based on density and temperature measurements performed by the Thomson scattering diagnostic and the x-ray imaging crystal spectrometer diagnostic. The resulting energy confinement times are similar to predictions of empirical scaling laws, like ISS04. For upcoming operational periods of Wendelstein 7-X, the diamagnetic energy measurement will be used to generate an interlock signal, which will turn off the main plasma heating systems in case of a sudden, unwanted plasma collapse.