The application of a directional electron probe in the measurement of W7-X scrape-off layer current

Abstract

An innovative configuration of the directional electron probe (DEP) has been recently implemented in the W7-X stellarator to accurately measure the scrape-off layer (SOL) electron current profile. This configuration employs dual radial arrays of electrodes oriented in contrasting toroidal directions. The probe is tilted by 10° in the toroidal direction such that each pair of electrodes is located on a common magnetic field line. Each DEP channel is equipped with a perforation through the boron nitride cover, terminating in a graphite collector. The perforation is designed in a sector shape with a central angle of 15°. Perforations are inclined at an angle of 18° with respect to the vertical direction in order to align with the shape of the magnetic flux surfaces at the probe position, featuring a radial width of 0.5 mm. By applying a positive biasing voltage to the graphite collector, such as +180 V, electrons are attracted and ions are repelled. Within the DEP setup, this voltage setup effectively repulses low-energy ions, while the perforation's dimensions impede most high-energy ions attributed to their substantial gyroradius in contrast to the smaller perforation width. Consequently, the current collected predominantly comes from electrons, and the differential current measured between the opposing collectors denotes the presence of a directional current driven by electrons. In the magnetic standard configuration with magnetic islands forming the SOL, the DEP measured SOL current profiles from close to the Last Closed Flux Surface (LCFS) to the far SOL (70 mm away). Net current follows in directions in the different regions of the SOL has been observed, with current densities reaching amplitudes up to 50 $\text{kA}/m^2$.