First measurements with the Faraday cup fast ion loss detector on Wendelstein 7-X.

Abstract

The first measurements made with a prototype Faraday cup fast ion loss detector (FC-FILD) on the Wendelstein 7-X (W7-X) device are presented and shown to be consistent with lost neutral beam fast ions. The FC-FILD is a small form factor, energy resolving detector designed to be mounted inside the first wall armor tiles of W7-X. Such mounting will allow for a future detector array to be installed, which has been shown to be critical for demonstrating improved fast ion confinement with plasma beta (β = 2μ0p/B2). The prototype diagnostic is composed of a movable armature with a water cooled diagnostic head, mimicking the first wall armor tile cooling structure. The movement of the armature allows for assessment of different detector positions relative to the plasma edge. Measurements of lost fast ions in the low shear magnetic configuration are correlated with neutral beam blips and the neutral beam energy spectrum. Analysis of the rise time of the signals presents a timescale much faster than the energy or particle confinement time and of the order of the predicted neutral beam fast ion slowing down time. Simulations predict a larger signal amplitude than measured, and possible reasons for this are discussed. Finally, a redesigned aperture is presented, which allows for fast ions to reach the detector in a larger set of magnetic configurations.