Pop-up divertor Langmuir Probe diagnostic in the High Heat Flux (HHF) divertor of Wendelstein 7-X (W7‑X)

The water-cooled HHF divertor, recently installed in the W7-X stellarator is equipped with 36 pop-up Langmuir probes (LP) for plasma parameter measurement starting from Operational Phase (OP) 2.1. These tungsten probes present oblique faces to the magnetic field, serving as an upgrade over the flush-mounted graphite probes used previously in W7-X. This paper focuses on the design and development of the pop-up LP diagnostic tailored for the water-cooled divertor in W7-X. Each pair of probes is connected to a "drive-coil" actuator. An upper and a lower divertor module each house 9 drive coils. By passing an appropriate current (j) through a drive coil in the magnetic field (B) of W7-X, a j×B force is applied on the coil, causing the probes to move. Such a pop-up LP concept was previously used in JET and Alcator C-Mod. Each drive coil includes a co/counterweight for passive retraction of the probes when j = 0. Before installing the drive coils in W7-X, extensive durability tests were conducted on prototypes under relevant experimental conditions (B ∼ 2.5T, ultra-high vacuum). The probe design is customized for each of the 36 probes to prevent the probe tips from posing leading edges to the flux tubes carrying high heat flux. An electronic bridge circuit is used for measurement to compensate for the effects of signal propagation time on the long cable lengths. The diagnostic system is seamlessly integrated in W7-X segment control system for an automated operation and control of the diagnostic. The system was successfully put into operation in the recent campaign and successfully measured plasma density, temperature and electric potential.