Flush-mounted Langmuir probes in the WEST tokamak divertor

Abstract

The design of the Langmuir probes for the lower divertor of the WEST tokamak is presented, in which uncooled, tungsten-coated graphite targets were installed for the first phase of operation in anticipation of full-tungsten, actively cooled, ITER-like divertor components. Due to the long plasma discharges and high heat loads, the probe tips are flush with the divertor tiles in which they are embedded, as it is foreseen and indeed required in ITER. Tantalum metal was chosen for the probe tips because of its high melting point, and ductility at room temperature which allows simple crimping to electrically connect them to cables, as well as being compatible with the full metal environment of WEST. High heat flux tests of Ta Langmuir probe prototypes confirmed the robustness of the design for energy flux densities up to 8 MW/m2 for 5.5 s, and at lower 3.5 MW/m² energy flux density for 40 s long discharges. The probes were operational from the first discharge in WEST and the good alignment of all probes with the surrounding surface was confirmed by a good match of each adjacent probe's data when the strike points were swept across the target.