Speaker
Description
Ion temperature measurements on the lower divertor of the WEST tokamak from attached plasmas to detached and X-point radiator regimes
J. P. Gunn,1 P. Ivanova,2 M. Dimitrova,2,3 J. Kovačic4
1 CEA, IRFM, F-13108 Saint-Paul-Lez-Durance, France.
2 Emil Djakov Institute of Electronics, Bulgarian Academy of Sciences, 72, Tsarigradsko Chaussee Blvd., 1784 Sofia, Bulgaria.
3 Institute of Plasma Physics of the Czech Academy of Sciences, U Slovanky 2525/1a, 182 00 Prague 8, Czech Republic.
4 University of Ljubljana, Faculty of Mechanical Engineering, Slovenia.
Ion temperature plays an important role in heat loading and sputtering processes at the surface of a tokamak divertor. A reciprocating retarding field analyzer was installed on the lower tungsten divertor of the WEST tokamak and made its first measurements of divertor ion temperature in 2025. Complementing the ion temperature measurements are arrays of flush-mounted Langmuir probes, and pop-up probes that provide local measurements of density and electron temperature, and which provide a basis for understanding the ion temperature measurements in terms of collision and ion-electron equipartition times. About 200 measurements were obtained for a range of core plasma density and lower hybrid heating power. In two dedicated experiments, strike point sweeping was employed to obtain the radial profile of ion temperature at the divertor target. When the plasma is attached, with electron temperature above 10 eV, the ion temperature is 3 to 6 times higher. Ion temperatures up to 80 eV have been observed. In detached conditions both at high core plasma density without additional heating power, and in the X-point radiator regime triggered by nitrogen seeding with additional lower hybrid heating, the ion temperature collapses to become about equal to the electron temperature, around 5 eV or lower on average (see Figure). SOLEDGE3X mean-field simulations show that observed Ti/Te ratios are challenging to recover and raise questions concerning the standard model and parametrization used in edge plasma codes. Simultaneous ion temperature measurements upstream as well as on the divertor are needed to address this issue. At the time of writing of this abstract, a reciprocating retarding field analyzer is being built that will provide simultaneous measurements of the ion temperature in the scrape-off layer at the top of the machine. We expect to have these fresh measurements before the conference.