Effect of boronization on impurity migration and particle balance in WEST long pulse operation*

Future fusion devices like ITER will have tungsten (W) plasma facing components and long plasma pulses. The W Environment Steady-state Tokamak (WEST) is one of very few long pulse (~364 s) machines with actively cooled, W plasma facing components in the world. Therefore, WEST is an ideal test bed to study the reactor relevance of Glow Discharge Boronization (GDB). During the 2023-24 WEST campaign, we carried out 5 GDB (2 at 170 °C, 3 at 70 °C) with, on average, an injected boron (B) mass of ~11 g, a duration of the B₂D₆ injection of ~4.25 h, a glow pressure of 0.55 Pa and 4 glow electrodes. We compare the radiated power fraction as well as photon fluxes of B, W, and oxygen (O) near the PFC of both divertors and outboard limiters in pulses right before and right after a GDB to study the formation and lifetime of B-rich layers from GDB and their effect on impurity sources. W spectral lines in the UV range instead, indicate the effect of increase B content on W transport towards the central plasma. The deuterium (D) photon flux, together with the D₂ injection rate, helps us evaluate the effect of GDB on recycling. We also carried out the first WEST experiment on the effect of GDB on particle balance, repeating a sequence of identically programmed pulses before and after a GDB. In the first pulse after GDB we injected 2 times more D₂ (14.7 Pam³) to obtain the same central density, indicating a pumping wall. Such effect, unlike the longer lasting one on impurities, fades quickly with wall retention becoming again as low as before GDB after 4 pulses (~100 s).