Improvements in wall conditioning and performance throughout the phase I of WEST operations

The tungsten (W) Environment in Steady-state Tokamak (WEST) is the only superconducting EU device with metallic plasma-facing components (PFC). The main goal of WEST is to test actively cooled, ITER-like, W diverter PFC under a steady state heat load of q_div~10 MWm^-2. In campaigns 1 and 2 no glow discharge boronization (GDB) was performed, to study W PFC all installed at once. After a challenging commissioning, frequent deuterium (D₂) glow discharge cleaning (GDC) led to pulses up to ~10 s, lower hybrid power up to P_LH~2.4 MW, and central density up to n~1.5e19 m^-2, but with high impurity content, radiated fraction f_rad>0.8, and q_div~0 MWm^-2. During campaign 3, besides D₂ and helium GDC, diborane (B₂D₆) GDB was first deployed, significantly opening the operational space: improved startup conditions and lower oxygen content allowed for long pulses (>30 s) at more than twice the density ( n~3.5e19 m^-2), with higher injected power (P_LH~5 MW), lower f_rad~0.6, and q_div~2.5 MWm^-2. Large use of GDC and GDB in campaign 4, with a renewed GD system in campaign 5, pushed performance further: WEST achieved minute-long pulses with better LH coupling and q_div~6 MWm^-2. W content remains high and the spectral brightness of light impurities sputtering W (mainly oxygen and carbon) rises back to pre-GDB levels after a few long pulses.