Experiments of plasma shape changes in WEST: contribution of primary separatrix on W and light impurity sourcing and plasma radiation.

WEST’s unique characteristics with nearly all W PFCs offer an ideal platform to study plasma operations, plasma scenario development and modeling validation for a long-pulsed, actively cooled, fully W PFC reactor, such as ITER and SPARC. In WEST, the radiated fraction does not correlate with the already measured impurity sources. When the plasma is in a lower single null (LSN), the upper divertor is assumed to be a low impurity source relative to the lower divertor and limiters. However, the upper divertor is more poorly screened, which may result in a non-negligible source of core pollution. Towards this end, a dedicated plasma shape change experiment was performed with 2 and 4 MW injected power at 0.5 MA of plasma current during which the primary separatrix was driven away from the upper divertor (5 / 35 / 110 / 165 mm) with a constant secondary X-point position (dXup = 30 mm) in the same shot. A comprehensive array of diagnostic measurements were taken at/near the upper and lower divertor targets utilizing the recently expanded measurement capabilities at WEST including W gross erosion (W I) and net erosion (W II) with the newly installed McPherson 1m C-T visible / near-UV spectrometer, light impurity fractions near the target (B, C, N, O), n_e and T_e. Experimental results show significant differences in the impurity sources at the upper divertor (-90%) and lower divertor inner (+250%) and outer (~10 – 15%) targets for light impurities and W for the primary separatrix scan. The total radiated power also varied from step to step (~5 – 10%). This increase in impurity sourcing from the inner lower divertor could be explained as open magnetic field lines coming from the boundary plasma stagnation point that intercepted the upper divertor prior to shifting the separatrix now intercepting the inner divertor. The expanded array of diagnostic measurements is providing an increased number of constraining parameters for ongoing scrape-off layer impurity transport modeling efforts.