Tungsten pump-out by n=1 MHD mode in a DIII-D/WEST similarity experiment

We report new experimental evidence from a DIII-D/WEST similarity experiment demonstrating that central tungsten accumulation can be effectively mitigated by saturated (m, n) = (1,1) MHD activity. The accumulation of high-Z impurities such as tungsten (W) in the plasma core is a critical challenge for ITER and future fusion reactors. In this work, a controlled W injection was performed using the Laser Blow-Off system (LBO) in a DIII-D hybrid scenario with ITER lower single-null shape matched to WEST parameters (elongation, triangularity and q₉₅). The scenario’s resilience to W injection was explored through a scan of core radiation levels. Soft X-ray diagnostics revealed hollow tungsten emission profiles, correlated with saturated (1,1) mode indicating MHD-induced core impurity screening. Similar effects have been observed in various tokamaks including DIII-D and ASDEX Upgrade in the presence of central Electron Cyclotron Heating (ECH). The observed results provide specific insight that an n=1 MHD mode, and not only the 3/2 mode as recently observed in DIII-D/KSTAR hybrid discharges, can contribute to MHD-induced screening of high-Z impurities in ITER-relevant plasmas. Ongoing analysis includes 2D-resolved W density reconstructions and modeling of neoclassical and turbulent W transport in the presence of 3D MHD structure.

ACKNOWLEDGEMENTS:

Work supported by US DOE under DE-FC02-04ER54698, DE-SC0014264, DE-AC52-07NA27344, DE-AC02-09CH11466 and DE-FG02-04ER54761.