Ferritic wall studies on HBT-EP

Low-activation ferritic steels are leading material candidates for use in next-generation fusion development experiments such as ITER and DEMO. Understanding the interaction of plasmas with a ferromagnetic wall will provide crucial physics for these experiments. Although the ferritic wall mode (FWM) was not observed in JFT-2M [1], it has been been studied in HBT-EP [2], while the effects of related error fields have been studied on DIII-D [3]. HBT-EP operates with a high-permeability tiled ferritic first wall, characterizing its MHD effects using high-resolution magnetic diagnostics. We report on our study of FWM dynamics comparing stainless and ferritic wall configurations, including increases in plasma response to RMPs, plasma disruptivity, and natural mode growth [2,4]; new results include differences in scrape-off layer (SOL) current dynamics and mode rotation frequency dependence of the FWM growth rate [4]. Additionally, we present the effects of toroidally asymmetric distribution of ferromagnetic material on mode rotation.\\ [1] K. Tsuzuki, et. al. Nucl Fus 46 (2006)\\ [2] J.P. Levesque, et al. Phys. Plasmas 22 (2015)\\ [3] M.J. Schaffer, et al. Nucl Fus 51 (2011)\\ [4] P.E. Hughes, Ph.D. Thesis (2016)