Boronization-enabled I-mode on EAST tokamak with an expanded density window and favorable-configuration access

Abstract

I-mode is a promising confinement regime for future fusion reactors because it combines enhanced energy confinement with L-mode-like particle transport and naturally ELM-free operation. Previous EAST I-mode studies were performed exclusively under lithium-conditioned wall conditions. Here we report the first systematic experimental investigation of I-mode under boronized wall conditions on EAST and compare it with an existing lithium-conditioned I-mode database at the same toroidal field, $B_t = 2.47$\,T. The boronized-wall dataset exhibits a substantially broader accessible density range, with the Greenwald fraction extending from $f_{\mathrm{GW}} = 0.26 - 0.77$ , compared with $f_{\mathrm{GW}} = 0.35 - 0.54$ under lithiation. A higher normalized $\mathrm{D}_\alpha$ emission suggests that enhanced edge recycling may contribute to this density extension. A striking increase in favorable-configuration I-mode is also observed: $51\%$ boronized-wall discharges are obtained in favorable-configuration, compared with only $8\%$ lithium-conditioned discharges. These favorable-configuration cases are concentrated at high density and exhibit a deeper radial electric-field($E_r$) well and stronger $\mathbf{E_r}\times\mathbf{B}$ velocity shear. When ETRO is present, the associated transition between electron and ion turbulence is similar under the two wall conditions, although ETRO occurs less frequently ($15\%$) under boronization. An empirical EAST I-mode energy confinement scaling at fixed $B_t$ is obtained, $\tau_E = 3.29 I_p^{0.51 \pm 0.10} P_{\mathrm{loss}}^{-0.53 \pm 0.05} \bar{n}_e^{0.08 \pm 0.07}$, indicating weaker power degradation than IPB98(y,2) H-mode scaling and a weak density dependence. These results show that boronization can broaden the operational space of EAST I-mode and support the development of reactor-relevant ELM-free scenarios.