Identification of I-mode with ion ITB in NBI-heated plasmas on the HL-2A tokamak

Improved energy confinement mode (I-mode) has been observed on the HL-2A tokamak. The I-mode features an edge transport barrier (ETB) in electron temperature and a low confinement mode like edge density. A weakly coherent mode (WCM) is observed in the edge region with the frequency of $f\sim 60{-}160$ kHz and the poloidal wavenumber of ${k_\theta }\sim 0.5{-}2.5{\text{cm}}{^{ - 1}}$ . The maximum of WCM amplitude is located near the top of electron temperature pedestal. A critical value of $E \times B$ velocity shear for triggering the L–I transition has been found, and is much lower than that for triggering the L–H transition. Additionally, ion internal transport barrier (ITB) has been observed in the I-mode. The formation of ion ITB is due to the increase of $E \times B$ velocity shear, leading to the suppression of turbulence. Transport analysis further confirms the existence of electron ETB and ion ITB. The coexistence of electron ETB and ion ITB leads to an improved plasma confinement, which is comparable to that in the high confinement mode, suggesting that I-mode with ITB regimes could be an interesting operation scenario for future fusion devices.