# I-mode investigation on the Experimental Advanced Superconducting Tokamak

## TL;DR

This study confirms I-mode operation in EAST tokamak, highlighting the formation of a high-temperature edge pedestal that improves energy confinement without affecting particle confinement, and identifies key edge plasma fluctuations.

## Contribution

First detailed analysis of I-mode characteristics in EAST, revealing edge electric field profiles, WCM behavior, and potential new oscillation modes unique to this device.

## Key findings

- Energy confinement improves during I-mode transition
- Deep edge $E_r$ well observed in I-mode
- Presence of weak coherent mode and low-frequency oscillations

## Abstract

By analyzing large quantities of discharges in the unfavorable ion $ \vec B\times \nabla B $ drift direction, the I-mode operation has been confirmed in EAST tokamak. During the L-mode to I-mode transition, the energy confinement has a prominent improvement by the formation of a high-temperature edge pedestal, while the particle confinement remains almost identical to that in the L-mode. Similar with the I-mode observation on other devices, the $ E_r $ profiles obtained by the eight-channel Doppler backscattering system (DBS8)\cite{J.Q.Hu} show a deeper edge $ E_r $ well in the I-mode than that in the L-mode. And a weak coherent mode (WCM) with the frequency range of 40-150 kHz is observed at the edge plasma with the radial extend of about 2-3 cm. WCM could be observed in both density fluctuation and radial electric field fluctuation, and the bicoherence analyses showed significant couplings between WCM and high frequency turbulence, implying that the $ E_r $ fluctuation and the caused flow shear from WCM should play an important role during I-mode. In addition, a low-frequency oscillation with a frequency range of 5-10 kHz is always accompanied with WCM, where GAM intensity is decreased or disappeared. Many evidences show that the a low-frequency oscillation may be a novel kind of limited cycle oscillation but further investigations are needed to explain the new properties such as the harmonics and obvious magnetical perturbations.