7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

First Demonstration of Stationary I-Mode Operationwith Hot-Ion Core in KSTAR

Y.M. Jeon1, H.S. Han1, J. Chung1, J.M. Park2, S.M. Yang3, and W.C. Kim1
1Korea Institute of Fusion Energy, Korea, 2Oak Ridge National Laboratory, USA, 3Princeton Plasma
Physics Laboratory, USAe-mail (speaker): ymjeon@kfe.re.kr
Recently, KSTAR has highlighted the successful
demonstration of stationary long pulse operation of hotion temperature plasma up to 10~15 keV without anydetrimental ELM events. This was achieved by operating
the plasma in a low density L-mode with anunfavourable ion grad-B configuration, instead of theconventional high density H-mode. In this report, wediscuss the first successful implementation of thisimproved L-mode, referred to as I-mode [1], and provide
an overview of its key features, particularly the highly
peaked hot-ion core mode [2-3]. Also, we describe
several recipes aimed at ensuring reliable access androbust sustainment of I-mode, which are linked todifferent physics mechanisms.
I-mode exhibits H-mode-like energy confinement witha 100~150% enhancement (up to H89y2~2.5) compared toconventional L-mode, but particle confinement remains
low, similar to typical L-mode. Moreover, I-mode canmaintain a stationary state for over 30 seconds, which isin contrast to conventional I-mode, which is considered
as a transient intermediate state between L- and H-modes.
Interestingly, the toroidal rotation in this I-mode issubstantially increased by ~100% or more, along with asignificant increase in beam-driven neutron rate. The
improvement in energy confinement is attributed to theformation of an edge transport barrier (ETB) with asimilar pedestal width but reduced gradient compared tothat of H-mode. However, a weakly coherent mode(WCM) observed at ~20 kHz is thought to be responsible
for the lack of ETB formation in particle transport. One
intriguing feature of this I-mode is its exceptionallypeaked hot ion temperature, reaching up to 10~15 keV,coupled with strong toroidal rotation shear, even at low
NBI heating power. This significant rise in core iontemperature seems to be associated with the strong
stabilization of sawtooth instability, which is highly
sensitive to plasma density. This phenomenon is similar
to the so-called hot-ion mode [2-3].
To ensure reliable access and robust sustainment of
this I-mode, several control techniques were investigatedunder diverse conditions. Achieving stable I-mode
operation without mode-locking was possible through anelaborate early-neutral-beam scenario or qmin control,
given the crucial role of sawtooth stabilization.
Furthermore, the use of an edge-localized resonant
magnetic perturbation (E-RMP) proved to be an effectivemeans of improving the robustness of I-mode access. Asa result, the I-mode operational regime was successfully
expanded with respect to heating power and plasma
density. Additionally, an alternative advanced scenario of
an integrated operation, combining both L-to-I andI-to-H transitions in a controlled sequence, is presented,
allowing for the exploration of extremely high beta
operation despite limited heating power.
This work was supported by R&D Program of"KSTAR Experimental Collaboration and Fusion Plasma
Research(EN2301-14)" through the Korea Institute ofFusion Energy(KFE) funded by Korea Ministry ofScience and ICT (MIST).
References[1] A.E. Hubbard et al, Nucl. Fusion 56, 086003 (2016)[2] J.F. Clarke, Nucl. Fusion 20 (1980) 563[3] M. Knolker, et al, Plasma Phys. Control. Fusion 63(2021) 025017

Figure 1. (Left) Comparison of H-, L-, and I-modes under similar conditions, (right) the correlation of highly peaked ion temperature
profile and the rotation shear, and (middle) the observed highly peaked ion temperature profile along with I-mode driven ETB.