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

Development of High-performance Long-pulse Scenario and Investigation ofPerformance Degradation over Long-time Scale in KSTAR
Hyun-Seok Kim1, YoungMu Jeon1, Hyunsun Han1, Kimin Kim1 and KSTAR Team1
Korea Institute of Fusion Energy (KFE)e-mail (speaker): hskim0618@kfe.re.kr

The primary goal of the KSTAR project is to attain an
pulse duration of 300 seconds while achieving ahigh-performance plasma [1]. To develop long-pulsedischarges, the approaches have been directed towards
accomplishing objectives: the attainment of an extremely
low loop voltage (Vloop) and the mitigation of surface
temperature increase in the plasma facing components(PFCs). Moreover, dedicated efforts have been
undertaken to sustain these desired conditions as long aspossible. Since KSTAR primarily employs NBI as theprimary H&CD system, the research on fast ion behavior
is crucial. In high-performance long-pulse discharges ofKSTAR, it was observed that the fast ion behavior wasrelated to the rapid temperature increase in the poloidal
limiter and the gradual plasma performance degradation
over a long time.
In order to mitigate the temperature increase in thepoloidal limiter, based on the analysis of fast ion orbit
loss[2], Rout of plasma shape was optimized andappropriate NBI sources were selected. Besides, theselection of an NBI source also reduced temperature
increase in other PFC such as the inboard limiter. Thetemperature increase in the central divertor or the outer
divertor was mitigated by adjusting the position of thestriking point over time. The optimization of the H&CD
injection scenario, especially ECH, was important forwithachieving
andsustaining
lower
Vloop
high-performance plasma. One or two ECH injections
enabled lower Vloop operation for the longer pulse
discharge by enhancing the NBI-driven current throughthe Te increase. The controlled injection of ECH helped
access the high βP state by improving the fast ion
confinement with stabilization of the Toroidal Alfvénic
Eigenmodes (TAEs) [3,4]. The higher fast ion
confinement in the high βP state contributed to themitigation of PFCs temperature increase as well as theimproved plasma performance. In addition, KSTAR
experienced nonlinear signal drift in the magnetic probes
in the long-pulse discharges. The nonlinear signal drift ina discharge impacted on accuracy and reliability of EFIT
analysis and led to unintentional errors in the real-time
plasma shape control. To effectively resolve this issue,
the thermal shielding protector was installed on themagnetic probes, which successfully reduced thenonlinear signal drift.
Based on these efforts, KSTAR has achieved a pulse
length of ~90 seconds with high-performance plasma.
The KSTAR long-pulse operation conditions were
IP~400 kA, BT~1.8 or ~2.5 T, PNBI≤5.2 MW, and PEC≤1.4
MW with the optimized plasma shape. The achieved
plasma parameters were 𝑛𝑛�e ≤2.8×1019 m-3, Te,core≤5.0keV, Ti,core~2.0 keV, βP≤2.7, H98y2~1.1, Vloop~34-100 mV,and fNI~0.69-0.96 with fBS~0.29-0.37 and fCD~0.40-0.59.
However, KSTAR long-pulse discharge has sufferedfrom gradual performance degradation over time. In thiswork, it was figured out that one of the causes ofperformance degradation is the degradation of fast ion
confinement coupled with relatively weak TAEs. The
global analysis through kinetic EFIT confirmed that thethermal stored energy remained almost constant overtime, but the fast ion stored energy decreased over a longtimescale, resulting in the performance degradation.
Despite attempts to mitigate or suppress the TAEs byutilizing ECH to obtain a high βP state, the decrease in
the fast ion pressure is still thought to be associated withlong-lasting TAEs that affected fast ion confinement overa long time. KSTAR high βP discharges are typicallyaccompanied by enhanced fast ion pressure. This fast ionpressure facilitated the easier activation of TAEs, leading
to the degradation in fast ion confinement. Furthermore,in higher βP plasma, as βP decreased by worsening thefast ion confinement along with TAE activities, thedischarge eventually reached a certain level of fast ionpressure, at which point TAEs spontaneously stabilized
themselves. Consequently, βP seemed to be converged to~2.0-2.2 in the KSTAR long-pulse discharges.
One notable observation is that the intensity of TAEs
in the long-pulse discharges was weaker by an order ofmagnitude than the intensity of TAEs typically observedin KSTAR. The TAEs excited in the long-pulse
discharges were not strong enough to degrade the plasma
performance during short pulse length, but theircumulative effect on the confinement degradation wasevident during long pulse length. The impacts of fast ion
confinement can be more significant in ITER, wherealpha particle confinement will be crucial for achieving
the burning plasma. As demonstrated in KSTAR, the fastion confinement and associated Alfvénic instabilities
should be effectively controlled for successful
high-performance long-pulse operation in ITER.

References[1] G.S. Lee, et al., Nuclear Fusion, 40, 575 (2012)[2] T. Rhee, et al., Phys. Plasmas, 26, 112504 (2019)[3] J. Kim, et al., Nuclear Fusion, 62, 026029 (2022)[4] Y.M. Jeon, et al., 16th H-mode Workshop, St.
Petersburg, Russia (2017)