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28th IAEA Fusion Energy Conference (FEC 2020)

May 10 – 15, 2021
Virtual Event
Europe/Vienna timezone
The Conference will be held virtually from 10-15 May 2021

Exploration of RMP ELM control on ITER similar shape (ISS) in KSTAR

May 11, 2021, 2:00 PM
4h 45m
Virtual Event

Virtual Event

Regular Poster Magnetic Fusion Experiments

Speakers

Sang-hee Hahn (National Fusion Research Institute) Yongkyoon In (Ulsan National Institute of Science and Technology)

Description

Recent control advances at KSTAR enabled us to not only sustain the ITER-similar shape (ISS) in a stationary manner but also experimentally demonstrate the ISS-compatible RMP-ELM control in KSTAR for the first time, using the n=2, +90-deg phasing RMP, matching the ITER-like dimensionless parameters in lower single null (LSN) configuration with vastly contrasting upper/lower triangularities. The kinetic parameters of the ISS are different from the typical KSTAR configurations seen in ELM-RMP suppression experiments [Y.M.Jeon PRL 2012, Y.In NF 2019].

Achieving ITER relevant parameters in KSTAR has been a challenging target. The main difficulty lies in the fact that low edge safety factor requirement ($q_{95}$ ~ 3.1 - 3.4) would result in kink-driven mode-locking easily, considering fundamental challenges of the independent control of highly up/down asymmetric triangularities with constraints by a large up/down symmetric central solenoid (CS). However, the establishment of the ITER relevant parameters in a superconducting device like KSTAR allows us to explore various ITER-related physics and engineering constraints, prior to the ITER-era.

In order to accomplish this ISS target, various advanced magnetic controllers have been implemented and executed in the real experiments, including enhanced vertical stabilization using inboard flux loop differences [Mueller FED 2019], a multi-input multi-output (MIMO) shape control design suitable for LSN shape and the real-time feedforward algorithm [Walker CCA 2016] that would minimize accumulation of integral gain errors.

Figure 1: ISS discharges obtained in KSTAR (a) major parameters in time: Used 2 NB ion sources of total 3.0 MW (#22889) or 1 beam + 2 gyrotrons (#21368). From top to bottom, Ip [MA], $q_{95}$, $\beta_N$, mid-RMP current [kA/turn], edge Thomson [1e19 $m^{-3}$], and $D_{\alpha}$. (b) Obtained shape at the shot #22889, t=5.5065s.

The experimental setup of the typical ISS discharge examples is shown in Figure 1. Using available combinations of 1-2 neutral beam sources and 2 ECH 105GHz gyrotrons, the ISS with 3 different levels of the toroidal rotation were created at the total power level of 3.0-3.6 MW. Two types of power combinations were frequently used in order to create different toroidal rotation levels: 1) Two neutral beam (NB) ion sources (up to 3.6 MW) or 2) one NB + 2 gyrotrons (up to 3.3 MW). As shown in Figure 1(a), the discharges match major ITER-like parameters, i.e. $q_{95}$= 3.2 - 3.4, $\beta_N$= 1.6 ~2.0, at the plasma current flattop Ip = 780 - 850 kA at $B_T$ = 1.7~1.8T. The measured upper triangularity ($\delta_u$) is around 0.5~0.55, and lower triangularity ($\delta_l$) is fixed at 0.45, matching $\delta = (\delta_u + \delta_l)/2$ ~ 0.52 in the ITER shape: a typical ISS example is shown in Figure 1(b). The kinetic properties are, however, very different from the typical RMP-driven, ELM control experiments reported in similar $q_{95}$ range in the ref [Y.In NF 2019]. For instance, the ISS shows relatively higher line averaged density $n_e$ ~ 5-7 e19 $m^{-3}$, accompanied by high edge density pedestals with edge Thomson $n_e$ ~ 2-3e19 $m^{-3}$. The edge electron collisionality is estimated as $\nu^*$ ~0.4-0.5 at the pedestal top $\rho$~ 0.89, using the definition at [Sauter PoP 1999] with the assumption of $Z_{eff}$~ 2.

At first the ELM control on the ISS by resonant magnetic perturbation (RMP) coils was attempted using n=1, +90-degree RMP configuration, but it disrupted the plasma instantly, as frequently observed in the typical low-q95 KSTAR configurations. However, from previous observations, the application of n=2 RMP is expected to be more manageable both for the typical low-$q_{95}$ and for the ISS. In fact, we have found the n=2 RMP-driven, ELM-crash control was greatly affected by the absolute density level: In the highest electron density range, with line-averaged $n_e$ ~ 7e19 $m^{-3}$, we were only able to observe a very low mode-locking threshold. Meanwhile, there were strong ELM-crash mitigations where the ELM frequency from 6-10 Hz to 100-120 Hz, even at relatively low level of RMP current = 1.3-1.5 kA/turn.

On the other hand, the ISS experiments with moderate level of density, at line-averaged $n_e$~5e19 $m^{-3}$, expecting lower edge collisionality than the cases above, showed more promising results: The RMP with n=2, +90-degree phasing showed a locking threshold of 3.1-3.3 kA/turn. Application of the RMP current below the locking threshold onto the ISS successfully accessed a marginal but clear ELM-crash suppression window for the first time at $q_{95}$=3.3-3.4, as shown in Fig. 2, for a short time of 200-600 milliseconds. The ELM-crash suppression was accompanied by 1) a global electron density pumpout (as measured at Thomson scatting density profiles and the line-averaged density), and 2) a characteristic $\beta_N$ drop at the level between that of H- and L-mode. The experimental results suggest that the sustainable ELM-crash suppression window for this ISS is likely to be at a level of RMP current = 2.5 – 2.9 kA/turn.

In the near future experimental exploration is planned to robustly access the sustained RMP-driven, ELM-crash suppression on the ISS in KSTAR, helping us to articulate the main advantages of a superconducting device that is capable of a much longer pulses over hundreds of energy confinement time.

Country or International Organization Korea, Republic of National Fusion Research Institute

Primary authors

Sang-hee Hahn (National Fusion Research Institute) Yongkyoon In (Ulsan National Institute of Science and Technology) Nicholas Eidietis (General Atomics) Dr June-Woo Juhn (National Fusion Research Institute) Jisung Kang (National Fusion Research Institute) Minwoo Kim (UNIST) Won Ha Ko (Korea, Republic of) Mr Jekil Lee (KrNFRI) Mr Yeong Ho Lee (National Fusion Research Institute) Mr Myungwon Lee (National Fusion Research Institute) Dr Giwook Shin (National Fusion Research Institute) Jayson Barr (General Atomics) Prof. Mike Walker (General Atomics) David Humphreys (General Atomics)