Bulletin of the American Physical Society
60th Annual Meeting of the APS Division of Plasma Physics
Volume 63, Number 11
Monday–Friday, November 5–9, 2018; Portland, Oregon
Session NO5: KSTAR |
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Chair: Jong-Kyu Park, Princeton Plasma Physics Lab Room: OCC B113-114 |
Wednesday, November 7, 2018 9:30AM - 9:42AM |
NO5.00001: Overview of recent KSTAR experimental results Si-Woo Yoon, Y. K. Oh, W. C. Kim, J. G. Kwak, Hyeon Keo Park Recent KSTAR experiments have been directed toward resolving ITER critical issues and physics understanding for extrapolation to future devices. The key parameters including shape and collisionality were clarified for ELM-crash suppression with Resonant Magnetic Perturbation and the predictability of the physical model is validated through the inter-machine comparison among KSTAR/DIII-D/AUG tokamaks. The contrasting influences of non-axisymmetric field is documented on L-H transition power threshold and its ITER projection. With the new off-axis NBI (~1.5 MW) and ECH (~1 MW) systems, the steady-state operation window is extended above no-wall limit and the confinement degradation of fast ions via Alfven eigen-modes is estimated quantitatively using TRANSP. The dynamics by Edge Harmonic Oscillation is investigated for edge transport regularization. The avalanche-like electron heat transport is discovered and turbulence structure near magnetic island is simulated and compared with 2D measurements in L-mode discharges. Finally, the impact of the real-time wall condition is documented for performance sustainment in stationary long-pulse plasmas. |
Wednesday, November 7, 2018 9:42AM - 9:54AM |
NO5.00002: Toward the universality of low-n RMP-driven, ELM-crash-suppression physics Yongkyoon In, Jong-Kyu Park, Todd E Evans, Young-Mu Jeon, Carlos Paz-Soldan, Dmitry Orlov, Theresa Wilks The onset of resonant magnetic perturbation (RMP)-driven, ELM-crash-suppression still lacks in the universal physics understanding. To address the commonalities and discrepancies among tokamaks, a series of inter-machine comparison experiments of RMP are in progress. A recent similarity experiment done in DIII-D using KSTAR-similar shape (KSS) plasma (with the triangularity of δ ~0.6) demonstrated a merit of highly shaped plasmas, which turned out to be quite resilient against mode-locking during n=1 RMP. Specifically, even up to the maximum available n=1 RMP currents in both in-vessel and ex-vessel coils, the discharge with KSS survived without detrimental mode-lockings, which allowed a systematic phasing scan of n=1 RMP in DIII-D. Despite the a-priori knowledge about effective modeling-based 3-D configuration for ELM-crash-suppression, the attempts of n=1 RMP in DIII-D have not led to full suppression of ELM-crashes yet. Currently, we speculate that non-optimal edge/divertor recycling might have been influenced by the inadequate divertor pumping position associated with the unusual high value of triangularity. |
Wednesday, November 7, 2018 9:54AM - 10:06AM |
NO5.00003: Contrasting non-axisymmetric field influences on L-H Transition in KSTAR Won Ha Ko, Yongkyoon In, H.S. Han, June woo Juhn, J.H. Lee, Young-Mu Jeon, J. Seol, Hyun Seok Kim, Hyungho Lee, Jayhyun Kim, Sang-hee Hahn, Katsumi Ida, Si-Woo YOON, Yeong-Kook Oh, Hyeon Keo Park An amazingly contrasting influence of resonant and non-resonant fields on L-H transition has been identified using high precision non-axisymmetric in-vessel control coils in KSTAR. Specifically, we have demonstrated a rather ‘non-flat’ dependence of low-n resonant magnetic field on the threshold power of L- to H-mode (PTH), while observing literally no variation of non-resonant magnetic field on PTH. We have found no sensitivity of non-resonant field on L-H transition even at the maximum affordable level of non-axisymmetric field current. Thus, we speculate that the L-H power threshold might not be determined solely by ExB shearing rate but that a turbulent transport driven by resonant magnetic field would be much more stochastically involved than that of non-resonant magnetic field. At the same time, this strongly suggests that the ITER might not need any dedicated correction of non-resonant magnetic fields, while requiring a high-quality resonant magnetic field control to secure the access to H-mode, in particular, at the marginal level of auxiliary heating sources. |
Wednesday, November 7, 2018 10:06AM - 10:18AM |
NO5.00004: Toward steady-state high beta-N operation based on low qmin approach in KSTAR YoungMu Jeon, Si-Woo Yoon, Jung-Hee Kim, Ji-Sung Kang, Hyun-Seok Kim, Tong-Yeol Rhee, Yeong-Soon Bae, J. G. Kwak, Yeong-Kook Oh Recently, various operation scenario with low q-min characteristics have been studied in KSTAR to achieve high beta-N, steady state operation. Particularly, the so-called high poloidal beta mode, characterized by its high value of poloidal beta, has been verified to be capable of fully non-inductive operation at high BT(~2.5T). Interestingly, we found that obtained high poloidal beta plasmas had a dominant change in fast ion confinement while little change in thermal confinement. The improved fast ion confinement was related to low-n high frequency MHD stabilization by central EC, with a narrow window of BT. In addition, a large effort was made to improve the plasma performance, while keeping the betap high (~3.0) for steady state operation. As a result the plasma performance was able to increase up to betaN~2.8 with reduced BT. In parallel, another effort has been made to improve the thermal confinement with improved fast ion confinement. We found that low frequency low-n MHD observed in high poloidal beta plasma could be responsible for the little change of thermal confinement. It was associated with density evolution and NBI scheduling in the early phase of discharges. |
Wednesday, November 7, 2018 10:18AM - 10:30AM |
NO5.00005: High beta stability research on KSTAR at high non-inductive fraction Steven Anthony Sabbagh, Young-Seok Park, John W Berkery, Jaeheon Ahn, Yanzheng Jiang, Juan Riquezes, Jun-Gyo Bak, Sang-hee Hahn, Hyunsun Hahn, Young-Mu Jeon, Jayhyun Kim, Hyun Seok Kim, Jinseok Ko, Won Ha Ko, J.H. Lee, ByungHo Park, Laurent Terzolo, Si-Woo Yoon, Alan Herbert Glasser, Zhirui Wang KSTAR plasmas have exceeded the ideal n = 1 no-wall MHD stability limit and reached βN > 4. Analysis of advanced operation regimes using TRANSP indicate that the non-inductive current fraction has reached 75 percent. Regions of weak safety factor q shear can form in different parts of the plasma dependent upon the broadness of the bootstrap current profile. Kinetic equilibrium reconstructions with MSE data to constrain the local magnetic field pitch angle are used as input to evaluate ideal and resistive stability using DCON. At present, high βN operation is limited by tearing instabilities rather than resistive wall modes (RWM) that are computed to be stabilized by kinetic MHD effects. TRANSP code predictive capability is used to examine the effects of the second (off-axis) NBI system installed for the 2018 run determining plasma parameters important for stability. Values of the global energy confinement quality (H98y2) and the Greenwald density fraction are set to match past performance for reliable extrapolation. Predictive analyses are used to design experiments yielding solutions with βN~4.5 and 100% non-inductive current drive. Active RWM feedback with both DC and AC field compensation has been implemented for mode control in 2018. |
Wednesday, November 7, 2018 10:30AM - 10:42AM |
NO5.00006: Control of Alfvén eigenmodes and its impact on fast-ion confinement in KSTAR high poloidal beta discharges Junghee Kim, Tong-Nyeol Rhee, Minhao Yu, Ji-Sung Kang, Minjun J. Choi, Young-Mu Jeon, Jun-Gyo Bak, Jungmin Jo, Mun-Seong Cheon, Yong-Su Na, JeongWon Yoo, Hogun Jhang, Kouji Shinohara, C.Z. Frank Cheng In high poloidal beta (βP) discharges accompanying the toroidal Alfven eigenmodes (TAE) with broad q-profile, degradations of plasma performance as well as neutron emission rate are often observed. It has been found on KSTAR that ECRH deposition on the optimal location where the TAE gap can be formed could mitigate the TAEs successfully. Modeled fast-ion pressure profile during the TAE mitigation period is matched well to the case of classical (Dfast ~ 0 m2/s) profile, and the modeling is supported by comparison with the neutron emission rate and the total stored energy. Mechanism of TAE mitigation in this high βP discharge with monotonic broad q-profile is to align the ECRH deposition location on the existing TAE gap location, and is verified by NOVA calculations. Another consideration for control of TAEs in high βP discharges is that mismatched ECRH application is able to excite different TAEs or tearing modes at different radial locations, and neutron emission rate does not elevate much as a result. It implies that the AE control actuators on high βP discharges should be optimized and depend on the q-profile and the beam-ion distributions. |
Wednesday, November 7, 2018 10:42AM - 10:54AM |
NO5.00007: Nature of Edge Harmonic Oscillation (EHO) and its Dynamics Jaehyun Lee, Jayhyun Kim, Young-Mu Jeon, Won Ha Ko, Gunsu Yun, Woochang Lee, Yong-Un Nam, Young-chul Ghim, Jaewook Kim, Gunyoung Park, Minwoo Kim, Hyeon Keo Park The spatial structure and temporal dynamics of edge harmonic oscillation (EHO) have been investigated using 2-D imaging diagnostics (ECEI, BES) in KSTAR ELM-free H-mode plasmas. During the presence of low-n (n≤4) EHO, edge-localized mode (ELM) filaments remained stable and the ELM crashes hardly occurred. Spectral and bispectral analysis using the imaging data show that there is a strong nonlinear interaction between EHOs, and the nonlinear interaction of EHOs has a significant effect on the ELM structure and dynamics. The EHO is highly related to the perpendicular rotation and its shear, especially the EHO appeared when the rotation shear is large compared to typical H-mode plasmas. Observed EHO appeared discontinuously and synchronized with quasi-periodic RF spikes, reducing the electron density of pedestal. These particle transport events, seen through the quasi-periodic RF spikes, seem to reduce the pressure gradient below the crash threshold and thereby play an important role in maintaining the ELM-free H-mode plasmas. |
Wednesday, November 7, 2018 10:54AM - 11:06AM |
NO5.00008: Long range avalanche-like electron heat transport events in the L-mode tokamak experiment Minjun J. Choi, Min Ho Woo, Jae-Min Kwon, Sehoon Ko, Lei Qi, Hogun Jhang, Hyeon Keo Park, Taik Soo Hahm, Hyun Seok Kim, Ji Sung Kang, Jaehyun Lee, Minwoo Kim, Gunsu Yun Long range avalanche-like electron heat transport events are observed and analyzed in the L-mode tokamak experiment. We investigate avalanche dynamics and characteristics of the associated electron temperature (Te) fluctuations in detail using the electron cyclotron emission imaging diagnostics on the KSTAR tokamak plasmas [1]. Initially, the mesoscale jet-like Te profile corrugations appear. The long range avalanche-like event occurs with joint reflection symmetry when the Te profile corrugations disappear and the associated Te fluctuation power increases. In particular, the Te gradient-correlated-fluctuation with the power law spectrum prevails during the avalanching periods. These observations suggest that the electron heat transport can be regulated by a mesoscale structure and dominated by the nondiffusive avalanche-like transport events in the L-mode plasma [2] when the significant MHD instabilities are absent. While the nonlinear gyrokinetic simulation is in progress, preliminary results of linear simulations will be provided. [1] M. Choi et al., arXiv:1806.04947 (2018) [2] G. Dif-Pradalier et al., Nucl. Fusion 57, 066026 (2017) |
Wednesday, November 7, 2018 11:06AM - 11:18AM |
NO5.00009: Characteristics of turbulence spreading and zonal flow near magnetic island in electrostatic gyrokinetic simulations E.S. Yoon, Jae-Min Kwon, Minjun J. Choi, Seung Hoe Ku, Choong Seock Chang We present characteristics of turbulence spreading and zonal flow near magnetic island in electrostatic simulations of a gyro-kinetic code, XGC1. This work was motivated by recent ECEI measurements in dedicated KSTAR experiment employing resonant magnetic field perturbation(RMP). From the experiment, it was found that magnetic island induced by RMP can impact fluctuations and flows, and consequently electron thermal transport around the island[1]. A subsequent simulation study showed that 3D RMP field can enhance equilibrium ExB flow strong enough to suppress ambient micro-instabilities[2]. In this talk, we present more comprehensive nonlinear simulations with all relevant neoclassical and turbulence physics in the KSTAR experimental condition. In the simulations, we explore turbulence evolution around and inside (2,1) magnetic island. Unlike linear analysis results, fluctuation penetrates the strong ExB shearing layer around the island and the electron temperature profile inside has a finite gradient. We also investigate how flow evolution around the island affect ambient transport. [1] M.J. Choi et al., NF 57, 126058 (2017) [2] Jae-Min Kwon et al., PoP 25, 052506 (2018) |
Wednesday, November 7, 2018 11:18AM - 11:30AM |
NO5.00010: Thermal energy transport modeling of KSTAR ITB discharge and performance projection to heating and current drive upgrade Jisung Kang, Jinil Chung, J. G. Kwak, Jin Myung Park, Young-Mu Jeon, Si-Woo Yoon The internal transport barrier formation is one of key KSTAR milestones but also expected to be the basis for future high performance operation. KSTAR has achieved internal transport barrier (ITB) formation with an early injection of the NBI power while maintaining a limited shape to avoid H-mode transition. In this study, kinetic profile of ITB plasma is reproduced by integrated modeling and performance projection with heating and current drive upgrade is presented. Thermal energy transport analyses of ITB shots have been conducted using the IPS-FASTRAN package with TGLF turbulent heat flux model and compared with neoclassical thermal transport, ExB shearing rate, and gradient scale length. The modeling results are quite consistent with the experimental data and predictive modeling according to the H & CD upgrade plan shows the prospect of achieving ITB with more peak kinetic profile and reduced turbulent transport. |
Wednesday, November 7, 2018 11:30AM - 11:42AM |
NO5.00011: Effects of momentum transport on error field threshold observed in KSTAR SeongMoo Yang, Jong-Kyu Park, Yongkyoon In, JeongWon Lee, Won Ha Ko, JeongWon Yoo, SeulChan Hong, KyuDong Lee, Yuejiang Shi, SangGon Lee, June woo Juhn, Si-Woo Yoon, Yong-Su Na Resonant error field can lead to a disruptive mode locking when its strength is greater than a threshold. This mode locking process is often accompanied with the changes in the sawtooth activity, plasma rotation, and the reduction of energy confinement before terminating discharges in KSTAR. On the other hand, error field threshold against mode locking increases along with density but much weaker in KSTAR than expected, with the exponential scaling factor ne0.3. This might be related to the Linear Ohmic Confinement (LOC) to Saturated Ohmic Confinement (SOC) in the range of density for the studied KSTAR plasmas, as it implies weaker density correlation of momentum diffusion and viscous torque. Plasma rotation can also play a significant role to determine resonant error field thresholds. This supposition is clearly supported by an interesting example in KSTAR; A resonant field can accelerate rotation globally in some conditions and sustain plasma against mode locking beyond an expected error field threshold, as will be discussed in detail. |
Wednesday, November 7, 2018 11:42AM - 11:54AM |
NO5.00012: ELM Suppression and Formation of Internal Transport Barrier by Krypton Gas Seeding in KSTAR Plasmas Juhyeok Jang, Joohwan Hong, Jayhyun Kim, Hyunsun Hahn, Kimin Kim, Jisung Kang, Byron Jay Peterson, Inwoo Song, Jae Sun Park, Seungtae Oh, Taemin Jeon, Suk-Ho Hong, Wonho Choe ELMs were successfully mitigated and suppressed by krypton (Kr) seeding in the KSTAR divertor. After Kr seeding with 1.7x1019 particles, ELM was mitigated with reduction in electron density and temperature, and stored energy. When Kr amount was increased (~3.5x1019 particles), ELM was suppressed and H-L back transition occurred. Energy confinement time remained almost unchanged during both Kr seeding cases while normalized beta decreased. A peeling-ballooning stability analysis showed that the decrease of pedestal plasma pressure gradient due to Kr plays a dominant role in ELM mitigation or suppression. Tangentially-reconstructed 2D radiation images suggest that pedestal pressure was reduced by radiative cooling and heating power loss by Kr. At a higher level of Kr seeding (~5.0x1019 particles), an internal transport barrier (ITB) was formed. At the plasma core (0 < r/a < 0.4), electron and ion temperatures and toroidal rotation increased distinctly. TRANSP calculation showed that both ion and electron heat diffusivities decreased significantly inside the ITB. |
Wednesday, November 7, 2018 11:54AM - 12:06PM |
NO5.00013: Discovery of a turbulent ExB mixing mechanism of the ohmic breakdown in a tokamak Min-Gu Yoo, Yong-Su Na, Jeongwon Lee, Young-Gi Kim, Jayhyun Kim Although the ohmic breakdown has generally been used to produce initial plasmas in a tokamak, its physical mechanism has been obscured due to complicated electromagnetic (EM) topologies. Previous researches have assumed the traditional Townsend theory is applicable to the ohmic breakdown. However, we found clear experimental evidence that the existing theories cannot explain. We developed a theoretical model and a particle simulation code BREAK [1] to systematically study the ohmic breakdown physics. As a result, we propose an entirely new type of breakdown mechanism, namely a turbulent ExB mixing avalanche [2], which successfully explains the experimental results of the KSTAR device. As the plasma responses to the externally driven EM fields, the self-electric fields produced by the plasma play crucial roles during the ohmic breakdown. They drastically decrease the plasma growth rate and cause dominant ExB transports to differ significantly from the Townsend theory. The comprehensive plasma dynamics in the complex EM topology sheds new light on a design strategy of robust breakdown scenarios in a tokamak fusion reactor.
References: [1] Min-Gu Yoo, et al., Computer Physics Communications 221, 143–159 (2017). [2] Min-Gu Yoo, et al., Nature Communications9, 3523 (2018) |
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