Bulletin of the American Physical Society
58th Annual Meeting of the APS Division of Plasma Physics
Volume 61, Number 18
Monday–Friday, October 31–November 4 2016; San Jose, California
Session JO9: KSTAR |
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Chair: Wayne Solomon, General Atomics Room: 211 CD |
Tuesday, November 1, 2016 2:00PM - 2:12PM |
JO9.00001: The Status of KSTAR and Research Plan H.K. Park, Y.K. Oh, S.W. Yoon The Korean Superconducting Tokamak Advanced Research (KSTAR) capable of steady state operation of high beta plasmas, has achieved a significant progress in operation such as the longest ($\sim55$ s) H-mode operation as well as newly discovered MHD and transport physics issues such as the interaction between ELMs and turbulence under RMP. The KSTAR is equipped with unique features for potentially unexplored tokamak physics and advanced control; 1) Lowest error field and magnetic ripple ideal for study of the influence of the tokamak plasma symmetry on stability and confinement study 2) Versatile magnetic perturbation tool with $n=1,2$ IVCC coils for harmful MHD control including the ELMs and rotation control through NTV 3) Advanced 2D/3D imaging diagnostics for undisputed measurements for theory and modeling. In this talk, advances in research and vision toward the high beta long pulse operation in KSTAR will be addressed. [Preview Abstract] |
Tuesday, November 1, 2016 2:12PM - 2:24PM |
JO9.00002: Advanced operation scenarios toward high-beta, steady-state plasmas in KSTAR Si-Woo Yoon, Y. M. Jeon, M. H. Woo, Y. S. Bae, H. S. Kim, Y. K. Oh, J. M. Park, Y. S. Park For the realization of the fusion reactor, solving issues for high-beta steady-state operation is one of the essential topics for the present superconducting tokamaks and in this regard, KSTAR has been focusing on maximizing performance and increasing pulse length simultaneously. Typically, study on high beta operation has been focusing on advanced scenario limited at relatively short pulse discharge and partial success has been reported previously. However, it must be stressed that it is critical to verify compatibility of the developed scenario to long-pulse operation and compared with that of the short-pulse, it is turned out stable long-pulse operation is possible only with a reduced level of beta. In this work, the results of recent approaches in long-pulse operation are presented focusing respectively on high betaN, high betap and high li scenarios. For high betaN, the achieved level is close to 3 with Ip$=$0.4 MA, BT$=$1.4T and Pext \textasciitilde 6MW and it is found to be limited by m/n$=$2/1 tearing mode and is also sensitive on the internal inductance. For high betap, conditions of the maximum betap is investigated mainly by parametric scans of plasma current (Ip$=$0.4-0.7 MA) and also neutral beam injection power (3-5MW). The achieved betap is also close to 3 with Ip$=$0.4 MA, BT$=$2.9T and Pext \textasciitilde 6MW and it is found to be limited by heating power and without indication of MHD activities. Finally, attempt for high li discharge will be addressed on scenario development and transient results. [Preview Abstract] |
Tuesday, November 1, 2016 2:24PM - 2:36PM |
JO9.00003: Robust control of long-pulse, high performance plasmas in KSTAR tokamak YoungMu Jeon, S.H. Hahn, H.S. Han, M.H. Woo, M. Joung, Jayhyun Kim, Y.S. Bae, H.-S. Kim, S.W. Yoon, Y.K. Oh, Y.S. Na, N.W. Eidietis, M.L. Walker, M.J. Lanctot, A.W. Hyatt, D.A. Mueller The goal of KSTAR is to achieve and demonstrate high performance, steady state tokamak operations in long pulse up to 300 s. In recent years, we made significant progresses on plasma control and performance for this advanced tokamak (AT) operation. First of all, the plasma equilibrium magnetic control has been substantially improved by applying fully decoupled multi-input-multi-output (MIMO) isoflux shape controllers [1]. The MIMO shape controllers were designed using a newly developed design method by taking the plasma equilibrium response into account self-consistently. More than eight shape control variables including plasma currents are controlled independently on each other with high accuracy (less than 1cm error on average) and with wide variations of plasma shape. By virtue of this robust control, various long pulse H-mode discharges have been operated up to 60 s, which was the maximum pulse length allowable in current KSTAR system. Also, plasma performance has been improved accordingly. A fully non-inductive H-mode operation [1] was achieved for the first time in KSTAR, through the so-called `high betap' operation with betap\textasciitilde 3.0. In addition, various experimental attempts for advanced scenario development have been conducted such as the `hybrid' [2] and `high li' scenarios [3]. [Preview Abstract] |
Tuesday, November 1, 2016 2:36PM - 2:48PM |
JO9.00004: L-H power threshold studies under non-axisymmetric magnetic field in KSTAR Won-Ha Ko, Y. In, H.S. Kim, H.H. Lee, J.H. Lee, Y.M. Jeon, J. Seol, K. Ida, S.W. Yoon, Y.K. Oh, H. Park An exceptionally low level of H-mode power threshold (P$_{\mathrm{TH}})$, as well as its dependence on non-axisymmetric magnetic field ($\delta $B), has been measured in KSTAR. While the application of resonant magnetic perturbation (RMP) is deemed necessary to suppress or mitigate edge-localized-mode (ELM) in ITER and future reactors, $\delta $B influence on P$_{\mathrm{TH}}$ in deuterium plasmas has been predicted to be mostly insensitive at low level, while linearly proportional at high level [1]. However, in similarly neutral-beam-heated deuterium plasmas, we have found that the P$_{\mathrm{TH}}$ of KSTAR was almost a factor of 2 lower than that of DIII-D, while revealing linear $\delta $B dependence even at its low level. Despite various differences between two devices in terms of RMP characteristics and configurations, such contrasting results are mostly attributable to an order of magnitude lower level of intrinsic error field [2] and toroidal field ripple [3] in KSTAR. Perhaps, a theory about L-H transition physics might be in better agreement with experimental results, when $\delta $B dependence is explicitly incorporated. [1] P. Gohil, et. al, Nucl. Fusion 51 103020 (2011), [2] Y. In et al, Nucl. Fusion 55 043004 (2015), [3] S.W. Yoon et al, IAEA-FEC (2014) [Preview Abstract] |
Tuesday, November 1, 2016 2:48PM - 3:00PM |
JO9.00005: Critical aspects of ELM crash suppression by magnetic perturbations in KSTAR Jayhyun Kim, Y.M. Jeon, G.Y. Park, M.J. Choi, Y. In, S.W. Yoon, C. Bae, J. Lee, J.-K. Park, J. Ahn ELM crash suppressions have been achieved by low $n$ ($n=$1, 2, and mixture of them) magnetic perturbations (MPs) with using various configurations of in-vessel perturbation coils in KSTAR [1][2]. So far, the suppressed periods are extended longer than 10 seconds. In KSTAR, the complete suppression of ELM crashes almost always accompany with the increase of edge fluctuations which are likely to be excited by applied MPs. The excitation of edge fluctuation exhibited the bifurcation-like feature depending on the strength of MPs. The conditions to excite edge fluctuations were investigated with including well known $q_{\mathrm{95}}$ window. On the other hand, ELM mitigation does not come with the increase of edge fluctuations. Instead, it seems that applied MPs directly trigger small frequent ELMs since the mitigated ELMs suddenly disappear when turning MPs off. The results stress the importance of stability analysis with the use of perturbed equilibrium since most stability studies have assumed unperturbed/undistorted equilibrium. [1] Y.M. Jeon \textit{et al.}, Phys. Rev. Letters \textbf{109}, 035004 (2012). [2] J. Kim \textit{et al.}, submitted to Nucl. Fusion. [Preview Abstract] |
Tuesday, November 1, 2016 3:00PM - 3:12PM |
JO9.00006: Effect of RMP spectrum on ELM suppression and the divertor plasma in KSTAR Joon-Wook Ahn, J.-K. Park, Y. In, A. Loarte, J. Kim, Y.M. Jeon, G.Y. Park, W. Choe, J.H. Hong, S.H. Hong, H.H. Lee, C.S. Kang, W.H. Ko, S.W. Yoon ELM suppression by n$=$1 and n$=$2 magnetic perturbations have been robustly obtained in KSTAR, and effects of various coil configurations for applied magnetic perturbations (MPs) on ELM suppression as well as divertor plasma conditions have been investigated. The 4 toroidal and 3 poloidal sectors of internal coils allow to fully scan the phase difference ($\Delta \varphi )$ of n$=$1 between different rows of coils, where it is shown that ideal plasma response can either shield or amplify applied MPs, depending on $\Delta \varphi $, which leads respectively to the weakening and strengthening of divertor footprint striations compared to the vacuum case. On the other hand, shielding is found to be the dominant plasma response for all possible cases of n$=$2 configuration ($\Delta \varphi $ $=$0$^{\mathrm{o}}$ and 90$^{\mathrm{o}}$, and mid-plane coil only), which weakens footprint striations. Spectra of applied MPs have been varied by changing $\Delta \varphi $ as well as modifying the ratio of coil currents between different row of coils, e.g. I$_{\mathrm{U}}$/I$_{\mathrm{L}}$, in order to find optimal conditions for ELM suppression and divertor heat and particle flux dispersal. Effects of divertor conditions in various density and impurity levels on the ELM behavior and footprint striations are also being investigated. Work supported by the U.S. DOE, contract {\#} DE-AC05-00OR22725. [Preview Abstract] |
Tuesday, November 1, 2016 3:12PM - 3:24PM |
JO9.00007: Extended MHD modeling of edge-localized mode suppression by three-dimensional magnetic perturbations in KSTAR G.Y. Park, J. Kim, T.E. Evans, B.C. Lyons, D.M. Orlov, N.M. Ferraro, Y. In, M.J. Choi, S.W. Yoon In this presentation, we report on numerical calculations of the linear response of a plasma to applied three-dimensional magnetic perturbations (MPs) in KSTAR. Simulations are implemented using the extended MHD code M3D-C1 [1]. Initial M3D-C1 calculation of the plasma response in KSTAR [2] has already produced results that are qualitatively consistent with some experimental characteristics observed during the application of MPs in KSTAR, i.e., amplified kink response and associated plasma displacements. Both of the tearing and kink responses are considered and used to explain the basic experimental characteristics of the suppression of edge-localized modes (ELMs) in KSTAR, i.e., q95, heating, and plasma shape dependences of the suppression. In particular, attention is focused on validation studies comparing the M3D-C1 plasma response results to the magnetic and imaging diagnostic measurements (i.e., ECEI data) in KSTAR. In addition, comparison study of the M3D-C1 results with KSTAR and DIII-D data will be presented to help understand a common ELM suppression mechanism which is expected to hold across the different tokamaks. [1] N. M. Ferraro, Phys. Plasmas \textbf{19}(5), 056105 (2012) [2] D. M. Orlov \emph{et al}., Plasma Phys. Control. Fusion \textbf{58}, 075009 (2016) [Preview Abstract] |
Tuesday, November 1, 2016 3:24PM - 3:36PM |
JO9.00008: Nonlinear interaction between edge-localized modes (ELMs) and edge turbulence during ELM-crash-suppression phase under n=1 RMP Jaehyun Lee, Gunsu Yun, Minjun Choi, Jae-Min Kwon, Young-Mu Jeon, Woochang Lee, Neville C. Luhmann, Jr., Hyeon K. Park Mutual interactions between edge-localized modes (ELMs) and turbulent eddies have been investigated in 2-D by using the KSTAR electron cyclotron emission imaging (ECEI) system. ECEI shows that ELM filaments still exist in the edge when the usual large scale collapse of the edge pedestal, i.e., the ELM crash, is completely suppressed by $n=1$ resonant magnetic perturbation (RMP). Correlation analysis among ECEI channels reveals that the RMP enhances turbulent fluctuations in the edge and that ELM crashes are suppressed when the RMP exceeds a certain threshold. The spectral power distribution of turbulence shows a clear dispersion for a wide range of wavenumber ($k_{\theta}<1$ cm$^{-1}$) and frequency ($f<70$ kHz). The radial velocity and ECE intensity fluctuations of the turbulent eddies are approximately in-phase and thus the turbulence involves a net radial energy transport. Bispectral analysis indicates the coexisting ELMs and turbulent eddies nonlinearly interact with each other. Both the enhancement of radial transport and the nonlinear interaction with ELMs may be the key to the physics mechanism of ELM-crash-suppression by low-n RMP. [Preview Abstract] |
Tuesday, November 1, 2016 3:36PM - 3:48PM |
JO9.00009: Role of the magnetic island and low-$k$ turbulence on radial electron heat transport M. J. Choi, H. K. Park, Y. In, S. H. Ko, H. S. Kim, C Bae, J. M. Kwon, W. Lee, K. D. Lee, H. H. Lee, W. H. Ko, S. H. Lee, J. H. Lee, J. Ko, J. Kim, M. H. Woo, M. Jeong, B. H. Park, G. S. Yun, J. Lee, M. Kim, N. C. Luhmann, Jr. Magnetic islands can enhance or reduce the radial transport either by reconnecting field lines or producing the poloidal flow shear across the rational surface. Both cases have been observed in the KSTAR L-mode plasmas. In the first case, the temperature inside the $q=2$ surface decreases severely ($\sim $25{\%}) with the enhanced transport by the rotating $m/n=2/1$ magnetic island. However, in the case where the 2/1 magnetic island is driven and locked by the $n=1$ resonant magnetic perturbation, the transport is reduced and the electron temperature (Te) gradient is increased across the island with a clear poloidal flow shear. The poloidal flow shear has been identified utilizing electron cyclotron emission imaging (ECEI) measurements of the low-k turbulent Te fluctuations driven by the increased Te gradient. In addition, the interaction between the Te turbulence and magnetic island causes the transient heat transport events and affects the transport characteristics near the $q=2$ region. [Preview Abstract] |
Tuesday, November 1, 2016 3:48PM - 4:00PM |
JO9.00010: Comparative study between ion-scale turbulence measurements and gyrokinetic simulations W. Lee, S. H. Ko, M. J. Choi, W. H. Ko, K. D. Lee, J. Leem, G. S. Yun, H. K. Park, W. X. Wang, R. V. Budny, Y. S. Park, N. C. Luhmann, Jr., K. W. Kim Ion gyroscale density fluctuations were measured with a microwave imaging reflectometer (MIR) in neutral beam injected L-mode plasmas on KSTAR. The spatial and temporal characteristic scales of the measured fluctuations were studied by comparing with the local equilibrium parameters relevant to the ion-scale turbulence. Linear and nonlinear gyrokinetic simulations predicted unstable modes with poloidal wavenumbers of $\sim $3 cm$^{\mathrm{-1}}$ (or $k_{\theta } \rho_{s} \sim 0.4)$ and the wavenumbers were also identified from the measured fluctuations. The poloidal wavenumber can be derived from the measured mode frequency and poloidal velocity. The dominant mode frequency and poloidal velocity were obtained from cross correlations among 16 poloidal channels. Both the mode frequency and poloidal velocity mostly are primarily due to the E x B flow velocity in fast rotating plasmas with neutral beam injection. [Preview Abstract] |
Tuesday, November 1, 2016 4:00PM - 4:12PM |
JO9.00011: A Mechanism of ELM Mitigation by External Magnetic Field Perturbations Raghvendra Singh, H Jhang, J.-H. Kim, T.S. Hahm We study the impact of external magnetic perturbations (EMP) on the stability of ballooning mode (BM). We use: 1) the two-step process; 2) standard four wave interactions. In two-step process, we consider EMP are long wave-length perturbations interacting with short scale BM and generating side-bands of higher harmonics. This calculates contributions from all the high toroidal mode numbers. EMP can modify the dispersion characteristics of BM - the growth spectrum becomes broader in$k^{BM}$space. The increase in high$k^{BM}$can lead to the mitigation of an ELM crash by increasing turbulent transport. New nonlinear instability is also found even below the BM threshold at large EMP amplitude. In four wave interaction, EMP act like a short scale pump wave interacting with BM and creating two sidebands. The side-bands couple with the pump and produce the ponderomotive force, magnetic stress at BM frequency. EMP may enhance the BM instability threshold if RMP $\vec{{k}}_{BM} \le \;\vec{{k}}_{RMP} $and reduce the threshold if$\vec{{k}}_{BM} >\;\vec{{k}}_{RMP} $. [Preview Abstract] |
Tuesday, November 1, 2016 4:12PM - 4:24PM |
JO9.00012: Enhanced understanding of momentum transport barrier observed in KSTAR H. H. Lee, J Seol, W. H. Ko, L. Terzolo, A. Y. Aydemir, Y. In, Y. C. Ghime, S. G. Lee It is expected that H-mode plasmas exhibit transport barriers not only for plasma particles and energy but also for toroidal angular momentum. Although density and temperature pedestals at the edge have been seen since the first observations of H-mode in tokamaks three decades ago, a toroidal rotation pedestal is not commonly observed except in some special cases such as QH-mode or is much weaker than those in the density and temperature profiles. But, in the KSTAR tokamak, H-mode plasma is always accompanied by the noticeable toroidal rotation pedestal. We show that the inherent nonaxisymmetric error fields and toroidal ripple can generate significant neoclassical toroidal viscosity (NTV), which damps the toroidal rotation at the edge and to a large extent remove the pedestal in the rotation profile. On the other hand, we demonstrate that the NTV torque induced by the intrinsic error fields and toroidal field ripple in the level of the KSTAR tokamak, which are expected to be smaller than most tokamaks by at least one order of magnitude, is negligible in determining the toroidal rotation velocity profile. Thus we conclusively show that H-mode provides a transport barrier against all three transport channels when turbulent transport is suppressed at the edge. [Preview Abstract] |
Tuesday, November 1, 2016 4:24PM - 4:36PM |
JO9.00013: Quiescence of magnetic braking and control of 3D non-resonance in KSTAR J.-K. Park, Y. In, Y.M. Jeon, N.C. Logan, Z.R. Wang, J.E. Menard, J.H. Kim, W.H. Ko Magnetic braking using non-axisymmetric (3D) field is a promising tool to control rotation in tokamaks and thereby micro-to-macro instabilities. Ideally magnetic braking should induce only neoclassical momentum transport, without provoking resonant instabilities or unnecessary perturbations in particle or heat transport. Indeed in KSTAR, it was shown that the 3 rows of internal coils could be used to generate highly non-resonant n$=$1 with backward-helicity field distribution, called -90 phasing, and to change rotation without any perturbations to other transport channels [1]. Recent KSTAR experiments, however, have also shown that the broad-wavelength field distribution, called 0 phasing, is rather more quiescent whereas -90 phasing can be highly degrading especially in high q$_{\mathrm{95}}$ plasmas. IPEC and NTV modeling are consistent with both observations, and further provide the optimal point in coil phasing and amplitude space. Additional experiments and comparisons with modeling all imply the sensitivity of plasma response to remnant resonant field, and thus importance of non-resonance control, to accomplish quiescent magnetic braking. This work was supported by DOE Contract DE-AC02-09CH11466. [1] J.-K. Park, Y.M. Jeon et al., Phys. Rev. Lett. \textbf{111}, 095002 (2013) [Preview Abstract] |
Tuesday, November 1, 2016 4:36PM - 4:48PM |
JO9.00014: MSE measurements for sawtooth and non-inductive current drive studies in KSTAR J Ko, H Park, Y.S Bea, J Chung, Y.M Jeon Two major topics where the measurement of the magnetic-field-line rotational transform profiles in toroidal plasma systems include the long-standing issue of complete versus incomplete reconnection model of the sawtooth instability and the issue with future reactor-relevant tokamak devices in which non-inductive steady state current sustainment is essential. The motional Stark effect (MSE) diagnostic based on the photoelastic-modulator (PEM) approach is one of the most reliable means to measure the internal magnetic pitch, and thus the rotational transform, or its reciprocal ($q$), profiles. The MSE system has been commissioned for the Korea Superconducting Tokamak Advanced Research (KSTAR) along with the development of various techniques to minimize systematic offset errors such as Faraday rotation and mis-alignment of the bandpass filters. The diagnostic has revealed the central $q$ is well correlated with the sawtooth oscillation, maintaining its value above unity during the MHD quiescent period and that the response of the $q$ profile to external current drive such as electron cyclotron wave injection not only involves the local change of the pitch angle gradient but also a significant shift of the magnetic topology due to the wave energy transport. [Preview Abstract] |
Tuesday, November 1, 2016 4:48PM - 5:00PM |
JO9.00015: Lesson from Tungsten Leading Edge Heat Load Analysis in KSTAR Divertor Suk-Ho Hong, Richard Anthony Pitts, Hyeong-Ho Lee, Eunnam Bang, Chan-Soo Kang, Kyung-Min Kim, Hong-Tack Kim An important design issue for the ITER tungsten (W) divertor and in fact for all such components using metallic plasma-facing elements and which are exposed to high parallel power fluxes, is the question of surface shaping to avoid melting of leading edges. We have fabricated a series of tungsten blocks with a variety of leading edge heights (0.3, 0.6, 1.0, and 2.0 mm), from the ITER worst case to heights even beyond the extreme value tested on JET. They are mounted into adjacent, inertially cooled graphite tile installed in the central divertor region of KSTAR, within the field of view of an infra-red (IR) thermography system with a spatial resolution to 0.4 mm/pixel. Adjustment of the outer divertor strike point position is used to deposit power on the different blocks in different discharges. The measured power flux density on flat regions of the surrounding graphite tiles is used to obtain the parallel power flux, q\textbar \textbar impinging on the various W blocks. Experiments have been performed in Type I ELMing H-mode with Ip $=$ 600 kA, BT $=$ 2 T, PNBI $=$ 3.5 MW, leading to a hot attached divertor with typical pulse lengths of 10 s. Three dimensional ANSYS simulations using q\textbar \textbar and assuming geometric projection of the heat flux are found to be consistent with the observed edge loading. [Preview Abstract] |
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