Bulletin of the American Physical Society
59th Annual Meeting of the APS Division of Plasma Physics
Volume 62, Number 12
Monday–Friday, October 23–27, 2017; Milwaukee, Wisconsin
Session NO4: KSTAR |
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Chair: Zheng Yan, University of Wisconsin-Madison Room: 201AB |
Wednesday, October 25, 2017 9:30AM - 9:42AM |
NO4.00001: Research Progress and Future Plan of the KSTAR Yeong-Kook Oh, H.K. Park, S.W. Yoon, J.G. Kwag, Y. Chu, K.R. Park Unique capabilities of the Korean Superconducting Tokamak Advanced Research (KSTAR) have been fully capitalized for steady state operation of high beta plasmas and fundamental physics research. The KSTAR is ideal for study of the tokamak plasma symmetry on stability and confinement study owing to the lowest error field and magnetic ripple. Versatile magnetic perturbation tool with n=1,2 in-vessel control coils has been extensively used in control of the harmful MHD such as the ELMs and toroidal rotation through NTV. Advanced 2D/3D microwave imaging diagnostics for undisputed measurements for theory and modeling. In 2016 and 2017 campaigns, KSTAR has achieved a record long operation (~34s) of a ELM-crash free as well as long ELMy (~70s) H-mode operation. A predictive capability as well as the underlying physics of the resonant magnetic perturbation (RMP) have been demonstrated. This talk will address the advances in research and vision toward the high beta long pulse operation in KSTAR together with the upgrade plan. [Preview Abstract] |
Wednesday, October 25, 2017 9:42AM - 9:54AM |
NO4.00002: Scenario development toward high beta steady-state operation at KSTAR S. W. Yoon, Y. M. Jeon, H. S. Kim, M. H. Woo, S. H. Hahn, Y. S. Bae, J. S. Kang, Y. S. Na, Y. K. Oh, H. K. Park Solving issues for high-beta long-pulse operation is one of the essential topics for superconducting tokamaks and sustainment of a fully non-inductive H-mode discharge with high performance is successfully demonstrated upto record-long \textasciitilde 70 seconds at KSTAR. Typical plasma parameters are 0.4MA(I$_{\mathrm{p}})$, max 5MW (NBI$+$ECH), $\beta _{\mathrm{p}}$\textasciitilde 3, f$_{\mathrm{BS}}$\textasciitilde 0.5, H$_{\mathrm{98}}$\textasciitilde 1.3 and in a wide range of q$_{\mathrm{95}}=$6\textasciitilde 12. Though an internal transport barrier is not identified yet, the developed scenario has many features in common with the so called “high $\beta_{\mathrm{p}}$ discharge” at DIII-D. The thermal confinement is sensitive on the deposition layer of the central ECH heating \textasciitilde 1MW and it correlates with MHD activities in the range of TAE frequency (100\textasciitilde 200kHz) suggesting strong interaction TAE with fast ion transport. Based on the transport/stability analysis on the present discharge, improved performance is also estimated with higher NBI$+$ECH heating power envisaged in near future. [Preview Abstract] |
Wednesday, October 25, 2017 9:54AM - 10:06AM |
NO4.00003: Distinctive dependence of RMP-ELM coupling on plasma shape, and its optimization for robust ELM control in KSTAR Y.M. Jeon, Y. In, J.-K. Park, Jay H. Kim, J.-W. Ahn, G.Y. Park, H.S. Han, S.W. Yoon In recent KSTAR experiments, a critical dependence of RMP-ELM coupling on plasma shape was found to be as important as q$_{\mathrm{95}}$. In application of low-n RMPs, small variations of lower triangularity ($\delta _{\mathrm{Lower}})$ made dramatic changes on RMP coupling. Specifically, as $\delta_{\mathrm{Lower}}$ increases, the coupling to plasma core appears weakened, while the edge coupling gets strengthened effectively for ELM suppression. So far, such $\delta_{\mathrm{Lower}}$ dependence window for RMP-ELM suppression is surprisingly narrow ($\Delta \delta _{\mathrm{Lower}}=$0.08), while the other shape dependence is unclear. In 2017, a further investigation reveals that such a strict condition can be relaxed by allowing a small up-down asymmetry on plasma shape. Applying this new optimized plasma shape made substantial improvements on the reliability and robustness of RMP-ELM control. As results, the ELM suppressions were successfully demonstrated in a wide range of q$_{\mathrm{95}}$ (4.0-6.4 for n$=$1 and 3.3-3.8 for n$=$2 RMPs) even with a fixed RMP phasing, achieving a record-long sustainment of ELM-suppression (\textgreater 30sec). [Preview Abstract] |
Wednesday, October 25, 2017 10:06AM - 10:18AM |
NO4.00004: Decoupling edge and core RMPs for ELM suppression in KSTAR J.-K. Park, Y. M. Jeon, Y. In, J.-W. Ahn, G. Y. Park, J. Kim, H. S. Kim, N. C. Logan, Z. Wang, R. Nazikian Resonant magnetic perturbations (RMPs) can suppress edge-localized-modes (ELMs) in tokamaks when carefully optimized, in particular for the resonant response in the edge pedestal while minimizing other unnecessary resonances or degradations in the core. KSTAR has been unique in testing these RMP decoupling and optimizing principles, by its 3 rows of in-vessel coils and independent n$=$1 control at each row. A recent experiment on a special subset of that multi-dimensional coil configuration space in KSTAR clearly suggests the importance of 3D MHD response to RMPs, as both IPEC and MARS codes were greatly successful in predicting ELM suppression windows whereas vacuum approximation was entirely misleading. The linear 3D MHD predictions were often surprisingly accurate in details, as validated by various dynamic RMP applications and ELM responses. This predictive RMP capability played an important role in guiding experiments later and finding non-standard RMPs, leading to the first demonstration of the n$=$1 ELM suppression without using the mid-plane coils and also the RMP ELM suppression in high q$_{\mathrm{95}}$\textgreater 6 in high-$\beta $ KSTAR plasmas. [Preview Abstract] |
Wednesday, October 25, 2017 10:18AM - 10:30AM |
NO4.00005: Evidence of perpendicular flow bifurcation at the onset of ELM-crash suppression Jaehyun Lee, YoungMu Jeon, Yongkyoon In, Gunsu Yun, Minwoo Kim, Minjun Choi, Gunyoung Park, Hyeon K. Park The evidence of perpendicular electron flow ($v_{\bot ,e} $ \begin{figure}[htbp] \centerline{\includegraphics[width=0.23in,height=0.18in]{140720171.eps}} \label{fig1} \end{figure} ) bifurcation at the onset of ELM-crash suppression has been measured using electron cyclotron emission imaging (ECEI) system [1] for the first time in KSTAR. The ECEI has shown that (1) resonant magnetic perturbation (RMP) enhances small scale turbulent fluctuations in the edge toward the ELM-crash suppression phase, (2) the induced turbulence regulates growth of the ELM filament via nonlinear interaction between them [2]. Cross spectra and correlation analysis among the ECEI channels revealed that the ELM crashes get suppressed along with a rapid reduction of $v_{\bot ,e} $ \begin{figure}[htbp] \centerline{\includegraphics[width=0.23in,height=0.18in]{140720172.eps}} \label{fig2} \end{figure} close to zero (small but finite value) together with decrease of its shear. The $v_{\bot ,e} \sim $0 km/s \begin{figure}[htbp] \centerline{\includegraphics[width=0.78in,height=0.18in]{140720173.eps}} \label{fig3} \end{figure} is sustained during the ELM-crash suppression even under a large variation of RMP current and external torque and when this condition is violated, ELM crashes are reappeared. [1] G.S. Yun et al., Rev. Sci. Instrum., 81 (2010) 10D930 [2] J. Lee et al., Phys. Rev. Lett., 117 (2016) 075001 [Preview Abstract] |
Wednesday, October 25, 2017 10:30AM - 10:42AM |
NO4.00006: Compatibility of RMP ELM control with divertor heat flux dispersal and detachment in KSTAR Joonwook Ahn, Yongkyoon In, Alberto Loarte, Jun Gyo Bak, Bin Cao, YoungMu Jeon, Jayhyun Kim, Hyung Ho Lee, Gunyoung Park, Jong-Kyu Park, Suk Ho Hong, Won Ha Ko, Si Woo Yoon RMP ELM suppression can help avoid transient peak heat flux problem in tokamaks but the steady state heat flux should be also effectively dispersed over the divertor surface, either by profile broadening or divertor detachment. Low-n RMPs were utilized to address the effect of B-field structure, including intentionally misaligned RMP configuration, and divertor gas puffing on both heat and particle flux footprints. Particularly, full ELM suppression for n$=$2 at q95$=$3.4 was successfully sustained even with the strong gas puffing and the subsequent density ramp up until the plasma finally disrupted due to the apparent radiation loss. On the other hand, as for the n$=$1 RMP ELM suppression at q95$=$5, both heat and particle fluxes were reduced significantly near the outer strike point (OSP) with divertor gas puffing (indicative of partial detachment) but the ELM suppression itself was not sustained. Overall, good progress in high-density ELM suppression for n$=$2 was made and this needs to be combined with the stable divertor detachment, which was demonstrated for n$=$1. Detailed analyses with various plasma parameters and response to RMPs will be discussed. Work supported by the U.S. DOE, contract {\#} DE-AC05-00OR22725. [Preview Abstract] |
Wednesday, October 25, 2017 10:42AM - 10:54AM |
NO4.00007: Observation of the Generalized Neoclassical Toroidal Viscosity Offset Rotation Profile in KSTAR* S.A. Sabbagh, Y.S. Park, J. Kim, W.H. Ko, S.H. Hahn, Y. In, Y.K. Oh, K.C. Shaing, Y. Sun A beneficial effect of Neoclassical Toroidal Viscosity [1] that may be important in slowly rotating plasmas as expected in ITER is the inherent or ``offset'' rotation created by an applied 3D field. Past experiments and associated theory have only considered that the NTV offset rotation can occur in the direction opposite to the plasma current. More recently, the NTV offset rotation profile, V$_{\mathrm{0}}^{\mathrm{NTV}}$, was directly measured and studied in the KSTAR tokamak that has shown for the first time strong, controlled rotation in the co-I$_{\mathrm{p}}$ direction at high electron temperature. This result is expected from generalized NTV theory that allows for electron and ion torques. A field with dominant n $=$ 2 component was applied to produce V$_{\mathrm{0}}^{\mathrm{NTV}}$. Rotation in the plasma outer region exceeded 12 krad/s, quite significant compared to projections for ITER of approximately 2 krad/s in the pedestal region. Also, the V$_{\mathrm{0}}^{\mathrm{NTV}}$ rotation profile shear is 15 times greater than measured in the intrinsic rotation profile (without 3D field). Experiments at higher T$_{\mathrm{e}}$ produced the strong co-rotation and rotation shear while higher density and lower T$_{\mathrm{e}}$ reduced these characteristics. [1] K.C. Shaing, K. Ida, S.A. Sabbagh, Nucl. Fusion \textbf{55} (2015) 125001. $^{\mathrm{\ast }}$Supported by US DOE Contracts DE-FG02-99ER54524 and DE-SC0016614. [Preview Abstract] |
Wednesday, October 25, 2017 10:54AM - 11:06AM |
NO4.00008: Comparison of MHD simulation codes for understanding nonlinear ELMs dynamics in KSTAR H-mode plasma M. Kim, J. Lee, H. K. Park, G. S. Yun, X. Xu, S. C. Jardin, M. Becoulet KSTAR electron cyclotron emission imaging (ECEI) systems have contributed to understanding the fundamental physics of ELMs by high-quality 2D and quasi-3D images of ELMs. However, in the highly nonlinear phase of ELM dynamics, the interpretation of ECE signals becomes complicated intrinsically. Theoretical and numerical approaches are necessary to enhance the understanding of ELM physics. Well-established MHD codes (BOUT$++$, JOREK, and M3D-C1) are introduced for comparative study with the observations. The nonlinear solutions are obtained using the same equilibrium of the KSTAR H-mode plasma. Each code shows the partial difference in mode evolution, probably, due to the difference in optimized operation window of initial conditions. The nonlinear simulation results show that low-$n$ ($n<5)$ modes becomes dominant close to pedestal collapse. The mode evolution in the simulations qualitatively matches with the recent ECEI observation just before ELM-crash, or excitation of non-modal solitary perturbation (typically, $n=1)$ [1] which is highly localized in poloidal and toroidal. Regardless of differences in details, qualitative similarity can provide inspiration to understand the triggering of ELM-crash. [1] J. E. Lee, et al., Scientific Reports, 7 (2017) 45075 [Preview Abstract] |
Wednesday, October 25, 2017 11:06AM - 11:18AM |
NO4.00009: Modulation method as a tool to measure three dimensional magnetic field structures in toroidal plasmas Katsumi Ida, Tatsuya Kobayashi, WonHa Ko, Hyungho Lee, Yongkyoon In, HyunSeok Kim Three dimension (3D) magnetic field applied by external coils is modified by the 3D current produced by plasma (plasma response). For example, the magnetic island produced by the external field and resonant or non-resonant magnetic perturbation (NRMP or RMP) field applied usually differ from the vacuum magnetic field due to the plasma response. Therefore, it is important to determine the 3D magnetic field experimentally, not based on the magnetic field calculation. Two modulation techniques applied to various toroidal plasmas to measure 3D magnetic field structure are discussed. One is the modulation electron cyclotron heating (MECH) to measure the size of magnetic island by taking advantage of bi-directional (inward and outward) heat pulse propagation excited by MECH inside magnetic island. Recently, the modulation of NRMP or RMP field is applied to measure the penetration length of the 3D perturbation magnetic field using the modulation amplitude profile of the toroidal rotation gradient. In this talk, 1) bifurcation phenomena of magnetic island in LHD [1] and DIII-D [2] plasmas and 2) non-linearity of the penetration of 3D magnetic field in KSTAR H-mode plasma will be presented. Ref [1] K.Ida et. al., New J. Phys. 15 (2013) 013061, [2] K.Ida et. al., Sci. Rep. 5 (2015) 16165. [Preview Abstract] |
Wednesday, October 25, 2017 11:18AM - 11:30AM |
NO4.00010: Control advances for achieving the ITER baseline scenario on KSTAR N.W. Eidietis, J. Barr, S.H. Hahn, D.A. Humphreys, Y.K. In, Y.M. Jeon, M.J. Lanctot, D. Mueller, M.L. Walker Control methodologies developed to enable successful production of ITER baseline scenario (IBS) plasmas on the superconducting KSTAR tokamak are presented: decoupled vertical control (DVC), real-time feedforward (rtFF) calculation, and multi-input multi-output (MIMO) X-point control. DVC provides fast vertical control with the in-vessel control coils (IVCC) while sharing slow vertical control with the poloidal field (PF) coils to avoid IVCC saturation. rtFF compensates for inaccuracies in offline PF current feedforward programming, allowing reduction or removal of integral gain (and its detrimental phase lag) from the shape controller. Finally, MIMO X-point control provides accurate positioning of the X-point despite low controllability due to the large distance between coils and plasma. Combined, these techniques enabled achievement of IBS parameters (q$_{\mathrm{95\thinspace }}=$ 3.2, $\beta_{\mathrm{N}}=$ 2) with a scaled ITER shape on KSTAR. n$=$2 RMP response displays a strong dependence upon this shaping.. [Preview Abstract] |
Wednesday, October 25, 2017 11:30AM - 11:42AM |
NO4.00011: Improvement of vertical stabilization on KSTAR D. Mueller, J.G. Bak, M.D. Boyer, N. Eideitis, S.H. Hahn, D.A. Humphreys, H.S. Kim, Y.M. jeon, M. Lanctot, M.L. Walker The successful control of strongly shaped plasmas on the Korea Superconducting Tokamak Advanced Research (KSTAR) device requires active feedback of fast motion of the plasma vertical position by the use of internal normal conducting coils (IVC). This has required new electronics to supply relative flux loop differences, for z$_{\mathrm{p,}}$ and voltage loop differences, for dz$_{\mathrm{p}}$/dt, as well as a novel technique (Zfast) to use a high-pass filter, typically 1 Hz, on the error in the signal in the feedback loop. Use of Zfast avoids the potential contention encountered when the internal coil attempts to perform control of the plasma shape which should be controlled by the slower and more powerful superconducting coils. A common problem of this contention is saturation of the IVC and loss of fast vertical control. This is eliminated by proper use of the Zfast. A Ziegler-Nichols relay feedback system was used to fine tune the required feedback gains. The selection of the magnetic sensors, filter time constants, control gains and of the Zfast control strategy which allowed vertically stable operation at a plasma elongation, kappa. of up to 2.16 at l$_{\mathrm{i}} \quad =$ 1.15 and Betap $=$ 2.4 will be discussed which is beyond the design reference of KSTAR of kappa $=$ 2.0 at l$_{\mathrm{i}}$ $=$ 1.2 and Betap $=$1.9. [Preview Abstract] |
Wednesday, October 25, 2017 11:42AM - 11:54AM |
NO4.00012: Perspectives of extremely low $q$ operation with internal transport barrier in KSTAR Jayhyun Kim, H.K. Park, J. Chung, M.J. Choi, H.S. Kim, Y,M. Jeon, J.S. Ko, H.H. Lee, K.D. Lee For an ideal fusion reactor, discharges with internal transport barrier (ITB) can be the most attractive operation mode due to high core temperature and low transient heat flux. However, the ITB mode inherently has problems such as collapse by MHD instability. Recently, it has been proved in KSTAR that stable operation of more than several seconds is possible even in the ITB mode due to the existence of benign bursting mode which prevents the formation of steep pressure gradient [1]. On the other hand, KSTAR also demonstrated the stabilization of potentially dangerous instabilities of low mode numbers through low $q$ (\textasciitilde 2) operation [2]. In this study, we will discuss the perspectives of low $q$ operation with ITB mainly considering their MHD characteristics. During a preliminary attempt in 2017 campaign, transient ITB of H$_{\mathrm{89L}}$\textasciitilde 1.9 was obtained in NBI heated L-mode discharge after entering relatively low $q$ (\textless 3) regime by late additional NB heating. [1] J. Chung \textit{et al.}, submitted to Nucl. Fusion. [2] T. Suzuki et al., IAEA FEC (2004) EX/1-3. [Preview Abstract] |
Wednesday, October 25, 2017 11:54AM - 12:06PM |
NO4.00013: Characteristics of hybrid scenarios in KSTAR Youngho Lee, Cheol-Sik Byun, Yong-Su Na We report the characteristics of hybrid scenarios under development in KSTAR. Firstly, detailed definition of the hybrid scenario in KSTAR is described and categorized according to the MHD activities. The discharges exhibiting H89 \textgreater 1.9, betaN \textgreater 2.2 sustained more than 5*tauE at q95 \textless 6.5 without or mild sawtooth are classified into the hybrid regime. Fishbones and neoclassical tearing modes are usually observed in this regime. Improved confinement in this regime is also confirmed with comparing general H-mode in KSTAR. Secondly, several experimental approaches are presented to access the hybrid regime. Here, four different recipes are described. Thirdly, the origin of the confinement enhancement is discussed. The role of the plasma rotation is found to be small in experiments where electron cyclotron heating is applied to reduce the toroidal rotation. The pedestal enhancement is thought to be the main reason for the confinement improvement in KSTAR hybrid scenarios. [Preview Abstract] |
Wednesday, October 25, 2017 12:06PM - 12:18PM |
NO4.00014: Study of quasi-coherent fluctuations in low-density ECH and NBI L-mode plasmas on KSTAR W. Lee, S. H. Ko, J. Leem, G. S. Yun, H. K. Park, K. W. Kim, N. C. Luhmann, Jr. Quasi-coherent fluctuations with $\Delta f\sim f_{\mbox{peak}} $ in the electron density were measured in stationary low-density ECH and NBI L-mode plasmas on KSTAR. Dominant poloidal wavenumbers of the fluctuations were estimated from measured peak frequencies and poloidal rotation velocities of the fluctuations, and compared to the poloidal wavenumbers with maximum growth rate obtained from linear gyro-kinetic simulations. The poloidal wavenumbers from the measurements and linear simulations are comparable with each other, and this seems to be due to low collisionality of the plasmas where the quasi-coherent fluctuations were measured. Group velocities of the fluctuations in the plasma frame are all in the ion diamagnetic drift direction for the NBI L-mode plasmas, and this agrees well with the linear simulation results. For ECH L-mode plasmas, however, the group velocities from the measurements are not clearly recognized. [Preview Abstract] |
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