Bulletin of the American Physical Society
56th Annual Meeting of the APS Division of Plasma Physics
Volume 59, Number 15
Monday–Friday, October 27–31, 2014; New Orleans, Louisiana
Session GO3: NSTX, Spherical Tori and KSTAR |
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Chair: Richard Buttery, DIII-D Room: Salon D |
Tuesday, October 28, 2014 9:30AM - 9:42AM |
GO3.00001: Overview of MAST results Ian Chapman MAST addresses key issues for ITER and DEMO. ELM mitigation with RMPs with n$=$2,3,4,6 has been demonstrated: at high and low collisionality; for the first ELM; during the current ramp; when in-vessel coils fail; and with rotating n$=$3 RMPs. n$=$4,6 RMPs cause less braking whilst H-mode access is easiest with n$=$4. Refuelling allows reduced peak heat flux but only 10{\%} drop in confinement. Gyrokinetic simulations of micro-tearing modes are consistent with ELM precursors strikingly observed with beam emission spectroscopy. Global gyrokinetic runs show kinetic ballooning modes mediate the pedestal width. A scan in beta at L-H transition shows that pedestal height scales strongly with core pressure. The observed tilt of low-k turbulent vortices increases with flow shear, due to a decrease in poloidal wave number. Highly efficient electron Bernstein wave current drive (1A/W) has been achieved in solenoid-free start-up. Langmuir probes and a high-speed camera suggest filaments play a role in particle transport in the private flux region whilst coherence imaging has measured scrape-off layer flows. MAST Upgrade is due to operate in 2015 to support ITER preparation and importantly to operate with a Super-X divertor to test extended leg concepts for particle and power exhaust. [Preview Abstract] |
Tuesday, October 28, 2014 9:42AM - 9:54AM |
GO3.00002: Towards a Predictive Capability for Local Helicity Injection Startup J.L. Barr, M.W. Bongard, M.G. Burke, R.J. Fonck, E.T. Hinson, B.T. Lewicki, J.M. Perry, A.J. Redd, D.J. Schlossberg Local helicity injection (LHI) is a non-solenoidal tokamak startup technique under development on the Pegasus ST. New designs of the injector cathode geometry and plasma-facing shield rings support high-voltage operation up to 1.5 kV. This leads to reduced requirements in injector area for a given helicity input rate. Near-term experiments in Pegasus are testing the gain in $I_{p} $ obtained with a $1.5\times $ increase in the helicity input rate and the efficacy of helicity injection in the lower divertor region. A predictive model for LHI is needed to project scalable scenarios for larger devices. A lumped-parameter circuit model using power and helicity balance is being developed for LHI on Pegasus-U and NSTX-U. The model indicates that MA-class startup on NSTX-U will require operating in a regime where the drive from LHI dominates the inductive effects arising from dynamically evolving plasma geometry. The physics of this new regime can be tested in Pegasus-U at $I_{p} \approx 0.3$ MA. The LHI systems on the proposed Pegasus-U will be expanded to provide $3-4\times $ helicity injection rate and the toroidal field doubled to reach this regime. Predictive models to be validated on Pegasus-U include the 0-D power balance model, NIMROD, and TSC. [Preview Abstract] |
Tuesday, October 28, 2014 9:54AM - 10:06AM |
GO3.00003: H-mode Characteristics and ELM Dynamics at Near-Unity Aspect Ratio K.E. Thome, G.M. Bodner, M.W. Bongard, M.G. Burke, R.J. Fonck, D.J. Schlossberg Ohmic H-mode is achieved at near-unity aspect ratio in the Pegasus Toroidal Experiment through the use of high-field-side fueling in both limited and diverted geometries. This regime is characterized by: increased edge rotation shear; increased central heating; and measured energy confinement consistent with the ITER98pb(y,2) scaling. In limited plasmas the power threshold is $\sim 10\times $ higher than predicted by the high-$A$ empirical tokamak scaling for $n_{G} =0.1-0.6$. No significant reduction in the power threshold has been observed in favorable $\nabla B$ SN plasma when compared to limited plasmas. Two classes of ELMs have been identified to date by their proximity to the power threshold and measured $n$ spectra. Small, Type III-like ELMs are present at input power $P_{OH} \sim P_{th} $ and have $n\le 4$. At $P_{OH} \gg P_{th} $, they transition to large, Type-I-like ELMs with intermediate 5 \textless $n $\textless 15. These general mode numbers are opposite those seen at large $A$ and reflect the increased peeling drive present at low $A. $ The unique operating characteristics available at $A\sim 1$ in Pegasus allow long-sought measurements of the time evolution of the $J_{edge} (R)$ pedestal collapse during an ELM event. They show a complex, multimodal pedestal collapse and the subsequent ejection of a current-carrying filament. [Preview Abstract] |
Tuesday, October 28, 2014 10:06AM - 10:18AM |
GO3.00004: Characteristics of Neoclassical Toroidal Viscosity in NSTX and KSTAR for Rotation Control and the Evaluation of Plasma Response S.A. Sabbagh, J.W. Berkery, Y.S. Park, R.E. Bell, D.A. Gates, S.P. Gerhardt, I. Goumiri, T.E. Evans, N. Ferraro, Y.M. Jeon, W. Ko, K.C. Shaing, Y. Sun Three-dimensional magnetic fields producing non-resonant magnetic braking allow control of the plasma rotation profile, $\omega_{\varphi}$, in tokamaks. Experimental angular momentum alteration created by 3D field configurations with dominant $n = $ 2 and $n$~$=$~3 components in NSTX is compared to theoretical neoclassical toroidal viscosity (NTV) torque density profiles, $T_{NTV}$. Large radial variations of $T_{NTV}$ are typically found when flux surface displacements are computed using ideal MHD assumptions. In contrast, experimentally measured $T_{NTV}$ does not show strong torque localization. This may be explained by ion banana width orbit-averaging effects. A favorable characteristic for $\omega_{\varphi}$ control clearly illustrated by KSTAR experiments is the lack of hysteresis of $\omega_{\varphi}$ when altered by non-resonant NTV. Results from a model-based rotation controller designed using NBI and NTV from the applied 3D field as actuators are shown. The dependence of $T_{NTV}$ on $\delta $\textbf{\textit{B}}$^{2}$ significantly constrains the allowable field amplification in plasma response models when compared to experiment. Initial analysis shows that the single fluid model in the M3D-C$^{1}$ resistive MHD code produces a flux surface-averaged $\delta $\textbf{\textit{B}} consistent with the experimentally measured $T_{NTV}$. [Preview Abstract] |
Tuesday, October 28, 2014 10:18AM - 10:30AM |
GO3.00005: The Potential Contribution of RF Sheaths to Field-Aligned SOL Losses on NSTX R.J. Perkins, R.E. Bell, N. Bertelli, S. Gerhardt, J.C. Hosea, M.A. Jaworski, G.J. Kramer, B.P. LeBlanc, R. Maingi, C.K. Phillips, G. Taylor, J.-W. Ahn, T.K. Gray, E.F. Jaeger, A. McLean, S.A. Sabbagh NSTX can exhibit a major loss of high-harmonic fast wave (HHFW) power to the upper and lower divertor regions along scrape-off layer (SOL) field lines passing in front of the antenna [1]. One contributing loss mechanism is RF sheaths forming at the divertor at strike points of the magnetic field lines. Here we compare calculations of the Langmuir probe sheath transmission factor to infrared camera (IR) measurement estimates. At the probe, the sheath transmission increases by a factor of two with RF showing that the RF sheath losses at the divertor are significant. We will compare sheath voltage determined using the AORSA code [2] with the RF voltage measured at the Langmuir probe. Future experimentation on NSTX-U will employ a wide-angle IR camera and coaxial Langmuir probes to quantify the HHFW sheath losses. \\[4pt] [1] R.J. Perkins et al., Phys. Rev. Lett. 109 (2012) 045001.\\[0pt][2] N. Bertelli et al., Nucl. Fusion 54 (2014). [Preview Abstract] |
Tuesday, October 28, 2014 10:30AM - 10:42AM |
GO3.00006: Application of 3D Synthetic Reflectometry Diagnostics to Comparing Results from PIC Simulations with Reflectometry Measurements in NSTX Lei Shi, Ahmed Diallo, Gerrit Kramer, Seung-Hoe Ku, William Tang, Ernest Valeo Synthetic diagnostics are powerful tools to connect advanced numerical simulations with experimental measurements. They can be used for validation studies of the simulations as well as providing insights for experimental observations. Since individual synthetic diagnostic codes are usually developed independently from predictive simulation codes, interfacing them is a significant task. In this talk, we will report on new results obtained from interfacing a recently-developed 3D synthetic reflectometry code (FWR3D) - as well as the well-established 2D version (FWR2D) -- against the global particle-in-cell (PIC) code - XGC1 - generally regarded as the most advanced integrated edge/core code that is characterized by high phase-space and coordinate-space resolution including complex separatrix geometry in the edge region of tokamak plasmas. The simulation results are compared with the reflectometry signals measured between an edge localized mode (ELM) cycle in an actual NSTX discharge. Associated findings regarding to what degree the features observed by the actual reflectometry diagnostic deployed in the experiment are reproduced will be reported. In addition, information from the XGC-1 code on the characteristics and spectral properties of the turbulence will be provided. [Preview Abstract] |
Tuesday, October 28, 2014 10:42AM - 10:54AM |
GO3.00007: Lithium wetting of stainless steel for plasma facing components C.H. Skinner, A.M. Capece, J.P. Roszell, B.E. Koel Ensuring continuous wetting of a solid container by the liquid metal is a critical issue in the design of liquid metal plasma facing components foreseen for NSTX-U and FNSF. Ultrathin wetting layers may form on metallic surfaces under ultrahigh vacuum (UHV) conditions if material reservoirs are present from which spreading and wetting can start. The combined scanning electron microscopy (SEM), Auger electron spectroscopy (AES) and ion beam etching capabilities of a Scanning Auger Microprobe (SAM) have been used to study the spreading of lithium films on stainless steel substrates. A small (mm-scale) amount of metallic lithium was applied to a stainless steel surface in an argon glove box and transferred to the SAM. Native impurities on the stainless steel and lithium surfaces were removed by Ar+ ion sputtering. Elemental mapping of Li and Li-O showed that surface diffusion of Li had taken place at room temperature, well below the 181$^{\circ}$C Li melting temperature. The influence of temperature and surface oxidation on the rate of Li spreading on stainless steel will be reported. [Preview Abstract] |
Tuesday, October 28, 2014 10:54AM - 11:06AM |
GO3.00008: Sputtering rates of lithium and lithium hydride due to low energy ion impact John Roszell, Angela Capece, Charles Skinner, Bruce Koel The presence of lithium coatings on plasma facing components (PFCs) has been shown to improve plasma performance through the reduction of hydrogen recycling. Understanding the interactions between plasma species and lithium-covered PFCs is important to the successful implementation of lithium in a tokamak environment. Fundamental surface science experiments performed in a controlled UHV environment are used to investigate the interactions between deuterium ions and lithium films. Sputtering yields of lithium from pure and deuterated lithium films due to the impact of low energy deuterium and rare gas ions are measured with a well characterized ion beam capable of achieving energies of \textless 10 eV/D$^{\mathrm{+}}$. Lithium films are deposited on a molybdenum single crystal substrate from an SAES getter source allowing for pure Li film growth with a highly reproducible thickness. These films are characterized with Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS), while sputtering yield is measured with temperature programmed desorption (TPD) to quantify the number of atoms remaining on the surface after ion bombardment. Sputtering yields are measured as a function of ion energy, film composition, and substrate temperature. [Preview Abstract] |
Tuesday, October 28, 2014 11:06AM - 11:18AM |
GO3.00009: Lithium vapor trapping at a high-temperature lithium PFC divertor target Michael Jaworski, T. Abrams, R.J. Goldston, R. Kaita, D.P. Stotler, G. De Temmerman, J. Scholten, M.A. van den Berg, H.J. van der Meiden Liquid lithium has been proposed as a novel plasma-facing material for NSTX-U and next-step fusion devices but questions remain on the ultimate temperature limits of such a PFC during plasma bombardment. Lithium targets were exposed to high-flux plasma bombardment in the Magnum-PSI experimental device resulting in a temperature ramp from room-temperature to above 1200$^{\circ}$C. A stable lithium vapor cloud was found to form directly in front of the target and persist to temperature above 1000$^{\circ}$C. Consideration of mass and momentum balance in the pre-sheath region of an attached plasma indicates an increase in the magnitude of the pre-sheath potential drop with the inclusion of ionization sources as well as the inclusion of momentum loss terms. The low energy of lithium emission from a surface measured in previous experiments ($<1$eV) is conducive to trapping within this modest potential well. The scale length derived from the ionizing pre-sheath model of 3mm is consistent with the observed neutral lithium emission found in the experiment. The strong trapping inferred from the lifetime of the coating indicates previously calculated temperature limits for lithium PFCs need to be re-evaluated. [Preview Abstract] |
Tuesday, October 28, 2014 11:18AM - 11:30AM |
GO3.00010: VUV/XUV measurements of impurity emission in plasmas with liquid lithium surfaces on LTX Kevin Tritz, M. Finkenthal, D. Stutman, R.E. Bell, D.P. Boyle, R. Kaita, T. Kozub, M. Lucia, R. Majeski, E. Merino, J.C. Schmitt, P. Biersdorfer, J. Clementson, S. Kubota The VUV/XUV spectrum has been measured on the Lithium Tokamak eXperiment (LTX) using a spatially-resolved transmission grating imaging spectrometer (TIGS) coupled to a direct-detection X-ray CCD camera. TGIS data show significant changes in the ratios between the lithium and oxygen impurity line emission during the discharges with different lithium wall conditions. Lithium coatings that have been semi-passivated by lengthy exposure to significant levels of impurities contribute to a large O/Li ratio measured during LTX plasma discharges. Furthermore, results from previous experiments have indicated that a passivated lithium film on the boundary shells can function as a stronger impurity source when in the form of a liquid layer compared to a solid lithium layer. However, recent TGIS measurements of plasma discharges in LTX with hot stainless steel boundary shells and a fresh liquid lithium coating show significantly lower O/Li impurity line ratios when compared to discharges with a solid lithium film on cooler shells. These new line ratio measurements help clarify the somewhat contradictory results of the effects of solid and liquid lithium on plasma confinement observed in previous experiments. [Preview Abstract] |
Tuesday, October 28, 2014 11:30AM - 11:42AM |
GO3.00011: Recent experimental results and future plan in KSTAR Jong-Gu Kwak, Sang-Gon Lee, Young-Sun Bae, Boung-Ho Park, Jin-Young Kim In this talk, the recent results of KSTAR will be presented focusing on extention of operational boundary in long-pulse discharges and highlights in experimental physics. H-mode discharges has been sustained longer and the operational regime of plasma parameters has been significantly extended in terms of heating power and plasma current. The long-pulse operation is in accordance with ITER requirement, i.e., in ITER similar shape, low safety factor (q95$\sim$3) and normalized beta ($\sim$2.0) with real-time control of density and power. Both ELM suppression and mitigation are discovered in wide range of RMP coil configuration and the suppression window in the edge safety factor has extended from 6.5 to 3.9 indicating the strong impact of resonant component. Beside RMP ELM suppression, it is also investigated the effect of other techniques on ELMs, such as edge heating by ECH and cooling by SMBI. Detailed evaluation of error field (EF) has been performed by 4 segment compass scan by the internal coils and the measured level of intrinsic error field is an order of magnitude lower than other tokamaks. In addition to the above topics, it is summarized the recent results on rotation \& transport physics, newly installed diagnostics, MHD and fast ion activities, followed by the near future plan. [Preview Abstract] |
Tuesday, October 28, 2014 11:42AM - 11:54AM |
GO3.00012: Long pulse and steady state operation activities at KSTAR Young-soon Bae The mission of Korea Superconducting Tokamak Advanced Research (KSTAR) is to develop a steady state capable advanced tokamak (AT) operation. The original AT operation mode at KSTAR is a reversed shear scenario with the plasma current of 2 MA, the toroidal magnetic field of 3.5 T, $\beta _{\mathrm{N}}$ of 5, safety factor q$_{95}$ of 3.7. Recently, the stationary long pulse H-mode discharge is sustained for maximum pulse duration of 20 s using heating of 2.5-MW NBI and 0.7-MW, X3 170 GHz ECH with low density level \textless n$_{\mathrm{e}}$\textgreater $\sim$ 0.3x10$^{20}$/m$^{3}$. The main activities of long pulse and steady state operation in KSTAR are the density feedback control, optimization of plasma shape and vertical control, real-time $\beta $ control, and steady state capable heating upgrade. For the longer pulse H-mode discharge at the increased plasma current upcoming KSTAR campaign, there have been improvements in plasma control system and upgraded heating systems. Meanwhile, steady state operation scenario in KSTAR next 4-year is being investigated using time-dependent integrated transport simulation code with possible heating upgrade-schemes. The promising steady state scenario near future is a reversed shear using a new 4 MW off-axis neutral beam injector for broad pressure profile peaked at off-axis, and using ECH for local current profile control aiming at $\beta_{\mathrm{N}}$ \textgreater 3 with Ip $\sim$ 1 MA. This paper present activities and plan for steady state operation in KSTAR as well as the long pulse H-mode discharge results in the recent KSTAR campaign. [Preview Abstract] |
Tuesday, October 28, 2014 11:54AM - 12:06PM |
GO3.00013: Research Activities on MHD and Energetic Particle Physics in KSTAR Byoung-Ho Park, Jong-Gu Kwak, San-Gon Lee, Si-Woo Yoon, Young-Sun Bae, Jin-Young Kim In this talk, the recent achievements in MHD and energetic particle physics in KSTAR will be presented. Throughout the 2014 campaign, strategically important works in achieving KSTAR milestone including NTM stabilization, error field measurement, establishing disruption mitigation system, and identification of Alfvenic eigenmode are conducted. Real time feedback control of 2/1 NTM is successfully demonstrated with the search and suppression algorithm and the improved ECCD mirror control system. 3-D structure of n=1 intrinsic error field are fully explored with L- and H-mode plasma aiming not only to complete MID IVCC compass scan but also to set a groundwork toward understanding of KSTAR's unique feature, ELM suppression by n=1 RMP. Elaborated $q_{95} \sim 2$ discharge regime is achieved without any error field correction by virtue of the extremely low intrinsic error field of KSTAR. The integrated disruption avoidance/mitigation system for the safety secured MA-class operation is well assessed. Further investigations of the energetic particle mode have been done with various control nobs of ECH, RMP and tailoring of NBI profile and mode identification efforts have been followed. Besides high priority works above, studies on sawtooth and run-away electron have made progresses. [Preview Abstract] |
Tuesday, October 28, 2014 12:06PM - 12:18PM |
GO3.00014: First Trial of Real-time Poloidal Beta Control in KSTAR Hyunsun Han, S.H. Hahn, J.G. Bak, M.L. Walker, M.H. Woo, J.S. Kim, Y.J. Kim, Y.S. Bae Sustaining the plasma in a stable and a high performance condition is one of the important control issues for future steady state tokamaks. In the 2014 KSTAR campaign, we have developed a real-time poloidal beta ($\beta$p) control technique and carried out preliminary experiments to identify its feasibility. In the control system, the $\beta$p is calculated in real time using the measured diamagnetic loop signal (DLM03) with coil pickup corrections, and compared with the target value leading to the change of the neutral beam (NB) heating power using a feedback PID control algorithm. To match the required power of NB which is operated with constant voltage, the duty cycles of the modulation were adjusted as the ratio of the required power to the maximum achievable one. This paper will present the overall procedures of the $\beta$p control, the $\beta$p estimation process implemented in the plasma control system, and the analysis on the preliminary experimental results. [Preview Abstract] |
Tuesday, October 28, 2014 12:18PM - 12:30PM |
GO3.00015: Toroidal rotation studies in KSTAR S.G. Lee, H.H. Lee, J.W. Yoo, Y.S. Kim, W.H. Ko, L. Terzolo, M. Bitter, K. Hill Investigation of the toroidal rotation is one of the most important topics for the magnetically confined fusion plasma researches since it is essential for the stabilization of resistive wall modes and its shear plays an important role to improve plasma confinement by suppressing turbulent transport. The most advantage of KSTAR tokamak for toroidal rotation studies is that it equips two main diagnostics including the high-resolution X-ray imaging crystal spectrometer (XICS) and charge exchange spectroscopy (CES). Simultaneous core toroidal rotation and ion temperature measurements of different impurity species from the XICS and CES have shown in reasonable agreement with various plasma discharges in KSTAR. It has been observed that the toroidal rotation in KSTAR is faster than that of other tokamak devices with similar machine size and momentum input. This may due to an intrinsically low toroidal field ripple and error field of the KSTAR device. A strong braking of the toroidal rotation by the n $=$ 1 non-resonant magnetic perturbations (NRMPs) also indicates these low toroidal field ripple and error field. Recently, it has been found that n $=$ 2 NRMPs can also damp the toroidal rotation in KSTAR. The detail toroidal rotation studies will be presented. [Preview Abstract] |
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