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 PO3: C-Mod and ASDEX Upgrade Tokamaks |
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Chair: Francesca Turco, Columbia University Room: Salon D |
Wednesday, October 29, 2014 2:00PM - 2:12PM |
PO3.00001: Overview of Recent Alcator C-Mod Results Earl Marmar Alcator C-Mod research currently emphasizes RF heating, current and flow drive, divertor/PMI issues, non-ELMing pedestal regimes with enhanced confinement, and disruption mitigation/runaway dynamics. Stability analysis of I-mode pedestals shows pressures well below the peeling-ballooning limit, as well as expected kinetic ballooning mode thresholds, consistent with the lack of ELMs. Results with the magnetic field aligned ICRF antenna show reductions in high-Z metallic impurities. Implementation of novel ``mirror-probe'' electronics has enabled simultaneous measurements of T$_{e}$, n$_{e}$ and $\varphi $ with 1 $\mu$s time response using a single probe tip, revealing important properties of the Quasi-Coherent-Mode (QCM) which regulates edge particle transport in EDA H-mode. An Accelerator-based In-situ Material Surveillance diagnostic has been deployed, providing the first between-shot measurements of surface evolution of the all-metal wall. We have observed suppression of boundary turbulence and $\tau_{E}$ improvement using LHRF into high-density H-modes, with H-factor increases up to 30{\%}. Upgrades which are ready for construction and near term installation on C-Mod include: an off-midplane LH launcher to test theories of improved current drive at high density and an actively heated (900 K) tungsten DEMO-like outer divertor. We are proposing a new facility, ADX, based on Alcator technology, to access advanced magnetic topologies to solve the divertor PMI problem, combined with high-field launch LHCD and ICRF to extend the tokamak to steady-state with reactor relevant tools. [Preview Abstract] |
Wednesday, October 29, 2014 2:12PM - 2:24PM |
PO3.00002: I-mode access and transitions in an expanded operating space on Alcator C-Mod A.E. Hubbard, S.-G. Baek, R.M. Churchill, T. Golfinopoulos, J.W. Hughes, Y. Lin, E.S. Marmar, J.L. Terry, C. Theiler, J.R. Walk, A.E. White, D.G. Whyte, S.M. Wolfe, S.J. Wukitch, I. Cziegler, E. Edlund New experiments in the I-mode regime on Alcator C-Mod have further expanded the wide range of conditions over which it has been studied. I-mode is attractive in that it combines the high energy confinement of H-mode, up to H$_{98}=$1.2, with the low particle/impurity confinement of L-Mode. Accessing and maintaining the regime, without transitions to L-mode or H-mode, is key to its extrapolation. The most robust conditions on C-Mod have been in lower null plasmas with unfavorable drift and B$_{\mathrm{T}}$ of 5-6 T, spanning the ITER value. In 2014, experiments at 2.8 T showed that the power range between L-I and I-H transitions was significantly lower, consistent with results on DIII-D and AUG. I-mode was also accessed in near double null. Fueling into I-modes can raise the density, up to 2x10$^{20}$ m$^{-3}$. The upper n$_{\mathrm{e}}$ increases with input power. Local pedestal parameters and evolution of turbulence at transitions are being assessed with the aim of understanding the physical mechanisms and for extrapolation. [Preview Abstract] |
Wednesday, October 29, 2014 2:24PM - 2:36PM |
PO3.00003: Stability and ELM Characterization in I-Mode Pedestals J.R. Walk, J.W. Hughes, P.B. Snyder, A.E. Hubbard, J.L. Terry, A.E. White, D.G. Whyte, S.G. Baek, I. Cziegler, E. Edlund The I-mode is a novel high-confinement regime explored on Alcator C-Mod, notable for its formation of an H-mode-like temperature pedestal without the accompanying density pedestal, maintaining L-mode particle confinement. I-mode exhibits a number of desirable properties for a reactor regime: among them, it naturally lacks large ELMs, avoiding the need for externally-applied ELM suppression. However, under certain conditions small, intermittent ELM-like events are seen. These events exhibit a range of phenomena in terms of edge and pedestal behavior, particularly for the ELM trigger - the majority of events are synchronized with the sawtooth heat pulse reaching the edge. The stationary pedestal structure is stable against peeling-ballooning MHD as calculated by ELITE in all cases, necessitating treatment of transient pedestal modification to characterize these events. We characterize these ELM events in terms of edge behavior, particularly the modification of the temperature pedestal, edge turbulence and fluctuations, and peeling-ballooning MHD stability. [Preview Abstract] |
Wednesday, October 29, 2014 2:36PM - 2:48PM |
PO3.00004: Core Impurity Transport in C-Mod L-, I- and H-mode Plasmas J. Rice, M. Reinke, C. Gao, N. Howard, M. Chilenski, L. Delgado-Aparicio, R. Granetz, M. Greenwald, A. Hubbard, J. Hughes, J. Irby, Y. Lin, E. Marmar, R. Mumgaard, S. Scott, J. Terry, J. Walk, A. White, D. Whyte, S. Wolfe, S. Wukitch Core impurity transport has been investigated for a variety of confinement regimes in C-Mod plasmas from x-ray emission following laser blow-off injection of medium and high Z materials. In Ohmic L-mode discharges, impurity transport is anomalous (D$_{\mathrm{eff}}$ \textgreater \textgreater D$_{\mathrm{nc}})$ and changes very little across the LOC/SOC boundary. In ICRF heated L-mode plasmas, the core impurity confinement time decreases with increasing ICRF input power (and subsequent increasing electron temperature) and increases with plasma current. Nearly identical impurity confinement characteristics are observed in plasmas with I-mode confinement. In EDA H-mode discharges the core impurity confinement time is much larger, but exhibits a similar scaling with plasma current, although there is a covariance with the density. There is a strong connexion between core impurity confinement time and the edge density gradient. Central impurity density profiles in these stationary regimes are often flat, in spite of large amplitude sawtooth oscillations, and there is little evidence of impurity convection inside of r/a $=$ 0.3. [Preview Abstract] |
Wednesday, October 29, 2014 2:48PM - 3:00PM |
PO3.00005: Comparison of sawtooth heat pulses across confinement regimes in Alcator C-Mod plasmas E.M. Edlund, E. Fredrickson, C. Fiore, C. Gao, J.E. Rice, A.E. White, S.J. Wukitch, M. Brookman, P.E. Phillips, W. Rowan, N.T. Howard Prior studies of heat pulses from sawteeth in TFTR and DIII-D experiments found that the thermal conductivities derived from heat pulses ($\chi^{hp}$) are increased by a factor of a few to an order of magnitude over the thermal conductivities derived from power balance ($\chi^{pb}$) and suggest a strong dependence of $\chi$ on the local temperature gradient $[1]$. This may have important consequences for turbulence modeling that ignores the role of sawteeth by using profiles representative of the mean. In this study, we investigate Alcator C-Mod experiments and apply methods similar to those used in {{ref.} [1]} to model the evolution of heat pulses propagating outward from the core. Calculations of thermal conductivities derived from sawtooth heat pulses will be presented as a function of density and confinement regime and compared with those derived from power balance. This work is supported by USDoE awards DE-FC02-99ER54512, DE-AC02-09CH11466 and DE-FG03-96ER-54373. \\[4pt] [1] E. Fredrickson \textit{et al.}, Physics of Plasmas \textbf{7}, 5051 (2000). [Preview Abstract] |
Wednesday, October 29, 2014 3:00PM - 3:12PM |
PO3.00006: Collisionality dependence of impurity transport in Alcator C-Mod H-modes M.A. Chilenski, M. Greenwald, N.T. Howard, M.L. Reinke, J. Rice, A.E. White, Y. Marzouk Understanding and actuating impurity transport is of particular interest for future machines because of the concern that core accumulation of heavy impurities will lead to radiative collapse and higher disruptivity. This problem is expected to be especially pronounced at low collisionality, where a strong peaking of the electron density profile has previously been observed (Greenwald et al. 2007, {\it Nucl. Fusion} {\bf 47}, L26). To investigate this issue several experiments have been performed in Alcator C-Mod to measure the behavior of mid-Z (Ar, Ca) and high-Z (Mo, W) impurities in H-mode plasmas of varying collisionality ($2<\nu_{eff}<40$). These plasmas are of particular interest to this problem because they are entirely RF heated and lack core particle sources. Impurities are injected using laser blow-off or gas injection and the evolution of the impurity density profile is constrained with an X-ray imaging crystal spectrometer. These diagnostics combined with analysis using STRAHL allows detailed study of the transport. Furthermore, analysis of the background $n_e$, $T_e$ profiles is conducted using advanced techniques including Gaussian process regression. An outline of this analysis scheme will be presented and recent results obtained from its application will be shown. [Preview Abstract] |
Wednesday, October 29, 2014 3:12PM - 3:24PM |
PO3.00007: The Effects of Dilution on Turbulence and Transport in C-Mod Ohmic Plasmas and Comparisons With Gyrokinetic Simulations Paul Ennever, Miklos Porkolab, John Rice, J. Chris Rost, Evan Davis, Darin Ernst, Catherine Fiore, Amanda Hubbard, Jerry Hughes, Jim Terry, Naoto Tsujii, Jeff Candy, Gary Staebler, Matthew Reinke Main ion dilution had been predicted by gyrokinetic simulations to decrease the turbulent ion thermal transport in C-Mod ohmic plasmas in the radial range of r/a $=$ 0.4-0.8 [1]. This predicted effect was tested with a set of experiments on C-Mod where ohmic plasmas across the LOC (linear ohmic) -SOC (saturated ohmic) transition densitywere seeded with Nitrogento dilute them. The seeding decreased the ion energy diffusivity and increased the electron density at which the toroidal rotation reversed direction. Experiments were also performed by injecting Arto increase Zeff without significantdilution to separate the two effects. GYRO, TGYRO, and TGLF simulations were performed on nitrogen seeded discharges and it was found that the simulations using experimental profiles agreed with the electron heat flux, but over-predict the ion flux. To get agreement between simulated and experimental ion flux, the experimental ion temperature profiles had to be flattened slightly but within experimental error.\\[4pt] [1] M. Porkolab, et. al., Plasma Phys. and Contr. Fusion, 54, 124029 (2002). [Preview Abstract] |
Wednesday, October 29, 2014 3:24PM - 3:36PM |
PO3.00008: Validation study of gyrokinetic simulation (GYRO) near the edge in Alcator C-Mod ohmic discharges C. Sung, A. White, N. Howard, D. Mikkelsen, C. Holland, J. Rice, M. Reinke, C. Gao, P. Ennever, M. Porkolab, R. Churchill, C. Theiler, J. Walk, J. Hughes, A. Hubbard, M. Greenwald A validation study of local gyrokinetic simulations (GYRO [J. Candy and R. E. Waltz, J. Comput. Phys. {\bf186}, 545 (2003)]) near the edge region ($r/a\sim0.85$) has been performed for two C-Mod ohmic discharges, namely one that is in the Linear Ohmic Confinement (LOC) regime and the other one in the Saturated Ohmic Confinement (SOC) regime. Comparing the simulated heat fluxes and synthetic $T_e$ fluctuations with the experiments, it is found that GYRO can reproduce the ion heat flux and the $T_e$ fluctuation level measured by the Correlation ECE (CECE) diagnostic within their uncertainties, while the simulated electron heat flux is under-predicted. Furthermore, the synthetic $T_e$ spectral shape is not matched with the measured spectrum in both LOC/SOC discharges. We have also performed global simulations to consider the interaction of turbulence within the sampling volume of the CECE diagnostic, enabling us to evaluate the importance of global simulations in applying a synthetic CECE diagnostic in this study. The LOC/SOC transition physics will be also explored. [Preview Abstract] |
Wednesday, October 29, 2014 3:36PM - 3:48PM |
PO3.00009: Assessment of parallel ion heat flux in the SOL of Alcator C-Mod with implications for heat-flux limiters D. Brunner, B. LaBombard, M. Churchill, J. Hughes, B. Lipschultz, R. Ochoukov, C. Theiler, J. Walk, D. Whyte, T. Rognlien, M. Umansky A systematic assessment of parallel ion heat flux in the Alcator C-Mod scrape-off layer is performed, revealing the role of kinetic effects. Experimental upstream ion-to-electron temperature ratios, as measured with charge exchange recombination spectroscopy and a reciprocating Langmuir probe near the separatrix, are compared to a simplified 1D, two-fluid model (benchmarked with UEDGE). At high collisionality with the divertor plasma nearly detached, the measured temperature ratio ($\sim$ 2) is matched by the fluid model. At low collisionality with the divertor sheath limited, the measured ratio ($\sim$ 4) is under-predicted by 2, suggesting that kinetic corrections are needed. Heat flux limiters are often used to approximate kinetic effects in fluid models; a coefficient of 0.21 for the free streaming heat flux is typical, informed by kinetic and fluid simulation comparisons. Using this correction brings modeled and measured temperature ratios into agreement at low and high collisionalities. This verifies the role of kinetic effects on ion heat transport and supports the coefficient of 0.21 as a first approximation. However, a more precise empirical prescription for heat flux limiter coefficients will require a more thorough examination of boundary ion temperature, both up- and down-stream. [Preview Abstract] |
Wednesday, October 29, 2014 3:48PM - 4:00PM |
PO3.00010: Spectral measurements of lower hybrid waves in the high-density multi-pass regime of Alcator C-Mod Seung Gyou Baek, R.R. Parker, S. Shiraiwa, G.M. Wallace, P.T. Bonoli, I.C. Faust, A.E. Hubbard, B.L. LaBombard, M. Porkolab, Y. Takase, T. Shinya, R. Vieira, N. Mucic Spectral measurements of lower hybrid waves have been performed on the diverted Alcator C-Mod tokamak with an aim of identifying the root cause of the observed anomalous loss of LH current drive efficiency in the high-density multi-pass regime. A recent experiment conducted in the reversed-field configuration confirms the previously observed magnetic-configuration dependent parametric decay instabilities (PDI) in the forward-field configuration at $\bar{n}_e \approx 1.1 \times 10^{20}$ m$^{-2}$, suggesting edge/scrape-off-layer plasmas are playing an important role in determining the PDI onset. As the plasma density is raised toward $\bar{n}_e \approx 1.5 \times 10^{20}$ m$^{-2}$, decay spectra are observed to be dominated by PDI that are excited at the low-field-side (LFS) of the tokamak, regardless of magnetic-configuration types. While the quantification of pump depletion due to PDI needs further investigations, the measured pump peak power at the high-field-side is observed to maintain its strength up to $\bar{n}_e \approx 1.5 \times 10^{20}$ m$^{-2}$, indicating multi-pass propagations of LH waves. This also implies that strong single-pass Landau absorption could help recover the expected current drive efficiency. A set of LH magnetic probes is being designed to further examine how much the launched parallel wavenumber spectrum is affected by nonlinear effects on the first pass from the launcher to the plasma at the LFS. [Preview Abstract] |
Wednesday, October 29, 2014 4:00PM - 4:12PM |
PO3.00011: Power balance of Lower Hybrid Current Drive in the SOL of High Density Plasmas on Alcator C-Mod Tokamak I.C. Faust, G.M. Wallace, S.G. Baek, D. Brunner, B. LaBombard, R.R. Parker, Y. Lin, S. Shiraiwa, J.L. Terry, D.G. Whyte Lower Hybrid Current Drive (LHCD) on Alcator C-Mod exhibits low efficiency for densities $ \sim \bar{n}_e > 1\cdot 10^{20}$ m$^{-3}$ for diverted discharges. Emissivity profiles of Hydrogenic Ly$_\alpha$ and H$_\alpha$ show significant changes during the application of LH wave as high density, along with enhanced parametric decay instability (PDI) and the generation of thermoelectric scrape-off-layer (SOL) currents. A corresponding reduction in X-ray emission from fast electrons in the confined plasma suggest damping of the LH waves in the SOL. A wide-viewing, absolutely-calibrated Hydrogen Ly$_\alpha$ camera was installed to characterize fast timescale ($\sim .1$ ms) poloidal dynamics of SOL during the application of LHCD. Analyses and results will be shown characterizing the absolute power deposition LHCD as it relates to various plasma null configurations. Ly$_\alpha$ emission is also correlated to various experimental parameters such as SOL electron density and temperature profiles and Lower Hybrid input power in order to elucidate possible damping mechanisms. [Preview Abstract] |
Wednesday, October 29, 2014 4:12PM - 4:24PM |
PO3.00012: Modeling of pedestal and core radiation in nitrogen seeded H-modes at ASDEX Upgrade Livia Casali, Emiliano Fable, Ralph Dux, Matthias Bernert, Francois Ryter This work presents the time dependent modeling of the radiation and impurities in the presence of ELMs using the ASTRA transport code coupled to the impurity and radiation code STRAHL. The modeling focuses on the nitrogen seeded discharges of ASDEX Upgrade which exploits the high radiation scenario necessary for next step devices. ASDEX Upgrade has a full tungsten wall and therefore the impurities considered in the model are N and W. The modeling results highlight the importance of non coronal effects induced by transport for low-Z impurities in the pedestal, while tungsten radiation is not affected by transport. Diffusive and convective ELM models are investigated and a comparison between the modeled and the measured radiation suggests a dominant diffusive contribution in the ELM crash. The different values of the neoclassical pinch for N and W result in different reactions to the ELM frequencies and explain the fact that a sufficiently high ELM frequency is required to prevent W accumulation in the confined region. [Preview Abstract] |
Wednesday, October 29, 2014 4:24PM - 4:36PM |
PO3.00013: Fast-ion transport and NBI current drive in ASDEX Upgrade Benedikt Geiger, Markus Weiland, Alexander Mlynek, Mike Dunne, Ralph Dux, Rainer Fischer, Joerg Hobirk, Christian Hopf, Matthias Reich, David Rittich, Francois Ryter, Philip Schneider, Giovanni Tardini, Manuel Garcia-Munoz Good confinement of fast ions is essential in fusion devices because these suprathermal particles are responsible for plasma heating, current drive and can, if poorly confined, damage surrounding walls. The degradation of the fast-ion confinement caused by large and small scale instabilities must consequently be investigated. In the ASDEX Upgrade tokamak, fast ions are generated by neutral beam injection (NBI) and their slowing down distribution can be studied using FIDA spectroscopy, neutral particle analyzers and neutron detectors. Neo-classical fast-ion transport is observed by these measurements in MHD-quiescent discharges with relatively weak heating power (less than 5 MW). The presence of sawtooth instabilities, in contrast, yields a strong internal fast-ion redistribution that can be modelled very well when assuming full reconnection of the helical magnetic field. The fast-ion current drive efficiency has been studied in discharges with up to 10 MW of heating power in which on-axis and off-axis NBI were exchanged. The radial shape of the fast-ion population, generated by the different NBIs, changes as predicted and a corresponding modification of the current profile is measured. [Preview Abstract] |
Wednesday, October 29, 2014 4:36PM - 4:48PM |
PO3.00014: Measurement of plasma response to 3D fields at high-$\beta $ in ASDEX Upgrade P. Piovesan, V. Igochine, A. Kirk, M. Maraschek, L. Marrelli, W. Suttrop, D. Yadykin, M. Cavedon, I.G.J. Classen, A. Gude, M. Reich, E. Viezzer, E. Wolfrum The plasma response to 3D fields is complex, possibly including both screening and amplification from MHD modes, and it must be well understood to optimize tokamak operation, e.g. to control ELMs. ASDEX Upgrade has unique and new capabilities to probe the plasma response, including two rows of 8 non-axisymmetric coils driven by AC power supplies and several high-resolution diagnostics. This recently allowed accurate measurements of the plasma response to low-n slowly rotating fields in high-$\beta $ plasmas up to the no-wall limit. The importance of 3D field amplification from a marginally stable kink with respect to resonant field screening was tested by continuously varying the pitch of the applied 3D field during the discharge, as $\beta $ increases, and by comparison with vacuum and plasma response calculations. It is found that the 3D field pitch determines the plasma response and affects ELM mitigation efficacy. [Preview Abstract] |
Wednesday, October 29, 2014 4:48PM - 5:00PM |
PO3.00015: Slow conversion of ideal MHD perturbations into a tearing mode after a sawtooth crash Valentin Igochine, Anja Gude, Sibylle G\"unter, Karl Lackner, Qingquan Yu, Laura Barrera Orte, Anton Bogomolov, Ivo Classen, Rachael McDermott, Neville C. Luhmann, Jr Optimization of the plasma performance with respect to beta normalized, $\beta_{N} $, is one of the main goals of fusion research. Unfortunately, the $\beta_{N} $ value in standard H-mode and advanced scenario discharges is limited by resistive instabilities, usually neoclassical tearing modes (NTMs). These modes are metastable and thus can be triggered by other MHD events at very low $\beta_{N} $ values for large perturbations. Sawteeth typically provide the strongest magnetic perturbations and are able to trigger the modes at the smallest normalized beta values. We have investigated the mechanism of the seed island formation by sawteeth in much detail. Careful analysis of electron cyclotron emission, magnetic probes and Soft X-ray measurements directly after the crash reveals the existence of an ideal (2,1) magnetic perturbation at the q$=$2 surface directly after the crash. This ideal perturbation converts into a tearing mode on a timescale much longer than the sawtooth crash time. [Preview Abstract] |
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