Bulletin of the American Physical Society
55th Annual Meeting of the APS Division of Plasma Physics
Volume 58, Number 16
Monday–Friday, November 11–15, 2013; Denver, Colorado
Session CO4: C-Mod Tokamak |
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Chair: Mickey Wade, General Atomics Room: Plaza D |
Monday, November 11, 2013 2:00PM - 2:12PM |
CO4.00001: Overview of Recent Alcator C-Mod Highlights Earl Marmar Analysis and modeling of recent C-Mod experiments has yielded significant results across multiple research topics. I-mode provides routine access to high confinement plasma (H$_{98}$ up to 1.2) in quasi-steady state, without large ELMs; pedestal pressure and impurity transport are regulated by short-wavelength EM waves, and core turbulence is reduced. Multi-channel transport is being investigated in Ohmic and RF-heated plasmas, using advanced diagnostics to validate non-linear gyrokinetic simulations. Results from the new field-aligned ICRF antenna, including significantly reduced high-Z metal impurity contamination, and greatly improved load-tolerance, are being understood through antenna-plasma modeling. Reduced LHCD efficiency at high density correlates with parametric decay and enhanced edge absorption. Strong flow drive and edge turbulence suppression are seen from LHRF, providing new approaches for plasma control. Plasma density profiles directly in front of the LH coupler show non-linear modifications, with important consequences for wave coupling. Disruption-mitigation experiments using massive gas injection at multiple toroidal locations show unexpected results, with potentially significant implications for ITER. First results from a novel accelerator-based PMI diagnostic are presented. What would be the world's first actively-heated high-temperature advanced tungsten divertor is designed and ready for construction. Conceptual designs are being developed for an ultra-advanced divertor facility, Alcator DX, to attack key FNSF and DEMO heat-flux challenges integrated with a high-performance core. [Preview Abstract] |
Monday, November 11, 2013 2:12PM - 2:24PM |
CO4.00002: X-point target divertor concept and the Alcator DX high power divertor test facility B. LaBombard, E. Marmar, J. Irby, R. Vieria, S. Wolfe, P. Bonoli, C. Fiore, R. Granetz, M. Greenwald, I. Hutchinson, A. Hubbard, J. Hughes, Y. Lin, B. Lipschultz, R. Parker, M. Porkolab, M. Reinke, J. Rice, S. Shiraiwa, J. Terry, C. Theiler, G. Wallace, A. White, D. Whyte, S. Wukitch Three critical challenges must be met before a steady-state, power-producing fusion reactor can be realized: (1) safely handle extreme plasma exhaust power, (2) completely suppress material erosion at divertor targets and (3) do this while maintaining a burning plasma core. Advanced divertors such as `Super X' and `X-point target' have the potential to solve all three challenges by producing a stable, fully detached, low temperature plasma in the divertor while maintaining a hot boundary layer around a clean plasma core. The X-point target divertor may be particularly effective. It places a second X-point in the pathway of the peak parallel heat flux with the intention of forming an X-point MARFE in the divertor volume, well away from the primary X-point that defines the last closed flux surface and at larger major radius, providing detachment front stability. Divertor heat dissipation is via volumetric processes (radiation, ion-neutral collisions), virtually eliminating erosion by ion bombardment and reducing peak heat flux and neutron fluence on remote divertor target components. Alcator DX is conceived as a national facility to test these ideas. It employs the high magnetic field technology of Alcator combined with high-power ICRH to investigate advanced divertors at reactor-level parallel heat flux densities. [Preview Abstract] |
Monday, November 11, 2013 2:24PM - 2:36PM |
CO4.00003: Reduction of Core Turbulence in I-mode Plasmas at Alcator C-Mod A.E. White, M. Barnes, A. Dominguez, M. Greenwald, N.T. Howard, A.E. Hubbard, J.W. Hughes, D.R. Mikkelsen, F.I. Parra, M.L. Reinke, C. Sung, J. Walk, D.G. Whyte Core turbulent fluctuations (0.40 \textless\ r/a \textless\ 0.95) are reduced in the high confinement regime, I-mode, at Alcator C-Mod. Long wavelength density fluctuation levels are observed to decrease from L-mode levels by up to 30{\%}, while long wavelength electron temperature fluctuation levels are observed to decrease by up to 70{\%}. This reduction in core turbulence is correlated with the increases in confinement in I-mode. During some L-I transitions, density fluctuations in the core (r/a $\sim$ 0.5) and near the top of the Te pedestal (r/a $\sim$ 0.95) are both reduced prior to the reduction of low-frequency edge turbulence (r/a $\sim$ 0.99-1.0), and prior to the onset of the edge-localized weakly coherent mode (WCM) (r/a $\sim$ 0.99-1.0). This result contrasts typical observations of confinement transitions (I-mode and H-mode) where changes in edge turbulence occur prior to changes in core turbulence [Preview Abstract] |
Monday, November 11, 2013 2:36PM - 2:48PM |
CO4.00004: Pedestal limiting instability between ELMs: observations and comparison with gyrokinetic calculations* Ahmed Diallo, J.W. Hughes, J. Canik, P. Snyder, M. Greenwald, J. Walk, C. Theiler, E. Davis, B. LaBombard, J. Terry, S-G. Baek, L. Delgado-Apricio, T. Golfinopoulos, A. Hubbard, M.L. Reinke, A. White On Alcator C-Mod, measurements of the inter-ELM fluctuations have been performed to determine the microinstabilities responsible for limiting the pedestal gradient increase in H-mode. The results show an onset of ion scale coherent density and magnetic fluctuations followed by a saturation of the edge temperature gradient. In addition, analysis shows that the mode is field-aligned consistent with a ballooning character. Preliminary stability analyses indicate that the discharges are near both the peeling ballooning and kinetic ballooning mode (KBM) thresholds. Gyrokinetic calculations indicate that the KBM is unstable in a region in the steep gradient. The results appear to be consistent with the EPED predictive model, which hypothesizes the existence of a violent KBM instability generating transport necessary to clamp the pedestal gradient. *Supported by US DOE contracts DE-AC02-09CH11466 and DE-FC02-99ER54512. [Preview Abstract] |
Monday, November 11, 2013 2:48PM - 3:00PM |
CO4.00005: Flux Surface Variation of Impurity Density and Flows in the Pedestal Region Michael Churchill, Bruce Lipschultz, Christian Theiler Measured impurity density and flows in the pedestal region of Alcator C-Mod can deviate significantly on a flux surface from current model predictions. Comparing localized measurements at the low-field side (LFS) midplane and the high-field side (HFS) midplane, boron (B5$+)$ impurity density asymmetries larger than 10x are observed in H-mode plasmas, with larger densities at the HFS. The LFS density pedestal varies in position and width with varying plasma conditions, while the HFS impurity density profile remains rather fixed. Impurity density asymmetries are not observed in plasmas with small gradients, i.e L-mode, suggesting the drive for the asymmetry may be the strong gradients in the H-mode pedestal region. However, impurity density asymmetries are also absent in I-mode plasmas, despite the presence of a strong radial gradient in temperature (with no main ion density pedestal). This indicates an interplay between the gradient scale lengths of the main ion density and temperature in the drive of the impurity density asymmetry. Impurity flows in the pedestal show the opposite behavior; flows measured in H-mode plasmas are close to the expected in-out variation, while in I-mode they deviate significantly. Supported by USDoE award DE-FC02-99ER54512. [Preview Abstract] |
Monday, November 11, 2013 3:00PM - 3:12PM |
CO4.00006: The Effect of ITG/TEM mix on Gyrokinetic Modeling of an Alcator C-Mod Current Scan Nathan Howard, Chris Holland, Anne White, Matt Reinke, Terry Rhodes, Martin Greenwald, Jeff Candy A gyrokinetic modeling study of an Alcator C-Mod I$_{\mathrm{p}}$ scan using the GYRO code has been conducted, to assess the ability of low-$k$ (k\textunderscore y rho\textunderscore s \textless~1) gyrokinetics to accurately reproduce the observed scaling of core transport with changes in the q-profile. The analysis focuses upon comparisons of gyrokinetic flux predictions with the experimental values at three radial locations (r/a $=$ 0.5, 0.65, 0.8) in four L-mode discharges (0.6, 0.8, 1.0, 1.2 MA). Linear gyrokinetic analysis of both dominant and subdominant modes is combined with sensitivity analysis of the nonlinear predictions to quantify differences in the mix of ITG and TEM turbulence at each condition. We find that these low-$k$ simulations accurately predict the experimental fluxes only when both ITG and TEM modes are unstable at low $k$. Discrepancies between simulated and experimental electron heat flux were robustly identified using sensitivity scans and appear correlated with the absence of low-k electron turbulence in the simulations. Initial experimental and modeling results from a sister scan performed on DIII-D will also be presented. [Preview Abstract] |
Monday, November 11, 2013 3:12PM - 3:24PM |
CO4.00007: The effects of main ion dilution on turbulence and transport in Alcator C-Mod and comparisons with gyrokinetic simulations Paul Ennever, Miklos Porkolab, Matthew Reink, John Rice, J. Chris Rost, Evan Davis, Darin Ernst, Catherine Fiore, Amanda Hubbard, Jerry Hughes, Jim Terry, Naoto Tsuii, Gary Staebler, Jeff Candy In previous studies of C-Mod experiments with gyrokinetic codes it was found that ion turbulence and transport was reduced when the main ions were diluted by introducing low-Z impurities. In recent experiments on C-Mod, nitrogen (Z$=$7) was injected into ohmic plasmas at a range of densities across the LOC-SOC transition. Experimentally it was observed that the ion thermal diffusivity decreased with nitrogen seeding, but the ion temperature gradient also increased such that the ion heat flux remained the same. It was also observed that the seeding induced a rotation reversal, similar to spontaneous reversals observed previously by lowering the density in unseeded ohmic plasmas. Simulations of these plasmas have been carried out with TGLF and non-linear GYRO. The energy transport, momentum transport, and turbulent density fluctuations simulated by these codes will be compared with experimental measurements. *Work supported by US DOE awards DE-FG02-94-ER54235 and DE-FC02-99-ER54512. [Preview Abstract] |
Monday, November 11, 2013 3:24PM - 3:36PM |
CO4.00008: Comparison of electron temperature fluctuations with gyrokinetic sumulations across the ohmic energy confinement transition in Alcator C-Mod C. Sung, A. White, N. Howard, D. Mikkelsen, J. Rice, M. Reinke, C. Gao, P. Ennever, M. Porkolab, R. Churchill, C. Theiler, A. Hubbard, M. Greenwald Long wavelength electron temperature fluctuations ($k_y\rho_s < 0.3$) near the edge ($r/a\sim0.85$) are reduced across the ohmic confinement transition from Linear Ohmic Confinement(LOC) regime to Saturated Ohmic Confinement(SOC) regime in Alcator C-Mod. Linear stability analysis shows that the dominant mode of long wavelength turbulence near the edge is changed from Trapped Electron Mode(TEM) to Ion Temperature Gradient(ITG) mode while the dominant mode is not changed deeper in the core ($r/a\sim0.5$). This indicates that local turbulence changes near the edge might be responsible for the change of global energy confinement in ohmic plasmas. Further study using nonlinear gyrokinetic simulations is being performed to clarify the relation between the change of local turbulence and global ohmic energy confinement. Through nonlinear gyrokinetic simulation (GYRO), we will investigate the change of fluctuating quantities ($\tilde T,\tilde n,\tilde\phi$) and their phase relations across ohmic confinement transitions, and relate them to the change of energy transport. A synthetic CECE diagnostic for C-Mod has been developed, and it will be used to validate the gyrokinetic simulations. [Preview Abstract] |
Monday, November 11, 2013 3:36PM - 3:48PM |
CO4.00009: Effects of Magnetic Shear on Toroidal Rotation in C-Mod Plasmas with LHCD John Rice, Yuri Podpaly, Matt Reinke, Bob Mumgaard, Steve Scott, Syun'ichi Shiraiwa, Greg Wallace, Pat Diamond, Chi Gao, Bob Granetz, Jerry Hughes, Ron Parker, Paul Bonoli, Luis Delgado-Aparicio, Martin Greenwald, Amanda Hubbard, Ian Hutchinson, Jim Irby, Jungpyo Lee, Earl Marmar, Steve Wolfe Application of lower hybrid current drive (LHCD) in Alcator C-Mod plasmas can induce both co- and counter-current directed changes in toroidal rotation, depending on the core q profile. For discharges with q$_{\mathrm{0}}$ \textless~1, rotation increments in the counter-current direction are observed. If the LH driven current is sufficient to suppress sawteeth and increase q$_{\mathrm{0}}$ above unity, the core toroidal rotation change is in the co-current direction. This observation unifies the results from several tokamaks. This change in sign of the rotation increment is consistent with a change in sign of the residual stress (the divergence of which constitutes an intrinsic torque that drives the flow) through its dependence on magnetic shear. [Preview Abstract] |
Monday, November 11, 2013 3:48PM - 4:00PM |
CO4.00010: Measurement of the lower hybrid driven current profile density dependence on Alcator C-Mod R.T. Mumgaard, S.D. Scott, S. Shiraiwa, G.M. Wallace, R.R. Parker, R.S. Granetz The plasma current profile has been reconstructed in plasmas with significant lower hybrid current drive (LHCD) using constraints from an improved motional Stark effect (MSE) diagnostic on Alcator C-Mod. The reconstructions demonstrate the application of LHCD significantly broadens the current profile, increasing q0 above 1 resulting in the suppression of sawteeth. Time resolved measurements of the current dynamics agree with the expected timescales. The current profile has a significant on-axis component in moderate density plasmas where LHCD drives 100{\%} of the current non-inductively for several current penetration times. The MSE-constrained reconstructions show a loss in current drive efficiency as the plasma density is increased consistent with previous 0-D observations. Additionally, the driven current profile moves radially outward as the density is increased. The current profile dependence on launched n$_{\vert \vert}$ is also discussed. These observations will constrain proposed theoretical models and simulations of LHCD and its observed degradation. This work is supported by USDoE awards DE-FC02-99ER54512 and DE-AC02-09CH11466. [Preview Abstract] |
Monday, November 11, 2013 4:00PM - 4:12PM |
CO4.00011: Characterization of onset of parametric decay instability of lower hybrid waves in ITER-relevant high-density plasmas Seung Gyou Baek, P.T. Bonoli, R.R. Parker, S. Shiraiwa, G.M. Wallace, M. Porkolab, Y. Takase, D. Brunner, I.C. Faust, A.E. Hubbard, B.L. LaBombard, C. Lau Lower hybrid (LH) current drive experiments on Alcator C-Mod have revealed that the density corresponding to the onset of parametric decay instability (PDI) is as low as $\overline n_{e} \approx 1\times 10^{20}m^{-3},$ suggesting that PDI may be a remaining parasitic loss mechanism to explain the observed loss of current drive efficiency in high density plasmas. Convective growth due to parallel coupling is most likely to explain the observed PDI. Depending on the magnetic configurations, PDI is excited at different locations with different strength, while a similar level of hard X-ray is observed as long as $\overline n_{e} $ is similar. PDI is excited at the high-field side edge in lower null plasmas with the decrease in the pump power, indicating that the single pass absorption is weak and pump depletion can occur below conventional PDI limit. In upper null plasmas, PDI is excited at the low-field side edge with no apparent indication of pump depletion. More extensive spectral measurements are necessary to fully understand the role of this seemingly weak PDI at the LFS to gauge the effect of the observed PDI in high single-pass absorption plasmas as will be in ITER. [Preview Abstract] |
Monday, November 11, 2013 4:12PM - 4:24PM |
CO4.00012: Time-domain simulation and benchmark of LHCD experiment at ITER relevant parameters S. Shiraiwa, P. Bonoli, F. Poli, R.W. Harvey, C. Kessel, R. Parker, G. Wallace LHCD experiments on Alcator C-Mod at ITER relevant parameters have stimulated recent improvements and new development in advanced LHCD codes including GENRAY/CQL3D with SOL and LHEAF and TORLH (full wave) codes. These codes have more comprehensive physics models, resulting in better agreement with experiments in particular at high density. In contrast, the physics model of LHCD used in previous time-domain discharge simulation was rather simple, and therefore, it is important to incorporate the result of these developments in order to better predict tokamak discharge scenarios. Among these new codes, GENRAY/CQL3D with SOL provides fast computation time while keeping parasitic absorption in SOL plasmas though it lacks diffraction. We have used the integrated plasma simulator (IPS) as a simulation framework and have integrated GENRAY/CQL3D with SOL in it. IPS simulations were performed for an LHCD discharge on C-Mod, showing good agreement with experiment. A prediction of LHCD discharge performance with proposed LHCD upgrade on C-Mod will also be discussed. [Preview Abstract] |
Monday, November 11, 2013 4:24PM - 4:36PM |
CO4.00013: Turbulence suppression and confinement improvements with LH power applied to high density H-modes J.L. Terry, J.W. Hughes, M.L. Reinke, J.E. Rice, R.M. Churchill, J. Irby, R.R. Parker, S. Shiraiwa, C. Theiler, G.M. Wallace, P. Xu Injecting LH power into high-density H-mode plasmas has a profound effect on the edge and SOL turbulence. Gas-Puff-Imaging of the outboard midplane region, as well as other diagnostics that measure density fluctuations in the boundary, show that the fluctuation power \textit{decreases significantly} in the edge/SOL when the LH power is applied to the high-density ICRF-heated H-modes, for which core penetration of LH waves is expected to be poor. In many of these discharges, increases in \textit{confinement} are also observed after the LH power is applied. The additional LH power can be quite efficient; for 20{\%} increases in auxiliary power, 30-40{\%} increases in global energy confinement time are observed (H$_{98}$ from 0.6 to 0.9). The confinement increases are best correlated with changes in rotation and increased pedestal temperature gradients. There is not perfect correlation with turbulence suppression. These results suggest the possibility of using LHRF as a tool to affect plasma rotation, confinement, and boundary turbulence via direct modification of quantities at the plasma edge. [Preview Abstract] |
Monday, November 11, 2013 4:36PM - 4:48PM |
CO4.00014: Lower hybrid and ICRF driven scrape-off-layer density modifications and their impact on lower hybrid power coupling on Alcator C-Mod Y. Lin, C. Lau, G. Wallace, S.J. Wukitch, B. LaBombard, R. Ochoukov, S. Shiraiwa, J. Terry, G. Hanson, J. Wilgen Scrape-off-layer (SOL) density profiles have been measured by a reflectometer at three poloidal locations adjacent to the lower hybrid (LH) launcher. Large modifications on SOL density profiles and strong poloidal density striations have been observed with the application of LH power. Such modifications in SOL have been shown to degrade LH power coupling. Adding ICRF power also modifies the SOL density and further degrades LH coupling, particularly when the ICRF antenna and LH launcher are magnetically connected. With density striations, calculating the LH reflection coefficient assuming a poloidally uniform density profile can have large mismatches vs. the experimental measurement. The density modifications with LH power can be reproduced by a 2-D diffusive-convective model that includes the dynamics of the LH induced ExB convective eddies. Adding ICRF power, the model shows that the overall density profile appears to be a superposition of both LH and ICRF driven convection. Based on our study, a gas puff system that aims at improving LH coupling will need to account for the poloidal variations in SOL density. [Preview Abstract] |
Monday, November 11, 2013 4:48PM - 5:00PM |
CO4.00015: The Role of Fast ICRF Waves in Enhancing the Plasma Potential in the SOL of Alcator C-Mod Roman Ochoukov, Dennis Whyte, Daniel Brunner, Daniel D'Ippolito, Brian LaBombard, Yijun Lin, Bruce Lipschultz, James Myra, James Terry, Stephen Wukitch An extensive experimental survey of the plasma potential ($\Phi _{\mathrm{P}})$ enhancement in the SOL of ICRF-heated discharges on Alcator C-Mod reveals that plasma facing surfaces that do not magnetically map to the active ICRF antennas experience $\Phi_{\mathrm{P}}$ enhancement of \textgreater 100 V. The key implication of this result is that surfaces that generally do not affect the core plasma performance may become impurity sources. The study reveals that the fast ICRF waves have a strong influence on $\Phi_{\mathrm{P}}$. A mechanism for $\Phi _{\mathrm{P}}$ enhancement is proposed that involves a fast-to-slow wave coupling at conducting surfaces. Several features of this coupling theory are observed experimentally. Examples include the effect of the shape of the conducting surface and the strength of the fast wave fields on $\Phi _{\mathrm{P}}$. Overall, we observe an increase in the enhanced $\Phi _{\mathrm{P}}$ in unmapped SOL regions with the strength of the fast wave amplitude and the correlation is most pronounced in monopole-heated discharges. It is concluded that strong single-pass absorption regime is crucial to minimizing the impact of ICRF power on plasma-material interactions and core plasma performance. [Preview Abstract] |
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