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 CO5: DIII-D Tokamak |
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Chair: Stanley Kaye, Princeton Plasma Physics Laboratory Room: Galerie 2/3 |
Monday, October 27, 2014 2:00PM - 2:12PM |
CO5.00001: Overview of Recent \mbox{DIII-D} Experimental Results M.E. Fenstermacher Recent DIII-D experiments have added to the ITER physics basis and to physics understanding for extrapolation to future devices. Physics mechanisms contributing to resonant magnetic perturbation ELM suppression and QH-mode were identified. The QH-mode operating space was extended to ITER-relevant parameters and predicted Super-H mode performance was observed at high shaping. Upgraded divertor Thomson data was combined with edge modeling to identify the core density limit at divertor detachment. Pedestal studies were done to determine the role of $\nu^*$, $Z_{eff}$ and kinetic ballooning mode instabilities in controlling pedestal structure. Injection of massive high-Z gas dissipates magnetic and kinetic energy of runaway electron beams. 3D magnetics data validate several linear MHD codes, including ability to predict neoclassical tearing viscosity torque. Feedback control of applied 3D fields facilitates access to increased $\beta_N$ values above the no-wall limit. The effect of test blanket module (TBM) fields on fast ion losses and momentum transport, and partial correction of TBM fields at high $\beta$ was achieved. Density gradient driven trapped electron modes and core $n_e$ peaking were controlled by electron cyclotron heating suggesting a possible burn control technique. [Preview Abstract] |
Monday, October 27, 2014 2:12PM - 2:24PM |
CO5.00002: Controlling DIII-D QH-Mode Particle and Electron Thermal Transport with ECH D.R. Ernst, K.H. Burrell, T.L. Rhodes, W. Guttenfelder, G.R. McKee, B.A. Grierson, C. Holland, A. Dimits, C.C. Petty, L. Schmitz, G. Wang, L. Zeng, E.J. Doyle, M.E. Austin Quiescent H-mode core particle transport and density peaking are locally controlled by modulated electron cyclotron heating (ECH) at $\rho\sim 0.2$. Gyrokinetic simulations show density gradient driven trapped electron modes (TEMs) are only unstable in the inner core, where the density profile flattens in response to ECH. Thus $\alpha$-heating could reduce density peaking, providing burn control. Density fluctuations from Doppler backscattering intensify at TEM wavenumbers $k_\theta\rho_s\sim 0.8$ during ECH, while new quasi-coherent modes are observed with adjacent toroidal mode numbers consistent with TEMs. Separately, ECH at two-deposition locations ($r/a\sim\rho =0.5$ \& 0.7) varied the electron temperature gradient. A jump in ``heat pulse'' diffusivity during the scan indicates a critical gradient was crossed. [Preview Abstract] |
Monday, October 27, 2014 2:24PM - 2:36PM |
CO5.00003: Experiments to Understand and Control Energetic Particle Transport by Alfv\'en Eigenmodes W.W. Heidbrink, C. Collins, D.C. Pace, M.A. Van Zeeland, C.T. Holcomb Alfv\'en eigenmodes (AE) cause appreciable fast-ion transport in both steady-state scenario and in L-mode current ramp plasmas. All fast-ion diagnostics that are sensitive to a populated portion of phase space observe reductions in signal relative to classical predictions in the presence of many, small-amplitude AEs. Theory indicates that the many wave-particle resonances in these plasmas results in stochastic transport and critical gradient behavior. Initial data from a modulation experiment is consistent with the hypothesis that the fast-ion transport becomes ``stiff.'' Another experiment investigates whether AE-induced transport from the core couples with edge losses induced by test-blanket module fields to enhance localized heating. Application of electron cyclotron heating to control the AEs gives mixed results: AEs are sometimes stabilized but the dependence on the fast-ion and $q$ profiles is complex [Preview Abstract] |
Monday, October 27, 2014 2:36PM - 2:48PM |
CO5.00004: Magnetic Measurements of MHD Toroidal Dynamics and Its Influence on Massive Gas Injection in DIII-D D. Shiraki, N. Commaux, N.W. Eidietis, E.M. Holmann, V.A. Izzo, R.A. Moyer Measurements from the DIII-D magnetic diagnostic system are used to characterize the low-order MHD activity leading up to the thermal quench during fast shutdowns induced by massive gas injection (MGI). The evolution of the 3D fields measured at the vessel wall are found to be consistent with the destabilization of modes by the inwardly propagating cold front passing low-order rational surfaces. This MHD activity, which is dominantly n=1, can be characterized by the n=1 magnetic signal measured on the low-field side of the plasma. The toroidal evolution of this MHD is found to be influenced by three factors: the injector location, pre-MGI plasma rotation, and large n=1 error fields. The effects of the toroidal phase of this n=1 mode on toroidal radiation asymmetries are discussed. Experimental results are compared with simulations of MGI in DIII-D equilibria using the NIMROD code [1]. The effects of existing MHD structures prior to MGI, such as locked islands, are compared.\par \vskip6pt \noindent [1] V.A. Izzo, Phys.\ Plasmas {\bf 20}, 056107 (2013). [Preview Abstract] |
Monday, October 27, 2014 2:48PM - 3:00PM |
CO5.00005: Simulations of DIII-D Rapid Shutdown Experiments with One and Two Gas Jets V.A. Izzo, E.M. Hollmann, R.A. Moyer, N. Commaux, D. Shiraki, N.W. Eidietis, P.B. Parks DIII-D has two massive gas jets for disruption mitigation, separated by 120$^\circ$ degrees toroidally and poloidally. Based on two radiated power measurements made at 90$^\circ$ and 210$^\circ$ degrees toroidally, little variation has been observed in the toroidal distribution of radiated energy between shots in which just one gas jet is fired, or both are fired. Three NIMROD simulations of massive neon injection - each jet individually and both simultaneously - are compared with the measurements. For each case, we calculate the radiation toroidal peaking factor (TPF) in two ways: 1) using only information from 90$^\circ$ and 210$^\circ$, and 2) using full toroidal information. The two-point TPF reproduces the experimental result of little variation depending on gas jet(s) used. But, the real TPF shows significant variation, with a logical trend in which two jets produces more symmetric radiated power than one. This comparison suggests that the lack of experimental trend may be a measurement artifact. [Preview Abstract] |
Monday, October 27, 2014 3:00PM - 3:12PM |
CO5.00006: Plasma Imaging of the DIII-D Tokamak S.L. Allen, C.J. Lasnier, W.H. Meyer, J. Howard, A.R. Briesemeister The LLNL IR/visible periscope views the entire DIII-D plasma cross section and a new beamsplitter enables simultaneous measurements with IR and visible cameras. In the lower divertor, a Coherence Imaging System (CIS) measures the 2-D flow of CIII ions with ~1 ms time resolution. Visible color movies obtained during Li dropper operations, $N_2$ puffing, and $D_2$ puffing provide insight into SOL flows; trends are seen with the direction of the toroidal field. Localized changes in recycling are compared with changes in heat flux. A second CIS is being installed to measure impurity ion flow on the periscope. The lower divertor CIS system has been upgraded to include a new in-situ calibration system and temperature control, which has significantly increased the quality of the flow measurements. Rapid inversion of the coherence phase shift data maps the toroidal plasma flow to magnetic flux surfaces. The CIS carbon flow measurements are compared with conventional spectroscopy and Mach probe measurements of the main ion flow.\par [Preview Abstract] |
Monday, October 27, 2014 3:12PM - 3:24PM |
CO5.00007: Advances in Simulating Detached Plasmas in \mbox{DIII-D} Using UEDGE M.E. Groth, E.T. Meyer, G.D. Porter, M.E. Fenstermacher, D.N. Hill, C.J. Lasnier, M.E. Rensink, T.D. Rognlien, N.H. Brooks, A.W. Leonard, J.G. Watkins Simulations of detached divertor plasmas in lower single-null L-mode discharges in DIII-D with the fluid edge code UEDGE show that the measured total radiated power, total ion current and total power to both the inner and outer targets can be reproduced by the simulations within the uncertainty of the measurements. These results were obtained by including cross-field drifts and assuming chemical sputtering yields twice as high as published by Davis et al. [1]. However, by assuming higher chemical sputtering yields, the divertor carbon source, as indicated by low charge-state carbon emission, is overestimated by almost an order of magnitude indicating that deuterium radiation may play a dominant role. The impact of these imposed radiative losses on the electron density and temperature, as well as deuterium radiation across the outer divertor leg will be presented.\par \vskip6pt \noindent [1] J.W.\ Davis, A.A.\ Haasz, J.\ Nucl.\ Mater.\ {\bf 241-243}, 37 (1997). [Preview Abstract] |
Monday, October 27, 2014 3:24PM - 3:36PM |
CO5.00008: Reduced Net Erosion of High-Z PFC Materials in DIII-D Divertor D.L. Rudakov, P.C. Stangeby, J.D. Elder, W.R Wampler, D.A. Buchenauer, J.G. Watkins, J.N. Brooks, A. Hassanein, T. Sizyuk, A.R. Briesemeister, A.G. McLean, C.P. Chrobak, H.Y. Guo, A.W. Leonard, C.P.C. Wong DiMES samples featuring 1 cm and 1 mm diameter W films deposited on a Si substrate were exposed in DIII-D near the attached outer strike point of LSN L-mode discharges. The measured net and gross erosion rates of W, determined from post-mortem ion beam analysis (IBA) of 1 cm and 1 mm samples, were 0.14 and 0.48 nm/s, respectively, giving net/gross erosion ratio of 0.29. REDEP/WBC modeling of this experiment yielded a very close ratio of 0.33. Projection of the modeling results to ITER shows very low net erosion of W. In another experiment Mo-coated samples were exposed with $^{13}$CH$_4$ gas injected $\sim$2 cm upstream of DiMES. Reduction of Mo erosion was evidenced $in-situ$ by the suppression of MoI line radiation. Post-mortem IBA showed that the net erosion of Mo was below the measurement resolution of 0.5 nm, corresponding to a rate of $\leq$0.07 nm/s. Compared to the previously measured erosion rates, this constitutes a reduction of more than 10X. [Preview Abstract] |
Monday, October 27, 2014 3:36PM - 3:48PM |
CO5.00009: H-mode Pedestal Enhancement and Improved Confinement in DIII-D with Lithium Injection G.L. Jackson, T.H. Osborne, R. Maingi, D.J. Battaglia, D.K. Mansfield, A.L. Roquemore, B.A. Grierson, C.P. Chrobak, A.G. McLean, G.R. McKee, Z. Yan Lithium has been injected into DIII-D discharges leading to larger density and temperature pedestal widths and pedestal pressure increases. The lithium injection allowed transitions from ELMing to ELM free \mbox{H-mode} with energy confinement improvements up to 70\%, compared to similar discharges without lithium. Lithium was injected directly into the plasma and SOL as an aerosol (44 $\mu$m dia particles) using a ``lithium dropper'' with no increase in radiated power. The lithium injection also led to density fluctuations of up to 8\% in the pedestal region in the frequency range $\approx$40 - 150 kHz, measured by the BES diagnostic [1]. We will discuss experiments to obtain ELM-free performance and enhanced pedestals with lithium, EPED modeling to determine proximity to the peeling-ballooning boundary, and conditions for obtaining reduced recycling. \\[4pt] [1] Z. Yan, et al., these proceedings [Preview Abstract] |
Monday, October 27, 2014 3:48PM - 4:00PM |
CO5.00010: Stability Limits in High Performance, Negative Central Shear Discharges J.M. Hanson, J. Bialek, G.A. Navratil, K.E.J. Olofsson, F. Turco, M. Clement, J.R. Ferron, A.M. Garofalo, R.J. La Haye, M.J. Lanctot, E.J. Strait, C.T. Holcomb Exploration of negative central shear equilibria in DIII-D has yielded discharges that transiently achieve $\beta_N\simeq 4$. The discharges exhibit broad current density profiles, leading to a significant separation in the no- and with-wall ideal kink stability limits predicted by MHD theory. As the no-wall limit is approached and exceeded in experiments, performance is often limited by n=1 resistive wall mode (RWM) instabilities that lead to abrupt collapses of the plasma stored energy. In addition, instabilities with n=1 rotating tearing precursors are observed when minimum $q$ value drops below 2. Theoretical calculations predict that magnetic feedback control using the in-vessel coils (internal coils) can provide RWM stabilization to $\beta_N$ values approaching the n=1 ideal-wall limit. In experiments, applying I-coil control indeed facilitates access to increased $\beta_N$ values above the no-wall limit. [Preview Abstract] |
Monday, October 27, 2014 4:00PM - 4:12PM |
CO5.00011: Coupling of Applied Non-axisymmetric Fields to Toroidal Torque N.C. Logan, J.-K. Park, J.E. Menard, E.J. Strait, C. Paz-Soldan, M.J. Lanctot Recent advances in the modeling of neoclassical toroidal viscosity (NTV) have enabled realistic predictions of the coupling between applied non-axisymmetric fields and the resultant toroidal torque in the DIII-D tokamak. The strong dependence of the NTV on the amplified plasma kink response reduces the control of the non-resonant torque to a single mode model, in which the torque optimization is equivalent to an optimization of the net non-axisymmetric field's overlap with the spatial structure of the dominant mode. This single mode model has enabled efficient feed-forward correction of the $n=1$ and $n=2$ intrinsic error fields and $n=\,$1-3 proxy error fields in NTV dominated scenarios. In addition, rotation drive toward a neoclassical offset using multiple coil sets has been optimized in accordance with the single mode model. Similar linear optimization techniques could be used to design future coil sets for rotation control, while inclusion of multimodal effects will be necessary for rotation profile control. [Preview Abstract] |
Monday, October 27, 2014 4:12PM - 4:24PM |
CO5.00012: 2D Measurements of TEM Structure at Varying Driven Toroidal Rotation on DIII-D S.E. Zemedkun, Y. Chen Jr, T. Munsat, S.E. Parker, W. Wan, S. Che, C.W. Domier, N.C. Luhmann, L. Yu, B.J. Tobias The first experimental 2D mapping of drift modes, trapped electron mode (TEM) spatial evolution, $T_e$ fluctuation levels, and dispersion relations are achieved using electron cyclotron emission imaging (ECEI) in a regime far from ITG parameter space in DIII-D. Linear gyrokinetic simulations with the GEM code find that the TEM is most unstable in the parameter regimes studied ($a/L_n=1.27$, $a/L_{Ti}=1.9$, $a/L_{Te}=3.3$), and exhibit a similar real frequency and eigenmode structure to that observed with ECEI. Measurements are made in L-mode discharges with neutral beam and electron cyclotron waves at fixed heating power over a range of driven toroidal rotation rates. 2D maps of the mode structure are determined using correlation techniques, and dispersion plots are constructed from the cross-phase and cross-spectral power. For different levels of NBI momentum input, $T_e$ fluctuation levels measured over a range of poloidal wavenumbers ($\sim$0.5\%, up to 200 kHz) decrease with increasing imposed toroidal rotation, which may be related to local shearing rates. [Preview Abstract] |
Monday, October 27, 2014 4:24PM - 4:36PM |
CO5.00013: Nonlinear Coupling Amongst Rotating Magnetic Islands in DIII-D B.J. Tobias, B.A. Grierson, M. Okabayashi, C.M. Muscatello, C.W. Domier, N.C. Luhmann Jr, S.E. Zemedkun, T. Munsat The appearance of magnetic islands at multiple rational surfaces limits performance and increases the risk of locked-mode disruption. These islands initially rotate independently, reflecting the differential flow of the background ion fluid. As the discharges progress, however, the phase-locking of two or more islands, e.g. 3/2 and 2/1, exacerbates confinement degradation by several mechanisms, including a flattening of the core rotation profile that increases the penetration depth of edge-localized modes (ELMs). However, neoclassical tearing mode structure can be manipulated to drive an inversion in the toroidal rotation profile, accelerating the edge plasma to maintain the local shearing rate, without additional neutral beam power. Although nonlinear 3-wave coupling is still observed, phase-locking is avoided and the thermonuclear neutron rate remains elevated, despite the discharge developing larger islands (at larger radii) that damp the toroidal angular momentum and reduce $\beta_N$. [Preview Abstract] |
Monday, October 27, 2014 4:36PM - 4:48PM |
CO5.00014: 3D Field-Induced Transport and Plasma Response Leading to ELM Suppression in DIII-D S.P. Smith, C. Paz-Soldan, R.J. Groebner, O. Meneghini, R. Nazikian, B.A. Grierson, M.E. Austin, J.D. Callen, E.M. Davis, R.A. Moyer, T.L. Rhodes, G. Wang, L. Zeng A clear increase in trapped electron mode (TEM) scale density fluctuation levels ne is seen at the top of the pedestal as the plasma transitions from edge localized mode (ELM)ing to ELM suppression with applied 3D resonant fields. Additional increases in $T_e$ fluctuations and line-integrated ne at the top of the pedestal are seen as the 3D field strength is increased. High resolution $T_e$ and $n_e$ profile measurements near the top of the pedestal show strong transport scaling with the applied field ($l/L_{Te}$, $1/L_{ne} \sim I_c^2$) during ELM suppression. These latter results are consistent with the magnetic flutter model regulating transport at the top of the pedestal, possibly driven by kink mode coupling, however the former results support a 3D modification of microturbulence stability as the process by which ELMs are suppressed. [Preview Abstract] |
Monday, October 27, 2014 4:48PM - 5:00PM |
CO5.00015: Using Quiescent H-mode to Access an Improved High Pressure Plasma Edge W.M. Solomon, B.A. Grierson, R. Nazikian, K.H. Burrell, A.M. Garofalo, T.H. Osborne, P.B. Snyder, A. Loarte, G.R. McKee, M.E. Fenstermacher Experiments on DIII-D have extended Quiescent H-mode (QH-mode) to high density through the use of strong shaping, overcoming a long-standing limitation in QH-mode operation, a high confinement state of the plasma that does not exhibit edge localized modes. These experiments have navigated a valley of improved edge peeling-ballooning stability dubbed ``Super H-mode,'' which opens up at high density with strong plasma shaping. The thermal energy confinement time increases due to improvements in both the pedestal height and the core transport. Theoretical calculations of the pedestal height and width as a function of density using the EPED model are in quantitative agreement with the measurements. Together with the achievement of high beta, high confinement and low $q_{95}$ for many energy confinement times, these results extend QH-mode as a potentially attractive operating scenario for ITER and point to a path for a new high performance regime that could improve the attractiveness of a fusion reactor. [Preview Abstract] |
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