Bulletin of the American Physical Society
58th Annual Meeting of the APS Division of Plasma Physics
Volume 61, Number 18
Monday–Friday, October 31–November 4 2016; San Jose, California
Session NO4: Alcator C-Mod Tokamak and DEMO |
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Chair: Francesca Turco, Columbia University Room: 230 A |
Wednesday, November 2, 2016 9:30AM - 9:42AM |
NO4.00001: Overview of Alcator C-Mod Research Earl Marmar C-Mod is the only divertor tokamak in the world capable of operating at B fields up to 8 T, equaling and exceeding that planned for ITER. C-Mod is compact, accessing regimes of extreme edge power density (q$_{\mathrm{\vert \vert }}$\textasciitilde 1 GW/m$^{\mathrm{2}})$. surpassing the design for ITER, and approaching the levels envisioned in power plants. C-Mod results are particularly important for providing the physics basis of the high-field, compact tokamak approach. Results of experiments and related modeling span the topics of core transport and turbulence, RF heating and current drive, pedestal physics, scrape-off layer, divertor and plasma-wall interactions. ICRF has been successfully applied to control and reverse accumulation of high Z impurities in the core plasma. For the first time ever, feedback of low Z seeding for divertor power dissipation has been tied directly to real-time plasma power fluxes measured on the high-Z metal PFCs in the divertor, and used to mitigate those fluxes with no degradation of the pedestal pressure or core confinement. The naturally ELM-less I-mode and EDA-H-mode regimes have been extended B$_{\mathrm{T}}=$8T. I-mode threshold scalings show a weak dependence on B, yielding a significantly broader window for I-mode operation at high field. Quiescence of the high-field side scrape-off layer makes this a potentially attractive location for placement of RF actuators to ameliorate plasma interactions with launchers; the wave physics for penetration and damping, for both ICRF and LHRF appears very favorable for high-field side launch [Preview Abstract] |
Wednesday, November 2, 2016 9:42AM - 9:54AM |
NO4.00002: Correlation Electron Temperature Fluctuation Measurements on Alcator C-Mod and ASDEX Upgrade: Cross Machine Comparisons and Transport Model Validation A. E. White, A. J. Creely, S. Freethy, N. Cao, G. D. Conway, T. Goerler, T. Happel, N. T. Howard, C. Inman, J. E. Rice, P. Rodriguez Fernandez, C. Sung, Alcator C-Mod, ASDEX Upgrade Correlation Electron Cyclotron Emission diagnostics have been developed for Alcator C-Mod and ASDEX Upgrade. Measurements of long wavelength (ktheta rhos \textless 0.5) electron temperature fluctuations have been measured in the core plasma (0.5 \textless r/a \textless 0.95) of a variety of discharges, including high performance I-mode and H-mode plasmas, during dedicated ITG/TEM transition L-mode plasma experiments, across intrinsic rotation reversals in both Ohmic L-mode LOC/SOC plasmas and RF L-mode plasmas, and in Helium L-mode plasmas. With nearly identical instrument set-ups at two tokamaks, it is possible to carry out direct cross-machine comparisons, as well as multi-machine transport model validation, using nonlinear simulations with the GENE and GYRO codes and reduced models such as TGLF. Electron temperature fluctuations, and the correlation with density fluctuations, which can be measured with coupled radiometer / reflectometer diagnostics, provide valuable constraints on gyrokinetic models. Recent results in transport model validation at both C-Mod and AUG are presented. [Preview Abstract] |
Wednesday, November 2, 2016 9:54AM - 10:06AM |
NO4.00003: The Role of Cross-Scale Coupling in the Saturation of Turbulence and Transport in ITER-Relevant, ELM-y H-mode Plasmas N.T. Howard, C. Holland, A.E. White, M. Greenwald, J. Candy, A.J. Creely New multi-scale gyrokinetic simulations which capture ITG/TEM/ETG turbulence and their interactions have been performed on an ITER-relevant, ELM-y H-mode Alcator C-Mod plasma. Recent multi-scale simulations indicate that ion and electron-scale turbulence strongly couple in the core of L-mode plasmas, driving significant electron heat flux at both turbulent scales. Building off of these results, cutting-edge multi-scale simulations were performed in reactor-relevant conditions (no external momentum input, dominant electron heating, and T$_{\mathrm{e}}$ \textasciitilde T$_{\mathrm{i}})$. These simulations include 3 gyrokinetic species, realistic geometry, collisions, rotation, finite beta, all experimental inputs, and required approximately 60M CPU hours on the NERSC Edison supercomputer. The presence of electron-scale turbulence is found to fundamentally change the saturation of the ion-scale turbulence in H-mode plasma conditions, calling into question reactor scenario predictions which are based on purely long wavelength turbulence models. To validate the gyrokinetic model in this reactor-relevant plasma regime, quantitative comparisons are made between simulated heat fluxes, incremental diffusivities, and density fluctuations with experiment. [Preview Abstract] |
Wednesday, November 2, 2016 10:06AM - 10:18AM |
NO4.00004: Perturbative transport modeling and comparison to cold-pulse and heat-pulse propagation experiments in Alcator C-Mod and DIII-D P. Rodriguez Fernandez, A. E. White, N. M. Cao, A. J. Creely, M. J. Greenwald, N. T. Howard, A. E. Hubbard, J. W. Hughes, J. H. Irby, C. C. Petty, J. E. Rice Possible ``non-local" transport phenomena are often observed in tokamak plasmas. Different models have been proposed to explain fast responses during perturbative transport experiments, including non-diffusive effects. Specific tools to characterize the dynamic behavior and power balance analysis using TRANSP and the quasi-linear trapped gyro-landau fluid code TGLF have been developed to analyze Alcator C-Mod experiments. Recent results from cold pulse experiments show that fast core temperature increases following edge cold-pulse injections (peak within $\sim10ms$, while $\tau_E\sim25ms$) are not correlated with the direction of intrinsic rotation, and instead the amplitude of the core response depends on density, plasma current and RF input power. The propagation of the cold pulse can be compared with propagation of heat pulses from sawteeth, and both may be used to probe changes in temperature profile stiffness. A Laser Blow Off (LBO) system is being developed for DIII-D that will allow further validation and cross-machine comparison of cold pulse experiments. LBO at DIII-D will also allow for direct comparisons with ECH perturbative heat pulse experiments. [Preview Abstract] |
Wednesday, November 2, 2016 10:18AM - 10:30AM |
NO4.00005: Observations of Intrinsic Rotation Reversal Hysteresis in Alcator C-Mod Plasmas Norman Cao, John Rice, Anne White, Seung Baek, Mark Chilenski, Alexander Creely, Paul Ennever, Amanda Hubbard, Jerry Hughes, Jim Irby, Pablo Rodriguez-Fernandez, Matthew Reinke, Patrick Diamond Intrinsic core toroidal rotation in Alcator C-Mod L-mode plasmas has been observed to spontaneously reverse direction when the normalized collisionality $\nu^*$, evaluated at the profile minimum, passes through a critical value around 0.4. In Ohmic plasmas, the low density linear Ohmic confinement regime exhibits co-current toroidal rotation, and the higher density saturated Ohmic confinement regime exhibits counter-current rotation. The reversal manifests a hysteresis loop in $\nu^*$, where the critical collisionalities for the forward and reverse transitions differ by 10-15\%. There appears to be memory associated with the rotation state, since reversals which do not begin from fully saturated rotation states do not manifest this hysteresis. In addition, high-k PCI fluctuation “wings” ($k_\theta \rho_s$ up to 1) at low density and high current appear only in the co-current rotation state, while density peaking and “non-local” heat transport behavior do not appear to change significantly with the rotation state. Results from fluctuation measurements and preliminary transport and stability analyses will also be presented. [Preview Abstract] |
Wednesday, November 2, 2016 10:30AM - 10:42AM |
NO4.00006: Reassessment of impurity transport coefficients in Alcator C-Mod M.A. Chilenski, M. Greenwald, Y. Marzouk, J.E. Rice, A.E. White Transport coefficients $D_{Z}$, $V_{Z}$ inferred from spectroscopic measurements of impurity injections are often used to validate simulations of turbulent transport. Existing methods for inferring $D_{Z}$, $V_{Z}$ fall short in one or more ways, leading to incorrect results and calling the use of such measurements for validation into question. For instance, the use of too simple of a function to describe the $D_{Z}$, $V_{Z}$ profiles leads to a dramatic underestimation of the uncertainty. This talk presents recent attempts to demonstrate some of these issues, estimate diagnostic requirements to determine impurity transport coefficients, and obtain rigorous uncertainty estimates on $D_{Z}$ and $V_{Z}$. In particular, transport coefficients are inferred using the MultiNest algorithm and the critical question of the level of complexity in the $D_{Z}$, $V_{Z}$ profiles which can be inferred is answered using the marginal likelihood. In addition to providing a useful framework for the solution of inverse problems relevant to the interpretation of experimental data, the use of these techniques will enable a reassessment of the ability of gyrokinetic simulations to match the impurity channel. [Preview Abstract] |
Wednesday, November 2, 2016 10:42AM - 10:54AM |
NO4.00007: C-Mod MHD stability analysis with LHCD Fatima Ebrahimi, A. Bhattacharjee, L. Delgado, S. Scott, J. R. Wilson, G. M. Wallace, S. Shiraiwa, R. T. Mumgaard In lower hybrid current drive (LHCD) experiments on the Alcator C-Mod, sawtooth activity could be suppressed as the safety factor q on axis is raised above unity. However, in some of these experiments, after applying LHCD, the onset of MHD mode activity caused the current drive efficiency to significantly drop. Here, we study the stability of these experiments by performing MHD simulations using the NIMROD code starting with experimental EFIT equilibria. First, consistent with the LHCD experiment with no signature of MHD activity, MHD mode activity was also absent in the simulations. Second, for experiments with MHD mode activity, we find that a core n=1 reconnecting mode with dominate poloidal modes of m=2,3 is unstable. This mode is a resistive current-driven mode as its growth rate scales with a negative power of the Lundquist number in the simulations. In addition, with further enhanced reversed-shear q profile in the simulations, a core double tearing mode is found to be unstable. [Preview Abstract] |
Wednesday, November 2, 2016 10:54AM - 11:06AM |
NO4.00008: Taming the ICRF Antenna – Plasma Edge Interaction via Novel Field-Aligned ICRF Antenna on Alcator C-Mod S.J. Wukitch, Y. Lin, J. Terry, A. Hubbard, R.T. Mumgaard, the Alcator C-Mod Team, M.L. Reinke Although ICRF is attractive for bulk plasma heating due to favorable wave propagation, ICRF antenna – edge plasma interaction remains a challenge. Recent experiments reveal that RF-induced potentials in the scrape-off layer and antenna impurity source are dependent on the power ratio between the inner and outer current staps, Pcent/Pout. Using a modified field aligned antenna, the transmission line network connected the center two straps at [0,pi] to one transmitter and the outer two straps another transmitter. This experiment was motivated by positive three strap antenna results from ASDEX-U [V. Bobkov et al 2016 Nucl. Fusion 56 084001]. With -30 dB decoupling, we scanned Pcent/Pout from zero to greater than 1000. A minimum in the RF enhanced potential and local impurity source is observed for Pcent/Pout greater than 1 and less than 4 with a gradual rise in impurity source for Pcent/Pout greater than 4. This minimum correlates where the image currents in the antenna limiters are expected to be smallest. We also tested antenna operation in [0,0,pi,pi] antenna phasing and found excessive local impurity production despite the antenna being field aligned. This antenna phasing excites low k and potentially have higher coupling. Latest results and analysis will be presented [Preview Abstract] |
Wednesday, November 2, 2016 11:06AM - 11:18AM |
NO4.00009: Evidence for Chaotic Edge Turbulence in the Alcator C-Mod Tokamak Ziyan Zhu, Anne White, Troy Carter, Jim Terry, Seung Gyou Baek Turbulence greatly reduces the confinement time of magnetic-confined plasmas; understanding the nature of this turbulence and the associated transport is therefore of great importance. This research seeks to establish whether turbulent fluctuations in Alcator C-Mod are chaotic or stochastic. This has an important impact on transport caused by turbulence in C-Mod: stochastic fluctuations sample all of phase space and can lead to diffusive transport, whereas chaotic fluctuations live in a restricted phase space (e.g. on attractors) and a diffusive description may not be valid. By analyzing the time series from an O-Mode reflectometer, turbulent edge density fluctuations in Ohmic plasmas and L-mode plasmas in the Alcator C-Mod tokamak are shown to be chaotic. Supporting evidence for chaos in the edge region includes: the observation of an exponential power spectra (which is associated with Lorentzian-shaped pulses in the time series) and the location of the signal in the Complexity-Entropy plane (C-H plane) and its corresponding Brandt-Pompe (BP) probability distribution [1]. These analysis techniques will be briefly introduced along with a discussion of the analysis results. Different diagnostic techniques, such as Gas Puff Imaging (GPI), could be used to confirm the results. [1] J. E. Maggs, T.L.Rhodes and G. J. Morales, Plasmas. Phys. Control. Fusion 57 (2015) 045004 (16pp) [Preview Abstract] |
Wednesday, November 2, 2016 11:18AM - 11:30AM |
NO4.00010: The Shoelace Antenna: Measurements of Driven Transport and Prospects for Active Edge Control Theodore Golfinopoulos, B. LaBombard, D. Brunner, J.L. Terry, S.G. Baek, P. Ennever, E. Edlund, W. Han, W.M. Burke, S.M. Wolfe, J.H. Irby, J.W. Hughes, E.W. Fitzgerald, R.S. Granetz, M.J. Greenwald, R. Leccacorvi, E.S. Marmar, S.Z. Pierson, M. Porkolab, R.F. Vieira, S.J. Wukitch The Shoelace antenna was built to drive edge fluctuations in the Alcator C-Mod tokamak, matching the wavenumber ($k=$1.5/cm) and frequency (50\textless $f$\textless 200 kHz) of the Quasi-Coherent Mode (QCM). This fluctuation is responsible for regulating transport across the plasma boundary in the steady-state, ELM-free Enhanced D$\alpha $(EDA) H-mode; the goal of the Shoelace antenna is to regulate edge transport actively via the same mechanism. Initial experiments demonstrated that the antenna drove a resonant response in the edge plasma in steady-state EDA and transient, non-ELMy H-modes, but transport measurements were unavailable. In 2016, the Shoelace antenna was relocated to enable direct measurements of driven transport by a reciprocating Mirror Langmuir Probe, while also making available gas puff imaging and reflectometer data to provide radial localization of the driven fluctuation. This talk will describe these measurements, and compare them to those of the intrinsic QCM in the context of assessing the feasibility of achieving active control of edge transport using direct coupling to edge modes. [Preview Abstract] |
Wednesday, November 2, 2016 11:30AM - 11:42AM |
NO4.00011: Fast Imaging of Filaments in the X-Point Region of Alcator C-MOD J.L. Terry, S. Ballinger, D. Brunner, B. LaBombard, A.E. White, S.J. Zweben A rich variety of field-aligned fluctuations has been revealed using fast imaging of D$_{\mathrm{\alpha }}$ emission from Alcator C-Mod's lower X-point region. Field-aligned filamentary fluctuations are observed along the inner divertor leg, within the Private-Flux-Zone (PFZ), in the Scrape-Off Layer outside the outer divertor leg, and, under some conditions, at or above the X-point. The locations and dynamics of the filaments in these regions are strikingly complex in C-Mod. Changes in the filaments' generation appear to be ordered by plasma density and magnetic configuration. In a Lower Single Null with 0.12 \textless $n/n_{Greenwald}$ \textless 0.45 and Bx$\nabla $B directed down, filaments typically move up the inner divertor leg toward the X-point. Reversing the field direction results in the appearance of filaments outside of the outer divertor leg. With the divertor targets ``detached'', filaments inside the LCFS are seen. These studies were motivated by observations of filaments in the X-point and PFZ regions in MAST, and comparisons with those observations will be made. [Preview Abstract] |
Wednesday, November 2, 2016 11:42AM - 11:54AM |
NO4.00012: Comparisons of Measured Gas Puff Emissions with DEGAS 2 Modeling of Alcator C-Mod Plasmas S. G. Baek, J. Terry, D. S. Stotler, D. Brunner, B. L. LaBombard Characterizing neutral particle transport in a tokamak and validating an available model against experimental measurements provide important predictive capabilities on neutral particle dynamics. On Alcator C-Mod, the gas-puff imaging system is useful in performing this validation study because both the gas puff rate and two systems detecting the light emission from the puff have been absolutely calibrated. This enables direct comparisons of the spatial distributions and absolute brightness with the synthetic diagnostic results from the Monte Carlo neutral transport code, DEGAS 2. An initial comparison between the gas puff imaging of a Helium-I line measured in a well-diagnosed L-mode plasma and the DEGAS 2 modeling of that puff has been performed. This result shows that the brightness profile shape generally agrees well with a small radial shift. While the simulation predicts a higher brightness, the peak brightness is also found to be in agreement within a factor of two. Based on this encouraging result, new experiments covering a wide range of density were conducted for He puffs and will be compared with DEGAS 2 simulations. A comparison of D\textunderscore alpha emission from a D2 puff with DEGAS 2 modeling will also be presented. [Preview Abstract] |
Wednesday, November 2, 2016 11:54AM - 12:06PM |
NO4.00013: Poloidal Structure of Disruption Halo Currents in Alcator C-Mod Robert Granetz, Alex Tinguely, Alexandra Berg, Adam Kuang, Dan Brunner, Brian LaBombard A new set of optimized Langmuir probes embedded in the outboard divertor of Alcator C-Mod has been run in a mode that effectively measures halo currents during disruptions. These data provide unprecedented poloidal spatial and temporal resolution of the halo current flowing into and out of the divertor surface. Reconstructions of the evolving plasma boundary during the disruption current quench gives us information on the point(s) of contact on the divertor surface as a function of time, and this has been compared to details of the halo current distribution, including the spatial profiles of halo current polarity and amplitude. In addition, the safety factor, qlim(t), of the plasma surface is also calculated from the boundary reconstructions, and its correlation with the halo current amplitude will be shown. [Preview Abstract] |
Wednesday, November 2, 2016 12:06PM - 12:18PM |
NO4.00014: Divertor conditions near double null in Alcator C-Mod Dan Brunner, Brian LaBombard, Adam Kuang, Jim Terry, Bob Mumgaard, Steve Wolfe Many tokamak reactor designs utilize a double-null equilibrium for the boundary plasma because of the expected benefits of heat flux sharing between the two outer divertor leg as well as the attractiveness of the high-field side scrape-off layer plasma in double-null for RF actuators. However, there has been very little reported on boundary plasma conditions near double null, especially at the divertor plate. And, due to the narrow boundary plasma width, there is concern of the precision to which a double-null equilibrium must be controlled to maintain divertor heat flux sharing. To this end, a series of experiments were performed varying the magnetic balance around double null. The magnetic balance~between~the two nulls was scanned shot-to-shot in L-, I-, and H-mode plasmas. In addition, current~and density scans were performed in L-mode plasmas. Results will be presented for relative balances of divertor particle and energy fluxes to the four divertors (inboard/outboard, upper/lower) as well as the sensitivity of changes in divertor conditions to the magnetic balance. Supported by USDoE award DE-FC02-99ER54512. [Preview Abstract] |
Wednesday, November 2, 2016 12:18PM - 12:30PM |
NO4.00015: A ‘Stepladder’ Approach to a Steady State Tokamak Fusion Power Plant Hartmut Zohm, Alexander Bock, Emiliano Fable, Joerg Stober, Frederik Traeuble In the EU strategy to an FPP, DEMO is the single step between ITER and an FPP. It is not obvious how to arrive at a DEMO design point in this strategy. We propose to avoid large scenario development steps in an ITER-DEMO-FPP step-ladder, since no other machines can qualify the scenarios. Thus, DEMO becomes a technology demonstrator, not a plasma physics experiment. We characterize the plasma scenario in terms of the quantities $\beta_N$, q, H and $f_{GW}$. To ensure adequate divertor performance, constant $n_e$ is chosen. Different from previous approaches, $\rho^*$ and $\nu^*$ will vary throughout the stepladder based on physics arguments that below minimum values, their variation is no longer important. This leaves open the choice of machine parameters A, R and B. Fixing A to the ITER value, constant $f_{GW}$ and absolute $n_e$ lead to B/R = const. At constant q, $\beta_N$ and A, B and R increase proportional to $P_{fus}^{1/7}$ in the stepladder. The power needed to drive the current in steady state varies similarly, so from DEMO to an FPP a significant decrease in recirculating power fraction occurs. A viable divertor solution and access to H-mode are considered explicitly. An example for such a stepladder is discussed, based on recent ASDEX Upgrade results in steady state. [Preview Abstract] |
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