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 TO4: Research in Support of ITER |
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Chair: Charles Greenfield, General Atomics Room: 230 A |
Thursday, November 3, 2016 9:30AM - 9:42AM |
TO4.00001: Role of plasma shape in access to ELM suppression at low collisionality: First observation of ELM suppression in ASDEX Upgrade in a shape-matching identity experiment with DIII-D Wolfgang Suttrop, Raffi Nazikian, Andrew Kirk Controlled plasma shape scans at low pedestal collisionality $\nu^*_\mathrm{ped} < 0.4$ in DIII-D reveal that the threshold of magnetic perturbation field strength for suppression of edge-localized modes (ELMs) depends on both upper and lower plasma triangularity. Similar plasmas with matching shape and matching plasma parameters have been performed in DIII-D and ASDEX Upgrade. In these discharges, stationary ELM suppression by magnetic perturbations is observed for the first time in ASDEX Upgrade.Despite different divertor geometry and different first wall materials in the two machines, these plasmas show many similarities: Complete ELM suppression occurs in a narrow windows around $q_{95} \approx 3.7$ with transitions to phases with ``fuzzy'' ELMs outside these windows, electron density and temperature profiles as well as the total pedestal pressure are well matched, while there are variations of other quantities such as impurity concentrations and impurity rotation frequencies. A first experiment with injection of tungsten shows that the tungsten impurity content in the plasma decays on the time scale of energy confinement. [Preview Abstract] |
Thursday, November 3, 2016 9:42AM - 9:54AM |
TO4.00002: Prediction and Optimization of the ITER Pedestal P.B. Snyder, O. Meneghini, M.N.A. Beurskens, J.W. Hughes, T.H. Osborne, H.R. Wilson The structure of the edge transport barrier, or pedestal, plays an important role in ITER performance, with fusion power predicted to scale roughly with the square of the pedestal pressure. Understanding the physics of the pedestal is also critical to reliably suppressing or mitigating ELMs. We present tests of the EPED model, which predicts pedestal structure based on the intersection of two calculated criticality constraints, on more than 800 cases on existing tokamaks, and assess model accuracy across a range of parameters including normalized gyroradius. The EPED model is found to predict observations significantly more accurately than existing empirical pedestal models. The model is then used both independently, and coupled to core transport predictions from TGLF and NEO, using OMFIT, to predict and optimize ITER performance, including exploring possible operation in the Super H-Mode regime. [Preview Abstract] |
Thursday, November 3, 2016 9:54AM - 10:06AM |
TO4.00003: ELM Mitigation in Low-rotation ITER Baseline Scenario Plasmas on DIII-D with Deuterium Pellet Injection L.R. Baylor ELM mitigation using high frequency D2 pellet ELM pacing has been demonstrated in ITER baseline scenario plasmas on DIII D with low rotation obtained with low NBI input torque. The ITER burning plasmas will have relatively low input torque and are expected to have low rotation. ELM mitigation by on-demand pellet ELM triggering has not been observed before in these conditions. New experiments on DIII-D in these conditions with 90 Hz $D_2$ pellets have shown that significant mitigation of the divertor ELM peak heat flux by a factor of 8 is possible without detrimental effects to the plasma confinement. High-Z impurity accumulation is dramatically reduced at all input torques from 0.1 to 2.5 N-m. Fueling with high field side injection of $D_2$ pellets has been employed to demonstrate that density buildup can be obtained simultaneously with ELM mitigation. The implications are that rapid pellet injection remains a promising technique to trigger on-demand ELMs in low rotating plasmas with greatly reduced peak flux while preventing impurity accumulation in ITER. [Preview Abstract] |
Thursday, November 3, 2016 10:06AM - 10:18AM |
TO4.00004: Energy Confinement Recovery in Low Collisionality ITER Shape Plasmas with Applied Resonant Magnetic Perturbations (RMPs) L. Cui, B. Grierson, N. Logan, R. Nazikian Application of RMPs to low collisionality ($\nu*_e\textless0.4$) ITER shape plasmas on DIII-D leads to a rapid reduction in stored energy due to density pumpout that is sometimes followed by a gradual recovery in the plasma stored energy. Understanding this confinement recovery is essential to optimize the confinement of RMP plasmas in present and future devices such as ITER. Transport modeling using TRANSP+TGLF indicates that the core a/LTi is stiff in these plasmas while the ion temperature gradient is much less stiff in the pedestal region. The reduction in the edge density during pumpout leads to an increase in the core ion temperature predicted by TGLF based on experimental data. This is correlated to the increase in the normalized ion heat flux. Transport stiffness in the core combined with an increase in the edge a/LTi results in an increase of the plasma stored energy, consistent with experimental observations. For plasmas where the edge density is controlled using deuterium gas puffs, the effect of the RMP on ion thermal confinement is significantly reduced. [Preview Abstract] |
Thursday, November 3, 2016 10:18AM - 10:30AM |
TO4.00005: Highlights of the KSTAR Research relevant to ITER Y.K. Oh, H.K. Park, S.W. Yoon The Korean Superconducting Tokamak Advanced Research (KSTAR) capable of steady state operation of high beta plasmas, is an ideal test bed for ITER relevant research such as a long pulse operation of the superconducting device. In this talk, highlights of the recent KSTAR campaigns (2015, 2016) that are relevant to ITER operation and physics will be discussed. In particular, implication of MHD control (i.e., sawtooth, NTM, ELM and Disruption), rotation control through NTV and L-H threshold power dependence on the error field will be addressed. [Preview Abstract] |
Thursday, November 3, 2016 10:30AM - 10:42AM |
TO4.00006: Progress and challenges in predictive modeling of runaway electron generation in ITER Dylan Brennan, Eero Hirvijoki, Chang Liu, Amitava Bhattacharjee, Allen Boozer Among the most important questions given a thermal collapse event in ITER is that of how many seed electrons are available for runaway acceleration and the avalanche process, how collisional and radiative mechanisms will affect the electron acceleration, and what mitigation techniques will be effective. In this study, we use the kinetic equation for electrons and ions to investigate how different cooling scenarios lead to different seed distributions. Given any initial distribution, we study their subsequent avalanche and acceleration to runaway with Adjoint and test particle methods. This method gives an accurate calculation of the runaway threshold by including the collisional drag of background electrons (assuming they are Maxwellian), pitch angle scattering, and synchrotron and Bremsstrahlung radiation. This effort is part of a new large collaboration in the US which promises to contribute substantially to our understanding of these issues. This talk will briefly review how this work contributes to this collaboration, and in particular discuss the technical challenges and open questions that stand in the way of quantitative, predictive modeling of runaway generation in ITER, and how we plan to address them. [Preview Abstract] |
Thursday, November 3, 2016 10:42AM - 10:54AM |
TO4.00007: Analysis of Runaway Electron Synchrotron Emission in Alcator C-Mod A. Tinguely, R. Granetz, A. Stahl Alcator C-Mod's high magnetic field allows relativistic ``runaway" electron (RE) synchrotron radiation (SR) to be observed in the visible wavelength range. Our aim is to determine the evolution of the RE energy distribution function, current, and density from measured SR spectra, providing insight into basic plasma physics as well as mitigation for fusion devices. Recent theoretical studies [1-3] predict that the SR reaction force and collisional friction will balance the electric force, forming a ``bump" on the tail of the energy distribution. However, both mono-energetic and monotonically-decreasing distributions fit the experimental data equally well. The COllisonal Distribution of Electrons [4-5] code is applied to C-Mod RE discharges and compared to experiment. In addition, a scan in magnetic field from 2.7 - 8 T explores the importance of SR as a power loss mechanism and limit on the maximum RE energy.\newline $[1]$ Aleynikov, et al. PRL 114 (2015).\newline [2] Decker, et al. PPCF 58 (2015).\newline [3] Hirvijoki, et al. JPP 81 (2015).\newline [4] M. Landreman, et al. CPC 185 (2014).\newline [5] A. Stahl, et al., to appear in NF. arXiv:1601.00898 [physics.plasm-ph] [Preview Abstract] |
Thursday, November 3, 2016 10:54AM - 11:06AM |
TO4.00008: Transient-Free Operations With Physics-Based Real-time Analysis and Control Egemen Kolemen, Keith Burrell, William Eggert, David Eldon, John Ferron, Alex Glasser, David Humphreys In order to understand and predict disruptions, the two most common methods currently employed in tokamak analysis are the time-consuming “kinetic EFITs,” which are done offline with significant human involvement, and the search for correlations with global precursors using various parameterization techniques. We are developing automated "kinetic EFITs" at DIII-D to enable calculation of the stability as the plasma evolves close to the disruption. This allows us to quantify the probabilistic nature of the stability calculations and provides a stability metric for all possible linear perturbations to the plasma. This study also provides insight into how the control system can avoid the unstable operating space, which is critical for high-performance operations close to stability thresholds at ITER. A novel, efficient ideal stability calculation method and new real-time CER acquisition system are being developed, and a new 77-core server has been installed on the DIII-D PCS to enable experimental use. [Preview Abstract] |
Thursday, November 3, 2016 11:06AM - 11:18AM |
TO4.00009: Secondary radiation effects during transient events on ITER divertor and nearby hidden components Ahmed Hassanein, Valeryi Sizyuk The secondary plasma developed as a result of disruptions and ELMs on divertor plate is composed mainly from high-Z divertor materials. This mini-plasma will greatly increase the radiation flux to nearby and hidden components. Our simulations showed significant increase in radiation fluxes and heat loads in the high-Z (i.e., tungsten) generated secondary plasma. These radiations could seriously damage hidden nearby components such as umbrella and dome structure. We have implemented enhanced models in our comprehensive integrated HEIGHTS package for 3D simulation of detailed photon and particle transport in the evolved secondary plasma during instabilities. HEIGHTS can simulate full 3D realistic ITER geometry, including the reflector plate, etc. to assess performance of all components resulting from plasma instabilities. HEIGHTS predicted, for the first time, details of heat loads and temperatures evolution of all nearby components due to transient events. Secondary radiation evolution and expansion cause serious damage to hidden internal components that were not directly exposed to DT plasma. Current ITER divertor design may require changes to mitigate such effects and protect interior components that is hard and costly to repair in case of transient disruptive events. [Preview Abstract] |
Thursday, November 3, 2016 11:18AM - 11:30AM |
TO4.00010: Tungsten divertor sourcing in DIII-D H-mode discharges and its impact on core impurity accumulation in different ELM regimes T. Abrams, R. Ding, J. Guterl, A. Briesemeister, E.A. Unterberg, H.Y. Guo, A.W. Leonard, D.M. Thomas, A.G. McLean, B. Victor, D. Rudakov, B. Grierson, J.G. Watkins, J.D. Elder, P.C. Stangeby Significant progress has been made understanding W sourcing during Type I ELMy H-mode on DIII-D using fast high-resolution measurements of W sourcing coupled with OEDGE/ERO and TRIM.SP modeling. ERO modeling of the inter-ELM phase, using a new OEDGE capability for charge state-resolved carbon ion fluxes and a material mixing model, shows measured W erosion is well explained by C-$>$W sputtering. Ion impact energies in the DIII-D divertor during ELMs, inferred from ratios of heat flux to ion flux, are 200-500 eV. Comparisons with TRIM.SP indicate C-$>$W sputtering dominates W sourcing during ELMs. This is in contrast to JET where ion impact energies are 3-5 keV during ELMs, predicted by the "free streaming model," and D-$>$W sputtering strongly contributes to W sourcing. Fast measurements of W erosion dynamics during ELMs agree well with TRIM.SP-based sputtering models assuming C/W surface concentrations of 0.5-0.8 and a 2$\%$ C$^{2+}$ ion flux fraction. Core W accumulation and SOL W density measurements made during the DIII-D high-Z tile array mini-campaign correlate with ELM frequency and W source rate. [Preview Abstract] |
Thursday, November 3, 2016 11:30AM - 11:42AM |
TO4.00011: Feed-back control of 2/1 locked mode phase: experiment on DIII-D and modeling for ITER W. Choi, K.E.J. Olofsson, R. Sweeney, F.A. Volpe A model has been developed for ITER to predict the dynamics of saturated $m/n$ = 2/1 tearing modes subject to various torques. The modes, with finite moment of inertia, are modeled as surface currents interacting with error fields, applied magnetic perturbations generated by internal and external non-axisymmetric coils, the vacuum vessel, and the first wall. Using this model, a feed-back controller has been designed to control the phase of locked modes. As predicted by simulation, experimental results on DIII-D show a simple fixed-gain controller can impose a desired constant phase or entrain the mode at a desired constant frequency (e.g. 20 Hz). For a given current in the control coils, a maximum entrainment frequency exists and is dependent on island width. The performance of such a controller in ITER is hereby simulated. The controller is expected to be useful in assisting island suppression with electron cyclotron current drive, as well as to prevent large amplitude locked modes and possible disruption. [Preview Abstract] |
Thursday, November 3, 2016 11:42AM - 11:54AM |
TO4.00012: Application of the three ion species ICRF scenario to ITER operations J. Wright, Y. Lin, M. Porkolab, S. Wukitch, Ye.O. Kazakov, D. Van Eester, J. Ongena, E.F. Jaeger Recent ICRF (ion cyclotron range of frequencies) heating experiments on C-Mod and JET confirm that using a third species as a second minority at fractions of less than a percent, eg D-($^3$He)-H, can increase heating efficiency and generate very energetic ions on the order of an MeV [Y. Kazakov, invited talk this meeting.] Together with 9Be-(4He)-H scenario, three-ion ICRF schemes are applicable to the pre-activation phase of ITER using trace concentrations of 3He or 4He ($\sim$0.1%) as a source of energetic ions for confinement studies of pseudo-alphas. During D-T operations, this scheme may be employed to heat intrinsic beryllium impurities for efficient bulk heating. We will briefly review experimental findings on Alcator C-Mod and JET with validated model comparison. Modeling using full wave solvers and Fokker-Planck will be used to determine heating efficiencies ICRF generated ion tail temperatures for ITER. [Preview Abstract] |
Thursday, November 3, 2016 11:54AM - 12:06PM |
TO4.00013: EC power management and NTM control in ITER Francesca Poli, E. Fredrickson, M. Henderson, N. Bertelli, D. Farina, L. Figini, S. Nowak, E. Poli, O. Sauter The suppression of Neoclassical Tearing Modes (NTMs) is an essential requirement for the achievement of the demonstration baseline in ITER. The Electron Cyclotron upper launcher is specifically designed to provide highly localized heating and current drive for NTM stabilization. In order to assess the power management for shared applications, we have performed time-dependent simulations for ITER scenarios covering operation from half to full field. The free-boundary TRANSP simulations evolve the magnetic equilibrium and the pressure profiles in response to the heating and current drive sources and are interfaced with a GRE for the evolution of size and frequency of the magnetic islands. Combined with a feedback control of the EC power and the steering angle, these simulations are used to model the plasma response to NTM control, accounting for the misalignment of the EC deposition with the resonant surfaces, uncertainties in the magnetic equilibrium reconstruction and in the magnetic island detection threshold. Simulations indicate that the threshold for detection of the island should not exceed 2-3cm, that pre-emptive control is a preferable option, and that for safe operation the power needed for NTM control should be reserved, rather than shared with other applications. [Preview Abstract] |
Thursday, November 3, 2016 12:06PM - 12:18PM |
TO4.00014: Experimental Evidence of Edge Fluctuation Broadening of ECH Deposition at DIII-D[1] M.W. Brookman, M.E. Austin, K.W. Gentle, C.C. Petty, Y. Peysson, J. Decker, K. Barada, D.E. Ernst This work provides experimental evidence for broadening of the ECH and ECCD deposition by edge density fluctuations. Results on the DIII-D tokamak suggest a deposition FWHM $\sim$1.7-2.8 times wider than TORAY-GA. A 1D ECH deposition profile was measured through gyrotron power modulation. From 500 kHz, 48-channel ECE measurements and trial ECH deposition functions, a Fourier transformed heat flux is found and fit to transport drive terms. Radially broader ECH deposition best fit calculated fluxes in discharges with higher levels of edge density turbulence. Broadening of deposition does not arise from anomalous transport, which is minimal on DIII-D. Simulation [2] and theory[3] suggest edge ($\rho\textgreater.9)$ turbulent n$\_$e fluctuations refract RF waves that pass through them, broadening radial deposition of ECH and ECCD. On ITER, this effect could hinder NTM suppression by broadening ECCD deposition outside the 3/2 island.\par \vskip9pt \noindent [1] Work supported by the U.S. DOE under Award DE-FC02-04ER54698 \par \noindent [2] Peysson Y. PPCF 53 (2011) \par \noindent [3] Kyriakos H. POP 17 (2010) [Preview Abstract] |
Thursday, November 3, 2016 12:18PM - 12:30PM |
TO4.00015: Extrapolation of the DIII-D high poloidal beta scenario to ITER steady-state using transport modeling J. McClenaghan, A.M. Garofalo, O. Meneghini, S.P. Smith Transport modeling of a proposed ITER steady-state scenario based on DIII-D high $\beta_P$ discharges finds that the core confinement may be improved with either sufficient rotation or a negative central shear q-profile. The high poloidal beta scenario is characterized by a large bootstrap current fraction($\sim$80$\%$) which reduces the demands on the external current drive, and a large radius internal transport barrier which is associated with improved normalized confinement. Typical temperature and density profiles from the non-inductive high poloidal beta scenario on DIII-D are scaled according to 0D modeling predictions of the requirements for achieving Q=5 steady state performance in ITER with "day one" H&CD capabilities. Then, TGLF turbulence modeling is carried out under systematic variations of the toroidal rotation and the core q-profile. Either strong negative central magnetic shear or rotation are found to successfully provide the turbulence suppression required to maintain the temperature and density profiles. [Preview Abstract] |
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