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 PO4: Stellarator and 3D Confinement Physics |
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Chair: David Gates, Princeton Plasma Physics Laboratory Room: 230 A |
Wednesday, November 2, 2016 2:00PM - 2:12PM |
PO4.00001: Global energy confinement in the first operational phase of Wendelstein 7-X Golo Fuchert, S.A. Bozhenkov, M. Beurskens, A. Dinklage, Y. Feng, J. Geiger, P. Helander, M. Hirsch, U. Hoefel, M.W. Jakubowski, J. Knauer, A. Langenberg, H.P. Laqua, H. Maassberg, D. Moseev, H. Niemann, D. Zhang, E. Pasch, K. Rahbarnia, T. Stange, H. Trimino Mora, J. Turkin, N. Pablant, G. Wurden, R.C. Wolf Wendelstein 7-X went into operation on Dec. 10th 2015 for a first operation phase (OP1.1), dedicated to testing and commissioning of device components and diagnostics before the uncooled test divertor is installed. Nevertheless, a first physics program could be conducted alongside the commissioning. One of the OP1.1 activities was the determination of the energy content and confinement time of the plasma, which were estimated independently from profile measurements and data from a diamagnetic loop. The results were combined with radiation losses and limiter heat fluxes to a global power balance, where the combined losses match the heating power within 10-40 {\%}. This gives confidence that reasonable experimental data is available for OP1.1. Typical energy confinement times were found to be roughly between 100 and 150 ms. These values are consistent with the ISS04 scaling and show power degradation and strong dependence on the radiated power. Clear signs of stellarator optimization were neither expected nor observed in OP1.1 due to the neoclassical $\surd \nu $ -transport in the high Te-low density core plasma. [Preview Abstract] |
Wednesday, November 2, 2016 2:12PM - 2:24PM |
PO4.00002: Helical self-organization in the reversed-field pinch: detection of barriers to transport. Marco Veranda, Daniele Bonfiglio, Susanna Cappello, Dominique Escande, Fulvio Auriemma, Dario Borgogno, Luis Chac\'{o}n, Daniela Grasso, Giulio Rubino, Alessandro Fassina, Paolo Franz, Marco Gobbin, Maria Ester Puiatti Self-organized helical states are an ubiquitous feature in astrophysical and magnetic fusion plasmas. In the reversed-field pinch plasmas quasi-helical states are observed both in high current experiments [1] and in nonlinear magnetofluid numerical simulations [2]. We show the 3D nonlinear MHD prediction that small edge helical magnetic perturbations can shape the whole plasma column with their helical twist [3], a result confirmed by dedicated experiments in RFX-mod. When a non-resonant twist is imposed chaos healing results in a higher area of conserved magnetic surfaces and in reduced diffusion of magnetic field lines. Barriers to the transport of magnetic field lines are detected: they can be identified as cantori of the magnetic field [4], and can explain the observation of electron temperature transport barriers in experiments [5]. 1 - R.Lorenzini, E.Martines, et al, Nature Physics \textbf{5}, 570 (2009) 2 - D.Bonfiglio, M.Veranda, et al, PRL \textbf{111}, 085002 (2013) 3 - M.Veranda, D.Bonfiglio, et al, PPCF \textbf{55}, 074015 (2013) 4 - G.Rubino, D.Borgogno, et al, NF \textbf{57}, 085004 (2015) 5 - M.Gobbin, D.Bonfiglio, et al, PRL \textbf{106}, 025001 (2011) [Preview Abstract] |
Wednesday, November 2, 2016 2:24PM - 2:36PM |
PO4.00003: Integrated Simulation Studies of Plasma Performances and Fusion Reactions in the Deuterium Experiment of LHD S Murakami, H Yamaguchi, M Homma, S Maeta, Y Saito, A Fukuyama, K Nagaoka, H Takahashi, H Nakano, M Osakabe, M Yokoyama, K Tanaka, K Ida, M Yoshinuma, M Isobe, H Tomita, K Ogawa The deuterium experiment project from 2017 is planned in LHD, where the deuterium NBI heating beams with the power more than 30MW are injected into the deuterium plasma. Principal objects of this project are to clarify the isotope effect on the heat and particle transport in the helical plasma and to study energetic particle confinement in a helical magnetic configuration measuring triton burn-up neutrons. We study the deuterium experiment plasma of LHD applying the integrated simulation code, TASK3D [Murakami, PPCF2015], and the 5-D drift kinetic equation solver, GNET [Murakami, NF2006]. (i) More than 20\% of ion temperature increment is obtained in the deuterium plasma ($n_{D}/n_{H}+n_{D}=0.8$) due to the isotope effect assuming the turbulent transport model based on the H/He plasma experiment of LHD. (ii) The triton burn-up simulation shows the triton slowing down distribution and the strong magnetic configuration dependency of the triton burn-up ratio in LHD. [Preview Abstract] |
Wednesday, November 2, 2016 2:36PM - 2:48PM |
PO4.00004: Characterization of error fields and their impacts on Wendelstein 7-X Samuel Lazerson, Matthias Otte, Marcin Jakubowski, Glen Wurden, Uwe Wenzel, Tamara Andreeva, Sergey Bozhenkov, Christoph Biedermann, Gábor Kocsis, Tamás Szepesi, Joachim Geiger, Thomas Sunn Pedersen, David Gates The measurement and correction of magnetic fields which destroy flux surfaces (error fields) is critical to long pulse high beta operation in Wendelstein 7-X (W7-X). Even small error fields are predicted to cause overloading of divertor plates, as well as the loss of confinement. In the early commissioning phase of the experiment, the trim coils were used to open an n/m=1/2 island chain in a specially designed magnetic configuration. The flux surfacing mapping diagnostic was then able to directly image the magnetic topology of the experiment, allowing the inference of a small 4 cm intrinsic island chain. Scaled to the planned operating field (2.5 T), such error fields would be correctable using less than 10\% the rated trim coil capacity. Numerical modeling confirms that a slight misalignment of the superconducting stellarator coil set is the source of this error field. Observations of the limiters temperatures in module 5 show a clear dependence of the limiter heat flux pattern as the perturbing n=1 fields are rotated. Thus the sources of error fields in W7-X are quantified and demonstrated to be correctable by the trim coil system. [Preview Abstract] |
Wednesday, November 2, 2016 2:48PM - 3:00PM |
PO4.00005: Inverse methods for stellarator error-fields and emission K. C. Hammond, A. Anichowski, P. W. Brenner, R. Diaz-Pacheco, F. A. Volpe, Y. Wei, Y. Kornbluth, T. S. Pedersen, S. Raftopoulos, P. Traverso Work at the CNT stellarator at Columbia University has resulted in the development of two inverse diagnosis techniques that infer difficult-to-measure properties from simpler measurements. First, CNT's error-field is determined using a Newton-Raphson algorithm to infer coil misalignments based on measurements of flux surfaces. This is obtained by reconciling the computed flux surfaces (a function of coil misalignments) with the measured flux surfaces [1]. Second, the plasma emissivity profile is determined based on a single CCD camera image using an onion-peeling method. This approach posits a system of linear equations relating pixel brightness to emission from a discrete set of plasma layers bounded by flux surfaces [2]. Results for both of these techniques as applied to CNT will be shown, and their applicability to large modular coil stellarators will be discussed. [1] K.~C.~Hammond \textit{et al.}, Plasma Phys.~Control.~Fusion \textbf{58}, 074002 (2016) [2] K.~C.~Hammond \textit{et al.}, Rev.~Sci.~Instrum., \textbf{87}, 11E119 (2016) [Preview Abstract] |
Wednesday, November 2, 2016 3:00PM - 3:12PM |
PO4.00006: Measurements of the 3D boundary distortion due to external n=2 magnetic perturbations in comparison to ideal MHD Matthias Willensdorfer, Wolfgang Suttrop, Erika Strumberger, Hartmut Zohm, Francois Orain, Andrew Kirk, David Ryan Best ELM mitigation/suppression at DIII-D and AUG are achieved by external magnetic perturbation (MP) fields, when the applied poloidal mode spectrum is aligned with the mode (kink) at the edge that is most strongly amplified by the plasma. This kink mode causes a 3D displacement of the plasma boundary, which is characterized at AUG using data from toroidally localized high resolution diagnostics and rigid rotating MP-fields with different applied poloidal mode spectra. Various profile and imaging diagnostics, e.g.~electron cyclotron emission (ECE), are used to determine the amplitude, the penetration and the poloidal mode structure of the displacement around the outer midplane. The displacement around the X-point/plasma top, which is related to ELM mitigation, is measured using a new steerable ECE and SOFT X-ray. These measurements are compared to MHD codes like JOREK, MARS-F and VMEC. As predicted by MHD, the measured amplitudes clearly exceed the vacuum field calculations. The displacement measured by imaging ECE indicates a resonant response, although the calculated magnetic structure of this edge kink peaks at poloidal mode numbers larger than the resonant components. [Preview Abstract] |
Wednesday, November 2, 2016 3:12PM - 3:24PM |
PO4.00007: Numerical analysis of the heat flux characteristics in the helical scrape-off layer of the limiter start up configuration at Wendelstein 7-X Florian Effenberg, Y. Feng, O. Schmitz, H. Frerichs, T. Barbui, J. Geiger, M. Jakubowski, R. Koenig, M. Krychowiak, H. Niemann, T.S. Pedersen, L. Stephey, G. Wurden A crucial topic for the stellarator W7-X is the power dissipation by impurities for future island divertor scenarios. The investigation of the related heat flux distribution and profiles including the radial power fall-off length $\lambda_q$ in the 3D stellarator SOL is less straight forward as in toroidally symmetric tokamaks. Studies with the 3D plasma edge transport code EMC3-Eirene predicted a modulation of plasma parameters with $L_C$ and correlated heterogenous heat and particle loads onto the limiters during start-up operation. The relative simple start up geometry at W7-X allows for a detailed analysis of the heat fluxes in separate helical transport channels featuring different $\parallel$ to $\perp$ transport ratios. It is shown that the SOL has two characteristic fall off domains - a near SOL and a far SOL domain which both have different power decay lengths. An increase of $\lambda_q$ with $L_C$ in the order of ~1-1.5cm in the near SOL and ~1.8-2.8cm in the far SOL for a power scan in the range of $P=$0.5-2MW at $n_{LCFS}=2\times$10$^{18}$m$^{-3}$ has been found. First comparisons with IR camera data will be discussed. [Preview Abstract] |
Wednesday, November 2, 2016 3:24PM - 3:36PM |
PO4.00008: Nonaxisymmetric modelling in BOUT++; toward global edge fluid turbulence in stellarators Jarrod Leddy, Brendan Shanahan, Peter Hill, Ben Dudson As Wendelstein 7-X has been optimized for neoclassical transport, turbulent transport could potentially become comparable to neoclassical losses. Furthermore, the imminent installation of an island divertor merits global edge modelling to determine heat flux profiles and the efficacy of the system. Currently, however, nonaxisymmetric edge plasma modelling is limited to either steady state (non-turbulent) transport modelling, or computationally expensive gyrokinetics. The implementation of the Flux Coordinate Independent (FCI) approach to parallel derivatives has allowed the extension of the BOUT++ edge fluid turbulence framework to nonaxisymmetric geometries. Here we first investigate the implementation of the FCI method in BOUT++ by modelling diffusion equations in nonaxisymmetric geometries with and without boundary interaction, and quantify the inherent error. We then present the results of non-turbulent transport modelling and compare with analytical theory. The ongoing extension of BOUT++ to nonaxisymmetric configurations, and the prospects of stellarator edge fluid turbulence simulations will be discussed. [Preview Abstract] |
Wednesday, November 2, 2016 3:36PM - 3:48PM |
PO4.00009: Turbulence investigations at Wendelstein 7-X Olaf Grulke, Thomas Windisch, Gavin Weir, Ralf Kleiber, Josefine Proll, Pavlos Xanthopoulos A key design aspect of the Wendelstein 7-X stellarator, which went into operation in 2015, has been its optimization with respect to MHD stability and neoclassical transport. Based on fundamental theoretical investigations and numerical simulation results, it is evident that the neoclassically optimized geometry impacts the evolution of turbulence, potentially reducing also the anomalous transport. The experimental verification of those results is, however, challenging due to the expected poloidal localization of fluctuations and toroidal ballooning effects. This paper presents initial results obtained in the limiter configuration of the first operation phase. For the upcoming operation phase, the full island divertor configuration will be realized and the capabilities of fluctuation diagnostics at Wendelstein 7-X will be greatly enhanced. They cover different regions in radial, poloidal and toroidal space and allow for a more detailed connection to the simulation results. The details of the diagnostics layout are presented and the priorities of the experimental program are highlighted.\\ O. Grulke, T. Windisch, G. Weir, R. Kleiber, J. Proll, P. X [Preview Abstract] |
Wednesday, November 2, 2016 3:48PM - 4:00PM |
PO4.00010: Gyrokinetic Studies of Resonant Magnetic Perturbation Effect on Microturbulence in DIII-D H-Mode Pedestal Ihor Holod, Zhihong Lin, Sam Taimourzadeh, Raffi Nazikian, Donald Spong, Andreas Wingen Vacuum Resonant Magnetic Perturbation (RMP) applied to otherwise axisymmetric plasmas for the purpose of ELM mitigation produce in general a combination of non resonant effects preserving closed flux surfaces (kink response) and resonant effects that introduce magnetic islands. The effect of the plasma kink response on the stability and transport of edge turbulence is studied using the gyrokinetic code GTC for a DIII-D discharge with applied n=2 vacuum RMP. Three reference equilibria were modeled using VMEC code, based on DIII-D shot 158103: axisymmetric (no RMP) equilibrium, n=2 RMP, and artificially amplified RMPx10 equilibria. Gyrokinetic simulations reveal no increase of growth rates for electrostatic driftwave instability and electromagnetic kinetic-ballooning mode in the presence of the RMP. The effect of RMP on zonal flow damping is found to be insufficient to modify turbulent transport. Therefore, the plasma kink response to the RMP cannot account for the change in the turbulence level seen in experiments with suppressed ELMs. These results demonstrate that other physics must be controlling the transition in confinement responsible for ELM suppression. [Preview Abstract] |
Wednesday, November 2, 2016 4:00PM - 4:12PM |
PO4.00011: Characterization of Expected Operating Conditions of the HIDRA Stellarator with EMC3-EIRENE Steven Marcinko, Davide Curreli The HIDRA stellarator (formerly WEGA) at the University of Illinois at Urbana-Champaign has begun operation in partially-ionized conditions. Using the coupled EMC3-EIRENE code the 3D edge plasma region may be self-consistently simulated including the effects of neutrals and impurities. By including Bohm-like cross-field diffusivity into EMC3, valid for classical stellarator designs like HIDRA, a preliminary consideration of expected operating conditions in HIDRA’s magnetic geometry may be presented. Axial magnetic field strengths between 87.5 mT and 0.5 T are considered with whole-device power levels scanned from 26 kW to 250 kW. An ITER-sized divertor tile is considered as a limiter positioned around the five-fold symmetry of the device with inboard, outboard, and 'trench' poloidal orientations. Divertor-relevant materials including carbon, lithium, and molybdenum have been simulated. Particle and energy confinement times, impurity and neutral energy sinks, and resulting particle and heat fluxes incident on potential limiter geometries are presented. [Preview Abstract] |
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