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 BI1: Edge Pedestal Physics |
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Chair: Anne White, Massachusetts Institute of Technology Room: Acadia |
Monday, October 27, 2014 9:30AM - 10:00AM |
BI1.00001: The effects of impurities and core pressure on pedestal stability in JET and MAST Invited Speaker: Samuli Saarelma The H-mode pedestal plays an important role in determining global confinement in tokamaks. In high triangularity H-mode experiments in JET with the ITER-like wall (JET-ILW), plasma purity is improved, but both the pedestal temperature height and global confinement are degraded compared with JET plasmas with a carbon wall (JET-C). The JET-C performance can be partially recovered with nitrogen seeding. The stability calculations show that edge localised impurity concentration can increase the electron pressure pedestal at the point of marginal stability, mainly through ion dilution but also due to changes in the bootstrap current profile, whilst not degrading the total core pressure. The edge stability in low triangularity plasmas is improved less by the edge impurities due to a different shape of the stability boundary, consistent with a weak improvement in pedestal height with impurity seeding. Significantly better confinement and pedestal height has been observed in JET plasmas when the global pressure is increased. The enhanced pedestal height is linked to an improvement in edge stability arising from an increase in the Shafranov-shift. In MAST a 20\% increase in the global $\beta$ at the LH-transition led to a doubling of the electron pressure pedestal height just before the first ELM. The increase in global $\beta$ results in an increase in the marginally stable pressure gradient. This increases the attainable pedestal height, but also results in increased edge impurity content due to a longer first ELM period, which enhances the electron pressure pedestal height by ion dilution. The JET and MAST experiments and modelling show the importance of impurities and core pressure in determining the pedestal height. [Preview Abstract] |
Monday, October 27, 2014 10:00AM - 10:30AM |
BI1.00002: Edge Instabilities Limiting the Pedestal Evolution Invited Speaker: A. Diallo Identifying the transport mechanism and instabilities limiting pedestal properties and global confinement are essential to predict and control the performance of ITER and future fusion devices. Measurements of the edge density and magnetic fluctuations on the DIII-D and Alcator C-Mod tokamaks provide direct evidence for the onset of quasi-coherent edge fluctuations limiting the pedestal temperature recovery after an edge-localized-mode (ELM). These instabilities onset at the critical pressure gradient for kinetic ballooning mode (KBM) instabilities, which is consistent with predictions of EPED model. On both C-Mod and DIII-D, the low-k coherent fluctuations are observed having magnetic signatures, localized near the pedestal top. At low current on DIII-D these fluctuations are observed to correlate well with the density gradient recovery (measured with high temporal resolution) suggesting that particle transport is responsible for limiting the pedestal. At higher plasma current, the density gradient recovers on the same time scale as in the low current case. However, the temperature gradient increases until saturation, which suggests a different transport mechanism compared to the low current case. This plasma current dependence is consistent with changes of heat flux from the core needed to replenish the pedestal after an ELM crash. This paper reports detailed measurements of the pedestal recovery dynamics and associated edge fluctuations in two fusion devices, which clearly indicate that quasi-coherent edge fluctuations with magnetic signatures limit the temperature pedestal evolution. These new measurements as well as the recovery time of the pedestal strongly suggest that the pedestal temperature is a potential control knob, if acted on early in the recovery phase, for optimizing the pedestal in future fusion devices. [Preview Abstract] |
Monday, October 27, 2014 10:30AM - 11:00AM |
BI1.00003: Poloidal Asymmetries in Edge Transport Barriers Invited Speaker: R.M. Churchill Investigations of the poloidal structure within edge transport barriers on Alcator C-Mod using novel impurity measurements are presented, revealing large poloidal variations of parameters within a flux surface in the H-mode pedestal region, and significantly reduced poloidal variation in L-mode or I-mode pedestals. These measurements provide complete sets of impurity density, temperature, flow velocity, and electrostatic potential at both the low- and high-field side midplane, utilizing the Gas Puff-CXRS technique\footnote{RM Churchill, et al, RSI 84 (9), 093505 (2013)}. Uncertainties in magnetic equilibrium reconstructions require assumptions to be made in order to properly align the LFS/HFS profiles. In H-mode plasmas, if profiles are aligned assuming impurity temperature is constant on a flux surface, large potential asymmetries would result ($e \Delta{\Phi}/T_e \approx 0.6$). If instead total pressure is assumed constant on a flux-surface, then the measured potential asymmetry is significantly reduced, but large in-out asymmetries result in the impurity temperature (${>}1.7$x) \footnote{C Theiler, et al, Nucl. Fusion 54 083017 (2014)}. This shows that impurity temperature and potential can not both be flux functions in the pedestal region. In both alignment cases, large asymmetries in impurity density (${>}6$x) are present in H-mode plasmas\footnote{RM Churchill, et al,Nucl. Fusion 53 122002 (2013)}. In I-mode plasmas, which lack an electron density pedestal but do have a temperature pedestal, the poloidal variation of impurity temperature is weaker (${\sim}1.3$x) and the impurity density nearly symmetric between the LFS and HFS. These measurements indicate that the sharp gradients in the pedestal region, particularly of main ion density, have a significant effect on the poloidal and radial distribution of impurities, which could have important implications for the prediction of impurity contamination in future fusion reactors such as ITER. Estimates of particle and heat transport timescales suggest that the radial and parallel transport timescales are of the same order in the pedestal region of C-Mod, supporting the idea that two-dimensional transport effects need to be retained in impurity modeling of the pedestal region. To this end, initial comparisons to global neoclassical transport codes in the pedestal region\footnote{CS Chang, et al, PoP, 16, 056108 (2009)} will be presented. [Preview Abstract] |
Monday, October 27, 2014 11:00AM - 11:30AM |
BI1.00004: Impact of inward turbulence spreading on energy loss of edge-localized modes Invited Speaker: Chenhao Ma BOUT++ six-field Landau-fluid simulations show that an ELM crash has two phases: fast initial crash of ion temperature profile on the order of Alfven time scale near the peak gradient region and slow electron inward turbulence spreading from the ELM crash event. Both of them contribute to the ELM energy loss. However, the conducted ELM energy loss dominates over the convected ELM energy loss, which remains almost constant after the initial crash. The total ELM energy loss is mainly determined by the MHD turbulence spreading when the pedestal temperature height is large. The inward front propagation of electron temperature perturbation spreads into the linearly stable zone, while the ion perturbation front has much less spreading. The electron temperature fluctuation peaks on the rational surfaces and the front jumps gradually inwards towards neighboring rational surfaces. The electron wave-particle resonances via Landau closure provide a relatively strong parallel damping effect on the electron temperature perturbation and induce a large cross-phase shift of about $\pi/2$ angle between ExB velocity and the ion temperature, which yields almost no spreading for ion temperature and density fluctuation. When pedestal temperature height increases, the cross-phase shift of electron decreases and is close to $\pi/4$ angle which yields a large turbulence spreading and generates the large electron conducted energy loss. The front propagation stops at the position where the radial turbulent correlation length is shorter than the magnetic surface spacing. The energy burst of an ELM is controlled by the magnetic shear profile, the characteristic front propagating velocity and the turbulence correlation time. The inward turbulence spreading is mainly driven by (1) a series of micro-crashes due to a localized steepening of profile and (2) the magnetic flutter. The impact of other kinetic effects, such as full FLR effect and toroidal resonance, will be presented via simulations of a newly developed electro-magnetic Gyro-Landau-Fluid extension. [Preview Abstract] |
Monday, October 27, 2014 11:30AM - 12:00PM |
BI1.00005: 4D Fokker-Planck calculations of neoclassical effects in tokamak pedestals and stellarators Invited Speaker: Matt Landreman In this work, techniques are discussed for efficiently solving time-independent 4D (2 space + 2 velocity) linear drift-kinetic equations with Fokker-Planck collisions, illustrating several applications in stellarators and tokamaks. Conventional calculations of neoclassical flows, current, and fluxes can become inadequate in the tokamak pedestal if strong gradients violate the thin-orbit-width ordering, yet accurate calculation of these quantities is important for understanding edge stability and confinement. We have therefore developed a new radially global continuum neoclassical code PERFECT [1] which allows some radial scale lengths to be as small as the poloidal ion gyroradius. Using this tool, we demonstrate the first precise verification of recent analytic theory that accounts for finite orbit width effects in the limit of large aspect ratio [2]. The main-ion and impurity flows predicted by the finite-orbit-width calculations can have significantly different magnitude and poloidal variation compared to conventional (local) calculations. PERFECT solves 4D drift-kinetic equations for each species, using a preconditioned Krylov-space algorithm to accelerate solution of the time-independent problem. A related new code SFINCS implements these same algorithms but with a toroidal angle in place of the radial coordinate, enabling radially local calculations for stellarators and rippled tokamaks [3]. For W7-X and LHD scenarios, we use SFINCS to compare the common incompressible-$E\times B$ drift-kinetic equation to more accurate drift-kinetic equations. For radial electric fields below roughly 1/3 of the resonant value, the different kinetic equations lead to similar predictions for the fluxes and flows, whereas the results can differ significantly for larger electric fields. In SFINCS calculations for expected W7-X conditions, the various kinetic models predict similar levels of bootstrap current, improving confidence in past predictions for this quantity that strongly impacts the W7-X divertor.\\[4pt] [1] Landreman et al, PPCF 56, 045005 (2014).\\[0pt] [2] Catto et al, PPCF 55, 045009 (2013).\\[0pt] [3] Landreman et al, Phys Plasmas 21, 042503 (2014). [Preview Abstract] |
Monday, October 27, 2014 12:00PM - 12:30PM |
BI1.00006: The effects of weakly 3D equilibria on the stability and turbulent transport of tokamak pedestals Invited Speaker: Thomas Bird It has been widely demonstrated experimentally that externally applied resonant magnetic perturbations (RMPs) can substantially modify the properties of tokamak pedestals. In this work we demonstrate that perturbations of experimentally relevant magnitudes can have an O(1) effect on the marginal stability boundaries for ideal MHD ballooning modes and the heat flux driven by ITG instabilities. It has recently been established that RMPs can induce 3D flux surface deformations by exciting stable kink modes to finite amplitude, even when strong screening of the resonant field occurs. These deformations are usually only a few percent of the minor radius, but have a potent and generally destabilizing effect on pedestal stability. Local 3D equilibrium theory is employed to show how the deformations modify the normal curvature and local magnetic shear. We will show how helical Pfirsch-Schl\"{u}ter currents can strongly modulate the local magnetic shear near rational surfaces. The sensitivity of infinite-n ideal MHD ballooning stability to these deformation-induced effects is calculated as a proxy for the onset of KBM turbulence. The effect on ITG turbulence is assessed using a newly developed ``full-surface'' version of the GENE code that has been designed for 3D configurations. We will show how 3D deformations enhance the ion heat flux driven by ITGs, which may be able to explain recent measurements of the sensitivity of turbulent density fluctuations to RMP fields. A hypothesis for ELM suppression will be presented, based on strong helical local shear modulations causing enhanced transport near the pedestal top. Using M3D-C1 linear response calculations to model experimentally relevant tokamak equilibria, we will quantify the magnitude of the 3D effects as well as their sensitivity to various experimental knobs. [Preview Abstract] |
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