Bulletin of the American Physical Society
57th Annual Meeting of the APS Division of Plasma Physics
Volume 60, Number 19
Monday–Friday, November 16–20, 2015; Savannah, Georgia
Session BI2: PedestalsInvited

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Chair: Masayuki Ono, Princeton Plasma Physics Laboratory Room: Chatham Ballroom C 
Monday, November 16, 2015 9:30AM  10:00AM 
BI2.00001: Access to a New Super Hmode Regime By Manipulation of Pedestal Stability Invited Speaker: Wayne Solomon A physics understanding of constraints on the Hmode pedestal has enabled access to higher pedestal pressure on DIIID and the potential for more favorable scenarios for future devices. The pedestal height is limited due to coupled peelingballooning modes (PBMs) and the highest pressure consistent with PBM stability is obtained at the transition between the peeling and ballooning branch. When PBM and kinetic ballooning mode (KBM) constraints are coupled in the EPED pedestal model, the effect of shaping on the maximum pedestal pressure is amplified and can lead to a splitting of predicted pedestal solutions into an Hmode and ``Super Hmode'' (SH) root, where the SH root with higher and wider pedestal can be reached following a specific density trajectory. On DIIID, a theoryguided search for SHmode has resulted in pedestal heights twice that of regular Hmode at the same density, accessed by controlling the edge bootstrap current with increasing density. EPED calculations of the pedestal height versus density are in quantitative agreement with experiment. SHmode was first achieved with a Quiescent Hmode edge, enabling a smooth trajectory through pedestal parameter space. While elimination of ELMs is beneficial for SHmode, it may not be a requirement, as recent experiments maintained high pedestals with ELMs triggered by lithium granule injection. Experiments exploiting SHmode by coupling it with a high performance core have resulted in plasmas with Hmode confinement factors $>1.2$, normalized beta$\sim$3 and normalized pedestal beta twice that required for ITER. With higher pedestals, SHmode improves prospects for steadystate scenarios with high bootstrap fraction and increased ideal wall stability limit, and may simultaneously provide a solution to maintaining high confinement at high density. [Preview Abstract] 
Monday, November 16, 2015 10:00AM  10:30AM 
BI2.00002: Toward integrated multiscale simulations for a full ELM cycle with ELM dynamics Invited Speaker: Xueqiao Xu The highfidelity BOUT$++$ twofluid and GyroLandauFluid code suites have demonstrated significant recent progresses toward integrated multiscale simulations for a full ELM cycle with ELM dynamics. In order to improve the computational efficiency for a full ELM cycle with ELM dynamics, the basic set of dynamical equations has been separated into equations in the fluctuating and averaged parts over binormal direction. The two parts are advanced together in time but with different time steps, and dynamically exchange the turbulence fluxes and averaged profiles. Nonlinear ELM simulations show three stages of an ELM event: (1) a linear growing phase; (2) a fast crash phase; and (3) a slow inward propagation phase lasting until the core heating flux balances the ELM energy loss and the ELM is terminated. To better understand the interELM pedestal dynamics during the pedestal recovery, BOUT$++$ simulations started from a kinetic equilibrium reconstruction using measured plasma profiles from DIIID show that quasicoherent fluctuations (QCFs) can provide the necessary transport to limit and saturate the Hmode pedestal gradient. The simulations predict that (1) QCFs are localized in the pedestal region as observed on DIIID; (2) the QCFs are near marginal instability for ideal ballooning modes combined with driftAlfven wave modes; (3) the dominant mode is around n$=$15, k$_{\mathrm{\theta }}\rho_{\mathrm{i}}=$0.034, comparable to the measured value of 0.04; (4) the frequency of the mode is around 80kHz, close to that of the measured QCF; and (5) particle transport is smaller than the heat transport. BOUT$++$ simulations have also been performed to elucidate the nature and underlying physics mechanisms of the weaklyquasicoherent mode (WCM) with higher collisionality, which causes particle transport in Imode pedestals of Alcator CMod. Key simulation results are that (1) there is no ideal peelingballooning mode instability for the Imode studied; (2) a strong instability exists at n $\ge $ 20; (3) the mode propagates in the electron diamagnetic direction; (4) the predicted frequency of the n$=$20 mode agrees with the measured WCM peaking around 300kHz; (5) the predicted $\chi_{\mathrm{e}}$ agrees with the $\chi _{\mathrm{eff}}$ from the experiment; and (6) the predicted particle transport is larger than the predicted heat transport. [Preview Abstract] 
Monday, November 16, 2015 10:30AM  11:00AM 
BI2.00003: Direct evidence of stationary zonal flows and critical gradient behavior for E$_{r}$ during formation of the edge pedestal in JET* Invited Speaker: Jon Hillesheim High spatial resolution measurements with Doppler backscattering in JET have provided new insights into the development of the edge radial electric field during pedestal formation. The characteristics of E$_{r}$ have been studied as a function of density at 2.5 MA plasma current and 3 T toroidal magnetic field. We observe finescale spatial structure in the edge E$_{r}$ well prior to the LH transition, consistent with stationary zonal flows. Zonal flows are a fundamental mechanism for the saturation of turbulence and this is the first direct evidence of stationary zonal flows in a tokamak. The radial wavelength of the zonal flows systematically decreases with density. The zonal flows are clearest in Ohmic conditions, weaker in Lmode, and absent in Hmode. Measurements also show that after neutral beam heating is applied, the edge E$_{r}$ builds up at a constant gradient into the core during Lmode, at radii where E$_{r}$ is mainly due to toroidal velocity. The local stability of velocity shear driven turbulence, such as the parallel velocity gradient mode, will be assessed with gyrokinetic simulations. This critical E$_{r}$ shear persists across the LH transition into Hmode. Surprisingly, a \textit{reduction} in the apparent magnitude of the E$_{r}$ well depth is observed directly following the LH transition at high densities. Establishing the physics basis for the LH transition is important for projecting scalings to ITER and these observations challenge existing models based on increased E$_{r}$ shear or strong zonal flows as the trigger for the transition. \\[4pt] *This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 20142018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. [Preview Abstract] 
Monday, November 16, 2015 11:00AM  11:30AM 
BI2.00004: The Role of Nonlinear Interactions in Causing Transitions into Edge TransportBarrier Regimes Invited Speaker: Istvan Cziegler Transitions of tokamak confinement regimes are studied with a focus on interactions between turbulence and zonal flows (ZF) or geodesicacoustic modes (GAM). Results show that access to improved confinement regimes is profoundly affected by these interactions and clarify the role of GAM and ZF in different types of transitions. In order to understand the underlying dynamics of these transitions, both their trigger mechanism and the parametric dependence of nonlinear transfer processes are studied using gaspuffimaging. For the LtoH transition, this work shows that the stress mediated transfer rate of kinetic energy from turbulence into ZF leads in the changes, the turbulence collapses, and finally the pressure gradient forms  establishing the trigger as flow organization. For the Imode, turbulence is studied with the aim of understanding /emph{access} to the improved confinement regime, which exhibits an edge temperature pedestal, but a relaxed density profile. LtoI and ItoH transitions are analyzed in a timeresolved manner analogous to the LH transition. For the LtoI transition there is a difference between the scaling of the regime's typical edge fluctuation, the Weakly Coherent Mode (WCM), and GAM, known to be essential in shaping the WCM. Both the WCM and the GAM are necessary for the regime, and regime access is found to be sensitive to the GAM drive and damping. Parametric dependences of nonlinearities are examined in steady state discharges from a range of toroidal field, plasma current, and density; and interactions between flows and turbulence in both Lmode and Imode are estimated using bispectral methods. The ZF drive increases monotonically with crossfield heat flux, i.e. approaches a transition, while GAM follow more complicated trends. These results advance our progress toward predicting the parametric dependences of transition conditions. [Preview Abstract] 
Monday, November 16, 2015 11:30AM  12:00PM 
BI2.00005: Observation, Identification, and Impact of MultiModal Plasma Responses to Applied Magnetic Perturbations Invited Speaker: Nikolas Logan Experiments on DIIID have demonstrated that multiple kink modes with comparable amplitudes can be driven by applied nonaxisymmetric fields with toroidal mode number n=2, in good agreement with ideal MHD models. In contrast to a singlemode model [1], the structure of the response measured using poloidally distributed magnetic sensors changes when varying the applied poloidal spectrum [2]. This is most readily evident in that different spectra of applied fields can independently excite inboard and outboard magnetic responses, which are identified as distinct plasma modes by IPEC modeling. The outboard magnetic response is correlated with the plasma pressure and consistent with the long wavelength perturbations of the least stable, pressure driven kinks calculated by DCON and used in IPEC. The models show the structure of the pressure driven modes extends throughout the bad curvature region and into the plasma core. The inboard plasma response is correlated with the edge current profile and requires the inclusion of multiple kink modes with greater stability, including opposite helicity modes, to replicate the experimental observations in the models. IPEC reveals the resulting mode structure to be highly localized in the plasma edge. Scans of the applied spectrum show this response induces the transport that influences the density pumpout, as well as the toroidal rotation drag observed in experiment and modeled using PENT. The classification of these two mode types establishes a new multimodal paradigm for n=2 plasma response and guides the understanding needed to optimize 3D fields for independent control of stability and transport.\\[4pt] [1] N.C. Logan, et al., submitted to Nucl. Fusion (2015).\\[0pt] [2] C. PazSoldan et al., Phys. Rev. Lett. 114, 105001 (2015). [Preview Abstract] 
Monday, November 16, 2015 12:00PM  12:30PM 
BI2.00006: Gyrokinetic Simulations of the ITER Pedestal Invited Speaker: Mike Kotschenreuther It has been reported that low collisionality pedestals for JET parameters are strongly stable to Kinetic Ballooning Modes\footnote{S. Saarelma, M.N.A. Beurskens, D. Dickinson, et. al., Nucl. Fusion 53 123012 (2013)} (KBM), and it is, as simulations with GENE\footnote{genecode.org} show, the drifttearing modes that produce the pedestal transport.\footnote{D. Hatch, M. Kotschenreuther, et. al., 2015 US/EU Transport Task Force Workshop, Salem, MA April 2015} It would seem, then, that gyrokinetic simulations may be a powerful, perhaps, indispensable tool for probing the characteristics of the Hmode pedestal in ITER especially since projected ITER pedestals have the normalized gyroradius $\rho^*$ smaller than the range of present experimental investigation; they do lie, however, within the regime of validity of gyrokinetics. Since ExB shear becomes small as $\rho^*$ approaches zero, strong drift turbulence will eventually be excited. Finding an answer to the question whether the ITER $\rho^*$ is small enough to place it in the high turbulence regime compels serious investigation. We begin with MHD equilibria (including pedestal bootstrap current) constructed using VMEC. Plasma profile shapes, very close to JET experimental profiles, are scaled to values expected on ITER (e.g., a 4 keV pedestal). The equilibrium ExB shear is computed using a neoclassical formula for the radial electric field. As with JET, the ITER pedestal is found to be strongly stable to KBM. Preliminary nonlinear simulations with GENE show that the turbulent drift transport is strong for ITER; the electrostatic transport has a highly unfavorable scaling from JET to ITER, going from being highly subdominant to electromagnetic transport on JET, to dominant on ITER. At burning plasma parameters, pedestals in spherical tokamak Hmodes may have much stronger velocity shear, and hence more favorable transport; preliminary investigations will be reported. [Preview Abstract] 
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