Bulletin of the American Physical Society
59th Annual Meeting of the APS Division of Plasma Physics
Volume 62, Number 12
Monday–Friday, October 23–27, 2017; Milwaukee, Wisconsin
Session BI3: Gyrokinetics and Plasma Turbulence |
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Chair: Christopher Holland, University of California, San Diego Room: 103ABC |
Monday, October 23, 2017 9:30AM - 10:00AM |
BI3.00001: Experimental investigations of turbulent temperature fluctuations and phase angles in ASDEX Upgrade. Invited Speaker: Simon Freethy A complete experimental understanding of the turbulent fluctuations in tokamak plasmas is essential for providing confidence in the extrapolation of heat transport models to future experimental devices and reactors.~~Guided by~``predict first'' nonlinear gyrokinetic simulations with the GENE code, two new turbulence diagnostics were~designed and have been installed on ASDEX Upgrade (AUG) to probe the fundamentals of ion-scale turbulent electron heat transport.~ The first, a 30-channel correlation ECE (CECE) radiometer, measures radial profiles (0.5 \textless r/a \textless 0.8) of low-k (ktheta rhos \textless 0.3) temperature fluctuations as well as frequency spectra and radial correlation lengths in unprecedented detail in both L- and H-mode. Typical L-mode levels are in the range 0.3 -- 0.8{\%}. The second is formed by the addition of a reflectometer on the same line of sight to enable measurements of the phase angle between turbulent density and temperature fluctuations. Design predictions are followed by a more traditional ``post-diction'' validation study with GENE. Using a cutting edge synthetic diagnostic GENE shows a factor 1.6 - 2 over-prediction of the fluctuation amplitude, while matching both ion and electron heat fluxes within experimental error.~Detailed sensitivity scans are underway to understand the robustness of this disagreement and a detailed assessment of the experimental errors has been carried out. The discrepancy opens questions about the~role of multi-scale turbulence~physics, but also indicates the need for the comparison of more experimental turbulence properties to have a more complete validation hierarchy. In an effort to understand the discrepancy, \textit{predictions} of~the nT-phase and the radial correlation length have been made along with an assessment of their sensitivity to experimental errors. Comparison to experimental measurements will be discussed. [Preview Abstract] |
Monday, October 23, 2017 10:00AM - 10:30AM |
BI3.00002: Recent gyrokinetic turbulence insights with GENE and direct comparison with experimental measurements Invited Speaker: Tobias Goerler Throughout the last years direct comparisons between gyrokinetic turbulence simulations and experimental measurements have been intensified substantially. Such studies are largely motivated by the urgent need for reliable transport predictions for future burning plasma devices and the associated necessity for validating the numerical tools. On the other hand, they can be helpful to assess the way a particular diagnostic experiences turbulence and provide ideas for further optimization and the physics that may not yet be accessible. Here, synthetic diagnostics, i.e. models that mimic the spatial and sometimes temporal response of the experimental diagnostic, play an important role. In the contribution at hand, we focus on recent gyrokinetic GENE simulations dedicated to ASDEX Upgrade L-mode plasmas and comparison with various turbulence measurements. Particular emphasis will be given to density fluctuation spectra which are experimentally accessible via Doppler reflectometry. A sophisticated synthetic diagnostic involving a fullwave code has recently been established and solves the long-lasting question on different spectral roll-overs in gyrokinetic and measured spectra as well as the potentially different power laws in the O- and X-mode signals. The demonstrated agreement furthermore extends the validation data base deep into spectral space and confirms a proper coverage of the turbulence cascade physics. The flux-matched GENE simulations are then used to study the sensitivity of the latter to the main microinstability drive and investigate the energetics at the various scales. Additionally, electron scale turbulence based modifications of the high-k power law spectra in such plasmas will be presented and their visibility in measurable signals be discussed. [Preview Abstract] |
Monday, October 23, 2017 10:30AM - 11:00AM |
BI3.00003: Non-linear isotope and fast ions effects: routes for low turbulence in DT plasmas Invited Speaker: Jeronimo Garcia The isotope effect, i.e. the fact that heat and particle fluxes do not follow the expected Gyro-Bohm estimate for turbulent transport when the plasma mass is changed, is one of the main challenges in plasma theory. Of particular interest is the isotope exchange between the fusion of deuterium (DD) and deuterium-tritium (DT) nuclei as there are no clear indications of what kind of transport difference can be expected in burning plasmas. The GENE code [1] is therefore used for computing DD vs DT linear and nonlinear microturbulence characteristics in the core plasma region of a previously ITER hybrid scenario at high beta obtained in the framework of simplified integrated modelling. Scans on common turbulence related quantitates as external ExB flow shear, Parallel Velocity Gradient (PVG), plasma beta, colisionality or the number of ion species have been performed. Additionally, the role of energetic particles, known to reduce Ion Temperature Gradient (ITG) turbulence has been also addressed [2,3]. It is obtained that the ITER operational point will be close to threshold and in these conditions turbulence is dominated by ITG modes. A purely weak non-linear isotope effect, absent in linear scans, can be found when separately adding moderate ExB flow shear or electromagnetic effects, whereas collisionality just modulates the intensity. The isotope effect, on the other hand, becomes very strong in conditions with simultaneously moderate ExB flow shear, beta and low q profile with significant reductions of ion heat transport from DD to DT [3]. By analyzing the radial structure of the two point electrostatic potential correlation function it has been found that the inherent Gyro-Bohm scaling for plasma microturbulence, which increases the radial correlation length at short scales form DD to DT, is counteracted by the concomitant appearance of a complex nonlinear multiscale space interaction involving external ExB flow shear, zonal flow activity, magnetic geometry and electromagnetic effects. The number of ion species and the fast ion population is also found to play a role in this non-linear process whereas a symmetry breaking between D and T, with systematic reduced heat and particle transport for T, is always obtained. [1] F. Jenko, W. Dorland, M. Kotschenreuther and B.N. Rogers, Phys. Plasmas 7, 1904 (2000). [2] J. Citrin et al., Phys. Rev. Lett. 111, 155001 (2013). J. Garcia et al., Nucl. Fusion 55, 053007 (2015). [3] J. Garcia et al., Nucl. Fusion 57, 014007 (2017) [Preview Abstract] |
Monday, October 23, 2017 11:00AM - 11:30AM |
BI3.00004: Transport Barriers in Bootstrap Driven Tokamaks Invited Speaker: Gary Staebler Maximizing the bootstrap current in a tokamak, so that it drives a high fraction of the total current, reduces the external power required to drive current by other means. Improved energy confinement, relative to empirical scaling laws, enables a reactor to more fully take advantage of the bootstrap driven tokamak. Experiments have demonstrated improved energy confinement due to the spontaneous formation of an internal transport barrier in high bootstrap fraction discharges. Gyrokinetic analysis, and quasilinear predictive modeling, demonstrates that the observed transport barrier is due to the suppression of turbulence primarily due to the large Shafranov shift. ExB velocity shear does not play a significant role in the transport barrier due to the high safety factor. It will be shown, that the Shafranov shift can produce a bifurcation to improved confinement in regions of positive magnetic shear or a continuous reduction in transport for weak or negative magnetic shear. Operation at high safety factor lowers the pressure gradient threshold for the Shafranov shift driven barrier formation. The ion energy transport is reduced to neoclassical and electron energy and particle transport is reduced, but still turbulent, within the barrier. Deeper into the plasma, very large levels of electron transport are observed. The observed electron temperature profile is shown to be close to the threshold for the electron temperature gradient (ETG) mode. A large ETG driven energy transport is qualitatively consistent with recent multi-scale gyrokinetic simulations showing that reducing the ion scale turbulence can lead to large increase in the electron scale transport. A new saturation model for the quasilinear TGLF transport code, that fits these multi-scale gyrokinetic simulations, can match the data if the impact of zonal flow mixing on the ETG modes is reduced at high safety factor. [Preview Abstract] |
Monday, October 23, 2017 11:30AM - 12:00PM |
BI3.00005: Multiscale Full Kinetics as an Alternative to Gyrokinetics Invited Speaker: Scott Parker Gyrokinetics has been extremely successful for modeling low frequency well-magnetized plasmas. However, for many plasmas, the expansion parameters in gyrokinetic theory are not so small. Until now, gyrokinetics was the only useful kinetic simulation model available for low frequency and weakly unstable plasmas even when its validity was questionable. Here, we report a new simulation model with full kinetic ions (Lorentz force dynamics) using implicit multi-scale techniques. This is the first six-dimensional model to accurately capture low-frequency physics, including finite Larmor radius (FLR) effects and weak gradient drive drift wave-type instabilities, operating comfortably within domain of gyrokinetics. Such a model allows for verification of gyrokinetics and can help identify the relative importance of higher order terms in gyrokinetic theory. Here we present full kinetic simulations of the toroidal ion-temperature-gradient (ITG) instability in tokamak plasma geometry. We will discuss the orbit averaging and sub-cycling techniques as well as the implicit variational integrator for the particle trajectories necessary to preserve adiabatic invariants. Results comparing the full kinetic model with gyrokinetics are reported. In slab geometry, excellent agreement is obtained linearly and nonlinearly including full FLR effects. High-frequency Ion Bernstein waves, which are present in the full kinetic model can easily be suppressed with the implicit time advance while maintaining FLR effects. The fully kinetic toroidal model uses field-line-following coordinates for the field quantities providing well-resolved field-aligned mode structure. Benchmarks with gyrokinetics within the domain of validity of gyrokinetics show excellent agreement. Nonlinear ITG simulations in slab geometry with both gyrokinetics and full kinetics will be compared in more marginal steep gradient regimes. Reference: ``An Implicit Delta-f Particle-in-Cell Method with Sub-Cycling and Orbit Averaging for Lorentz Ions’’, B.J. Sturdevant, S.E. Parker, Y. Chen, and B. Hause, J. Comput. Phys., {\bf 316} 519 (2016). [Preview Abstract] |
Monday, October 23, 2017 12:00PM - 12:30PM |
BI3.00006: Physics of thermal transport and increased electron temperature turbulence in the edge pedestal of ELM-free, H-mode regimes on DIII-D Invited Speaker: Choongki Sung It has been observed, for the first time, that suppression of Edge Localized Modes (ELMs) in tokamak plasmas is accompanied by an increase in electron temperature turbulence. A correlation electron cyclotron emission technique has been utilized to quantify the observed increase: 40$\%$ increase in Quiescent H-mode (QH-mode) and 70$\%$ increase in 3D field ELM suppressed H-mode. Since reliable ELM-free H-mode operation is essential for future burning plasma experiments, it is crucial to develop a validated predictive capability for these plasmas. Linear stability analysis using TGLF has provided an explanation for the observations and has indicated that the underlying physical mechanisms are different in the two regimes. In QH-mode, profile gradients and the associated linear growth rate are decreased compared to ELMing H-mode. However, the ExB shearing rate is reduced by an even greater factor such that turbulent transport is no longer suppressed by flow shear. In contrast, during 3D field ELM suppressed H-mode, gradients are increased and TGLF predicts that a large increase in linear growth rate is primarily responsible for the increased turbulence. Power balance analysis using ONETWO is also consistent with the changes in electron thermal transport being due to the increased turbulence. These new findings are significant since they i) provide a physics explanation of these changes via TGLF analysis and enable validation of the model in the key pedestal region, and ii) support the hypothesis that turbulent transport partially replaces ELM-dominated transport during ELM-free operation. These results form a basis to develop a predictive understanding of pedestal regulation in ELM suppressed regimes. [Preview Abstract] |
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