Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Plasma Physics
Volume 66, Number 13
Monday–Friday, November 8–12, 2021; Pittsburgh, PA
Session CO07: MFE: Turbulence and Transport IOn Demand
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Chair: Ian Abel, IREAP, University of Maryland, College Park Room: Rooms 315-316 |
Monday, November 8, 2021 2:00PM - 2:24PM |
CO07.00001: Impurity Transport Experiments at the HSX Stellarator J. Fernando F Castillo, Benedikt Geiger, Aaron Bader, Santhosh T Kumar, Konstantin M Likin, David T Anderson The laser blow-off technique is used to inject aluminum atoms into the confined region of HSX. To study the radial propagation of the injected impurities, photodiode arrays take time-resolved measurements of the impurity radiation. The spatially one-dimensional impurity transport code STRAHL is used to calculate a time-dependent plasma emissivity profile. A synthetic diagnostic code then integrates the modeled emissivity profile along the experimental lines-of-sight, providing simulated intensity signals. An optimization algorithm with the impurity diffusivity (D) acting as free parameters fits the model to experimental data. Results show D values two orders of magnitude more than neoclassically-predicted calculations from the PENTA code. For plasmas with an absorbed ECH power (P) of ~11.3 kW, the average impurity confinement time (τ) is ~1.3 ms while an average τ of 3.0 ms is observed when reducing P to 6.3 kW. A systematic study of the power shows a τ ∝ P-1 dependence, similar to the ISSO4 scaling. Thus, neoclassical diffusion alone is insufficient to explain these results and suggest a substantial impact of turbulence on the impurity confinement. |
Monday, November 8, 2021 2:24PM - 2:48PM |
CO07.00002: Experimental Inference and Modeling of Particle Transport in Alcator C-Mod and DIII-D Regimes without ELMs Francesco Sciortino, Nathan T Howard, Earl S Marmar, Igor Bykov, Colin Chrystal, Adam R Foster, Shaun R Haskey, Jerry W Hughes, Jeremy D Lore, Alessandro Marinoni, Saskia Mordijck, Tomas Odstrcil, Thomas Pütterich, Richard M Reksoatmodjo, Matthew L Reinke, John E Rice, Kathreen E Thome We present inferences of radial profiles of experimental impurity transport coefficients following laser blow-off (LBO) injections on both DIII-D and Alcator C-Mod. In most cases, we find agreement in diffusion profiles to within a factor of 2 with respect to NEO, TGLF and nonlinear CGYRO predictions, but significant discrepancies when hollow impurity profiles are experimentally observed. This suggests that turbulent transport models may be missing critical physics. We make use of an advanced Bayesian framework, leveraging a range of spectroscopic diagnostics and recent development of the Aurora package. Our investigation spans regimes without ELMs on both devices. On C-Mod, a novel forward model for the entire K-alpha spectrum of calcium has been combined with VUV spectroscopy. On DIII-D, analogous VUV measurements complement local CER density estimates for both intrinsic C and LBO-injected impurities. In all cases, the effect of charge exchange between background neutrals and impurities is extremely important for charge state balance in the pedestal, as indicated by C-Mod experimental D Lyman-alpha midplane measurements and SOLPS-ITER modeling. |
Monday, November 8, 2021 2:48PM - 3:00PM |
CO07.00003: Energy confinement in ohmic discharges in H, D and T on JET-ILW Ephrem Delabie, M.M.F. Nave, Pablo Rodriguez-Fernandez, Bart Lomanowski, Morten Lennholm The energy confinement of ohmic discharges is investigated in JET-ILW using discharges with steady density plateaus and neutral beam blips to obtain accurate profiles of the intrinsic rotation and ion temperature using main ion charge exchange spectroscopy. A comparison between matched hydrogen and deuterium pulses has experimentally demonstrated a shift from electron to ion dominated transport at the LOC-SOC transition and a reduction in energy confinement with reduced isotope mass. The latter is largely due to an increase in electron-ion equipartition power at lower isotope mass, but when accounting for the difference in heat flux at matched density we still find a higher effective ion heat diffusivity at lower isotope mass. |
Monday, November 8, 2021 3:00PM - 3:12PM |
CO07.00004: Investigation of Charge Dependency in impurity transport in Aditya-U tokamak Nandini Yadava, Joydeep Ghosh, Malay B Chowdhuri, Ashoke De, Amrita Bhattacharya, Ranjana Manchanda, Nilam Ramaiya, Kajal Shah, Sharvil Patel, Shishir Purohit, Manoj Gupta, Umesh Nagora, Surya Pathak, Tanmay Macwan, Kumarpalsinh A Jadeja, Kaushal M Patel, Ankur Pandya, Rohit Kumar, Suman Aich, Kaushlender Singh, Suman Dolui Impurity transport in tokamak mostly anomalous, driven by fluctuations in plasma parameters. However, depending upon machine parameters and plasma conditions, transport can follow neo-classical theory. In neo-classical transport, diffusion coefficient in classical and Pfirsch-Schlüter regimes has no Z dependence, while in Banana-Plateau regime shows weak dependence on Z. Also, turbulent diffusion coefficient remains primarily independent of Z. In Aditya-U tokamak, along with intrinsic low Z impurities (C, O), many low and medium Z impurities (He, Ne, Ar) are introduced via gas puffing and wall coating (Li) into the plasma for different diagnostic and operation purposes. Then, transport studies for different Z present in Aditya-U are interesting due to the fact that Z dependencies of diffusion coefficients vary with machines. For this purpose, the experimental brightness profile of spectral lines 464.7 nm (C2+), 650 nm (O4+) and 377.7 nm (Ne1+) are measured with 1m long multi-track spectrometer. An indigenously developed impurity transport code by using semi-implicit numerical method is modeled experimental profiles to determine the transport coefficients of these impurity ions and a comparison is done to understand the charge dependency of impurity transport in tokamak plasma. |
Monday, November 8, 2021 3:12PM - 3:24PM |
CO07.00005: The Study of Multi-Z Impurity Transport in DIII-D ITER Similar Shape Discharges Nathan T Howard, Tomas Odstrcil, Brian A Grierson, Francesco Sciortino, Tyler Abrams, Alessandro Bortolon, Edward T Hinson, Filippo Scotti The diffusion and convection of impurities arising from neoclassical and turbulent transport mechanisms has been studied in a set of DIII-D discharges operated with the ITER shape, RMP ELM suppression, and approximately equal ion and electron heat loss (Qi~Qe). Neoclassical and nonlinear, flux-matched (Qi, Qe, Ge) gyrokinetic simulations (NEO and CGYRO) were performed from the plasma axis to near the pedestal top (rho = 0.0 – 0.8) to assess the relative impact of collisions and turbulence on the transport of 7 impurities (He, Li, C, F, Al, Ca, and W). Inside of rho = 0.45 simulation predicts a negative correlation of diffusion with Z, in contrast to a strong positive correlation found outside of this region. Linear simulations suggest these trends may be linked to the dominant local instabilities (ITG/TEM) but the physical origin is the subject of ongoing investigations. We will present a summary of the experiments, gyrokinetic and neoclassical simulations, and comparisons with experimentally determined impurity transport in the target discharges. |
Monday, November 8, 2021 3:24PM - 3:36PM |
CO07.00006: Gyrokinetic modeling of electron temperature gradient turbulence in a tokamak pedestal Justin Walker, David R Hatch In H-mode tokamak plasmas, transport barriers in the pedestal allow for better confinement and heating of core plasmas. Additionally, the steep gradients of the pedestal region provide a source of free energy to drive microscale instabilities and turbulence. Understanding the operation of these instabilities and their saturation in the nonlinear regime is key to discovering better performing equilibria. |
Monday, November 8, 2021 3:36PM - 3:48PM |
CO07.00007: Non-Ballooning Electron Temperature Gradient Turbulence in the Tokamak Pedestal Jason F Parisi, Felix I Parra, Colin M Roach, Michael R Hardman, Michael Barnes, William D Dorland, Denis A St-Onge, Justin Ball, David R Hatch, David Dickinson, Samuli Saarelma, Benjamin Chapman, Carine Giroud, Jon C Hillesheim, Noboyuki Aiba Tokamak pedestal electron temperature gradient (ETG) turbulence has a complex three-dimensional structure that is highly inhomogeneous in the poloidal direction, particularly at ion scales. Nonlinear multiscale gyrokinetic simulations of Joint European Torus pedestals reveal that ETG pedestal turbulence has maximum fluctuation amplitudes and transport near the top and bottom of a flux surface, rather than the outboard midplane. Its parallel distribution is determined by the magnetic field geometry and steep equilibrium gradients, with magnetic drift and finite Larmor radius effects being particularly important. Nonlinear simulations must run sufficiently long for the slower ion-scale ETG turbulence to saturate and interact with electron-scale ETG turbulence. |
Monday, November 8, 2021 3:48PM - 4:00PM |
CO07.00008: Modelling AUG scrape-off-layer plasma with full-f continuum Electromagnetic Gyrokinetic simulation Rupak Mukherjee, Noah R Mandell, Manaure Francisquez, Tess Bernard, Ammar Hakim, Gregory W Hammett Divertor heat load reduction is a key issue yet to be resolved for all next-generation tokamaks. However, a thorough understanding of the behavior of plasma in the SOL demands first-principle simulation. We employ the Gkeyll computational framework, the first successful full-f continuum electromagnetic gyrokinetic code on open field lines to explore AUG-SOL turbulence. Gkeyll has been efficiently employed to analyze the SOL turbulence in NSTX device. Here, we have carried out similar simulations for AUG-like parameters. The ion and electron density and temperature profiles in the outer-midplane region are reconstructed in our simulation and compared with the experimental measurements. The available experimental data from various radial measurements as well as our simulation show generation and propagation of ion-density filaments in the SOL region. Additionally, we identify and track these coherent structures within the simulation domain. The radial and poloidal velocity of these blobs, auto-correlation function, and packing-fraction of the blobs are measured at a statistically steady-state region. We compare heat load onto the lower divertor as well as the turbulence statistics in the SOL region for two different experimental parameters. |
Monday, November 8, 2021 4:00PM - 4:12PM |
CO07.00009: Intermittent electromagnetic transport in hot low-β tokamak edge plasma Alexander Stepanenko Intermittent transport of plasma at the edge of fusion devices is known to contribute considerably to overall energy and particle fluxes coming to the first wall of installations. Plasma ejections driven by turbulence come in the form of blobs, thin filaments strongly elongated along the direction of the magnetic field lines. In this contribution, we analyze electromagnetic dynamics of blobs in hot, yet sufficiently dilute edge plasma, characterized by values of the plasma parameter β < me/mi. The physical and mathematical models describing blob dynamics are presented. The impact of the collisional skin effect, excitation of the Alfvén waves, and their reflection from the conducting surfaces (located at the ends of open magnetic field lines intersecting the installation plasma-facing components) on parameters of blob transport are analyzed. Possible modifications of the sheath boundary conditions for closing the system of MHD equations are considered. The qualitative conclusions on peculiarities of filament dynamics in hot edge plasma are compared with results of numerical modeling of seeded blob dynamics under conditions similar to the DIII-D and T-15MD tokamaks. |
Monday, November 8, 2021 4:12PM - 4:24PM |
CO07.00010: Intrinsically Multi-Scale Microturbulence in a Stochastic Magnetic Field Mingyun Cao, Patrick H Diamond Nowadays, due to the fact that resonant magnetic perturbation can raise the L-H power threshold, we need to reconcile good confinement with good power handling. To determine how stochastic magnetic fields affect L-H transition, it is essential to study their influence on the instability process such as the resistive interchange in this work. The highlight of this work is the appearance of microscopic convective cells when div J=0 is maintained at all scales. These small-scale convective cells can generate a turbulent viscosity and a turbulent diffusivity, which may explain RMP pump-out. By using method of averaging and quasi-linear theory, an integro-differential equation describing the evolution of the potential profile is obtained. From this equation, we can see small-scale convective cells can electrostatically scatter the large-scale resistive interchange mode in addition to the stochastic scattering of stochastic magnetic fields. As the large-scale mode is the drive of small-scale cells, micro and macro scales are thus connected. The modified growth rate of the large-scale convective cell and the turbulent viscosity are calculated by exploiting perturbation theory. |
Monday, November 8, 2021 4:24PM - 4:36PM |
CO07.00011: The fate of the edge shear layer and the density limit Rameswar Singh, Patrick H Diamond We elucidate the physics of edge shear layer collapse and consequential emergence of the density limit phenomenology. Neoclassical screening is one of the key factors fixing the strength of the zonal flow shear. Predator - prey dynamics with neoclassical screening shows that for weak enough turbulence drive, the zonal flow growth by negative turbulent viscosity fails to overcome the zonal flow damping. This threshold condition is linked to critical value of dimensionless parameter ρs/(ρscLn)1/2 which underpins the Greenwald density limit ng∼Ip, where Ip is plasma current. Here, ρs is ion sound radius, ρsc is zonal flow screening length and Ln is density scale length. Smaller ρsc i.e., higher Ip expands the regime of zonal flow persistence. The limiting local edge density nc for shear layer collapse scales favorably with Ip and integrated particle source S as nc∼S1/3Ip in the viscous damping regime and as nc∼S2/3I2p in the charge exchange friction dominated regime. While the critical edge density clearly depends on S, it is also known that edge shear strength depends on the input power. Hence, the critical density might also have a power scaling. This (work in progress) will be discussed in detail in the meeting. |
Monday, November 8, 2021 4:36PM - 4:48PM |
CO07.00012: Theory of plasma blob formation and its experimental validation Stewart J Zweben, Nirmal Bisai, Santanu Banerjee, Abhijit Sen Plasma blobs, that arise in the turbulent edge and scrape-off layer regions (SOL) of a tokamak, play an important role in the anomalous nature of plasma transport. A plasma blob can form from a radially elongated density structure when it breaks due to differential stretching in the radial and poloidal directions. Our model calculation shows that such a breakup can occur when the combined strength of the radial and poloidal electric field shear exceeds the growth rate of the interchange modes. This theoretical criterion has been validated against three-dimensional (3D) numerical simulation in the SOL region and is applicable for both L-mode and H-mode plasmas. In the presence of an electron temperature gradient the poloidal gradient of the poloidal electric field is necessary to satisfy the criterion for H-mode plasmas. Experimental validation of the criterion is carried out using NSTX (shot # 141746) gas-puff imaging diagnostics data in the SOL region. The averaged values of the measured velocity shear at various locations as a function of time are scanned to look for coincidences between the occurrence of a blob due to the break-up of the radially elongated structure and the fulfillment of the criterion. |
Monday, November 8, 2021 4:48PM - 5:00PM |
CO07.00013: Advanced Energy and Enstrophy Conserving FEM for Drift-Reduced MHD Milan Holec, Ben S Southworth, Will Pazner, Ilon Joseph, Ben Zhu, Alejandro Campos, Chris J Vogl, Andris M Dimits, Alex Friedman, Tzanio Kolev, Mark L Stowell In this work, we explore a high-order time and space discretization for edge plasma turbulence simulations. A drift-reduced extended magnetohydrodynamics model describes turbulence driven by the Kelvin-Helmholtz and drift wave instabilities coupled to the shear Alfven wave. The theory of finite element spaces and collocation integration schemes provides a high-order space and time discretization that satisfies physical constraints including energy and enstrophy conservation and ensuring divergence-free magnetic and drift velocity fields up to machine precision. In particular, we show how a specific choice of time and space discretization tackles the conservation while eliminating numerical artifacts such as dissipation of energy and enstrophy. Moreover, the schemes do not only conserve integrated quantities, but also show noticeable effects on the spatial distribution of the flow itself. The performance and advantages of our proposed discretization are demonstrated using simulations of plasma flows, such as decaying and forced Hasegawa-Mima turbulence. |
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