Session TI2: Invited MF: Core Turbulence, Transport, RF Scattering

Validation of gyrokinetic simulations in NSTX including comparisons with a synthetic diagnostic for high-k scattering.

A new extensive validation study performed for a modest beta NSTX NBI-heated H-mode predicts that electron thermal transport can be entirely explained by short-wavelength electron-scale turbulence fluctuations driven by the electron temperature gradient mode (ETG), both in conditions of strong and weak ETG turbulence drive. For the first time, local, nonlinear gyrokinetic simulations carried out with the GYRO code [Candy JPP 2003] reproduce the experimental levels of electron thermal transport while simultaneously matching the frequency spectrum of electron-scale turbulence, the shape of the wavenumber spectrum and the ratio of fluctuation levels between strongly driven and weakly driven ETG turbulence conditions. Ion thermal transport is shown to be very close to neoclassical levels predicted by NEO [Belli PPCF 2008], consistent with stable ion-scale turbulence predicted by GYRO. Comparisons between high-k fluctuation measurements [Smith RSI 2008] and simulations are enabled via a novel synthetic high-k diagnostic developed for GYRO. The frequency spectra characteristics of electron-scale turbulence (spectral peak and width) can be reproduced by the synthetic spectra, but prove not to be critical constraints on the simulations. However, the shape of the high-k wavenumber spectrum and the fluctuation level ratio between the strong and weak ETG conditions can also be simultaneously matched by electron-scale simulations within sensitivity scans about the experimental profile values, and prove to be great discriminators of the simulations analyzed. Electron-scale simulations were also able to isolate the effect of safety factor and magnetic shear to match the shape of the measured fluctuation wavenumber spectrum. This work is the strongest experimental evidence to date that ETG-driven turbulence can dominate in the outer-core of modest beta NSTX H-modes.   

Core to Edge Variation of Multiscale Turbulent Transport in ITER Baseline Discharges at DIII-D

High fidelity gyrokinetic simulations and dedicated experiments that measured the heat and impurity transport from r/a = 0.5 to 0.9 in reactor relevant, ITER baseline discharges find compelling evidence of multi-scale turbulence. Ion-scale (ITG/TEM), electron-scale (ETG), and multiscale (coupled ion and electron-scale) simulations have been used to probe the radial variation of cross-scale coupling and its role in setting experimental levels of transport. Validation quality profile and fluctuation data (BES, DBS, & PCI) were collected in conditions predicted to exhibit measurable characteristics of cross-scale coupling in intermediate scale density fluctuations (k$_\theta$ $\rho_s$ $\sim$ 3.0). Over 70 nonlinear CGYRO simulations were performed to study ion and electron-scale turbulence and probe the sensitivity of results within experimental uncertainty at 5 radial locations. The simulations were compared directly with experimental heat transport levels, low and intermediate-k density fluctuations, and predicted trace impurity transport (D and V) across the profile. Simulated ion-scale turbulence reproduces experimental ion heat flux levels but under-predicts electron heat flux in many radial locations, pointing to a likely role of electron and multi-scale turbulence. To confirm the multiscale nature of the turbulence and validate the gyrokinetic model, cutting-edge simulation was performed at r/a = 0.7 on the Titan Supercomputer that spans ion and electron scales (up to k$_\theta$ $\rho_s$ = 54.0) with unprecedented physics fidelity (E&M turbulence, realistic electron mass, rotation effects, collisions, all experimental inputs). The presented comparisons of ion, electron, and multi-scale simulation with experimental fluxes & fluctuations reveal the physical mechanisms dictating radial variation of heat and particle transport in reactor-relevant conditions.   

Hysteresis as a Probe of Turbulent Bifurcation in Intrinsic Rotation Reversals on Alcator C-Mod

Analysis and modeling of a set of rotation reversal hysteresis experiments unambiguously show that changes in turbulence are responsible for the intrinsic rotation reversal and the Linear to Saturated Ohmic Confinement (LOC/SOC) transition on Alcator C-Mod\footnote{N.M. Cao et. al. Nucl. Fusion (2019) \textit{submitted}}. Plasmas on either side of the reversal exhibit different toroidal rotation profiles and therefore different turbulence characteristics despite profiles of density and temperature that are indistinguishable within measurement uncertainty. The deactivation of subdominant (in linear growth rate and heat transport) ITG and TEM-like instabilities in a mixed-mode state is identified as the only possible change in turbulence within a quasilinear transport approximation which is consistent with the measured profiles and the inferred heat and particle fluxes across the reversal. This indicates an explanation for the LOC/SOC transition that provides a mechanism for hysteresis through the dynamics of subdominant modes and changes in their relative populations, and does not involve a change in the most (linearly) unstable ion-scale drift-wave instability.   

Theory for control of sub-cyclotron Alfv\'{e}n instabilities and implications for anomalous electron energy transport in tokamaks

New numerical and analytic results significantly advance understanding of the destabilization and suppression of beam-driven sub-cyclotron compressional (CAE) and global (GAE) Alfv\'{e}n eigenmodes in tokamaks. These instabilities have been experimentally linked to the anomalous flattening of electron temperature profiles at high beam power in NSTX [1]. In particular, CAEs/GAEs can effectively channel energy away from the core [2], resulting in presently unpredictable modifications to the core plasma heating. For this reason, CAEs and GAEs, which are routinely excited in spherical tokamaks such as NSTX(-U) and MAST, which have been observed on DIII-D, and which may be excited in ITER, must be considered when planning future burning plasma scenarios. A detailed understanding of CAE/GAE excitation, therefore, is vital to predicting and controlling their effects on plasma confinement. A comprehensive set of 3D hybrid MHD/particle simulations has been performed for a wide range of beam parameters, providing a wealth of information on CAE and GAE stability in realistic scenarios. Furthermore, a new analytic theory and stability conditions have been derived for CAEs and GAEs [3]. It describes key properties of the mode spectra generated in simulations and measured experimentally, explains the recent experimental discovery of GAE stabilization in NSTX-U [4], and resolves puzzling observations in DIII-D. New mechanisms for stabilization of CAEs and GAEs via multi-beam distributions are suggested and demonstrated numerically. The combined numerical and analytical approach presents a powerful predictive capability for effective control of these modes and the energy transport they induce. [1] Stutman PRL 102, 115002 (2009) [2] Belova PRL 115, 015001 (2015) [3] Lestz POP (submitted 2019) [4] Fredrickson PRL 118, 265001 (2018)   

Microtearing Turbulence: Properties of Instability, Saturation, and Transport

Transport driven by microtearing (MT) modes has been found to be significant in an array of applications, ranging from large-scale modes in tokamak core plasmas to spherical tokamaks to the H-mode pedestal. A brief overview of linear MT physics is given, distinguishing between different drives and mode branches, such as collisionless vs.~collisional and toroidal vs.~slab-like. Nonlinearly, the role of zonal flows and zonal fields -- i.e., zonal shear-magnetic fluctuations -- is investigated. While zonal flows can reach very large amplitudes, it is found that the zonal fields are responsible for a non-commensurately larger share of the energy transfer and thus saturation of the instability, providing an essential ingredient in transport modeling. An analysis of a DIII-D-relevant pedestal scenario is conducted, with particular focus on resonant magnetic perturbations (RMPs). For common electrostatic instabilities, zonal flows are a key regulator of turbulence and transport. In the presence of RMPs, zonal flows are eroded due to radial streaming of electrons, leading to increased fluxes, as can be illustrated via simulations of DIII-D L-mode plasmas. For MT turbulence, however, the RMP response to transport changes, as no zonal-field erosion occurs. Consequences for pedestal evolution are discussed in light of the transport fingerprint model.   

Microwave Scattering Due to Density Fluctuations in the DIII-D Tokamak

Heat pulse propagation experiments and beam propagation simulations have demonstrated a long-suspected connection between millimeter-scale turbulence and deposition profile broadening of electron cyclotron (EC) waves[1] on the DIII-D tokamak[2]. Over a variety of edge conditions produced in DIII-D experiments, Doppler backscattering measures an order of magnitude variation in density fluctuation level, which correlates linearly with the factor of 1.4-2.7 broadening of EC deposition as compared with equilibrium ray tracing. A self-consistent, profile-driven turbulence model coupled to a full wave rf simulation generates a level of broadening comparable to that seen in these experiments. The EC deposition profile is determined from transport analysis of the electron temperature modulation in response to EC power modulation at 20-70 Hz. Converting the temperature modulation to heat flux modulation and fitting across the harmonic structure to diffusive, convective, and coupled transport terms can resolve the EC deposition width from the temperature perturbation to within 25 percent in each case. The degree of beam broadening by scattering off simulated turbulence and the experimental evaluation of the deposition broadening agree across the full range of conditions studied. Quantifying the effect of edge fluctuation broadening on EC current drive on ITER, which can increase the power requirement for suppression of neoclassical tearing modes, will require 3D full wave codes that can be validated on the current generation of machines. These DIII-D experiments provide a quantitative measure of fluctuation effects, and demonstrate the power of 3D full wave simulations to model phenomena missed by 1D equilibrium beam and ray tracing. [1] A. Kohn et al PPFC 60 (2018) [2] M.W. Brookman et al submitted to PRL (2019)