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 VI2: Transport |
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Chair: Orso Meneghini, General Atomics Room: 102ABC |
Thursday, October 26, 2017 3:00PM - 3:30PM |
VI2.00001: Experimental Challenges to Stiffness as a Transport Paradigm Invited Speaker: T.C. Luce Transport in plasmas is treated experimentally as a relationship between gradients and fluxes in analogy to the random-walk problem. Gyrokinetic models often predict strong increases in local flux for small increases in local gradient when above a threshold, holding all other parameters fixed. This has been named `stiffness'. The radial scalelength is then expected to vary little with source strength as a result of high stiffness. To probe the role of ExB shearing on stiffness in the DIII-D tokamak, two neutral beam injection power scans in H-mode plasmas were specially crafted---one with constant, low torque and one with increasing torque. The ion heat, electron heat, and ion toroidal momentum transport do not show expected signatures of stiffness, while the ion particle transport does. The ion heat transport shows the clearest discrepancy; the normalized heat flux drops with increasing inverse ion temperature scalelength. ExB shearing affects the transport magnitude, but not the scalelength dependence. Linear gyrofluid (TGLF) and nonlinear gyrokinetic (GYRO) predictions show stiff ion heat transport around the experimental profiles. The ion temperature gradient required to match the ion heat flux with increasing auxiliary power is not correctly described by TGLF, even when parameters are varied within the experimental uncertainties. TGLF also underpredicts transport at smaller radii, but overpredicts transport at larger radii. Independent of the theory/experiment comparison, it is not clear that the theoretical definition of stiffness yields any prediction about parameter scans such as the power scans here, because the quantities that must be held fixed to quantify stiffness are varied. A survey of recent literature indicated that profile resilience is routinely attributed to stiffness, but simple model calculations show profile resilience does not imply stiffness. Taken together, these observations challenge the use of local stiffness as a paradigm for explaining global transport behavior. [Preview Abstract] |
Thursday, October 26, 2017 3:30PM - 4:00PM |
VI2.00002: Drift waves in the turbulence of reversed field pinch plasmas Invited Speaker: Derek Thuecks Turbulence is one of the principal mediators of energy exchange in natural and laboratory plasma settings, for example wave-particle interactions that lead to collisionless heating and acceleration. The turbulent cascade carried by Alfvenic fluctuations is especially important in magnetized plasmas, operating on a wide range of scales larger than the ion gyroradius. The MST laboratory plasma exhibits a robust turbulent cascade driven by tearing instability, which is likely connected to powerful non-collisional ion heating that is also observed. New electric and magnetic field fluctuation measurements in the plasma edge reveal a broadband cascade that is anisotropic relative to the mean $B_0$. Magnetic fluctuations dominate at the tearing scale, as expected, but energy equipartition is not observed at smaller scales. Instead, the kinetic energy, $\frac{1}{2}m_i n_i (\tilde{E}\times B_0)^2$, begins to dominate at $k_{perp} \rho_i>0.2$. Statistical coherency between density, parallel magnetic field, and floating potential fluctuations reveals previously unobserved features at this energy-crossing scale that are consistent with electron-branch drift waves with a phase velocity comparable to the electron drift speed. The edge region contains a strong density gradient, and either drift-Alfven coupling or unstable modes could be responsible for the excess kinetic energy. The turbulent energy rises and falls in concert with the tearing mode amplitudes, which suggests nonlinear wave coupling powers the cascade, but the coherency at small scales is more persistent than at the tearing-scale during sawtooth relaxation cycles, which suggests possible independent drift wave instability. Gradient regions are a universal feature of plasma interfaces, and similarities may be exploited to better understand turbulent dynamics in other space and laboratory plasmas, e.g., the corona-wind interface. [Preview Abstract] |
Thursday, October 26, 2017 4:00PM - 4:30PM |
VI2.00003: From core to coax: extending core RF modelling to include SOL, Antenna, and PFC Invited Speaker: Syun'ichi Shiraiwa A new technique for the calculation of RF waves in toroidal geometry enables the simultaneous incorporation of antenna geometry, plasma facing components (PFCs), the scrape off-layer (SOL), and core propagation [1,2]. Traditionally, core RF wave propagation and antenna coupling has been calculated separately both using rather simplified SOL plasmas. The new approach, instead, allows capturing wave propagation in the SOL and its interactions with non-conforming PFCs permitting self-consistent calculation of core absorption and edge power loss, as well as investigating far and near field impurity generation from RF sheaths and a breakdown issue from antenna electric fields. Our approach combines the field solutions obtained from a core spectral code with a hot plasma dielectric and an edge FEM code using a cold plasma approximation via surface admittance-like matrix. Our approach was verified using the TORIC core ICRF spectral code and the commercial COMSOL FEM package [2], and was extended to 3D torus using open-source scalable MFEM library. The simulation result revealed that as the core wave damping gets weaker, the wave absorption in edge could become non-negligible. Three dimensional capabilities with non axisymmetric edge are being applied to study the antenna characteristic difference between the field aligned and toroidally aligned antennas on Alcator C-Mod, as well as the surface wave excitation on NSTX-U. [1] J. Wright and S. Shiraiwa,”Coupling an ICRF core spectral solver to an edge FEM code”, AIP Conference Proceedings,1689 (2015). [2] S. Shiraiwa, J. Wright, et. al., "HIS-TORIC: Extending core ICRF wave simulation to include realistic SOL plasmas", Nucl. Fusion (2017) in press. [Preview Abstract] |
Thursday, October 26, 2017 4:30PM - 5:00PM |
VI2.00004: Lower Hybrid Wave Electric Field Vector Measurements Using Non-Perturbative Dynamic Stark Effect Optical Spectroscopy on Alcator C-Mod Invited Speaker: E.H. Martin Plasma-wave interactions near the lower hybrid (LH) wave launcher can have a major impact on driven LH current, especially in the high-density regime. To identify the relevant physics responsible for this interaction a correlated effort of experimental measurements and simulations of the LH wave electric field vector, \textbf{E}$_{\mathrm{LH}}$, were carried out on Alcator C-Mod using the SELHF (Stark Effect Lower Hybrid Field) diagnostic and COMSOL modeling. For a range of plasma parameters observations show that: 1) The polarization \textbf{E}$_{\mathrm{LH}}$ resides primarily in the radial-poloidal plane and becomes increasingly poloidal for locations away and to the top of the LH launcher. 2) Saturation of the radial component of \textbf{E}$_{\mathrm{LH}}$ is observed at an LH power density of approximately 12 MW/m$^{\mathrm{2}}$. 3) Reflectometry phase fluctuations were found to be correlated with \textbar \textbf{E}$_{\mathrm{LH}}$\textbar . These results suggest that the LH resonance cone and power spectrum may be substantially modified near the LH launcher in the high-density regime from the expected radial polarization and square root scaling of the magnitude with LH power. Simulation of the experimental data was carried out through development of a synthetic diagnostic using a full wave cold plasma COMSOL model. Density fluctuations and reflectometry measured density profiles were incorporated. Without density fluctuations, the synthetic \textbf{E}$_{\mathrm{LH}}$ signal is dominantly in the radial direction and scales with the square root of LH power, as expected. Increasing density fluctuations in the model can cause the magnitude of \textbf{E}$_{\mathrm{LH}}$ to decrease substantially and greatly vary the direction of \textbf{E}$_{\mathrm{LH}}$. The observations and results outlined above will be presented in detail and the applicability of density fluctuations as a mechanism behind the behavior of \textbf{E}$_{\mathrm{LH}}$ will be discussed. [Preview Abstract] |
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