Bulletin of the American Physical Society
58th Annual Meeting of the APS Division of Plasma Physics
Volume 61, Number 18
Monday–Friday, October 31–November 4 2016; San Jose, California
Session PI3: MFE: Turbulence & Transport IInvited

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Chair: George McKee, University of WisconsinMadison Room: 210 ABEF 
Wednesday, November 2, 2016 2:00PM  2:30PM 
PI3.00001: Scattering of radio frequency waves by turbulence in fusion plasmas Invited Speaker: Abhay K. Ram In tokamak fusion plasmas, coherent fluctuations in the form of blobs or filaments and incoherent fluctuations due to turbulence are routinely observed in the scrapeoff layer. Radio frequency (RF) electromagnetic waves, excited by antenna structures placed near the wall of a tokamak, have to propagate through the scrapeoff layer before reaching the core of the plasma. While the effect of fluctuations on RF waves has not been quantified experimentally, there are telltale signs, arising from differences between results from simulations and from experiments, that fluctuations can modify the spectrum of RF waves. Any effect on RF waves in the scrapeoff layer can have important experimental consequences. For example, electron cyclotron waves are expected to stabilize the deleterious neoclassical tearing mode (NTM) in ITER. Spectral and polarization changes due to scattering will modify the spatial location and profile of the current driven by the RF waves, thereby affecting the control of NTMs. Pioneering theoretical studies and complementary computer simulations have been pursued to elucidate the impact of fluctuations on RF waves. From the full complement of Maxwell's equations for cold, magnetized plasmas, it is shown that the Poynting flux in the wake of filaments develops spatial structure due to diffraction and shadowing. The uniformity of power flow into the plasma is affected by sidescattering, modifications to the wave spectrum, and coupling to plasma waves other than the incident RF wave. The Snell's law and the Fresnel equations have been reformulated within the context of magnetized plasmas. They are distinctly different from their counterparts in scalar dielectric media, and reveal new and important physical insight into the scattering of RF waves. The Snell’s law and Fresnel equations are the basis for the Kirchhoff approximation necessary to determine properties of the scattered waves. Furthermore, this theory is also relevant for studying backscattering of waves from density fluctuations in the core  for example, for millimeter wave reflectometry used to determine the wave numbers of fluctuations. All of these studies apply to the scattering of RF waves in any frequency range and for arbitrary variations in density. [Preview Abstract] 
Wednesday, November 2, 2016 2:30PM  3:00PM 
PI3.00002: Electron turbulence and transport in large magnetic islands Invited Speaker: Lucas Morton Magnetic islands, observed in both reversedfield pinches (RFPs) and tokamaks, often display unexpected turbulence and transport characteristics. For the first time in an RFP, the high repetition rate Thomson scattering diagnostic on MST has captured a 2D image of the rotating electron temperature structure of a magnetic island in a single discharge. MHD modeling using edge magnetic signals implies a 16 cm wide m,n$=$1,6 tearing mode island which completely overlaps a 5.5 cm n$=$7 island (12 cm between island centers). The 3D field is partially chaotic, but still reflective of the n$=$6 island structure. The measured temperature structure matches the shape and location of the n$=$6 partially chaotic (or `remnant') island. Contrary to the usual assumption that islands have flat internal temperature, the electron temperature is peaked inside the remnant magnetic island due to ohmic heating. The temperature peaking implies a local effective perpendicular conductivity 1040 m$^{\mathrm{2}}$/s inside the remnant island. This agrees quantitatively with an effective perpendicular conductivity of 16 m$^{\mathrm{2}}$/s estimated using the magnetic diffusion coefficient (evaluated at the electron mean free path) calculated from the modeled chaotic field. Statistical analysis of measurement ensembles with lower time resolution implies that remnant island heating is common in MST discharges. To investigate the role of turbulence near a magnetic island, the 2D structure of longwavelength density turbulence has been mapped around a large applied static m,n$=$2,1 Lmode island in the DIIID tokamak. The turbulence exhibits intriguing spatial structure. Fluctuations are enhanced severalfold (compared to the noisland case) on the inboard side of the Xpoint, but not on the outboard side of the Xpoint and are also reduced near the Opoint. [Preview Abstract] 
Wednesday, November 2, 2016 3:00PM  3:30PM 
PI3.00003: Multifield/scale interactions of turbulence with neoclassical tearing modes and impact on plasma confinement in the DIIID tokamak Invited Speaker: L. Bardoczi We present the first localized measurements of ITG scale temperature and density fluctuations and TEM scale density fluctuations modified by an m=2, n=1 magnetic island. These islands are formed by a Neoclassical Tearing Mode (NTM) deep in the core plasma at the q=2 surface. NTMs are important as they often degrade confinement and lead to disruption. This is the first experimental confirmation of a longstanding theory prediction [1] of decreased local smallscale turbulence levels across largescale magnetic islands. Our measurements capture a mean reduction of turbulence inside (and enhancement just outside) the island region during island evolution. Additionally, in the island saturated state, the fluctuations at the Opoint are observed to be reduced compared to the Xpoint [2]. These measurements allow the determination of the turbulence length scale at the island separatrix that is predicted to affect NTM stability [3]. A novel, nonperturbative measurement technique finds reduced crossfield electron thermal diffusivity (by 12 orders of magnitude) at the Opoint, consistent with the local turbulence reduction. Initial comparisons to the GENE nonlinear gyrokinetic code are promising with GENE predicting the observed turbulence reduction inside the island and increase just outside the island and replicating the observed scaling with island size. These results allow the validation of gyrokinetic simulations modeling the interaction of multiscale phenomena as well as have potential implications for improved NTM control.\par \vskip6pt \noindent [1] McDevitt and Diamond, PoP, 13, 032302 (2006)\par \noindent [2] L. Bardóczi et al, PRL 116 215001 (2016)\par \noindent [3] Hornsby et al, PPCF, 57, 05418 (2015) [Preview Abstract] 
Wednesday, November 2, 2016 3:30PM  4:00PM 
PI3.00004: Core Radial Electric Field and Transport in Wendelstein 7X Plasmas Invited Speaker: Novimir Pablant Results from the investigation of core transport and the role of the radial electric field profile ($E_r$) in the first operational phase of the Wendelstein 7X (W7X) stellarator are presented. In stellarator plasmas, the details of the $E_r$ profile are expected to have a strong effect on both the particle and heat fluxes. Neoclassical particle fluxes are not intrinsically ambipolar, which leads to the formation of a radial electric field that enforces ambipolarity. The radial electric field is closely related to the perpendicular plasma flow ($u_\perp$) through the force balance equation. This allows the radial electric field to be inferred from measurements of the perpendicular flow velocity from the xray imaging crystal spectrometer (XICS) and correlation reflectometry diagnostics. Large changes in the perpendicular rotation, on the order of $\Delta u_\perp \sim 5 km/s$ ($\Delta E_r \sim 12 kV/m$), have been observed within a set of experiments where the heating power was stepped down from $2 MW$ to $0.6 MW$. These experiments are examined in detail to explore the relationship between, heating power, response of the temperature and density profiles and the response of the radial electric field. Estimations of the core transport are based on power balance and utilize electron temperature ($T_e$) profiles from the ECE and Thomson scattering, electron density profiles ($n_e$) from interferometry and Thomson scattering, ion temperature ($T_i$) profiles from XICS, along with measurements of the total stored energy and radiated power. Also described are a set core impurity confinement experiments and results. Impurity confinement has been investigated through the injection of trace amount of argon impurity gas at the plasma edge in conjunction with measurements of the density of various ionization states of argon from the XICS and High Efficiency eXtremeUV Overview Spectrometer (HEXOS) diagnostics. Finally the inferred $E_r$ and heat flux profiles are compared to initial neoclassical calculations using measured plasma profiles. [Preview Abstract] 
Wednesday, November 2, 2016 4:00PM  4:30PM 
PI3.00005: Predicting Rotation via Studies of Intrinsic Torque and Momentum Transport in DIIID Invited Speaker: C. Chrystal Experiments at DIIID using dimensionless parameter scans to study momentum transport and intrinsic (selfgenerated) torque have yielded a predicted average toroidal rotation in ITER of ~10 krad/s and shown that intrinsic torque is relevant for large tokamaks. Intrinsic torque can generate toroidal rotation via various mechanisms (residual stress, orbit loss, field ripple, etc.), and rotation is important for determining turbulence suppression, MHD stability, and highZ impurity transport. The 10 krad/s prediction is ~2x higher than when only neutral beam torque is accounted for, an increase that is predicted to benefit ITER’s performance. This work employs scans of normalized gyroradius ($\rho_*$), normalized collision frequency ($\nu_*$), $T_e/T_i$, and q. Intrinsic torque normalized by $T_i$ has been found to scale as $\rho_*^{1.5}$, yielding significant intrinsic torque in ITER. The measurements disagree with theoretical predictions and suggest that residual stress is not the primary source of intrinsic torque. These results are consistent with a companion scan in JET. The $\nu_*$ scaling of normalized intrinsic torque is smaller ($\nu_*^{0.3}$). Momentum confinement time was measured to have gyroBohm like scaling ($\rho_*^{0.7}$, similar to ITB98(y,2) energy confinement time scaling), and weaker $\nu_*$ scaling ($\nu_*^{0.4}$). Intrinsic torque and momentum confinement time results are found by analyzing the time history of the angular momentum. The time variation of mainion and impurity rotation were found to be the same, verifying a key assumption in the analysis. The same intrinsic torque was measured when canceling the intrinsic torque with neutral beam torque, suggesting that the Mach number is not an important parameter. The beneficial level of rotation in ITER implied by these results is encouraging. [Preview Abstract] 
Wednesday, November 2, 2016 4:30PM  5:00PM 
PI3.00006: Understanding and Predicting Profile Structure and Parametric Scaling of Intrinsic Rotation Invited Speaker: Weixing Wang It is shown for the first time that turbulencedriven residual Reynolds stress can account for both the shape and magnitude of the observed intrinsic toroidal rotation profile. Nonlinear, global gyrokinetic simulations using GTS of DIIID ECH plasmas indicate a substantial ITG fluctuationinduced nondiffusive momentum flux generated around a midradiuspeaked intrinsic toroidal rotation profile. The nondiffusive momentum flux is dominated by the residual stress with a negligible contribution from the momentum pinch. The residual stress profile shows a robust antigradient, dipole structure in a set of ECH discharges with varying ECH power. Such interesting features of nondiffusive momentum fluxes, in connection with edge momentum sources and sinks, are found to be critical to drive the nonmonotonic core rotation profiles in the experiments. Both turbulence intensity gradient and zonal flow ExB shear are identified as major contributors to the generation of the k$_{\mathrm{\parallel }}$asymmetry needed for the residual stress generation. By balancing the residual stress and the momentum diffusion, a selforganized, steadystate rotation profile is calculated. The predicted core rotation profiles agree well with the experimentally measured mainion toroidal rotation. The validated model is further used to investigate the characteristic dependence of global rotation profile structure in the multidimensional parametric space covering turbulence type, qprofile structure and collisionality with the goal of developing physics understanding needed for rotation profile control and optimization. Interesting results obtained include intrinsic rotation reversal induced by ITGTEM transition in flatq profile regime and by change in qprofile from weak to normal shear.. Fluctuationgenerated poloidal Reynolds stress is also shown to significantly modify the neoclassical poloidal rotation in a way consistent with experimental observations. Finally, the firstprinciplesbased model is applied to studying the $\rho \ast $scaling and predicting rotations in ITER regime. [Preview Abstract] 
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