Bulletin of the American Physical Society
56th Annual Meeting of the APS Division of Plasma Physics
Volume 59, Number 15
Monday–Friday, October 27–31, 2014; New Orleans, Louisiana
Session NI1: Energy and Particle Transport |
Hide Abstracts |
Chair: Ron Waltz, General Atomics Room: Acadia |
Wednesday, October 29, 2014 9:30AM - 10:00AM |
NI1.00001: Weimer Award: Reduction of core turbulence and transport in I-mode and comparisons with nonlinear gyrokinetic simulations Invited Speaker: Anne White Understanding transport in high performance ELM-suppressed tokamak plasmas is of great interest for ITER and other future experiments. `I-mode' regime on Alcator C-Mod, also known as `improved L-mode' on ASDEX Upgrade, has several favorable characteristics: pedestals in electron and ion temperature, with ITER98y2 H-factors similar to and exceeding H-mode [Hubbard et al Phys. Plasmas 18, 056115 (2011)], but without a density pedestal and without impurity accumulation and without ELMs. Most research on I-mode focuses on changes in edge and pedestal turbulence/transport and stability. In this work, transport in I-mode is probed by measuring changes in \textit{core} turbulence across L-I transitions at Alcator C-Mod and comparing with nonlinear gyrokinetic simulations. Long wavelength (k$_{\mathrm{\theta }}\rho_{\mathrm{s}}$ \textless 0.5) density fluctuation levels decrease from L-mode levels by up to 30{\%} in I-mode, and long wavelength (k$_{\mathrm{\theta }}\rho_{\mathrm{s}}$ \textless 0.3) electron temperature fluctuation levels decrease by up to 70{\%}, reaching the instrumental sensitivity limit. Gyrokinetic simulation results suggest that ExB shear in the core of these intrinsically rotating plasmas can reduce the fluctuation amplitude in I-mode. As the pedestal temperature increases across slow L-I transitions, core density fluctuations (0.40 \textless $\rho $ \textless 0.95) are reduced prior to the onset of the edge-localized (0.99\textless $\rho $ \textless 1.0) weakly coherent mode (WCM) and prior to the reduction of low-frequency turbulence in the edge/pedestal region (0.99\textless $\rho $ \textless 1.0), which suggests that effects of profile stiffness across the radius can also lead to reduced core turbulence. By comparing experimental measurements from Alcator C-Mod to nonlinear gyrokinetic simulations and to different models of profile stiffness, this talk will explore the impact of core turbulence and transport on overall I-mode confinement and on the separation of particle and heat transport in I-mode. [Preview Abstract] |
Wednesday, October 29, 2014 10:00AM - 10:30AM |
NI1.00002: Impurity Particle Transport in High Confinement Regimes Without ELMs on DIII-D Invited Speaker: B.A. Grierson Recent experiments on DIII-D using trace levels of fluorine gas injection have shown that high confinement regimes without ELMs can achieve rapid transport of impurity ions. Much attention has recently been given to regimes with H-mode energy confinement without edge-localized modes (ELMs), accessed either through Resonant Magnetic Perturbations (RMPs) or MHD such as edge harmonic oscillations or quasi-coherent edge oscillations. Experiments on DIII-D have used gas puffing of trace levels of fluorine to introduce this fully-stripped, non-intrinsic and non-recycling impurity that can be easily measured with charge-exchange recombination spectroscopy. Trace fluorine is used because the time-history of the fluorine density profile permits direct extraction of the confinement time, particle diffusivity and convective velocity without relying on atomic modeling or assumptions about the source recycling. Results indicate impurity accumulation is more pronounced in RMP ELM suppressed plasmas with a pure n=3 spectrum compared with mixed n=1 and n=3 RMP fields with reduced number of control coils. In cases where strong central carbon impurity accumulation occurs, trace fluorine analysis reveals a strong inward impurity pinch. Conversely, in plasmas with weak central carbon accumulation, the fluorine pinch is significantly lower. These measurements of impurity influx are consistent with TGLF modeling of the ELM-suppressed phase of the discharge revealing that strong impurity influx occurs when the ratio V/D is between -1 to -3. In this work, the dependencies of impurity transport on local driving gradients will be presented, and the means of increasing the impurity diffusion to recover high purity plasmas will be discussed providing a basis for achieving low-dilution, stationary ELM-free operation in ITER and future devices. [Preview Abstract] |
Wednesday, October 29, 2014 10:30AM - 11:00AM |
NI1.00003: Tungsten Transport in the Core of JET H-mode Plasmas, Experiments and Modelling Invited Speaker: Clemente Angioni The physics of heavy impurity transport in tokamak plasmas plays an essential role towards the achievement of practical fusion energy. Reliable predictions of the behavior of these impurities require the development of realistic theoretical models and a complete understanding of present experiments, against which models can be validated. Recent experimental campaigns at JET with the ITER-like wall, with a W divertor, provide an extremely interesting and relevant opportunity to perform this combined experimental and theoretical research. Theoretical models of both neoclassical and turbulent transport must consistently include the impact of any poloidal asymmetry of the W density to enable quantitative predictions of the 2D W density distribution over the poloidal cross section. The agreement between theoretical predictions and experimentally reconstructed 2D W densities allows the identification of the main mechanisms which govern W transport in the core of JET H-mode plasmas. Neoclassical transport is largely enhanced by centrifugal effects and the neoclassical convection dominates, leading to central accumulation in the presence of central peaking of the density profiles and insufficiently peaked ion temperature profiles. The strength of the neoclassical temperature screening is affected by poloidal asymmetries. Only around mid-radius, turbulent diffusion offsets neoclassical transport. Consistently with observations in other devices, ion cyclotron resonance heating in the plasma center can flatten the electron density profile and peak the ion temperature profile and provide a means to reverse the neoclassical convection. MHD activity may hamper or speed up the accumulation process depending on mode number and plasma conditions. Finally, the relationship of JET results to a parallel modelling activity of the W behavior in the core of ASDEX Upgrade plasmas is presented. [Preview Abstract] |
Wednesday, October 29, 2014 11:00AM - 11:30AM |
NI1.00004: First Experimental Evidence of Turbulence-driven Main Ion Flow and ExB Flow Triggering the L-H Transition Invited Speaker: L. Schmitz Simultaneous measurements of main ion flow, $E\times B$ flow, and turbulence level $\tilde{n}/n$ inside the separatrix (LCFS) show for the first time that the initial turbulence collapse preceding the L-H transition is due to turbulence-driven ion flow and $E\times B$ flow in the ion diamagnetic direction, opposing the pressure-gradient-driven equilibrium $E\times B$ flow in the L-mode phase. Low to high confinement (L-H) transitions characterized by limit cycle oscillations (LCO, [1]) allow probing the trigger dynamics and synergy of turbulence-driven meso-scale flows, and pressure-gradient driven flows with high spatio-temporal resolution. A density/plasma current scan indicates that the LCO is triggered at a critical value of turbulence-driven flow shear. Near the minimum of the electric field well, turbulence-driven flow in the electron diamagnetic direction is observed. The radial flow (shear) reversal is consistent with the direction of the ($\tilde{n}$, $E_r$) limit cycle observed just inside the LCFS in DIII-D (and recently in LCO-H-mode transitions in HL-2A and JFT-2M), and the reversed limit cycle direction observed in the inner shear layer. Causality of shear-flow generation has been established: early during LCO, the $E\times B$ shearing rate leads the ion pressure gradient increase; during the final phase of the LCO, the edge pressure gradient and ion diamagnetic flow are modulated and increase, and the shearing rate lags the ion pressure gradient. Pressure-gradient-driven shear then becomes sufficiently large to secure the final LCO-H-mode transition. A two-predator, one-prey model, similar to a previously developed model [2] but retaining arbitrary polarity of turbulence-driven flow with to respect pressure-gradient-driven $E\times B$ flow, captures essential aspects of the transition dynamics, including the magnitude and direction of the driven poloidal main ion flow.\par \vskip6pt \noindent [1] L.~Schmitz, et al., Phys.\ Rev.\ Lett.\ {\bf 108}, 155002 (2012).\par \noindent [2] K.~Miki and P.H.\ Diamond, Phys.\ Plasmas {\bf 19}, 092306 (2012). [Preview Abstract] |
Wednesday, October 29, 2014 11:30AM - 12:00PM |
NI1.00005: Testing the High Turbulence Level Breakdown of Low-Frequency Gyrokinetics Against High-Frequency Cyclokinetic Simulations Invited Speaker: Zhao Deng Gyrokinetic simulations of L-mode near edge tokamak plasmas with the GYRO code underpredict both the transport and the turbulence levels by 5 to 10 fold [1], which suggest either some important mechanism is missing from current gyrokinetic codes like GYRO or the gyrokinetic approximation itself is breaking down. It is known that GYRO drift-kinetic simulations with gyro-averaging suppressed recover most of the missing transport [2]. With these motivations, we developed a flux tube nonlinear cyclokinetic [3] code rCYCLO with the parallel motion and variation suppressed. rCYCLO dynamically follows the high frequency ion gyro-phase motion (with no averaging) which is nonlinearly coupled into the low frequency drift-waves thereby interrupting and possibly suppressing the gyro-averaging. By comparison with the corresponding gyrokinetic simulations, we can test the conditions for the breakdown of gyrokinetics. rCYCLO nonlinearly couples $\nabla$B driven ion temperature gradient (ITG) modes and collisional fluid electron drift modes to ion cyclotron (IC) modes. As required, rCYCLO cyclokinetic transport recovers gyrokinetics at high relative ion cyclotron frequency ($\Omega^\ast$) and low turbulence levels. However, because the IC modes are stable and act as a turbulence sink, we have found that at high turbulence levels and low-$\Omega^\ast$ cyclokinetic transport is lower (not higher) than gyrokinetic transport. Work is in progress with unstable IC modes to explore the possibility of driving cyclokinetic transport higher than gyrokinetic transport.\par \vskip6pt \noindent [1] C.~Holland, A.E.\ White, et al., Phys.\ Plasmas {\bf 16}, 052301 (2009).\par \noindent [2] R.E.\ Waltz, Bull.\ Am.\ Phys.\ Soc.\ {\bf 57}, 105 (2012).\par \noindent [3] R.E.\ Waltz and Zhao Deng, Phys.\ Plasmas {\bf 20}, 012507 (2013). [Preview Abstract] |
Wednesday, October 29, 2014 12:00PM - 12:30PM |
NI1.00006: Zonal Field Generation by Toroidal Alfven Eigenmode Invited Speaker: Zhixuan Wang Zonal fields (zonal flow and zonal current) have been shown to spontaneously generate and regulate microturbulence. The generation of zonal fields by Alfven eigenmodes has attracted intense attention recently. Global hybrid-MHD and local gyrokinetic simulations of toroidal Alfven eigenmode (TAE) indeed find zonal flow generation by mode coupling. However, a nonlinear gyrokinetic theory finds spontaneous generation of zonal fields by TAE modulational instability, arguing the need for kinetic simulations in realistic geometry. Here we use gyrokinetic toroidal code (GTC) to study the TAE in DIII-D discharge {\#}142111 near 525ms by using experimental geometry. Global linear simulation finds a strongly unstable TAE driven by energetic particles (EP) for the dominant toroidal mode (n$=$4). The radial position of EP-driven TAE peaks at and moves with the location of the strongest EP pressure gradients as EP profile moves outward radially. Experimental data confirms the fast outward drift of the TAE eigenfunction, which is found to be caused by EP non-perturbative contribution. Global GTC nonlinear simulation finds that zonal fields are driven by TAE mode coupling such that the growth rate of zonal fields is twice of the TAE growth rate. Revisiting the nonlinear simulation model reveals a missing nonlinear term, which proves to be especially important for the zonal current. Although zonal current has little effect on the TAE saturation, zonal flow significantly reduces the TAE saturation amplitude. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700