Bulletin of the American Physical Society
53rd Annual Meeting of the APS Division of Plasma Physics
Volume 56, Number 16
Monday–Friday, November 14–18, 2011; Salt Lake City, Utah
Session CI2: Pedestal Control With 3D Fields |
Hide Abstracts |
Chair: Jon Menard, Princeton Plasma Physics Laboratory Room: Ballroom BD |
Monday, November 14, 2011 2:00PM - 2:30PM |
CI2.00001: Mitigation of Edge Localized Modes with new active in-vessel saddle coils in ASDEX Upgrade Invited Speaker: One of the challenges for ITER and a fusion reactor is the potential of severe life-time limitations of the first wall and divertor due to excessive thermal loads by Edge Localized Modes (ELMs). While ELMs have been sucessfully mitigated or even suppressed by application of non-axisymmetric magnetic perturbations [1], the physics base for this technique is still sparse and extrapolation towards ITER uncertain. In order to broaden the experimental data base, ASDEX Upgrade is being extended with a set of 24 in-vessel saddle coils [2]. A first set of eight in-vessel saddle coils, four coils at the low field side above and four coils below midplane, has been operational since the 2011 experimental campaign, together with a fully tungsten-coated first wall. This configuration allows for $n=1$ and $n=2$ perturbation fields with zero or 90 degrees toroidal phase shift between upper and lower arrays (even or odd parity). Application of stationary $n=2$ perturbations leads to mitigation of type-I ELMs in plasmas with moderate to high edge densities [3]; Greenwald density fraction $n/n_{GW} \geq 0.65$, and neoclassical pedestal electron collisionalities $\nu_{\mathrm{e,neo}} \geq 1.2$. With saddle coils off, the ELMs are of type I, with stored energy loss per ELM ranging from 30 to 100 kJ, and peak power loads to the inner divertor of up to $10$~MW (area-integrated). With saddle coils (coil current $4.5$~kA$\times$turns), the frequency of type-I ELMs gradually decreases and eventually they completely disappear. In between or instead of these large ELMs intermittent high frequency transport events are observed, with similarities to those in small ELM regimes and more continuous power load. The inner divertor remains completely detached. Plasma density and stored energy with coils on is not reduced compared to unmitigated type~I ELM phases. The tungsten concentration is lower in ELM-mitigated phases than in unmitigated type~I ELM phases. Pellets of different size have been injected into an ELM-mitigated phase; no large ELMs are triggered. So far, ELM mitigation has been observed with in a wide range of edge safety factors, $q_{95} = 3.7 - 6.2$. Direct comparison of optimum resonant and non-resonant fields (odd and even parity at $q_{95} = 5.5$) shows no difference of coil current threshold to access the ELM mitigated regime. The properties of these discharges are comparable to the high collisionality regime in DIII-D [4]. Current experimental work in ASDEX Upgrade aims to also reproduce and study the low collisionality ELM suppression regime. \\[4pt] [1] Editorial, Nucl. Fusion {\bf 49} (2009) 010202 \\[0pt] [2] SUTTROP, W. et~al, Fusion Eng. Design {\bf 84} (2009) 290, and references therein \\[0pt] [3] SUTTROP, W. et~al, Phys. Rev. Lett. {\bf 106} (2011) 225004 \\[0pt] [4] EVANS, T. et~al, Nucl. Fusion {\bf 45} (2005) 595 [Preview Abstract] |
Monday, November 14, 2011 2:30PM - 3:00PM |
CI2.00002: Particle Transport Modification Due to Resonant Magnetic Perturbations on the DIII-D Tokamak Invited Speaker: We present the first direct measurements of particle transport changes due to $n=3$ resonant magnetic perturbations (RMPs) in both H-mode and L-mode DIII-D plasmas, along with associated measurements of turbulence changes and transport modeling. RMP application is at present the most successful technique for suppressing edge localized modes (ELMs), but also has the undesired effect of reducing the operating density by 5\%-30\%. Understanding and limiting this density reduction is a key issue for achieving high performance on ITER. In this paper, we present the first direct confirmation of an increase in particle diffusivity (D), and reduction in inward pinch velocity (v), with RMP application, in both L- and H-mode plasmas. In H-mode, the changes in D and v, determined using gas puff modulation techniques, are consistent with the observed decrease in density with RMP, and extend into the plasma core. The increase in D and decrease in v are also correlated with an increase in density fluctuation levels, and a decrease in $E\times B$ shearing levels. Calculations with the linear TGLF code indicate that the measured profile changes increase the maximum linear growth rate from about 0.1$\,$MHz in ELMing H-mode to about 0.25$\,$MHz in RMP ELM suppressed H-mode, with the peak growth rate occurring at higher $k_\theta\rho_s$ in agreement with the measured fluctuation changes, and a shift from ion to electron propagation. This indicates that the changes in core density are the result of changes in gradients and not a direct result of the applied RMPs. On the contrary, in the pedestal area fluctuations decrease whereas the density gradient scale length increases and the $E\times B$ shearing levels increase, indicating that changes in turbulent transport cannot explain the density changes in this area of the plasma. [Preview Abstract] |
Monday, November 14, 2011 3:00PM - 3:30PM |
CI2.00003: Enhanced Superbanana Transport Caused by Chaotic Scattering across an Asymmetric Separatrix Invited Speaker: This talk discusses a novel ``chaotic'' form of superbanana transport, and compares theory to experiments on nonneutral plasmas.\footnote{D. Dubin and Y. Tsidulko, Phys. Plas. {\bf 18}, 062114 (2011); A.A. Kabantsev {\it et al.}, Phys. Rev. Lett. {\bf 105}, 205001 (2010).} Magnetically-confined plasmas often have one or more locally-trapped particle populations, partitioned by separatrices from one another and from passing particles. Strong superbanana transport is caused by particles that cross these separatrices in the presence of field ``errors'' (such as toroidal magnetic curvature), since trapped and passing particles respond to the field error differently.\footnote{H. Mynick, Phys. Plasmas {\bf 13}, 058102 (2006); H. Mynick, Phys. Fluids {\bf 26}, 2609 (1983).} Collisional scattering (at rate $\nu$) is one mechanism driving the separatrix crossings; theory predicts a collisional boundary layer at the separatrix energy, and collisional transport that scales as $\nu^{1/2} B^{-1/2}$. The chaotic transport of interest here occurs when the separatrix is ``ruffled'' in the direction of plasma drift; then, collisionless particle orbits (tp orbits) cross the separatrix, giving essentially random trapping and de-trapping, with transport scaling as $\nu^0 B^{-1}$. Prior theory assumed a symmetry such that these tp orbits become trapped and detrapped on the same flux surface, thereby giving zero chaotic transport and reduced collisional transport.$^3$ Here, we characterize chaotic transport without the assumed symmetry, and find quantitative agreement with pure electron plasma experiments and simulations in cylindrical geometry. A global field error consisting of a small tilt of the trap magnetic field is applied, to play the role of large-scale curvature in tokamaks or stellarators. Also, a separatrix with two trapped particle populations is produced by applying a ``squeeze potential'' to the middle section of the plasma column. When the separatrix is $\theta$-symmetric, radial transport is observed to scale as $1/ \sqrt{B}$ in agreement with standard $\sqrt{\nu}$ superbanana theory. When the separatrix is not $\theta$ symmetric, some particles transit chaotically from trapped to passing and back as they ExB drift in $\theta$ (the tp orbits). Typical field errors then cause tp orbits to trap and detrap on different flux surfaces, and enhanced transport scaling as $1/B$ is observed in the experiments, in quantitative agreement with our theory and simulations. [Preview Abstract] |
Monday, November 14, 2011 3:30PM - 4:00PM |
CI2.00004: Kinetic Understanding of RMP Penetration and Pedestal Transport in Diverted Tokamak Geometry Invited Speaker: A new understanding of self-organized RMP penetration effects on the pedestal plasma response has emerged from the XGC0 particle code with the inclusion of Monte Carlo neutrals and heat/torque sources. XGC0 provides a self-consistent evolution of RMP fields, E$_{r}$, plasma profiles, and toroidal current perturbation, which are essential in the RMP self-organization. Results are validated against DIII-D pedestal experiments, including n, T, E$_{r}$, U$_{i}$, and U$_{e \perp}$ profiles. The coil-induced magnetic islands and stochasticity are substantially reduced in the outer part (``skin-depth layer'') of the pre-RMP pedestal. However, islands and stochasticity survive at the inner part of the pre-RMP pedestal and into the core. As a result, RMPs enhance electron heat transport Q$_{e}$ in the inner part of the pre-RMP pedestal and into the core, but preserve the Q$_{e}$ barrier at the outer pre-RMP pedestal, as seen in DIII-D. Particle transport is enhanced in both regions, albeit less in the skin-depth layer. Q$_{e}$ enhancement in the stochastic region is not as catastrophic as that predicted by Rechester-Rosenbluth, since the trapped electrons have limited contribution to parallel heat conduction. Experiments in DIII-D show the existence of a finite ELM suppression q-window. XGC0 finds that the stochasticity suppression by plasma response is noticeably weaker inside the window. Q$_{e}$ is thus sensitive to the q- window, but density pump-out is not, well matching experiment. This suggests that the ``vacuum Chirikov$>$1 in the whole edge'' is only a necessary condition for the plasma to be in the ELM suppression window. [Preview Abstract] |
Monday, November 14, 2011 4:00PM - 4:30PM |
CI2.00005: The EPED Pedestal Model: Gyrokinetic Extensions, Experimental Tests, and Application to ELM-suppressed Regimes Invited Speaker: Accurate prediction of the pressure at the top of the edge transport barrier (or ``pedestal height") is a key element of the assessment and optimization of fusion performance in tokamaks. We develop and test a model, EPED, for the H-mode pedestal height and width based upon two fundamental and calculable constraints: 1) onset of non-local peeling-ballooning modes at low to intermediate mode number, 2) onset of nearly local kinetic ballooning modes (KBM) at high mode number. Gyrokinetic and MHD calculations in realistic edge geometry are used to accurately assess both constraints and employ them in a predictive pedestal model with no adjustable parameters. Detailed studies of the KBM and related instabilities with GYRO and TGLF are described. Extensive tests of the EPED model on several tokamaks have been completed and additional tests are planned. On Alcator C-Mod, a series of experiments testing the model over a wide range of magnetic field (3.5, 5.4 and 8$\,$T) have recently been completed, finding good agreement with EPED predictions of pedestal height and width. On DIII-D, experiments have been conducted to further test the EPED model and KBM physics with a new higher resolution edge Thomson system and turbulence diagnostics. On JET, the model has been compared to a large dataset of 137 discharges, allowing detailed statistical comparisons and finding good ($\sim$20\% or better) agreement with the model. Studies of ELM suppression with 3D magnetic fields suggest that a combination of the EPED model and realistic plasma response calculations may yield a plausible model for ELM suppression, including prediction of the required range of $q_{95}$. Pedestal prediction and optimization for ITER and reactor scale devices are also discussed. [Preview Abstract] |
Monday, November 14, 2011 4:30PM - 5:00PM |
CI2.00006: Effect of Resonant Magnetic Perturbations on secondary structures in Drift-Wave turbulence Invited Speaker: In this work, we study the effects of RMPs on turbulence, flows and confinement, in the framework of two paradigmatic models, resistive ballooning and resistive drift waves. For resistive ballooning turbulence, we use 3D global numerical simulations, including RMP fields and (externally-imposed) sheared rotation profile. Without RMPs, relaxation oscillations of the pressure profile occur. With RMPs, results show that long-lived convection cells are generated by the combined effects of pressure modulation and toroidal curvature coupling. These modify the global structure of the turbulence and eliminate relaxation oscillations. This effect is due mainly to a modification of the pressure profile linked to the presence of residual magnetic island chains. Hence convection-cell generation increases for increasing $\frac{\delta B_r}{B_0}$. For RMP effect on zonal flows in drift wave turbulence, we extend the Hasegawa-Wakatani model to include RMP fields. The effect of the RMPs is to induce a linear coupling between the zonal electric field and the zonal density gradient, which drives the system to a state of electron radial force balance for large $\frac{\delta B_r}{B_0}$. Both the vorticity flux (Reynolds stress), and particle flux are modulated. We derive an extended predator prey model which couples zonal potential and density dynamics to the evolution of turbulence intensity. This model has both turbulence drive and RMP amplitude as control parameters, and predicts a novel type of transport bifurcation in the presence of RMPs. We find a novel set of system states that are similar to the Hmode-like state of the standard predator-prey model, but for which the power threshold is now a function of the RMP strength. For small RMP amplitude and low collisionality, both the ambient turbulence and zonal flow energy increase with $\frac{\delta B_r}{B_0}$. For larger RMP strength, the turbulence energy increases, but the energy of zonal flows decreases with $\frac{\delta B_r}{B_0}$, corresponding to a damping of zonal flows. At high collisionnality, zonal flow damping occurs even at small RMP amplitude. Finally, for very strong values of $\frac{\delta B_r}{B_0}$, the system bifurcates back to an Lmode-like state. [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