Bulletin of the American Physical Society
54th Annual Meeting of the APS Division of Plasma Physics
Volume 57, Number 12
Monday–Friday, October 29–November 2 2012; Providence, Rhode Island
Session TI3: Pedestal ELMs and ELM mitigation |
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Chair: Chris Hegna, University of Wisconsin Room: Ballroom BC |
Thursday, November 1, 2012 9:30AM - 10:00AM |
TI3.00001: Reduction of ELM Intensity on DIII-D by On-demand Triggering With High Frequency Pellet Injection and Implications for ITER Invited Speaker: L.R. Baylor Deuterium pellet injection was used on the DIII-D tokamak to successfully demonstrate for the first time the on-demand triggering of ELMs at a 10x higher rate, and with much smaller intensity, than natural edge localized modes (ELMs). The triggering of small ELMs by high frequency pellet injection has been proposed as a method to prevent large ELMs that can erode the ITER plasma facing components [1]. The demonstration was made by injecting slow ($<$200 m/s) 1.3 mm diameter deuterium pellets at 60 Hz from the low field side in an ITER similar plasma with 5 Hz natural ELM frequency. The input power was only slightly above the H-mode threshold. Similar non-pellet discharges had ELM energy losses up to 55 kJ ($\sim$8\% of total stored energy), while the case with pellets demonstrated ELMs with an average energy loss less than 3 kJ ($<$1\% of the total). Total divertor ELM heat flux was reduced by more than a factor of 10. Central accumulation of Ni was significantly reduced in the pellet triggered ELM case. No significant increase in density or decrease in energy confinement was observed. Stability analysis of these discharges shows that the pedestal parameters are approaching the peeling unstable region just before a natural ELM crash. In the rapid pellet small ELM case, the pedestal conditions are well within the stable region with a narrower pedestal width observed. This narrower width is consistent with a picture in which the pellets are triggering the ELMs before the width expands to the critical ELM width. Nonlinear MHD simulations of the pellet ELM triggering show destabilization of ballooning modes by a local pressure perturbation. The implications of these results for pellet ELM pacing in ITER will be discussed.\par \vskip6pt \noindent [1] P.T.\ Lang {\em et al.}, Nucl.\ Fusion {\bf 44}, 665 (2004). [Preview Abstract] |
Thursday, November 1, 2012 10:00AM - 10:30AM |
TI3.00002: Gyro-Landau-Fluid Theory and Simulations of Edge-Localized-Modes Invited Speaker: X.Q. Xu We report on the theory and simulations of edge-localized-modes (ELMs) using a gyro-Landau-fluid (GLF) extension of the BOUT++ code. Consistent with the two-fluid model (including 1st order FLR corrections), large ELMs, which are low-to-intermediate toroidal mode number (n) peeling-ballooning (P-B) modes, are suppressed by finite Larmor radius (FLR) effects as the ion temperature increases, while small ELMs (at intermediate n's) remain unstable. This result is good news for high ion temperatures in ITER due to the large stabilizing effects of FLR. Because the FLR effects are proportional to both Ti and n, the maximum growth rate is inversely proportional to Ti and the P-B mode is stabilized at high n. Nonlinear gyro-fluid simulations show results similar to those from the two-fluid model, namely that the P-B modes trigger magnetic reconnection, which drives the collapse of the pedestal pressure. Hyper-resistivity limits the radial spreading of ELMs by facilitating magnetic reconnection. The gyro-fluid ion model further limits the radial spreading of ELMs due to FLR-corrected nonlinear ExB convection of the ion gyro-center density. A gyro-fluid ETG model is being developed to self-consistently calculate the hyper-resistivity. Zonal magnetic fields arise from an ELM event and finite beta drift-wave turbulence when electron inertia effects are included. These lead to current generation and self-consistent current transport as a result of ExB convection in the generalized Ohm's law. Because edge plasmas have significant spatial inhomogeneities and complicated boundary conditions, we have developed a fast non-Fourier method for the computation of Landau-fluid closure terms based on an accurate and tunable approximation. The accuracy and the fast computational scaling of the method are demonstrated. [Preview Abstract] |
Thursday, November 1, 2012 10:30AM - 11:00AM |
TI3.00003: Advancing the predictive capability for pedestal structure through experiment and modeling Invited Speaker: Jerry Hughes Prospects for predictive capability of the edge pedestal in magnetic fusion devices have been dramatically enhanced due to recent research, which was conducted jointly by the US experimental and theory communities. Studies on the C-Mod, DIII-D and NSTX devices have revealed common features, including an upper limit on pedestal pressure in ELMy H-mode determined by instability to peeling-ballooning modes (PBMs), and pedestal width which scales approximately as $\beta_{\mbox{pol}}^{1/2}$. The width dependence is consistent with a pedestal regulated by kinetic ballooning modes (KBMs). Signatures of KBMs have been actively sought both in experimental fluctuation measurements and in gyrokinetic simulations of the pedestal, with encouraging results. Studies of the temporal evolution of the pedestal during the ELM cycle reveal a tendency for the pressure gradient to saturate in advance of the ELM, with a steady growth in the pedestal width occurring prior to the ELM crash, which further supports a model for KBMs and PBMs working together to set the pedestal structure. Such a model, EPED, reproduces the pedestal height and width to better than 20\% accuracy on existing devices over a range of more than 20 in pedestal pressure. Additional transport processes are assessed for their impact on pedestal structure, in particular the relative variation of the temperature and density pedestals due, for example, to differences in edge neutral sources. Such differences are observed in dimensionlessly matched discharges on C-Mod and DIII-D, despite their having similar calculated MHD stability and similar edge fluctuations. In certain high performance discharges, such as EDA H-mode, QH-mode and I-mode, pedestal relaxation is accomplished by continuous edge fluctuations, avoiding peeling-ballooning instabilities and associated ELMs. Progress in understanding these regimes will be reported. [Preview Abstract] |
Thursday, November 1, 2012 11:00AM - 11:30AM |
TI3.00004: Towards understanding ELM mitigation by resonant magnetic perturbations in MAST Invited Speaker: Ian Chapman MAST is equipped with 18 in-vessel coils for use in Resonant Magnetic Perturbation (RMP) ELM control experiments. These coils give considerable flexibility since they allow a range of toroidal mode numbers (up to n=6) and also allow improved alignment of the magnetic perturbations with the plasma equilibrium by allowing the phase of the applied field to be varied during the shot. This is complemented by modelling advances to understand the plasma response to applied fields, the resultant torque and three-dimensional displacement. The application of n$\ge $3 RMPs in MAST results in up to a factor of eight increase in ELM frequency and the released energy per ELM dropping four-fold. The benefits of high-n RMPs include reduced core rotation braking and reduced effect on the L-H transition power with RMPs. During ELM mitigation, lobe structures near the X-point are observed for the first time in visible-light imaging of the plasma edge. These lobes, that were previously predicted, are correlated with RMP penetration and only appear when enhanced particle transport or increased ELM frequency is observed. The number and location of the lobes is well described by vacuum modelling. The toroidal corrugation of the plasma edge due to n=3 RMPs is also measured and found to be 5{\%} of the minor radius. The electron pressure gradient drops and the pedestal width increases when RMPs are applied, which would normally suggest improved stability to peeling-ballooning modes, yet the ELMs are more frequent, or destabilised. This dichotomy is resolved with a model which suggests that the critical pressure gradient to trigger an ELM is degraded by the RMPs, due to both the presence of the lobes and the non-axisymmetric plasma corrugation. A quasi-linear code, MARS-Q code has been used to investigate the effects of the penetration process and plasma response on the observed structures. These computations quantify several factors affecting the dynamics of the RMP field penetration, in particular that the plasma response induces a larger j$\times $b torque than the NTV torque and the penetration time is consistent with the time scale observed for the appearance of the lobe structures. \textit{This work was funded by the RCUK Energy Programme under grant EP/I501045 and the European Communities under the contract of Association between EURATOM and CCFE. The views and opinions expressed herein do not necessarily reflect those of the European Commission.} [Preview Abstract] |
Thursday, November 1, 2012 11:30AM - 12:00PM |
TI3.00005: Comparisons of Linear and Nonlinear Plasma Response Models for Non-Axisymmetric Perturbations Invited Speaker: A.D. Turnbull With the installation of non-axisymmetric coil systems on major tokamaks for the purpose of studying the prospects of ELM-free operation, understanding the plasma response to these fields is a crucial issue, particularly for ITER. Application of different response models, using standard tools, to DIII-D discharges with applied non-axisymmetric fields from internal coils is shown to yield distinctly different results. To resolve the discrepancies, the problem is posed from a more general point of view intended to identify and highlight the assumptions made in each approach so that they can be fleshed out in a comparative study, with the aim of identifying the conditions under which they are valid. The plasma response to non-axisymmetric field perturbations can be treated as an initial value (or dynamic) stability problem, following the system dynamically from an initial unperturbed state, or from a nearby perturbed equilibrium approach, and using both linear and nonlinear models. The different approaches, and even the same approach in many cases, can yield different responses in principle and criteria are discussed under which each of the approaches can yield a valid response. In the DIII-D cases studied, these criteria show a breakdown in the linear theory despite the small 10$^{-3}$ applied perturbations. For the nonlinear response, the nearby equilibrium approach bypasses the detailed evolution and search for the appropriate final state but to assure accessibility one needs to relate the two-dimensional (2-D) and nearby three-dimensional (3-D) system through some set of invariants or constraints on the global parameters and symmetries and the profiles. While a universally valid set of constraints is not presently known, some general principles for setting the right constraints that lead to the dynamically accessible solution to the plasma response are discussed. [Preview Abstract] |
Thursday, November 1, 2012 12:00PM - 12:30PM |
TI3.00006: Plasma Rotation and Radial Electric Field Response to Resonant Magnetic Perturbations in DIII-D Invited Speaker: R.A. Moyer Analysis of DIII-D experiments have revealed a complex picture of the evolution of the toroidal rotation $v_{tor}$ and radial electric field $E_r$ when applying edge resonant magnetic perturbations (RMPs) in H-mode plasmas. Measurements indicate that RMPs induce changes to the plasma rotation and $E_r$ across the plasma profile, well into the plasma core where islands or stochasticity are not expected. In the pedestal, the change in $E_r$ comes primarily from the $v\times B$ changes even though the ion diamagnetic contribution to $E_r$ is larger. This allows the RMP to change $E_r$ faster than the transport timescale for altering the pressure gradient. For $n=3$ RMPs, the pedestal $v_{tor}$ goes to zero as fast as the RMP current rises, suggesting increased toroidal viscosity with the RMP, followed by a slow rise in co-plasma current $v_{tor}$ (pedestal ``spin-up'') as the pedestal density pumps out. This spin-up could result from a reduction in ELM-induced momentum transport or a resonant $j\times B$ torque due to radial current. As $v_{tor}$ becomes more positive and the pressure pedestal narrows, the electron perpendicular rotation $\sim$0 point moves out toward the top of the pedestal; increasing the RMP current moves this crossing point closer to the top of the pedestal. These changes reduce the mean $E\times B$ shearing rate across the outer half of the discharge from several times the linear growth rate for intermediate-scale turbulence to less than the linear growth rate, consistent with increased turbulent transport. Full-f kinetic simulations with self-consistent plasma response and $E_r$ using the XGC0 code have qualitatively reproduced the observed profile and $E_r$ changes. These results suggest that similar to their role in regulating H-mode plasma transport and stability, plasma rotation and $E_r$ play a critical role in the effect of RMPs on plasma performance. [Preview Abstract] |
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