Bulletin of the American Physical Society
51st Annual Meeting of the APS Division of Plasma Physics
Volume 54, Number 15
Monday–Friday, November 2–6, 2009; Atlanta, Georgia
Session PI2: Tokamak Physics: Edge, Scrape-off Layer and Wall |
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Chair: Andrea Garofalo, General Atomics Room: Centennial I |
Wednesday, November 4, 2009 2:00PM - 2:30PM |
PI2.00001: ELMs, Magnetic X-points, and Chaotic Fields Invited Speaker: Edge Localized Modes (ELMs) in a magnetically confined plasma are shown to be a new type of nonlinear plasma instability, in which a coherent plasma structure couples to part of a chaotic magnetic field. Numerical simulation using the M3D code [1] provides a detailed picture. Toroidal magnetic fields can be described as Hamiltonian systems. Under small perturbations, a plasma boundary magnetic surface containing an X-point splits into two, defined asymptotically by the limits of the field lines traced infinitely in each direction. The limiting surfaces overlap to form a homoclinic tangle [2]. The steep pressure gradient near the edge of an H-mode fusion plasma drives ballooning-type plasma instabilities, aligned along the equilibrium magnetic field. These can couple nonlinearly to the ``unstable'' magnetic surface that ripples the equilibrium boundary. Fingers of plasma grow outwards, above and below the midplane on the outboard side, and the alternating low density regions may penetrate deep into the plasma. The field becomes chaotic over the affected region. Field lines are contained for many toroidal transits, but develop significant radial excursions. Many are eventually lost through the X-point region. The original magnetic boundary is preserved. Over several hundred Alfv\'en times the plasma heals back towards the original shape, but with relaxed profiles of density and temperature. A complex nonlinear interaction between the plasma instability and a magnetic homoclinic tangle leads to distinct stages in the ELM crash, that are similar to experimental observations. This new picture may help to explain the large range of ELM and ELM-free behavior seen in experiments and suggests a re-examination of H-mode edge confinement and the L-H transition.\\[4pt] [1] W. Park, et al., \emph{Phys. Plasmas} \textbf{6} 1796 (1999).\\[0pt] [2] T. Evans, et al,, \emph{J. Phys. Conf. Series} \textbf{7} 174 (2005). [Preview Abstract] |
Wednesday, November 4, 2009 2:30PM - 3:00PM |
PI2.00002: ELM mitigation studies in JET and implications for ITER Invited Speaker: Type I edge localized modes (ELMs) remain a serious concern for ITER because of the high transient heat and particle flux that can lead to rapid erosion of the divertor plates. This has stimulated worldwide research on exploration of different methods to avoid or at least mitigate the ELM energy loss while maintaining adequate confinement. ITER will require reliable ELM control over a wide range of operating conditions, including changes in the edge safety factor, therefore a suite of different techniques is highly desirable. In JET several techniques have been demonstrated for control the frequency and size of type I ELMs, including resonant perturbations of the edge magnetic field (RMP), ELM magnetic triggering by fast vertical movement of the plasma column (``vertical kicks'') and ELM pacing using pellet injection. In this paper we present results from recent dedicated experiments in JET focusing on integrating the different ELM mitigation methods into similar plasma scenarios. Plasma parameter scans provide comparison of the performance of the different techniques in terms of both the reduction in ELM size and on the impact of each control method on plasma confinement. The compatibility of different ELM mitigation schemes has also been investigated. The plasma response to RMP and vertical kicks during the ELM mitigation phase shares common features: the reduction in ELM size (up to a factor of 3) is accompanied by a reduction in pedestal pressure (mainly due to a loss of density) with only minor ($<$ 10\%) reduction of the stored energy. Interestingly, it has been found that the combined application of RMP and kicks leads to a reduction of the threshold perturbation level (vertical displacement in the case of the kicks) necessary for the ELM mitigation to occur. The implication of these results for ITER will be discussed. [Preview Abstract] |
Wednesday, November 4, 2009 3:00PM - 3:30PM |
PI2.00003: Modification of Edge Profiles and Stability with Lithium Wall Coatings in NSTX Invited Speaker: Rapidly growing instabilities known as Edge Localized Modes (ELMs) are commonly observed in high-confinement (H-mode) regimes in many toroidal confinement devices. The reduction or elimination of ELMs with high confinement is essential for ITER, which has been designed for H-mode operation. Large ELMs are thought to be triggered by exceeding either edge current density limits (peeling modes) and/or edge pressure gradient limits (ballooning modes) [1]. Edge stability calculations have indicated that spherical tori should have access to higher pressure gradients and H-mode pedestal heights than higher aspect ratio tokamaks, owing to high magnetic shear and possible access to second stability regimes [2]. Such a regime was recently discovered in the National Spherical Torus Experiment (NSTX) following the application of Lithium onto the graphite plasma facing components [3]. ELMs were eliminated in phases [4], with the resulting pressure gradients and pedestal widths increasing substantially [5]. The modification of the pressure profile originated mainly from reduced recycling and edge fueling, which relaxed the edge density gradients. PEST and ELITE calculations have confirmed that the resulting pressure profiles were further from the stability boundary than reference discharges. The resulting discharges are ELM-free with a 50{\%} increase in normalized energy confinement, up to the global $\beta _{N} \sim$ 5.5-6 limit. While the ELM-free discharges ultimately suffer radiative collapse, pulsed 3-d magnetic fields are used to trigger ELMs and purge impurities [6]. \\[4pt] [1] P. B. Snyder, et. al., \textit{Physics of Plasmas} \textbf{9} (2002) 2037. \\[0pt] [2] P. B. Snyder, \textit{Plasma Physics Controlled Fusion} \textbf{46} (2004) A131. \\[0pt] [3] H. W. Kugel, et. al., \textit{Physics of Plasmas} \textbf{15} (2008) 056118. \\[0pt] [4] D. M. Mansfield, et. al., \textit{J. Nucl. Materials} \textbf{390-391} (2009) 764 \\[0pt] [5] R. Maingi et. al., \textit{Physical Review Letters} (2009) at press. \\[0pt] [6] J.M. Canik, et. al, \textit{Nucl. Fusion} (2009) submitted. [Preview Abstract] |
Wednesday, November 4, 2009 3:30PM - 4:00PM |
PI2.00004: Detached divertor plasma physics with a stochastic magnetic field Invited Speaker: Stable detachment control is demonstrated by superposing resonant magnetic perturbation of m/n=1/1 on the stochastic boundary of LHD, which introduces a remnant magnetic island outside the last closed flux surface (LCFS). A strongly radiating region is stabilized in the edge, without core plasma degradation. The diagnostics show, a significant reduction of divertor particle/heat flux, formation of very dense ($\sim $10$^{20}$ m$^{-3})$ and cold (a few eV) plasma outside of LCFS, volume recombination in the hydrogen line emission, and strongly decreased intensity of iron emission (Fe VIII$\sim $XII). It is also demonstrated that the discharge is compatible with the internal diffusion barrier (IDB) type plasma, and that the detach-attach transition is readily controlled by gas puff adjustment. The field line structure of a remnant island outside the LCFS is considered responsible for stabilizing the radiating region with the core plasma being unperturbed, because of selective cooling at either O or X-point, where the thermal instability can favorably set in. Simulation of 3D edge transport shows a possibility of the radiative condensation around a remnant island structure at the detachment transition. The simulation also indicates that the geometry of low order (m=7$\sim $5) remnant islands effectively reduces the parallel temperature gradient by increased cross-field energy transport at high collisionality. This results in suppression of the thermal force that usually drives impurities towards the confinement region. Together with increased outward friction from background parallel plasma flow at high collisionality, the high density edge plasma with stochastic field leads to effective impurity screening. The measurements also indicate better screening for discharges with the m/n=1/1 perturbation. [Preview Abstract] |
Wednesday, November 4, 2009 4:00PM - 4:30PM |
PI2.00005: Quantification of Chemical Erosion in the DIII-D Divertor Invited Speaker: Chemical erosion (CE) yield at the graphite divertor target in DIII-D was measured to be substantially lower in cold near-detached plasma conditions compared to well-attached ones, with major implications for ITER. Current estimates of tritium retention by co-deposition with hydrocarbons (HCs) in ITER place potentially severe restrictions on operation. However, calculations done to date have been based on excessively conservative assumptions, due to limited understanding of cold divertor plasmas (1-5eV) which bridge energy thresholds for complex atomic and molecular processes not present in attached conditions. Hydrocarbon injection through a unique porous graphite plate which realistically simulates secondary reactions of HCs with a graphite surface has been used to measure CE\textit{ in-situ}. For the first time in a divertor, measurements were made at extrinsic CH$_{4}$ injection rates comparable to the expected intrinsic CE rate of C, with the resulting spectroscopic emissions separated from those of the intrinsic sources. Under cold plasma conditions the contribution of CE-produced C relative to total C sources in the divertor declined dramatically from $\sim $50{\%} to $<$15{\%}. Photon efficiencies for products from the breakup of injected CH$_{4}$ were greater than previous measurements at higher puff rates, indicating the importance of minimizing perturbation to the local plasma. At 350K, the measured CE yield near the outer strike point was $\sim $2.6{\%} in attachment dropping to only $\sim $0.5{\%} in cold plasma; results are consistent with some theoretical predications and lab studies. Under full detachment, near total extinction of the CD band occurred, consistent with suppression of net C erosion. These findings have potentially major impact on projected target lifetime and tritium retention in future reactors, and for the PFC choice in ITER. [Preview Abstract] |
Wednesday, November 4, 2009 4:30PM - 5:00PM |
PI2.00006: Runaway Electron Transport and Disruption Mitigation Optimization on Alcator C-Mod Invited Speaker: Experiments and modeling on Alcator C-Mod have provided new insights into the physics of runaway electron transport and disruption mitigation optimization, both critical issues for ITER. Avalanche amplification of runaway current in disruptions is a concern for ITER since the amplification gain scales exponentially with plasma current. While stochastic transport losses of the fast electrons can effectively eliminate the amplification, such losses have not previously been quantified. C-Mod has investigated runaway production and transport by seeding a suprathermal electron tail using Lower Hybrid heating and then promptly terminating the discharge with a massive gas injection (MGI). This innovative combination assures, through the Dreicer mechanism, a sufficient ``seed'' population of runaways, $\sim$0.1 of the initial plasma current, such that their amplification and transport can be measured. In the experiment electrons accelerate to relativistic velocities but are all lost due to transport on a timescale less than a millisecond, effectively resulting in runaway electron suppression before the current quench. The runaway loss mechanism is confirmed to be stochastic transport by using electron tracing in 3-D resistive MHD simulations (NIMROD), which have also shown the importance of MHD activity in MGI disruption mitigation. Large parallel electric fields and stochasticity appear simultaneously in the model, suggesting that the development of strongly stochastic regions through sufficient impurity radiation cooling is key to both mitigation and runaway avoidance. Particle delivery, mitigation effectiveness, runaway avoidance and overall response time of the system is optimized using mixed-gases of a fast low-Z gas carrier with a trace high-Z radiating species. Mitigation options and optimization for ITER are discussed. [Preview Abstract] |
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