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 BI3: Advances in Tokamaks: Core and Edge Physics |
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Chair: Robert Granetz, Massachusetts Institute of Technology Room: Centennial II |
Monday, November 2, 2009 9:30AM - 10:00AM |
BI3.00001: Advanced Tokamak Research with Integrated Modeling in JT-60U Invited Speaker: Researches on advanced tokamak (AT) characterized by high normalized beta, high bootstrap current fraction and high confinement have been progressed in JT-60U with integrated modeling. In AT, simultaneous achievement of high performances is important, because physical factors are strongly coupled with each other, resulting in complexity and autonomy. Edge pedestal has coupling physics of core plasma and SOL-divertor-wall characteristics. Its coupling is further enhanced by ELMs. The linkage among them has been systematically clarified in JT-60U experiments [1], in which a plasma rotation also affected the linkage. Although plasma performances may be controlled by the rotation, the rotation was varied not only by an external input but also by the intrinsic mechanism due to plasma itself. To predict and control these complex and autonomous plasmas and to achieve simultaneously high performances, new models are constructed and integrated based on JT-60U experimental analyses. In this paper, we report researches of JT-60U experiments with integrated models of core, pedestal, SOL-divertor-wall, which are important for AT and have coupling physics. It is found that a toroidal rotation with perpendicularly-injected NB is caused by fast ion orbit and driven most efficiently by horizontal injection. This indicates the possibility of producing and controlling an intrinsic rotation in AT. Steeping a pressure gradient in the core beyond the pedestal is found to enhance the ELM energy loss. The pressure gradient optimization is required for AT with high beta and low ELM energy loss. 2D SOL-divertor-wall integrated code is developed and further integrated with 1.5D core-pedestal transport code for understanding the whole coupling physics. \\[4pt] [1] N. Oyama and the JT-60U team., ``Overview of JT-60U results toward establishment of advanced tokamak operation'', Nucl. Fusion, in print (2009). [Preview Abstract] |
Monday, November 2, 2009 10:00AM - 10:30AM |
BI3.00002: Power and particle exhaust: recent progress on JET and implications for ITER Invited Speaker: Fusion burn, on the one hand, and particle and power exhaust, on the other, impose different constraints on plasma scenarios -- the former requiring sufficient levels of fuelling, heating, and confinement to achieve and maintain ignition, the latter for the helium ash, impurity ions and total power to be removed without undue damage to the plasma facing components (PFCs). Recent experiments on JET have significantly progressed our understanding of tokamak exhaust physics, specifically the steady-state and transient heat loads on divertor and main chamber PFCs, as well as first wall material migration and hydrogenic fuel retention. As part of these experiments, various strategies of reducing plasma loads onto PFCs have been investigated. The reduction of steady-state heat loads was achieved by a combination of fuelling and extrinsic impurity (nitrogen and neon) seeding, thereby increasing the energy radiated from the plasma edge, cooling the divertor/SOL plasma and facilitating its `detachment' from the divertor target PFCs. Transient heat loads associated with edge localized modes (ELMs) were reduced by increasing the frequency (reducing the size) of ELM events, by fuelling, seeding and active techniques, while those associated with plasma termination (disruption) were partly mitigated by massive gas injection (mixtures of deuterium, argon and neon). Finally, a series of dedicated, day-long experiments were performed to measure the fuel retention under various plasma conditions. In this contribution, the results of the recent JET experiments are summarized and their implications for ITER are discussed. [Preview Abstract] |
Monday, November 2, 2009 10:30AM - 11:00AM |
BI3.00003: Recent Progress on EAST Superconducting Tokamak Invited Speaker: EAST is the first fully superconducting tokamak with divertor configurations starting operation since 2006. It has now been upgraded from initial full metal wall to actively cooled graphite plasma facing components (PFC) and bakable to 350 \r{ }C. Stable double null (DN) divertor plasma discharges over 60 seconds have been achieved with the actively cooled graphite PFCs and the new internal divertor cryo-pump. Plasma current up to 600 kA, electron density $\sim $5x10$^{19}$m$^{-3}$, electron temperature $\sim $2.5keV have been obtained with lower hybrid current drive (LHCD) and auxiliary heating. Low loop voltage breakdown at 0.15V/m and plasma ramping rate between 0.1MA/s and 0.5MA/s have been obtained with assistance of LHCD. Various start-up scenarios have been explored for operating PF superconducting coils with a large safety margin. Up to 1.2MW LHCD power has been coupled to both SN (single null) and DN plasmas. Divertor performance under long pulse operating conditions has been assessed for both SN and DN configurations. DN operation led to stronger asymmetry in power loading, favoring the outer divertor. Novel wall conditioning techniques, including RF wall conditioning and high frequency RF wall conditioning in the presence of toroidal magnetic fields have also been successfully tested. The details of these recent advances are presented. [Preview Abstract] |
Monday, November 2, 2009 11:00AM - 11:30AM |
BI3.00004: Pedestal regulation techniques for enhanced confinement regimes on Alcator C Mod Invited Speaker: Recent research on Alcator C-Mod has achieved greater leverage on global confinement through both optimization and active modification of the edge pedestal. Pedestal scalings that are quite robust in typical H-mode operation can be broken, and particle and thermal transport in the edge barrier can be decoupled substantially. In H-mode, pedestal parameters show a striking sensitivity to the ion $B\times \nabla B$ drift direction, relative to the active x-point position, with considerable variability observed when the distance between separatrices is on the order of the pedestal width ($\approx$5mm) or less, $i.e.$ very near double null (DN). Near DN H-modes can have improved confinement factors ($H_{98}>1$) as a result of elevated pedestal temperature ($T_{ped}$), with the edge regulated by benign small ELMs or continuous modes, regimes desirable for ITER and other future devices. Operating with a single null and with $\nabla B$ drift away from the x-point allows the formation of discharges with L-mode-like particle confinement, yet with excellent energy confinement. This enhanced confinement regime has demonstrated $H_{98}\approx 1$, $T_{ped}\approx $1keV, and can be maintained steady-state with no ELMs by operating with high current and strong shaping, while holding input power below the L-H threshold to suppress particle barrier formation. Additional pedestal modification has also been obtained in H-modes by application of lower hybrid (LH) waves. Strong relaxation of the density pedestal is observed and accompanied by increases in $T_{ped}$, providing a substantial reduction in overall collisionality and somewhat improved confinement. Direct interaction of the LHRF with the edge plasma appears to play a role in enhancing the pedestal particle transport, which conveniently relaxes the discharge to a less dense and hotter state, one more conducive to core LH penetration and damping. In all cases, strongly modified pedestals affect core properties, often including surprising effects on core rotation. [Preview Abstract] |
Monday, November 2, 2009 11:30AM - 12:00PM |
BI3.00005: Turbulence Structures and Velocities in the Edge of Alcator C-Mod Invited Speaker: The work to be presented describes the velocity fields and scale structure of the turbulent edge regions in Alcator C-Mod plasmas. Strong turbulence phenomena are routinely observed in the plasma edge and Scrape-Off-Layer regardless of the quality of global confinement. These turbulence layers have been characterized using Gas-Puff-Imaging measurements. Observations of short $(\sim 10\:\mu$s) timescale features and variations of the velocity fields will be presented for plasmas with a range of $n_e, I_p$, in both L and H-mode regimes. Radial profiles of poloidal propagation velocities have been constructed using direct Fourier methods. The observed experimental dispersion relations show a clear radial structure with turbulence propagating in the ion-diamagnetic direction $(1.5-2$ km/s$)$ around and outside the separatrix, and moving in the electron-diamagnetic direction $(3.5-4$ km/s$)$ around and inside the separatrix. In the crossover region around the separatrix two counter-propagating velocities are registered as long as the observation duration is $\ga 1$ ms. However for observation durations $(\sim100\mu$s$)$ coinciding with the characteristic time for the ejection of coherent features (blobs), rapid changes in the propagation direction are revealed, while the speed of propagation in the two regions remains largely invariant. Data obtained from both classical and spatio-temporal wavelet analysis on the statistical behavior of the propagation velocities and spectral characteristics will also be presented. These results suggest a view of the plasma edge qualitatively different from the one previously considered. Possible interpretations and implications will be discussed in relation to the formation of blobs and the structure of the high-shear region. [Preview Abstract] |
Monday, November 2, 2009 12:00PM - 12:30PM |
BI3.00006: Kinetic simulations of plasma sheath with parallel to the wall magnetic field Invited Speaker: Plasma-wall interactions can play an important role in plasma transport and confinement. The sheath serves as a boundary between the interior quasi-neutral plasma and the wall's surface and can impact the energy and particles fluxes striking the vessel, and consequently can influence the choice for the wall material. As hotter and denser plasmas are confined by almost parallel to the wall magnetic field in current fusion experiments, the need to understand this particular type of plasma sheath is apparent. For example, in magnetized target fusion the confining B field remains mostly parallel to the converging liner surface, while on ITER, the magnetic field intercepts the main chamber and the divertor plates at no more than a couple degrees. The physics of such plasma sheath is fundamentally different from the conventional one, as it is the ions that are positively charging the wall due to their large Larmor radii. Detailed 1D and 2D kinetic simulations using VPIC[1] suggest that the particles' distribution functions in the sheath significantly deviate from a local Maxwellian (contrary to common assumption in analytical theory[2]). The anomalies in the distribution functions are quantified by analyzing the particles drift velocities and the roles of different parts that make up the sheath force balance, such as electric field, Lorentz force, pressure gradient, and viscosity tensor. Scanning of plasma parameters yields a scaling of the sheath width with magnetic field and electron and ion temperatures that differs from conventional Debye-length scaling. The instabilities due to non-Maxwellian particles distribution functions, pressure gradient, and sheared ExB flow are also considered and compared with the observed dynamical fluctuations in both 1D and higher dimensional simulations.\\[4pt] [1] K. J. Bowers, et al. Phys. Plasmas 15, 055703 (2008).\\[0pt] [2] U. Daybelge, et al. Phys. Fluids 24 (6) (1981). [Preview Abstract] |
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