Bulletin of the American Physical Society
50th Annual Meeting of the Division of Plasma Physics
Volume 53, Number 14
Monday–Friday, November 17–21, 2008; Dallas, Texas
Session TI2: Boundaries, Fluctuations and Transport |
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Chair: Robert La Haye, General Atomics Room: Landmark B |
Thursday, November 20, 2008 9:30AM - 10:00AM |
TI2.00001: A study of hydrogenic retention in a tokamak with reactor-like plasma-facing surfaces; Alcator C-Mod Invited Speaker: Tritium retention is an important safety concern for ITER; Operation for 1000 discharges without a major stoppage will require the fraction of ion fluence to Plasma Facing Components (PFCs) that is retained, R, to be $<$ 0.001{\%}. One year operation of a reactor, where tungsten (W) PFCs are envisioned, requires R to be 100x smaller! Co-deposition of H with carbon projects to unacceptably high T retention in ITER. We present the results of the first in-depth study of fuel retention for high-Z PFCs with ITER divertor n$_{e}$, T$_{e}$, particle and heat fluxes. We utilize molybdenum (Mo, with a small fraction of W), which is very similar to tungsten in terms of hydrogenic retention. The retention observed in a series of disruption-free C-Mod discharges is high, R$\sim $1{\%}, 1000x than expected from inherent Mo properties. These retention characteristics are exhibited regardless if the Mo surfaces are bare or partially covered by B films; D co-deposition with B is not contributing significantly to retention. Retention appears linear in fluence up to the limit of the discharge sequence, $\sim $20s, approaching one ITER discharge. Comparison of He- and D-fueled discharges gives support to a model of retention site creation in the lattice (`traps') due to D neutral buildup and accompanying lattice distortion driven by recombination-limited release (D-$>$D$_{2})$ from the front surface. Disruptions can be used to rapidly heat surfaces, releasing the H/D for recovery, potentially applicable to ITER. Naturally-occurring disruptions appear to balance single-discharge retention reducing the campaign-integrated retention by at least 100. Comparisons to laboratory-based retention studies indicate that the tokamak environment leads to additional enhancements of retention. This work is supported by U.S. Dept. of Energy Coop. Agreement DE-FC02-99ER54512. [Preview Abstract] |
Thursday, November 20, 2008 10:00AM - 10:30AM |
TI2.00002: Super-X Divertor High Power Density Devices Invited Speaker: The Super-X divertor (SXD), a new magnetic geometry that isolates the divertor from the main plasma, can solve the severe heat exhaust problem of fusion reactors as well as next-generation high power density devices. Using axisymmetric PF coils with currents comparable to standard divertor configurations, the SXD places divertor plates at the largest possible radius inside TF coils. This increases the plasma-wetted area by 2-3 times over flux-expansion alone (e.g., plate near main X-point, extreme plate tilting, X-divertor, snowflake divertor). The line length is also increased by 5-10 times. The reduction in core radiation enabled by SXD allows a high-power-density fusion experiment (HPDX), with beta 3 times and power density 10 times that of ITER. The HPDX could operate in the AT mode with R=2.2 m, A=2.5, and elongation 2.4-2.7, and produce 300-400 MW of fusion power during short periods of DT operation (like JET), with Q$_{xt}$ = (fusion power) / (total electrical input) =1-2. HPDX with a Super-X divertor could demonstrate integrated performance with simultaneous high beta, good confinement, stability (including thermal stability for a self-heated plasma), and appropriate heat exhaust, all by means of reactor-pertinent methods. [Preview Abstract] |
Thursday, November 20, 2008 10:30AM - 11:00AM |
TI2.00003: ELM filament structure in the National Spherical Torus Experiment Invited Speaker: The large pressure gradients near the edge of tokamak experiments during H-mode operation are predicted to drive periodic instabilities known as Edge Localized Modes (ELM). Numerical simulations have identified peeling modes and/or ballooning modes as the candidate instabilities, giving rise to filaments that burst radially outward across flux surfaces during the non-linear growth phase [1]. Fast-frame visible imaging, as well as other diagnostics, are used in NSTX to study the evolution and characteristics of the post-ELM filaments. SOL structures evolve from a perturbation of the edge topology that within 30-40~$\mu $s develops into strong ``primary'' filaments that propagate both radially and poloidally/toroidally, although long lived precursors are sometimes observed. These filaments are then followed by an increased level of edge turbulence (and blobs) momentarily resembling that observed during L-mode phases. The later blob filaments are clearly distinct from the initial primary ELM structures, with the early filaments being much denser (approx. pedestal densities), larger in cross-field dimensions (up to 5~cm), and moving at higher radial velocities (up to 7~km/s) than the later, ``secondary'' blob filaments. The edge turbulence subsides to H-mode levels within 1~ms. The correlation between filament size and speed is qualitatively consistent with analytic theory under the high collisionality regime [2]. These observations are consistent with the idea that the ELMs cause a temporary break down of the transport barrier, allowing a transient increase in turbulence close to L-mode levels. [1] P.~B. Snyder, H.~R. Wilson and X.~Q. Xu, Phys. Plasmas \textbf{12}, 056115 (2005). [2] J.~R. Myra, D.~A. D'Ippolito, et al., Phys. Plasmas \textbf{13}, 092509 (2006). [Preview Abstract] |
Thursday, November 20, 2008 11:00AM - 11:30AM |
TI2.00004: Fast Imaging of Transients and Coherent MHD Modes in DIII-D Invited Speaker: Fast framing cameras and new signal processing techniques yield images of plasma instabilities in DIII-D with unprecedented detail, including the first visible-light images of neoclassical tearing modes (NTMs). These images of both transient and coherent MHD activity are providing new understanding of the physical processes and new opportunities for comparison to theory. During edge-localized modes (ELMs), rotating filaments following helical magnetic field lines with effective toroidal mode number n ranging from $<$10 to 35 are rapidly ejected with radial velocity $\sim$500$\,$m/s into the region outside the separatrix. Visible images of the midplane region show that multiple ejected filaments interact with the outer wall in rapid succession during a single ELM. In high-density plasmas, the outer midplane ELM signal always precedes the divertor ELM signal, which is consistent with the model of a coupled peeling-ballooning instability driving ELMs. In lower density plasmas, however, the midplane and divertor ELM signals appear simultaneously, showing qualitatively different ELM behavior. The fast camera is used in separate studies to record rotating $m/n=2/1$ NTMs using visible bremsstrahlung emission. High-resolution 2D images of the mode amplitude and phase are obtained by Fourier filtering each pixel's time series at the mode frequency. Inside the $q=2$ surface, the measurements show excellent agreement with NTM modeling, and the magnetic island width is found by comparing to a synthetic camera diagnostic applied to simulations. Additional structure seen in the camera measurements outside the $q=2$ surface may require nonlinear, multi-mode modeling to explain. [Preview Abstract] |
Thursday, November 20, 2008 11:30AM - 12:00PM |
TI2.00005: Suppression of turbulent transport in NSTX internal transport barriers Invited Speaker: Electron transport will be important for ITER where fusion alphas and high-energy beam ions will primarily heat electrons. In the NSTX, internal transport barriers (ITBs) are observed in reversed (negative) shear discharges where diffusivities for electron and ion thermal channels and momentum are reduced. While neutral beam heating can produce ITBs in both electron and ion channels, High Harmonic Fast Wave (HHFW) heating can produce electron thermal ITBs under reversed magnetic shear conditions without momentum input. Interestingly, the location of the electron ITB does not necessarily match that of the ion ITB: the electron ITB correlates well with the minimum in the magnetic shear determined by Motional Stark Effect (MSE) [1] constrained equilibria, whereas the ion ITB better correlates with the maximum E$\times $B shearing rate. Measured electron temperature gradients can exceed critical linear thresholds for ETG instability calculated by linear gyrokinetic codes in the ITB confinement region. The high-k microwave scattering diagnostic [2] shows reduced local density fluctuations at wavenumbers characteristic of electron turbulence for discharges with strongly negative magnetic shear versus weakly negative or positive magnetic shear. Fluctuation reductions are found to be spatially and temporally correlated with the local magnetic shear. These results are consistent with non-linear gyrokinetic simulations predictions showing the reduction of electron transport in negative magnetic shear conditions despite being linearly unstable [3]. Electron transport improvement via negative magnetic shear rather than E$\times $B shear highlights the importance of current profile control in ITER and future devices. [1] F.M. Levinton, H. Yuh et al., PoP \textbf{14}, 056119 [2] D.R. Smith, E. Mazzucato et al., RSI \textbf{75}, 3840 [3] Jenko, F. and Dorland, W., PRL \textbf{89} 225001 [Preview Abstract] |
Thursday, November 20, 2008 12:00PM - 12:30PM |
TI2.00006: Drift-Kinetic Simulations of Neoclassical Transport Invited Speaker: A new $\delta f$ Eulerian kinetic code NEO has been developed for numerical studies of neoclassical transport. NEO serves a dual role: in addition to its practical value as a tool for high-accuracy neoclassical calculations, NEO also functions as a stepping-stone (together with the nonlinear GK code GYRO) toward a full-F gyrokinetic code which integrates neoclassical transport and microturbulence. NEO solves a hierarchy of equations derived by expanding the drift-kinetic equation in powers of $\rho_{*i}$, the ratio of the ion gyroradius to system size, and thus provides a first-principles calculation of the neoclassical transport coefficients for general plasma shape directly from solution of the distribution function. NEO extends previous numerical studies by including the self-consistent coupling of electrons and multiple ion species, the calculation of the first-order electrostatic potential via coupling with the Poisson equation, and rapid toroidal rotation effects. Systematic calculations of the second-order particle and energy fluxes and first-order plasma flows, poloidal rotation, and bootstrap current and comparisons with analytical theories are presented for multispecies plasmas. The ambipolar relation, which requires complete cross-species collisional coupling, is confirmed, and finite mass-ratio corrections due to this collisional coupling are identified. Parameterized studies of the effects of shaping are performed, and the application of analytic formulae obtained for circular plasmas to shaped cases is discussed. Results using DIII-D experimental profiles and the effects of strong rotation are also presented. Finally, finite orbit width effects are studied via solution of the higher-order drift-kinetic equations, and the implications of non-local transport on the validity of the $\delta f$ formulation for steep gradients in the H-mode edge are discussed. [Preview Abstract] |
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