Bulletin of the American Physical Society
55th Annual Meeting of the APS Division of Plasma Physics
Volume 58, Number 16
Monday–Friday, November 11–15, 2013; Denver, Colorado
Session YI2: Edge and Pedestal |
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Chair: Philip Snyder, General Atomics Room: Plaza E |
Friday, November 15, 2013 9:30AM - 10:00AM |
YI2.00001: The Heuristic Drift Model of the Scrape-Off Layer: Physics Issues and Implications Invited Speaker: Robert Goldston A heuristic drift-based (HD) model [1] has recently been developed for the power scrape-off width in H-mode tokamaks, predicting a SOL width $\sim$ 2(a/R) $\rho_{p,i}$. It agrees both in value and scalings with scrape-off width measurements [2] on ASDEX-U, C-MOD, DIII-D, JET, MAST and NSTX. Even the projected aspect ratio scaling is consistent with the data. The implications for ITER and beyond are daunting, projecting SOLs in the range of 2mm, including additional broadening in the divertor region. As a result, the ITER divertor could operate in the sheath-limited regime at unacceptable power density and target temperature - but realistic upstream pressure [3] - unless a very large fraction of the alpha power is dissipated by radiation and charge-exchange. Using the HD model for the SOL width, it is found that the SOL ballooning stability limit has value and scalings similar to the Greenwald limit. The predicted MHD $\alpha$ is shown to rise with n/n$_{\mathrm{GW}}$, as observed experimentally [4]. Interestingly, the narrow high-heat-flux regions observed in TEXTOR [5] and JET [6] limiter discharges are in the range of the HD projection, suggesting that the same mechanism could function in L-mode.\\[4pt] [1] R.J. Goldston, Nuclear Fusion 52 (2012) 013009\\[0pt] [2] T. Eich et al., Nuclear Fusion, submitted\\[0pt] [3] D. Whyte, Journal of Nuclear Materials, accepted\\[0pt] [4] LaBombard et al., Physics of Plasmas 18 (2010) 056104 (figure 14), R.J. Goldston IAEA 2012\\[0pt] [5] T. Denner et al., Nuclear Fusion 39 (1999) 83\\[0pt] [6] G. Arnoux et al., Nuclear Fusion, submitted [Preview Abstract] |
Friday, November 15, 2013 10:00AM - 10:30AM |
YI2.00002: The impact of peeling-ballooning turbulence on ELMs Invited Speaker: Pengwei Xi Although the onset of ELMs has possibly been determined by linear peeling-ballooning (P-B) instabilities and, the nonlinear BOUT$++$ simulations show that nonlinear mode coupling starts before the onset of ELMs, which can lead to finite amplitude peeling-ballooning (P-B) turbulence at the H-mode pedestal and play a crucial role in ELM dynamics in two aspects: (1) since the P-B turbulence can suppress ELM crash, for a given power input, pedestal can keep evolving to a state with larger pedestal pressure and current gradients. Accordingly, the drives of P-B modes also keep increasing. Therefore the onset of ELM is determined by the competition between linear drive and nonlinear mode coupling. We find that only when a single mode can overcome the nonlinear damping to become dominant, an ELM crash is triggered by this mode. This means with the P-B turbulence, the onset of ELM is determined by a nonlinear criterion $\gamma >\gamma_{c} $ rather than the previous linear criterion$\gamma >0$, where $\gamma _{\mathrm{c}}$ is the critical growth rate which depends on the P-B turbulence. (2) We find that the P-B turbulence can generate enough self-constant hyper-resistivity needed in ELM simulations when electron inertial is included in Ohm's law. This hyper-resistivity represents anomalous current transport and can set the limit of the narrow current layer width resolved in the simulations. Except the P-B turbulence, the impact of other micro-turbulence, such as KBM turbulence, will be presented via a newly developed electro-magnetic Gyro-Landau-Fluid extension of BOUT$++$ code. [Preview Abstract] |
Friday, November 15, 2013 10:30AM - 11:00AM |
YI2.00003: Kinetic Neoclassical Transport in the H-mode Pedestal Invited Speaker: D.J. Battaglia This paper presents the first quantitative comparison between the multi-species transport rates in H-mode pedestals on DIII-D and NSTX and the kinetic neoclassical transport calculated using XGC0, a full-f particle-in-cell drift-kinetic solver with self-consistent neutral recycling and sheath potentials. The best quantitative agreement between the simulation and measurement of the pedestal density, temperature and flow profiles during the ELM-free period following the L-H transition is achieved when assuming ion transport is reduced to the kinetic neoclassical level within the steep-gradient region while additional turbulent transport ($D \sim 0.5\,$m$^2$/s) exists in regions of low $E_r\times B$ shear such as the pedestal top and in the scrape-off layer (SOL). The non-Maxwellian ion distributions from kinetic effects lead to a co-$I_p$ intrinsic torque that matches the measurements in the pedestal on DIII-D. The kinetic neoclassical mean $E_r\times B$ shear is strongly dependent on the plasma boundary shape, and the predicted dependence of L-H transition conditions versus X-point radius is consistent with experiments on NSTX. In QH-modes on DIII-D, $T_i$ is larger than in the early ELM-free H-modes, and the drift-kinetic effects that are absent in fluid models become more pronounced. The ion distributions are calculated to be non-Maxwellian through the entire pedestal, driving $T_i$ anisotropy, poloidal asymmetries and intrinsic flows. For example, $T_i^{\perp} > T_i^{||}$ in the pedestal, consistent with the orthogonal measurements of $T_i^{C6+}$. Also, the observation that $T_i^{C6+} \geq T_{i,ped}$ in the far-SOL is reproduced in the simulations and attributed to long-lived collisionless ion orbits at low SOL densities. These studies indicate that kinetic neoclassical transport will play an important role in the L-H transition, H-mode transport and interpretation of measurements in the high-$T_i$ pedestals expected in ITER. [Preview Abstract] |
Friday, November 15, 2013 11:00AM - 11:30AM |
YI2.00004: Edge ambipolar potential in toroidal fusion plasmas Invited Speaker: Gianluca Spizzo A series of issues with toroidally confined fusion plasmas are related to the generation of 3D flow patterns by means of edge magnetic islands, embedded in a chaotic field and interacting with the wall. These issues include the Greenwald limit in Tokamaks and reversed-field pinches (RFP), the collisionality window for ELM mitigation with the resonant magnetic perturbations (RMPs) in Tokamaks, and edge islands interacting with the bootstrap current in stellarators. Measurements of the 2D map of the edge electric field $E^r(r=a,\theta,\phi)$ in the RFX RFP show that $E^r$ has the same helicity of the magnetic islands generated by a $m/n$ perturbation: in fact, defining the helical angle $u= m \theta - n \phi + \omega t$, maps show a sinusoidal dependence as a function of $u$, $E^r = \tilde{E}^r \sin u$. The associated $\mathbf{E} \times \mathbf{B}$ flow displays a huge convective cell with $\mathbf{v}(a) \neq 0$ which, in RFX and near the Greenwald limit, determines a stagnation point for density and a reversal of the sign of $E^r$. Similar reversal of $E^r$ at high collisionality is found also in Alcator C-mod. From a theoretical point of view, the question is how a perturbed toroidal flux of symmetry $m/n$ gives rise to an ambipolar potential $\Phi = \tilde{\Phi} \sin u $. On the basis of a model developed with the guiding center code ORBIT and applied to RFX and TEXTOR, we will show that the presence of a $m/n$ perturbation \textit{in any kind of device} breaks the toroidal simmetry with a drift proportional to the gyroradius $\rho$, thus larger for ions $(\rho_i \gg \rho_e)$. Immediately an ambipolar potential arises to balance the drifts, with the same symmetry as the original perturbation. [Preview Abstract] |
Friday, November 15, 2013 11:30AM - 12:00PM |
YI2.00005: External Excitation of a Drift-Alfv\'{e}n Wave Response in the Alcator C-Mod Edge Plasma and its Relationship to the Quasi-Coherent Mode Invited Speaker: Theodore Golfinopoulos Experiments indicate that short-wavelength, $k_{\perp} \rho_s \sim 0.1$, drift-Alfv\'{e}nic turbulence plays an important role in C-Mod edge plasma transport. A Quasi-Coherent Mode (QCM, $50 < f < 150$ kHz, $k_{\perp} \sim 1.5$ cm$^{-1}$) regulates particle and impurity transport in C-Mod's EDA H-modes. A Weakly Coherent Mode (WCM, $150 < f < 500$ kHz, $k_{\perp}\sim1.5$ cm$^{-1}$) plays a similar role in I-mode discharges, suppressing the formation of a density pedestal while maintaining a temperature pedestal. ELMs are not present in either confinement regime. With the idea of exciting, probing, and perhaps exploiting this transport behavior, we have developed a novel antenna system to excite drift-Alfv\'{e}n-like modes at the outer midplane. A winding with a ``shoelace'' geometry is placed $\sim3-5$ mm from the LCFS. The principal design parameters, $k_{\perp}=1.5\pm0.1$ cm$^{-1}$ and $45 < f < 300$ kHz, match the QCM and WCM properties, so that the antenna induces parallel currents in the boundary plasma that mimic those observed for the intrinsic modes. Phase-locking to intrinsic modes is also accomplished via a custom circuit. The antenna produces perturbations in density and field comparable to amplitudes of the intrinsic QCM. The plasma response exhibits a resonance near the natural QCM frequency, which generally satisfies the drift wave dispersion relation. While a driven $\tilde{B}_{\theta}$ fluctuation is visible throughout the discharge, the driven $\tilde{n}_e$ is only observed during H-mode, though it precedes the onset of the intrinsic QCM. Like the QCM, the driven mode propagates in the electron diamagnetic drift direction and is approximately field-aligned. Recent mirror probe measurements show the intrinsic QCM structure is predominantly drift-Alfv\'{e}nic, and we might expect the same of the driven mode. However, the induced perturbation is not global, but is localized to field lines which map to the antenna, suggesting a damped response, and direct measurements of the damping rate indicate $\gamma/\omega_0\sim5$\%. If the antenna response is, indeed, a linearly-stable drift wave, this may suggest that additional interchange physics and curvature drive are involved to make the QCM unstable. [Preview Abstract] |
Friday, November 15, 2013 12:00PM - 12:30PM |
YI2.00006: New insights on boundary plasma turbulence and the Quasi-Coherent Mode in Alcator C-Mod using a Mirror Langmuir Probe Invited Speaker: Brian LaBombard A ``Mirror Langmuir Probe'' (MLP) diagnostic has been used to interrogate edge plasma profiles and turbulence in Alcator C-Mod with unprecedented detail, yielding fundamental insights on the Quasi-Coherent Mode (QCM) -- a mode that regulates plasma density and impurities in EDA H-modes without ELMs. The MLP [1] employs a fast-switching, self-adapting bias scheme, recording density, electron temperature and plasma potential simultaneously at high bandwidth ($\sim$1 MHz) on each of four separate electrodes on a scanning probe. Temporal dynamics are followed in detail; wavenumber-frequency spectra and phase relationships are readily deduced. Poloidal field fluctuations are recorded separately with a two-coil, scanning probe. Results from ohmic L-mode and H-mode plasmas are reported, including key observations of the QCM: The QCM lives in a region of positive radial electric field, with a mode width ($\sim$3 mm) that spans open and closed field line regions. Remarkably large amplitude ($\sim$30{\%}), sinusoidal bursts in density, electron temperature and plasma potential fluctuations are observed that are in phase; potential lags density by at most 10 degrees. Propagation velocity of the mode corresponds to the sum of local ExB and electron diamagnetic drift velocities -- quantities that are deduced directly from time-averaged profiles. Poloidal magnetic field fluctuations project to parallel current densities of $\sim$5 amps/cm$^{2}$ in the mode layer, with significant parallel electromagnetic induction. Electron force balance is examined, unambiguously identifying the mode type. It is found that fluctuations in parallel electron pressure gradient are roughly balanced by the sum of electrostatic and electromotive forces. Thus the primary mode structure of the QCM is that of a drift-Alfven wave.\\[4pt] [1] B. LaBombard and L. Lyons, Rev. Sci. Instrum. \textbf{78} (2007) 073501. [Preview Abstract] |
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