Bulletin of the American Physical Society
52nd Annual Meeting of the APS Division of Plasma Physics
Volume 55, Number 15
Monday–Friday, November 8–12, 2010; Chicago, Illinois
Session PI2: Energy and Momentum Transport |
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Chair: Carl Sovinec, University of Wisconsin-Madison Room: Grand Ballroom CD |
Wednesday, November 10, 2010 2:00PM - 2:30PM |
PI2.00001: Response of Tokamaks to Non-axisymmetric Magnetic Perturbations Invited Speaker: Tokamaks are sensitive to non-axisymmetic perturbations. Non-axisymmetry as small as $\delta $B/B$\approx $10$^{-4}$ can deform or destroy flux surfaces causing significant non-ambipolar transport and possibly plasma disruption. Important progress in understanding tokamak plasma responses has been made through the applications of Ideal Perturbed Equilibrium Code (IPEC), which solves self-consistent ideal equilibria in the presence of non-axisymmetry in tokamaks. Non-axisymmetric perturbations can be amplified or shielded with strong poloidal harmonic coupling, and such plasma responses are shown to be essential to explain error field correction. Consistency over different machines, parameters, and fields including DIII-D mock-ups of ITER Test Blanket Modules (TBMs) has been demonstrated. A salient feature of the theory and modeling is the dominant distribution of non-axisymmetric field to which tokamaks respond most strongly by an order of magnitude relative to other orthogonal distributions. The design of correction coils in tokamaks including ITER can be greatly improved based on the new findings. Ideal plasma responses are already strong as shown by IPEC applications, but non-ideal plasma responses can be also important in high-performance tokamaks, as demonstrated in recent Resonant Field Amplification (RFA) experiments. One of the most important non-ideal plasma responses is the non-ambipolar transport along with the non-axisymmetric variation in the field strength, which can induce large friction forces and change the perturbed equilibrium itself. The progress in understanding non-ambipolar transport and eventually more general perturbed equilibria will be discussed. This work was supported by the US DOE Contract {\#}DE-AC02-09CH11466. [Preview Abstract] |
Wednesday, November 10, 2010 2:30PM - 3:00PM |
PI2.00002: Effects of Global Alfven Eigenmodes on Electron Thermal Transport in NSTX Invited Speaker: Very high levels of electron thermal transport (power balance $\chi _{e}\ge $10's m$^{2}$/s) are correlated with strong Global Alfven Eigenmode (GAE) activity in the deep core of beam heated NSTX plasmas [Stutman, et al., \textit{PRL} \textbf{102}, 115002 (2009)]. New measurements and recently proposed theoretical mechanisms offer possible explanations for this process. Overlapping large-amplitude GAE modes can induce stochastic particle trajectories in the bulk, trapped electron population resulting in rapid radial diffusion. However, numerical calculations using the ORBIT guiding center code and measured GAE amplitudes, obtained from a single line-integrated measurement of density fluctuations, have so far under-predicted the inferred $\chi _{e}$ by a factor of 5-10. Recent measurements of the GAE amplitude obtained from line-integrated density fluctuations at multiple tangential locations confirm the central location of the modes, in agreement with predictions of the initial value Hybrid and MHD (HYM) simulation code. The region of strong GAE amplitude also coincides with that of large $\chi _{e}$, further supporting a GAE induced transport mechanism. Furthermore, the dynamics of GAE activity (f$\sim $500-1000 kHz) demonstrate the bursting nature of these modes. Coupled with the strongly non-linear relation predicted between the mode amplitude and induced $\chi _{e}$, these results suggest that the \textit{peak} amplitude of the GAEs may be controlling the magnitude of stochastic electron transport, rather than their time-averaged amplitude, as previously used in computations. The ORBIT predictions would then be much closer to the experimentally inferred $\chi _{e}$'s. Finally, the experimental observations are examined from the perspective of a recent theory which claims that the central T$_{e}$ flattening occurs in NSTX through GAE mediated ``energy-channeling'' of the neutral beam power [Kolesnichenko, et al., \textit{PRL} \textbf{104}, 075001 (2010)]. [Preview Abstract] |
Wednesday, November 10, 2010 3:00PM - 3:30PM |
PI2.00003: Evidence of an Edge Momentum Source in DIII-D H-mode Plasmas and Role of the Reynolds Stress for Intrinsic Rotation Invited Speaker: Obtaining a predictive understanding of intrinsic rotation in tokamaks is an important issue for ITER. DIII-D experiments have inferred an ``intrinsic torque" in the plasma edge region [Solomon et~al., Nucl. Fusion {\bf 49} (2009) 085005], but a precise identification is still missing. Theory suggests an important role of the turbulent Reynolds stress [G\"urcan et~al., Phys.\ Plasmas {\bf 14} (2007) 042306], but insufficient experimental data exists to clarify its role. Using a reciprocating multi-tip Langmuir probe, we present the first measurements of the toroidal-radial Reynolds stress in a tokamak H-mode pedestal and report the discovery of a strong co-current rotation layer. The 1-cm-wide layer, whose peak is located just inside the separatrix, forms independently of the injected torque within less than 50~ms after the L-H transition from a much smaller feature in L-mode. In pure ECH plasmas with zero applied torque, the core rotation profile is flat and spins up over 600~ms until the velocity of the edge rotation layer of 35~km/s is matched. This proves that the layer is the cause --- not an effect --- of the core rotation and suggests that viscous transport down the layer's gradient can slowly spin up the core. A simple orbit loss model was applied to a representative discharge and good agreement with the layer was found, suggesting a role of ion orbit losses in the formation of the layer. The total toroidal-radial Reynolds stress is essentially zero outside the layer's peak and becomes increasingly positive further inward. It thus acts to oppose the spin-up of the core by transporting momentum back up the layer's gradient, thereby helping to maintain the peaked shape over such long timescales. This indicates that the stress also plays a key role in the physics of the edge rotation layer. [Preview Abstract] |
Wednesday, November 10, 2010 3:30PM - 4:00PM |
PI2.00004: Discoveries From the Exploration of Gyrokinetic Momentum Transport Invited Speaker: Gyrokinetic momentum transport can be driven by a variety of mechanisms that break the parity along the magnetic field: parallel and $E\times B$ velocity shear, parallel velocity, up/down flux surface asymmetry. In this work, the discovery of interesting properties of these mechanisms and a new mechanism will be reported. The first result is that the Kelvin Helmholtz (KH) mode driven by parallel velocity shear can drive a net negative energy flux when the temperature and density gradients are below the threshold for drift-wave instabilities. The signature of a negative ion energy flow from turbulence would be a power balance effective diffusivity that is below the neoclassical ion thermal diffusivity. The second result is the prediction that the effective momentum transport should depend on the relative sign between the toroidal magnetic field and the toroidal rotation. This follows from the relative sign between the $E\times B$ velocity shear in the Doppler shift of the gyro-kinetic equation and the parallel velocity shear term. This is a corollary effect to the property that the toroidal viscous stress can be zero (e.g.\ for no external torque) even when both the velocity shears are not zero. The two terms try and break the linear mode parity and can cancel each other out giving a net zero stress. A practical solution to the longstanding problem of including $E\times B$ velocity shear in linear driftwave eigenmodes in toroidal geometry has recently been developed for the TGLF gyro-fluid transport model. Simulations of momentum transport with TGLF will be compared with DIII-D data. Finally, when the $E\times B$ velocity is balance by the ion diamagnetic velocity, as in the H-mode edge, it has been discovered that the net stabilizing effect of the $E\times B$ shear is far stronger. The shear in the diamagnetic velocity is yet another symmetry breaking mechanism driving momentum transport. [Preview Abstract] |
Wednesday, November 10, 2010 4:00PM - 4:30PM |
PI2.00005: Peak Neoclassical Toroidal Viscous Force in DIII-D Invited Speaker: A predicted peak in the neoclassical toroidal viscous (NTV) force at ITER-relevant low toroidal plasma rotation has recently been observed [1] in the DIII-D tokamak using its unique capabilities of nearly balanced neutral beam injection and 3D fields applied by internal (I-)coils. The peak was predicted by a theoretical model [1] that smoothly connects the relevant low-collisionality asymptotic NTV regimes [2]. NTV originates from nonambipolar radial particle drifts driven by nonaxisymmetric (NA) magnetic fields that modify the ion parallel stress tensor; it drives the toroidal plasma rotation toward a diamagnetic-type offset flow opposite to the plasma current direction, as observed in DIII-D H-mode plasmas [3]. The maximum NTV force in low collisionality tokamaks occurs where the radial electric field vanishes, and depends critically on the poloidal ion flow [1]; this type of behavior is also applicable to quasi-symmetric stellarators. Using the I-coils to apply a static $n=3$ magnetic perturbation to an H-mode plasma, the NTV torque as a function of rotation was measured in DIII-D by comparing the beam power required to maintain a preprogrammed toroidal rotation before and after application of the $n=3$ fields. A localized peak NTV force was observed which is reasonably consistent with neoclassical theory for the damping of poloidal and toroidal flows. NTV has the potential to alter rotation profiles in low external torque configurations for a variety of applications in ITER.\par \vskip3pt \noindent [1] A.J.\ Cole et al., Report UW-CPTC 10-1 via http://www.cptc.wisc.edu (to be submitted to PRL).\par \noindent [2] K.C.\ Shaing et al., Plasma Phys. Control. Fusion {\bf 51}, 035009 (2009) and references cited therein.\par \noindent [3] A.M.\ Garofalo et al., Phys. Plasmas {\bf 16}, 056119 (2009) and references cited therein. [Preview Abstract] |
Wednesday, November 10, 2010 4:30PM - 5:00PM |
PI2.00006: I-Mode regime with an edge energy transport barrier but no particle barrier in Alcator C-Mod Invited Speaker: A regime of operation has been investigated on C-Mod which features a strong edge thermal transport barrier and H-mode-like energy confinement, but with little or no particle barrier. There is generally no increase in density or impurities or sudden drop in D$_{\alpha}$; impurity particle confinement is at L-mode levels. This ``I-mode'' regime is normally obtained by operating with the ion grad-B drift away from the active X-point and with ICRF heating. While the name was originally an abbreviation for ``Improved L-mode'' [1], it is now clear that this is a high confinement regime, with H$_{98(y,2)}$ up to 1.2 achieved. I-mode discharges have now been obtained over a wide parameter range, B=3-6 T, I$_{p}$=0.7-1.3 MA, q$_{95}$=2.5-5, and maintained in steady state for many $\tau _{E}$. Most discharges are ELM-free; small ELMs are in some cases triggered by large sawtooth heat pulses. I-modes are obtained at powers comparable to the L-H threshold for density barrier formation. This is 1.5-3 times above that in the favorable configuration, up to 6 MW, and scales quite differently, in particular increasing at low q$_{95}$. The I-mode regime is of considerable interest for transport barrier studies since it separates particle and energy transport channels. An edge E$_{r}$ well develops [2], and in many cases a clear bifurcation from L-mode edge temperatures is seen; pedestals up to 1 keV and edge $\nu $* as low as 0.15 have been obtained. Changes in edge turbulence are observed as the T barrier forms. Broadband fluctuations in the 50-200 kHz band decrease, while a broad peak at higher frequencies appears. This new mode apparently helps to regulate particle transport. \\[4pt] [1] F. Ryter et al, Plasma Phys. Control. Fusion \textbf{40} 725 (1998) \\[0pt] [2] R. McDermott et al, Phys. Plasmas \textbf{16} 056103 (2009) [Preview Abstract] |
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