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 PO6: Waves, Turbulence, and Boundary Effects in Laboratory and Space Plasmas II |
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Chair: Bill Dorland, University of Maryland Room: Hanover FG |
Wednesday, November 4, 2009 2:00PM - 2:12PM |
PO6.00001: Physics of turbulent transport and zonal flows in the edge of fusion plasmas Ulrich Stroth The understanding of turbulent transport at the plasma edge and on transition to the scrape-off layer is of greatest importance for fusion plasmas. Edge transport sets the values of temperature and density at the pedestal top, parameters which are needed as input for core-turbulence simulations. And SOL transport ultimately defines the peak power density on the divertor plates and the first wall. Furthermore, the interplay between flows and turbulence is one of the most fascinating and rich topics of fusion research. The talk will give an overview of experimental studies on turbulent transport and the interaction of fluctuations with poloidal flows. The studies were carried out on the low-temperature plasma in the TJ-K stellarator which is dimensionally similar to fusion edge plasmas and, hence, a kind of ``windtunnel'' experiment for fusion devices and testbed for turbulence code. Turbulence is studied with high spatial and temporal resolution and a close comparison of the data with results from the gyro-fluid code GEM shows a great structural similarity. This comparison indicates that drift-waves dominate turbulent transport in the edge plasma. It is also shown how drift waves couple to zonal flows and how they generate intermittent structures (blobs) at the natural edge shear layer. Results are compared with findings from ASDEX Upgrade and other devices. [Preview Abstract] |
Wednesday, November 4, 2009 2:12PM - 2:24PM |
PO6.00002: Control of drift wave turbulence by driven plasma currents Olaf Grulke, Christian Brandt, Stefan Ullrich, Kian Rahbarnia, Thomas Klinger The anomalous transport in the plasma edge of fusion devices is believed to be dominated by the drift wave instability, which is inevitable because of the relatively steep radial plasma pressure gradient. Due to its three-dimensional nature the associated parallel plasma currents are a characteristic feature of the drift wave instability. In the present paper we demonstrate that drift waves nonlinearly interact with driven plasma currents in the linear, homogeneously magnetized helicon plasma device VINETA. The intrinsic parallel drift wave currents have an amplitude of typically $100\,mA/cm^2$. If currents are driven mode-selectively by an array of eight electrodes or eight saddle coils on a plasma circumference, respectively, single drift wave modes can be synchronized in frequency, whereas the frequency range of synchronization depends nonlinearly on the amplitude of the driven current. The synchronization is of spatiotemporal nature, i.e.\ is only achieved if also the propagation direction of the driven current pattern is parallel to the drift wave's phase velocity. In drift wave turbulence the energy of the turbulent fluctuations can be synchronized to a single predefined drift wave mode, which results in an almost complete suppression of the associated fluctuation-induced transport. [Preview Abstract] |
Wednesday, November 4, 2009 2:24PM - 2:36PM |
PO6.00003: Drift-wave convection in a temperature filament at large Peclet number J.E. Maggs, M. Shi, D.C. Pace, G.J. Morales, T.A. Carter Heat transport in a narrow elongated temperature filament is governed by conduction at classical rates, and convection due to the ExB flows produced by the potential associated with drift-Alfven waves driven by the filament pressure gradient [Shi, et. al., Phys. of Plasmas, 16, 062306 (2009)]. The results of a study of temperature transport in a model of the temperature filament system due to pure convection (the limit of very large Peclet number) are reported. The model system uses two drift waves, one with azimuthal mode number, m=1 and the other with m=6. The drift wave amplitudes are increased slowly from zero to their maximum value. It is found that convection leads to very fine-scale spatial structures, the larger the final potential amplitude the finer the scale. The power spectrum of a temporal signal taken at a fixed spatial location is found to be exponential at all scale sizes. These spectra arise from Lorentzian-shaped temporal pulses whose temporal widths are consistent with the scale size of associated spatial structures. Although temperature fluctuations produced by pure convection can be quite large, the difference between the initial temperature profile and the azimuthal average of the modified temperature profile is rather modest. [Preview Abstract] |
Wednesday, November 4, 2009 2:36PM - 2:48PM |
PO6.00004: On the role of an inverse cascade in ion-scale turbulence Gabriel Plunk, Tomo Tatsuno, Bill Dorland Theoretical works using simplified models of magnetized plasma turbulence have brought a deeper understanding of mechanisms for zonal flow generation and saturation of ion-scale turbulence, and inverse cascade in particular is sometimes cited as an important player. However, these works investigate the Hasegawa--Mima (HM) equation or the Generalized Hasegawa--Mima (GHM) equation that assume cold ions and are thus not applicable for most plasmas. In this work we find a rigorously derived two-dimensional fluid system, valid at finite ion temperature, for turbulence at wavelengths larger than the ion Larmor radius. It is shown that the theoretical basis for an inverse cascade within this model is not present. We also perform the modulational/secondary instability analysis to investigate other mechanisms that are believed to be linked to the growth of a large-scale zonal component. Finally, the results are investigated by direct numerical simulation with AstroGK. [Preview Abstract] |
Wednesday, November 4, 2009 2:48PM - 3:00PM |
PO6.00005: Non-diffusive transport in collisionless trapped electron mode turbulence Yong Xiao Gyrokinetic simulations of collisionless trapped electron mode (CTEM) turbulence find that the electron heat transport shows a gradual transition from Bohm to gyroBohm scaling when the device size is increased. Radial correlation function shows that CTEM turbulence eddies are mainly microscopic but with a significant mesoscale tail. The mesoscale streamers result from a dynamical process of radial streamers breaking by zonal flows and merging of microscopic eddies. It is further found that the radial profile of the electron heat conductivity only follows the global profile of the fluctuation intensity, whereas the ion transport tracks more sensitively the local fluctuation intensity. This suggests the existence of a nondiffusive component in the electron heat flux, which arises from the ballistic radial ExB drift of trapped electrons due to a combination of the presence of mesoscale eddies and the weak detuning of the toroidal precessional resonance. In contrast, the ion radial excursion is not affected by the mesoscale eddies due to the parallel wave-particle decorrelation, which is not operational for trapped electrons because of the fast electron bounce motion. This is confirmed by our comprehensive analysis of kinetic and fluid time scales. [Preview Abstract] |
Wednesday, November 4, 2009 3:00PM - 3:12PM |
PO6.00006: Magnetic island induced ITG instability in multi-scale MHD and micro-turbulence Yasuaki Kishimoto, Jiquan Li, Z.X. Wang, J.Q. Dong, Miho Janvier, Kenji Imadera Multi-scale fluctuations due to different drive force can coexist and interplay with each other in magnetic fusion plasmas such as in tokamak. The cross-scale interaction may provide new energy channels as a drive or sink. As a common and typical multi-scale problem, we have proposed a gyrofluid model to numerically simulate the evolution of mixed-scale electromagnetic (EM) turbulence involving resistive MHD and ion-scale micro-instability[1]. In this work, we investigate the stability of ion temperature gradient (ITG) in a plasma with the excitation of resistive tearing mode (RTM). Our gyrofluid simulations are performed for linear excitation and nonlinear evolution of both RTM and ITG instabilities in a slab geometry. It is observed that due to the nonlinear interaction between RTM and ITG modes, the mixed scale EM fluctuation is first saturated at a lower level. Afterwards, the magnetic island continuously grows with a slow time scale. It is interestingly found that a new fluctuation propagating along ion diamagnetic direction becomes growing when the magnetic island width approaches some size. It is identified as a new ITG instability induced by magnetic island, which is referred to as MITG. [1] Jiquan Li, Y. Kishimoto, et al. Nucl Fusion 49, in printing (2009). [2] Z X Wang, Jiquan Li, J.Q.Dong, Y.Kishimoto, Phys. Plasmas 16, 060703(2009) [Preview Abstract] |
Wednesday, November 4, 2009 3:12PM - 3:24PM |
PO6.00007: Low-Frequency Fluctuations Driven by Electron Temperature Gradient in Magnetized Plasma Chanho Moon, Shuichi Tamura, Toshiro Kaneko, Rikizo Hatakeyama We investigate low-frequency instabilities driven by an electron temperature gradient (ETG), which is generated by using a magnetized electron cyclotron resonance (ECR) plasma. The ECR plasma produced by a microwave (6 GHz, 10 W) passes through mesh grids [grid 1 (\textit{$\phi $ }6 cm, 10 mesh/inch) and grid 2 (doughnut shape, OD:\textit{ $\phi $ }6 cm, ID: \textit{$\phi $ }3 cm, 30 mesh/inch)] into an experimental region where low-temperature thermionic electrons (0.2 eV) emitted from a tungsten hot plate (3 kW) are superimposed upon high-temperature electrons (2.5 eV) of the ECR plasma. By applying a bias voltage to the grid 2, a density of the high-energy electrons only in the peripheral region is controlled, and therefore the ETG can be formed easily. In addition, we observe density fluctuations in the frequency domain to find a relation with the ETG and low-frequency instabilities. As a result it can be figured out experimentally that the large ETG excites the fluctuations, but a space potential gradient is also necessary to destabilize the fluctuations. [Preview Abstract] |
Wednesday, November 4, 2009 3:24PM - 3:36PM |
PO6.00008: GAM shearing feedback loop in turbulence spreading and transport bifurcation Kazuhiro Miki, Patrick H. Diamond, Zhihong Lin To theoretically discuss the impact of the GAM on turbulence, two synergistic processes are elucidated; zonal flow modulation and the effect of secular wave group propagation on spreading. Using wavekinetic modulational analysis, the response of turbulence to the GAM is calculated. This group speed differs from that for zero-frequency zonal flows due to resonance between drift wave group speed and the GAM strain field, which allows secularity. Finite real frequency and radial group velocity are intrinsic to the GAM, so non-local phenomena at the edge region are likely. To understand the effect of the GAM on turbulence dynamics, a predator-prey model incorporating the turbulence and the GAMs is constructed and analyzed for stability. Three possible states are identified, namely on an L-mode-like stationary, an H-mode-like stationary, and an H-mode-like oscillatory state which reproduces a regime where the GAM shearing regulates the turbulence level. The system is attracted to the state with the minimum turbulence level for the given control parameters. The Doppler-shifted GAM frequency is found as the result of the GAM radiative dissipation. This material is based upon work supported by the Department of Energy under Award Numbers DE-FG02-04ER54738 and DE-FC02-08ER54959. [Preview Abstract] |
Wednesday, November 4, 2009 3:36PM - 3:48PM |
PO6.00009: Control of turbulent transport by GAMs Klaus Hallatschek In principle, geodesic acoustic modes (GAM), poloidal flows in tokamaks oscillating at the system sound frequency, are able to control turbulent transport in a similar way as the better known zonal flows - if certain conditions are met. For once, the GAMs must be weakly damped, restricting the safety factor to the relatively high values of the tokamak edge. The GAM frequency should be significantly lower than the turbulence frequencies to avoid averaging effects, and to allow effective feedback amplification of the GAMs, e.g., by diamagnetic drive. Lastly, the drive is significantly boosted in highly nonlinear situations. Using numerical turbulence studies, the effectivity of GAM drive and the dependence of transport on the GAM properties have been analyzed for varying magnetic geometry, plasma parameters, gradients, and magnitude of nonlinear terms. In particular, in the limit of high gradients and favorable circumstances the GAM activity can become so strong as to virtually suppress the transport in comparison to standard scenarios. The transport is then organized in individual bursts associated with the zero crossings of the flow oscillation. [Preview Abstract] |
Wednesday, November 4, 2009 3:48PM - 4:00PM |
PO6.00010: Photonic Beam-Plasma Instabilities and Imaging Dmitry V. Dylov, Jason W. Fleischer We consider an all-optical version of the bump-on-tail instability and show that signal-noise interactions can be modeled as a beam-plasma instability. Theoretically, the mapping follows by treating partially-coherent light using a wave-kinetic approach$^2$. We analytically derive a Bohm-Gross dispersion relation, showing that optical speckles interact via Langmuir-type modulation waves. Experimentally, we confirm the theory by demonstrating single$^2$ and multiple$^3$ bump-on-tail instabilities in a self-focusing photorefractive crystal. We then observe the recovery and amplification of noise-hidden images, showing that the coherent-incoherent coupling is a photonic beam-plasma instability. Remarkably, the plasma formula recovers a formula from information theory describing stochastic resonance, extended to include the dynamical coupling of transverse modes. The results link the fields of optics, plasma, and information theory in unanticipated ways and suggest new uses for beam shaping in material plasma. [Preview Abstract] |
Wednesday, November 4, 2009 4:00PM - 4:12PM |
PO6.00011: Orbital angular momentum of photons, plasmons and neutrinos in a plasma J.T. Mendonca, Bo Thid\'e, H. Then, S. Ali We study the exchange of angular momentum between electromagnetic and electrostatic waves in a plasma, due to the stimulated Raman and Brillouin backscatering processes [1]. Angular momentum states for plasmon and phonon fields are introduced for the first time. We demonstrate that these states can be excited by nonlinear wave mixing, associated with the scattering processes. This could be relevant for plasma diagnostics, both in laboratory and in space. Nonlinearly coupled paraxial equations and instability growth rates are derived. The characteristic features of the plasmon modes with finite angular momentum are also discussed. The potential problem is solved and the angular momentum is explicitly calculated [2]. Finally, it is shown that an electron-neutrino beam, propagating in a background plasma, can be decomposed into orbital momentum states, similar to that of photon states. Coupling between different neutrino states, in the presence of a plasma vortex, is considered. We show that plasma vorticity can be transfered to the neutrino beam, which is relevant to the understanding of the neutrino sources in astrophysics. [1] J.T. Mendonca et al., PRL 102, 185005 (2009). [2] S. Ali and J.T. Mendonca, PoP (2009) submitted. [3] J.T. Mendonca and B. Thide, Europhys. Lett. 84, 41001 (2008). [Preview Abstract] |
Wednesday, November 4, 2009 4:12PM - 4:24PM |
PO6.00012: Turbulence aspects of nonlinearly interacting ion waves in a nonuniform quantum plasma P.K. Shukla, B. Eliasson, Dastgeer Shaikh By using the inertialess electron momentum equation with the quantum statistical pressure and the quantum Bohm potential, as well as the ion continuity and momentum equations, we derive a pair of nonlinear equations for studying the turbulence properties of 2D nonlinearly interacting ion oscillations in a nonuniform quantum plasma. Computer simulations of the nonlinear quantum ion fluid equations reveal spectrum cascading and the formation of vortical structures. The resulting turbulent spectrum scaling involving nanostructues do not follow the Kolmogorov law. The relevance of our investigation to nanoscale turbulence in the interior of white dwarfs and other compact superdense astrophysical objects is highlighted. [Preview Abstract] |
Wednesday, November 4, 2009 4:24PM - 4:36PM |
PO6.00013: Spectral Dependence of Modulation Instability Can Sun, Dmitry V. Dylov, Jason W. Fleischer We consider the spectral dependence of modulation instability. In addition to the usual Maxwell-Boltzmann, or Gaussian, distribution, we consider Lorentzian, exponential and rectangular power spectra. We show that each distribution gives its own threshold for instability, and that the rectangular spectrum gives the counterintuitive trend of increasing perturbation period as the profile narrows (becomes more coherent). Theoretical results are confirmed experimentally by mapping the plasma problem to the equivalent statistical optics system $^{2,3}$. [Preview Abstract] |
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