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 KI3: Transport and Turbulence |
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Chair: David E. Newman, University of Alaska Room: Plaza F |
Tuesday, November 12, 2013 3:00PM - 3:30PM |
KI3.00001: Observation of multi-scale turbulence and non-local transport in LHD plasmas Invited Speaker: Tokihiko Tokuzawa Recent studies of relation between heat/particle/momentum flux and temperature/density/flow gradient show that the flux is not necessarily determined by the local plasma parameters [1]. This violation of local closure in transport is accompanied by peculiar change in the amplitude of turbulent fluctuation. This talk reports recent experimental observations of multi-scale turbulence by the microwave diagnostics with high temporal and spatial resolution in LHD, and discusses its relation to non-local transport. These observations suggest that the micro turbulence in the plasma is coupled with meso- and macro-scale turbulence and it is strongly influenced by the plasma parameters at different and separated radii, which causes ``non-local'' transport. In order to investigate the response of multi-scale turbulence at the event of non-local transport phenomenon, (1) accurate measurements of the distinctive multi-scale turbulence, and (2) identification of the causal relationship between turbulence with different scale length, have been developed in LHD. In this paper, the following experimental results are discussed: (a) long range fluctuation, (b) hysteresis in the flux-gradient relation, and turbulence propagation after (c) an ELM collapse and (d) limiter biasing. Based on the high temporal spatial resolved turbulence measurements in LHD, how the multi scale turbulence plays a role on non-local of ballistic transport is documented.\\[4pt] [1] K. Ida, et al., Proc. 24th IAEA Fusion Energy Conference 2012 San Diego OV/3-4. ``Towards an Emerging Understanding of Nonlocal Transport'' [Preview Abstract] |
Tuesday, November 12, 2013 3:30PM - 4:00PM |
KI3.00002: Electromagnetic gyrokinetic turbulence in high-beta helical plasmas Invited Speaker: Akihiro Ishizawa Gyrokinetic simulation of electromagnetic turbulence in finite-beta plasmas is important for predicting the performance of fusion reactors. Whereas in low-beta tokamaks the zonal flow shear acts to regulate ion temperature gradient (ITG) driven turbulence, it has often been observed that the kinetic ballooning mode (KBM) and, at moderate-beta, the ITG mode continue to grow without reaching a physically relevant level of saturation. The corresponding problem in helical high-beta plasmas, the identification of a saturation mechanism for microturbulence in regimes where zonal flow generation is too weak, is the subject of the present work. This problem has not been previously explored because of numerical difficulties associated with complex three-dimensional magnetic structures as well as multiple spatio-temporal scales related to electromagnetic ion and electron dynamics [1]. The present study identifies a new saturation process of the KBM turbulence originating from the spatial structure of the KBM instabilities in a high-beta Large Helical Device (LHD) plasma. Specifically, the most unstable KBM in LHD has an inclined mode structure with respect to the mid-plane of a torus, i.e. it has finite radial wave-number in flux tube coordinates, in contrast to KBMs in tokamaks as well as ITG modes in tokamaks and helical systems. The simulations reveal that the growth of KBMs in LHD is saturated by nonlinear interactions of oppositely inclined convection cells through mutual shearing, rather than by the zonal flow shear. The mechanism is quantitatively evaluated by analysis of the nonlinear entropy transfer.\\[4pt] [1] A. Ishizawa, et.al., Nuclear Fusion 53, 053007 (2013). [Preview Abstract] |
Tuesday, November 12, 2013 4:00PM - 4:30PM |
KI3.00003: An Explanation for the High-$\beta$ Runaway: the Non-Zonal Transition Invited Speaker: M.J. Pueschel During the so-called high-$\beta$ runaway, heat and particle fluxes grow to extremely large values after a transient saturation phase---this can occur well below the kinetic ballooning threshold. It is shown that this growth is driven by an ion temperature gradient mode which is no longer saturated by zonal flows; instead of {\it runaway}, this process is thus termed {\it non-zonal transition} [M.J.~Pueschel et al., Phys.~Rev.~Lett.~{\bf 110}, 155005 (2013)]. Changes in zonal flow drive and tertiary modifications of the driving gradients are excluded as potential causes; whereas zonal flow decay due to magnetic fluctuations is singled out as the responsible mechanism. An important contribution stems from field line decorrelation: magnetic field lines, on their way from the inboard to the outboard side, are displaced radially by a perturbed field $B_x$. This displacement can reach the radial correlation length of $B_x$, causing the non-resonant fluctuations to contribute to the magnetic stochasticity; suddenly subjecting the zonal flows to much stronger decay. The non-zonal transition provides a new critical $\beta$ that, for sufficiently strong background pressure gradients, can be significantly more restrictive than the ballooning threshold. [Preview Abstract] |
Tuesday, November 12, 2013 4:30PM - 5:00PM |
KI3.00004: Predictions of the Transport-limited Fusion Alpha Profile in ITER Invited Speaker: E.M. Bass We present a simple 1D transport model prediction for the alpha density profile in an ITER burning plasma. Thermal species profiles, which provide the basis for the alpha birth and collisional slowing-down rates, come from a recent ITER H-mode prediction using the EPED model for the pedestal and a quasilinear microturbulent model in the core [1]. The model includes the fusion source, an effective sink into a population of helium ash, and diffusive transport due to both microturbulence and alpha-driven Alfv\'en eigenmodes (AEs). Where applicable, we assume the local distribution maintains the classical slowing-down form. The microturbulent (passive) contribution to alpha transport is given by combining the known absolute energy flux appropriate for a $Q=10$ scenario with the GYRO simulation-fitted model for the quasilinear transport ratio given in Ref.~[2]. A ``stiff" transport model gives the alpha-driven AE component. In this model, {\it local} AEs drive the alpha gradient to the {\it local} mode stability threshold determined by fully realistic GYRO simulations. Both the stiffness [3] and locality [4] of AE transport are supported by previous work. We compare the impact on the AE stability threshold of using Maxwellian versus slowing-down forms for the alpha distribution. In general, the AEs are found to re-distribute fusion alphas within the plasma core, but the Alfv\'en avalanche does not appear to propagate to the loss boundary. Microturbulence can drive modest alpha losses at the edge.\par \vskip3pt \noindent [1] J.E.\ Kinsey et al., Nucl.\ Fusion {\bf 51}, 083001 (2011)\par \noindent [2] C.~Angioni et al., Nucl.\ Fusion {\bf 49}, 055013 (2009)\par \noindent [3] E.M.\ Bass and R.E.\ Waltz, Phys.\ Plasmas {\bf 17}, 112319 (2010)\par \noindent [4] E.M.\ Bass and R.E.\ Waltz, Phys.\ Plasmas {\bf 20}, 012508 (2013) [Preview Abstract] |
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