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 NI2: Energetic Particles, 3D Physics |
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Chair: Nikolai Gorelenkov, Princeton Plasma Physics Laboratory Room: Plaza E |
Wednesday, November 13, 2013 9:30AM - 10:00AM |
NI2.00001: Energetic-particle-driven instabilities and induced fast-ion transport in a reversed field pinch Invited Speaker: Liang Lin Multiple bursty energetic-particle (EP) modes with fishbone-like structures are observed during 1 MW tangential neutral-beam injection into MST reversed field pinch (RFP) plasmas. The distinguishing features of the RFP, including large magnetic shear (tending to add stability) and weak toroidal magnetic field (leading to large fast ion beta and stronger drive), provide a complementary environment to tokamak and stellarator configurations for exploring basic understanding of these instabilities. Detailed measurements of the EP mode characteristics and temporal-spatial dynamics reveal their influence on fast ion transport and interaction with global tearing modes. Internal magnetic field fluctuations associated with the EP modes are directly observed for the first time by Faraday-effect polarimetry (frequency $\sim 90$ kHz and amplitude $\sim 2$ G). Simultaneously measured density fluctuations exhibit a dynamically evolving and asymmetric spatial structure that peaks near the core where fast ions reside and shifts outward as the instability evolves. Furthermore, the EP mode frequencies appear at $\sim k_{\parallel } V_{A} $, consistent with continuum modes destabilized by strong drive. The fast-ion temporal dynamics, measured by a neutral particle analyzer, resemble a classical predator-prey relaxation oscillation. It contains a slow-growing phase arising from the beam fueling followed by a rapid drop ($\sim 15\% )$ when the EP modes peak, indicating the fluctuation-induced transport maintains a stiff fast-ion density profile. The inferred transport rate is strongly enhanced ($\times 2)$ with the onset of multiple nonlinearly-interacting EP modes. The fast ions also impact global tearing modes, reducing their amplitudes by up to 65{\%}. This mode reduction is lessened following the EP-bursts, further evidence for fast ion redistribution that weakens the suppression mechanism. Possible tearing mode suppression mechanisms will be discussed. [Preview Abstract] |
Wednesday, November 13, 2013 10:00AM - 10:30AM |
NI2.00002: Enhanced Localized Energetic Ion Losses Resulting from First-orbit Linear and Nonlinear Interactions with Alfv\'en Eigenmodes in DIII-D Invited Speaker: X. Chen First observations of enhanced prompt neutral beam-ion losses due to non-resonant scattering by Alfv\'en eigenmodes (AEs) in DIII-D provide a novel measure of the displacement of fast-ions due to individual modes. The coherent losses are from full energy beam-ions born on unperturbed trapped orbits that pass close to a fast-ion loss detector (FILD) within one poloidal transit. However, the perturbing effect of AEs can cause the particles to be expelled from the plasma before completing their first poloidal orbit. The FILD signals emerge within 100$\,\mu$s after the beam switch-on, which is the time scale of a single poloidal transit, and oscillate at the mode frequencies. This loss mechanism can account for a large fraction of fast-ion losses observed in some DIII-D discharges (even for mode amplitudes as low as $\delta B/B\leq 1\times 10^{-3}$). Time-resolved loss measurements show a linear dependence on the AE amplitude. Simulations with the full-orbit-following SPIRAL code reveal that the dominant losses are due to a change of the magnetic moment rather than the toroidal canonical momentum. A fast-ion radial displacement of $\sim$10 cm is directly measured, which provides a unique experimental validation of SPIRAL simulations and suggests a new diagnostic method. The FILD signals also probe nonlinear interactions, needed to develop a self-consistent model of AE induced transport. Oscillations in the beam-ion losses are observed at the sum and difference frequencies of two AEs as well as at the second harmonics--these oscillations are absent in magnetic, density, and temperature fluctuations. The nonlinear features seen by FILD can be explained by an analytical model, where changes in phase caused by deflections of the orbit at fundamental frequencies generate the additional frequencies. SPIRAL simulations that include the toroidal geometry, beam deposition profiles, and realistic eigenmodes show good agreement with the observations. [Preview Abstract] |
Wednesday, November 13, 2013 10:30AM - 11:00AM |
NI2.00003: Fast-ion energy loss during TAE avalanches in the National Spherical Torus Experiment Invited Speaker: Eric Fredrickson Increasingly, advanced operating scenarios are dependent on non-inductive control of the current profile with neutral beam injection often used as the tool to modify the current profile. It is then important to understand and predict the impact of energetic particle driven modes, such as fishbones and Alfvenic instabilities on the distribution of fast ions. The redistribution of fast ions is an inherently non-linear problem involving mode saturation physics, non-linear changes in mode structure, frequency chirping and coupling between modes. Direct 3-wave coupling and coupling of different modes through the fast ion distribution together with other non-linear behaviors are seen in the National Spherical Torus Experiment. From a database constructed of over 700 TAE avalanche events on NSTX it was found that avalanches occur only when $\beta_{\mathrm{fast}}$/$\beta _{\mathrm{total}}$ greater than 0.3 and mode amplitude $\delta $B$_{\mathrm{\theta }}$/B greater than 5x10$^{-4}$. The avalanches are correlated with drops in neutron rate of up to $\approx $25 percent, indicating an enhancement in fast ion transport. The fast ion transport is modeled with the ORBIT code using measured TAE frequency and mode amplitude evolution to scale the linear eigenmodes calculated with NOVA. This modeling found that the fast ion losses from the plasma were negligible at the measured mode amplitudes, however, the simulations predicted significant drops in neutron rate. The estimated transfer of energy from the fast ion distribution to the TAE, comparable to the estimated energy loss through wave damping, results in a predicted neutron rate of about half that observed. The fast ions were also moved outward in minor radius to a region of lower plasma density, accounting for the other half of the observed neutron rate drop. [Preview Abstract] |
Wednesday, November 13, 2013 11:00AM - 11:30AM |
NI2.00004: Unified parametric dependence, control, and reconstruction of 3D equilibria in the RFP Invited Speaker: Brett Chapman A helical, stellarator-like equilibrium emerges in the core of RFP plasmas when the normally broad tearing mode spectrum spontaneously condenses -- the innermost resonant mode grows to large amplitude, while the other, secondary mode amplitudes are reduced. This quasi-single-helicity (QSH) transition is not fully understood, but it likely hinges on the nonlinear MHD that governs the tearing mode spectrum. Here we report (1) progress in understanding the transition in terms of the Lundquist number, S, a key dimensionless parameter in nonlinear MHD, (2) improved energy confinement in MST with QSH and inductive current profile control, and (3) progress in developing 3D equilibrium reconstructions for QSH plasmas. In MST, the likelihood and duration of QSH spectra increase strongly with the plasma current, Ip, similar to the trend in RFX-mod, but the Ip at which QSH emerges in MST is 3x smaller. However, given that MST can, for a given Ip, access lower density and higher Te, the two devices share a common range of S, which varies as Ip*Te$^{\mathrm{1.5}}$n$^{\mathrm{-0.5}}$, and the tearing spectra from the two devices exhibit a common dependence on S. The above results accrued in plasmas with largely Maxwellian electrons and ions. With the addition of neutral-beam-injected fast (25 keV) ions in MST, the likelihood of QSH in low-S plasmas decreases further. In high-S plasmas, the likelihood of QSH is largely unaffected by the fast ions. The dominant mode in MST can reach 8{\%} of the equilibrium field. This, combined with the reduced secondary modes, leads to a locally enhanced Te in the core and a 50{\%} improvement in energy confinement. The secondary modes are further reduced by slowly ramping down Ip, a form of current profile control. This leads to a larger Te \textgreater 1 keV and a tripling of the energy confinement. These results were achieved with zero applied Bt (infinite toroidal beta). The 3D magnetic topology was measured directly for the first time in MST via Faraday rotation. This and other advanced diagnostics are being included in the V3FIT equilibrium reconstruction code through a multi-institution collaboration. The varying orientation of the 3D structure relative to the diagnostics will help in V3FIT optimization. [Preview Abstract] |
Wednesday, November 13, 2013 11:30AM - 12:00PM |
NI2.00005: Magnetic Flutter Plasma Transport Induced by 3D Fields in DIII-D Invited Speaker: S.P. Smith New combined MHD and transport modeling show that the recently developed magnetic flutter model of plasma transport [1] predicts an electron thermal diffusivity ``hill" at the top of the pedestal comparable to that seen in DIII-D discharges where edge localized modes (ELMs) are suppressed by the application of 3D fields. It is hypothesized that this ``hill" prevents the inward growth of the pedestal to avoid reaching the peeling-ballooning limit that precipitates an ELM. The magnetic perturbations in the plasma are modeled with the two fluid MHD M3D-C1 code, which results in perturbation harmonics that are flow-screened or amplified (relative to vacuum calculations) at their corresponding rational surface. Despite any screening, the flutter predicted diffusivity peaks at the rational surfaces in the plasma. However, in integrating the inverse of the diffusivity to obtain an electron temperature profile, the average predicted temperature gradient is dominated by the smaller but finite diffusivity between rational surfaces. The magnitude of the flutter predicted diffusivity is decreased by a factor of about 4/13 when the effects of the pressure gradient and radial electric field (off-diagonal transport matrix terms) are taken into account, and this lower predicted diffusivity is much closer to the experimentally inferred diffusivity. On the other hand, adding a large diffusivity in the approximate island width regions around the rational surfaces leads to a diffusivity much higher than inferred experimentally. Flutter effects on the electric field and particle transport are also currently being investigated. These flutter model studies provide a promising new approach for developing a predictive capability for achieving ELM suppression in future devices.\par \vskip6pt \noindent [1] J.D.\ Callen, A.J.\ Cole, and C.C.\ Hegna, Phys.\ Plasmas {\bf 19}, 112505 (2012). [Preview Abstract] |
Wednesday, November 13, 2013 12:00PM - 12:30PM |
NI2.00006: Plasma Response Measurements of Non-Axisymmetric Magnetic Perturbations on DIII-D Invited Speaker: M.W. Shafer Measurements of three-dimensional (3D) perturbations due to the application of non-axisymmetric fields in DIII-D show experimental evidence of helical distortions that modify the tokamak boundary during edge localized mode (ELM) suppression experiments. Two distinct 3D features localized in minor radius are imaged via tangential filtered soft x-ray (SXR) emission in the X-point region: (i) the formation of lobes extending from the unperturbed separatrix at the plasma boundary with a low energy filter ($T_e \ga 40\,$eV), and (ii) helical kink-like displacements in the steep-gradient region inside the separatrix with a higher energy filter ($T_e \ga 400\,$eV). These measurements are used to test and to validate plasma response models, which are crucial for providing predictive capability of ELM control. In particular, vacuum and two-fluid resistive MHD responses are tested in the regions of these measurements. At the plasma boundary, measurements compare well to vacuum-field calculations that predict lobe structures created by intersecting manifolds based on a Hamiltonian formulation of the perturbed magnetic field line structure. This is corroborated by previous measurements of heat and particle flux strike-point splitting. Yet in the steep gradient region, displacement measurements agree better to calculations with the linear resistive two-fluid MHD code, M3D-C1. Displacements measured via Thomson scattering, poloidally and toroidally separated from the SXR imaging, also match this modeling. The calculations show partial resonant screening and non-resonant amplification compared to vacuum model, leading to a stronger kink response. This is largely dependent on rotational screening from large perpendicular electron flow. These results indicate that while the vacuum approach describes measurements in the edge region well, it is important to include two-fluid resistive MHD effects for the H-mode pedestal. [Preview Abstract] |
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