Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Plasma Physics
Volume 66, Number 13
Monday–Friday, November 8–12, 2021; Pittsburgh, PA
Session JI01: MFE II: Turbulence and TransportInvited Live
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Chair: David Hatch, University of Texas at Austin Room: Ballroom B |
Tuesday, November 9, 2021 2:00PM - 2:30PM |
JI01.00001: Confinement and collisionality in I-mode and L-mode decoupled from edge Weakly Coherent Mode at ASDEX Upgrade Invited Speaker: Rachel Bielajew The I-mode confinement regime is a promising operational scenario for future fusion reactors because it features high energy confinement without high particle confinement [1]. The edge-localized Weakly Coherent Mode (WCM) in I-mode is often used to contrast I-mode and L-modes. However there has been observation of WCM precursors in L-modes preceding I-modes [2], which call into question the role of the WCM in determining the transport properties of I-mode. Novel I-mode experiments with higher than typical collisionality (νei) at ASDEX Upgrade have been performed that unambiguously show that the WCM is present in both L-mode and I-mode. Furthermore, the quality of the confinement of the discharge phases is found to be independent of the presence and amplitude of the WCM or WCM-like feature, but inversely dependent on pedestal density and collisionality. Electron temperature fluctuation (δTe/Te) measurements in the plasma outer core and pedestal (ρpol=0.85-1.0) are made with a new 24-channel high radial resolution Correlation Electron Cyclotron Emission radiometer [3]. The WCM in I-mode is located in the pedestal region (ρpol=0.97-1.0) with δTe/Te=2-4%. A lower frequency and more coherent WCM-like feature also appears in the L-mode. Linear gyrokinetic analysis performed with the CGYRO code suggests that dominant turbulent modes in the outer core are electrostatic and ion directed while the edge and pedestal modes nearer the WCM location are electron directed and electromagnetic in nature. |
Tuesday, November 9, 2021 2:30PM - 3:00PM |
JI01.00002: Multi-field Ion Distribution Fluctuation Measurements of Te/Ti-dependent Transport in DIII-D Invited Speaker: Dinh Truong The electron-ion temperature ratio (Te/Ti) has minimal impact on measured long-wavelength ion temperature fluctuation amplitudes in DIII-D L‑mode plasmas, while parallel velocity fluctuations are found to decrease at higher Te/Ti, accompanied by large increases in local ion thermal transport and reduced energy confinement. These carbon ion fluctuation measurements are obtained with the novel Ultra‑Fast Charge Exchange Recombination Spectroscopy (UF-CHERS) diagnostic at two line-averaged densities; neutral beam (NBI) heated and neutral beam plus electron cyclotron (ECH) heated phases are examined to systematically vary collisionality (ν*) and Te/Ti. In addition, low and intermediate wavenumber density fluctuation (ñ/n) measurements show virtually no change. Increasing Te/Ti is expected to excite drift wave instabilities which typically drive most turbulent transport. TGLF modeling using experimental profiles shows decreased ion mode growth rates and increased electron mode growth rates when increasing Te/Ti at both densities. The changes in growth rates are, in part, consistent with observations of increased electron temperature fluctuations. TGLF results are also consistent with poloidal turbulence velocity measurements, which show mode propagation trending from the ion diamagnetic towards the electron diamagnetic direction in the plasma frame as Te/Ti is increased. TGLF, however, predicts a significant decrease in relative Ti fluctuations with increasing Te/Ti at low density, which is not observed. This discrepancy provides an excellent test case for nonlinear gyrokinetic turbulence codes at burning plasma relevant parameters with Te~Ti and low rotation. |
Tuesday, November 9, 2021 3:00PM - 3:30PM |
JI01.00003: Fast ion-induced transport barriers in global gyrokinetic simulations Invited Speaker: Alessandro Di Siena In this contribution, we discuss global gyrokinetic simulations predicting a possible new high-confinement regime in fusion plasmas, sustained via a novel type of transport barrier called F-ATB (fast ion-induced anomalous transport barrier) [1]. More specifically, we performed global GENE [2, 3] simulations with realistic ion-to-electron mass ratio, collisions, electromagnetic effects, and fast ions modelled with realistic background distributions and profiles of a properly designed ASDEX Upgrade plasma discharge with substantial ion-cyclotron-resonance-heating (ICRH). The trigger mechanism responsible for the F-ATB generation is shown to be a recently discovered wave-particle resonance interaction between supra-thermal particles and ion-scale plasma turbulence [4, 5, 6]. This interaction can strongly affect the free energy content of the ITG instabilities and lead to a transport barrier [1]. Signatures of these beneficial fast ion effects have been observed at ASDEX Upgrade [1], JET [7] and are also expected for W7-X [6] and during the ramp-up phase of an ITER [4] standard scenario. |
Tuesday, November 9, 2021 3:30PM - 4:00PM |
JI01.00004: Observations of Magnetic Turbulence During Local Helicity Injection on PEGASUS Invited Speaker: Nathan J Richner Local helicity injection (LHI) uses small localized current sources for DC helicity injection to provide high plasma current non-solenoidal tokamak startup, which is critically needed for the spherical tokamak. Helicity-conserving instabilities relax the injected electron streams to form a tokamak-like state with Ip >> Iinj. To better understand the relaxation and accompanying current drive processes, characterization of the magnetic activity present during LHI was conducted on the PEGASUS spherical tokamak. Frequency spectra of broadband turbulence during LHI follow power law behaviors (∼ -5/3 at MHD scales and ∼ -8/3 at sub-ion scales) similar to those of Alfvén wave turbulence observed in astrophysical systems. Such turbulence transfers magnetic helicity from small to large scales, providing a candidate mechanism for magnetic relaxation. This activity is localized within the plasma edge where the injected current streams entrain to the tokamak-like core. The streams are both suprathermal and super-Alfvénic, and the turbulence shows a strong dependence on the inferred beam energy. Nonlinear spectral analyses indicate unstable modes in the intermediate, MHD frequency regime. Coarse estimates of current drive from dynamo EMFs are comparable with the current density from equilibrium reconstructions. These observations provide a working picture of a relaxation and current drive mechanism active during LHI. Beam-driven instabilities in the injected current streams drive MHD Alfvénic activity that nonlinearly couples to drive broadband turbulence. Small-scale dynamo activity from the associated cascades drives net plasma current. This paradigm imposes requirements on applications of LHI to fusion-scale, specifically that the injected streams must satisfy requirements for excitation of edge magnetic turbulence. |
Tuesday, November 9, 2021 4:00PM - 4:30PM |
JI01.00005: Zonally dominated dynamics and the transition to strong turbulence in ion-scale plasma turbulence Invited Speaker: Plamen G Ivanov Since the discovery of the Dimits shift (Dimits et al. 2000), the role of zonal flows (ZFs) for the turbulence in tokamak plasmas has been an area of intense research. We attempt to shed some light on this problem by studying the transition to turbulence in a cold-ion fluid model for ion-scale turbulence. Our equations are obtained in an asymptotic cold-ion limit of the ion gyrokinetic equation and capture the two ion-temperature-gradient (ITG) instabilities driven by either magnetic curvature or parallel compression. We find that this model has a well-defined Dimits (low-transport, ZF-dominated) state characterised by a staircase-like arrangement of ZFs that suppresses turbulence. Viscous decay of the ZFs leads to turbulent bursts that reconstitute the staircase by providing a negative zonal turbulent viscosity. In 2D, at large equilibrium temperature gradients, the zonal turbulent viscosity switches sign and the Dimits state is destroyed. In 3D, the Dimits state is much more resilient and can always be sustained provided sufficient parallel extent of the system. This is because the large-scale curvature-driven perturbations go unstable to small-scale "parasitic" 3D slab-ITG modes that always give rise to a negative zonal turbulent viscosity. If we restrict the parallel extent of the system, a strongly turbulent, high-transport state emerges. In this state, energy is injected into large-scale perturbations by the curvature-ITG instability, then transferred into the parasitic small-scale modes, and finally dissipated by the finite collisionality. We also find that insufficient parallel resolution can result in a non-physical breakup of the Dimits regime. |
Tuesday, November 9, 2021 4:30PM - 5:00PM |
JI01.00006: Observation of turbulence spreading into edge stochastic magnetic layer caused by MHD mode excitation in LHD Invited Speaker: Masahiro Kobayashi The theoretical models of turbulence spreading have been proposed since 1990’s [1,2]. Experimentally, however, direct observation of the turbulence spreading is difficult, because it requires to distinguish the fluctuations excited locally from those propagated remotely from somewhere else. While there are several examples [3,4], in this contribution we present new observation where the spreading is induced by MHD instability. |
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