Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Plasma Physics
Volume 65, Number 11
Monday–Friday, November 9–13, 2020; Remote; Time Zone: Central Standard Time, USA
Session NI02: Invited: Magnetic Fusion: ELMsLive
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Chair: Ahmed Diallo, PPPL |
Wednesday, November 11, 2020 9:30AM - 10:00AM Live |
NI02.00001: Experimental observations on energy flow from low frequency electromagnetic components to broadband turbulence via nonlinear couplings during ELM crashes Invited Speaker: Young-chul Ghim It is well known that transport levels during ELM crashes are much larger compared to the inter-ELM phases, which have been explained based on linear stability criteria, e.g. reduction of shear flow rates, and formation of stochastic magnetic fields. In this work, we show that nonlinear coupling is an additional factor causing increased fluctuation levels during the ELM crashes. More specifically, we find that low frequency (\textless 15 kHz) density fluctuations contain electromagnetic components (based on second-order spectral analyses with magnetic field fluctuations), which transfer energies to broadband (\textless 100 kHz) density fluctuations via nonlinear interactions. Such nonlinear interactions are not observed during the inter-ELM phases. Density and magnetic field fluctuations are measured by a 2D beam emission spectroscopy and a Mirnov coil, respectively, in KSTAR. Existence of nonlinear couplings between two different frequencies are identified based on bicoherence, while the direction of energy flow is found using the Volterra series. These findings are based on a conditional ensemble average scheme applied to 2,660 ELMs obtained from three different long ELMy H-mode KSTAR discharges. The results of this study indicate that one must include the effect of nonlinear couplings, which exist only during ELM crashes and not during inter-ELM phases, between low frequency electromagnetic component and broadband fluctuations to understand the transport phenomena during ELM cycles. [Preview Abstract] |
Wednesday, November 11, 2020 10:00AM - 10:30AM Live |
NI02.00002: On-demand destabilization of edge turbulence and robust ELM elimination with Boron powder injection in EAST Invited Speaker: Zhen Sun We report the achievement of highly reproducible H-mode discharges with elimination of edge-localized modes (ELMs) with boron (B) powder injection in EAST. A key ingredient of the observed ELM elimination is the destabilization of a continuous edge harmonic oscillation above a threshold B edge concentration; the enhanced turbulence drives transport to maintain constant line-average density with modest confinement improvement. The observed mode exists in the pedestal region $\rho $ \textasciitilde 0.89-1, peaking in amplitude inside the separatrix, while also being manifest on the open field lines at the divertor targets. We hypothesize that the density perturbation induced by the B ablation generates an oscillation with features of a geodesic-acoustic mode that increase pedestal particle transport via orbit loss. This ELM-stable regime has controlled core impurity concentration, and no strong wall conditioning or hysteresis: ELMs resume within 0.5 s of termination of B powder injection. The operating window for this scenario persists over a wide range of plasma conditions, wider by far than any other ELM control method, and seemingly insensitive to normalized pedestal electron collisionality $\nu^{\mathrm{\ast }}_{\mathrm{e,\thinspace ped}}$, edge safety factor q$_{\mathrm{95}}$, plasma toroidal rotation that was varied by the mix of auxiliary heating schemes, and even main ion species. In contrast to ELM elimination by lithium powder injection in tokamaks, B powder injection does not result in reduced recycling and enhanced wall retention. The applicability to future devices includes projecting the minimum B injection rate for ELM elimination, which was measured to increase with heating power in EAST, and control of injected particle inventory; modeling-based assessment to future devices has commenced. [Preview Abstract] |
Wednesday, November 11, 2020 10:30AM - 11:00AM Live |
NI02.00003: Pellet ELM triggering with low collisionality, peeling-limited pedestals in DIII-D Invited Speaker: Robert Wilcox ELMs triggered using large deuterium pellets injected from the low-field side of the DIII-D tokamak into ITER-relevant low collisionality plasmas have a peak ELM energy fluence that is reduced by as much as 50% relative to a natural ELM, as measured by infrared camera. ELM energy deposition is reduced with respect to naturally occurring ELMs at the inner strike point (ISP), where it is naturally largest, while energy deposition is less affected at the outer strike point (OSP). The reduction of peak ELM energy fluence to the divertor may enable a reduction in the required pellet pacing frequency and an increase in the peak and time-averaged pedestal pressure during pellet ELM pacing in ITER and next-step devices with low collisionality pedestals. D-alpha fast camera images show that the ISP ELM particle flux is reduced in the pellet-triggered case as well, suggesting that the reduction in heat flux is not simply due to pellet mass being carried by divertor drifts to the inner leg and protecting the ISP by enhanced ionization / radiation. ELITE calculations show that without pellet injection, these low collisionality pedestals were limited by current density (peeling-limited) rather than pressure gradient (ballooning-limited), which is the condition expected in ITER and any other next-step tokamak with high edge temperatures and large bootstrap currents. These are novel conditions for pellet ELM triggering experiments in DIII-D, as it has been historically difficult to achieve low collisionality while injecting pellets that raise the density. In many cases here, a large pellet (1.8 mm) triggered an ELM shortly after a smaller pellet (1.3 mm) failed to do so, suggesting that a larger pressure perturbation is required to trigger an ELM in low collisionality discharges that are far from the ballooning stability boundary. [Preview Abstract] |
Wednesday, November 11, 2020 11:00AM - 11:30AM Live |
NI02.00004: Predicting operational windows of ELMs suppression by Resonant Magnetic Perturbations in the DIII-D and KSTAR tokamaks. Invited Speaker: Qiming Hu A newly developed plasma response model, combining the nonlinear two-fluid MHD code TM1 and toroidal ideal MHD code GPEC, quantitatively predicts the narrow isolated $q_{\mathrm{95}}$ windows ($\Delta q_{\mathrm{95}}\approx $ 0.1) of ELM suppression by $n=$ 1, 2 and 3 resonant magnetic perturbations (RMPs) in both DIII-D and KSTAR tokamaks across a wide range of plasma parameters. The key physics that unites both experimental observations and our simulations is the close alignment of key resonant $q$-surfaces and the location of the top of the pedestal prior to an ELM. This alignment permits an applied RMP to produce field penetration rather than being screened due to the lower \textbf{E}x\textbf{B} rotation at the pedestal top. The model successfully predicts that narrow magnetic islands form when resonant field penetration occurs at the top of pedestal, and these islands are easily screened when $q_{\mathrm{95}}$ moves off resonance, leading to very narrow windows of ELM suppression (typically $\Delta q_{\mathrm{95}}\approx $ 0.1). Furthermore, the observed reduction in the pedestal height is also well captured by the calculated collisional transport across the island. We recover the $q_{\mathrm{95}}$, $\beta_{\mathrm{N}}$ and plasma shape dependence of ELM suppression due to the effect of magnetic islands on pedestal transport and Peeling-Ballooning-Mode (PBM) stability. Importantly, experiments do occasionally observe wide windows of ELM suppression ($\Delta q_{\mathrm{95}}\ge $ 0.5). Our model reveals that at low density multiple islands open at the pedestal top, leading to wide operational windows of ELM suppression consistent with experiment. The model indicates that wide $q_{\mathrm{95}}$ windows of ELM suppression can be achieved at substantially higher pedestal pressure with less confinement degradation in DIII-D by operating at higher toroidal mode number ($n=$ 4) RMPs. This can have significant implications for the operation of the ITER ELM control coils for maintaining high confinement during ELM suppression. [Preview Abstract] |
Wednesday, November 11, 2020 11:30AM - 12:00PM Live |
NI02.00005: Reactor-friendly core and boundary of DIII-D diverted negative triangularity plasmas with persistent L-mode edge Invited Speaker: Max Austin At high heating power DIII-D negative triangularity (NT) discharges are characterized by high confinement ($H_{98y2} \sim 1.3$) and significant $\beta_N \sim 3.0$ while maintaining an L-mode edge and being robustly ELM free. Recent experiments were carried out with a new shape that permitted diverted operation for the first time with strong negative triangularity $\delta_u = -0.4$ in the upper half and neutral triangularity $\delta_l \sim 0.0$ on the lower half. These discharges retained an L-mode edge with up to 14 MW of auxiliary heating power, exceeding the expected L-H transition power by a factor of 5 while still exhibiting H-mode confinement globally ($H_{98y2} = 1.0$). In one shot, an accidental relaxation of $\delta_u$ to -0.2 resulted in an H-mode transition at $P_{aux} = 4$ MW. Comparison of the heat flux width of this H-mode edge discharge with the L-mode edge ones indicates the L-mode cases have up to 50\% wider divertor heat flux width $\lambda_q$. Impurity transport experiments demonstrate that the NT discharges have $\tau_p /\tau_E \sim 1$, as opposed to typical H-modes with $\tau_p /\tau_E \sim$ 2-3. Additional experiments and modeling are giving new insights into the good performance of up-down symmetric NT shape. Nonlinear CGYRO runs confirm experimental observations of reduced energy transport in NT compared to matched positive triangularity (PT) discharges; the H-mode-level confinement is attributable to a combination of shape-related effects and weakened turbulent transport. Both global and perturbative transport analysis indicate improved confinement in NT compared to matched PT cases. The features of the negative triangularity plasma represent significant advantages in achieving core-edge integration via high radiation fraction and provide a simpler technical path to divertor construction in a reactor. [Preview Abstract] |
Wednesday, November 11, 2020 12:00PM - 12:30PM Live |
NI02.00006: Favorable Core and Pedestal Transport Properties of the Wide Pedestal QH-Mode Regime Invited Speaker: D. R. Ernst The high confinement Wide Pedestal Quiescent H-Mode regime ($H_{98y2}$ up to 1.6) is promising for steady burning plasma operation without ELMs and associated divertor damage, at ITER collisionalities, with nearly equal ion and electron temperatures and no net torque injected. In recent DIII-D experiments, unlike other H-Modes, confinement improves when electron cyclotron heating (ECH) replaces neutral beam power, promising for burning plasma operation. We have sustained Wide Pedestal QH-Mode for several confinement times with up to 77\% ECH power (23\% NBI) with $T_{e0} > 12$ keV.\footnote{D. R. Ernst et al., in Proc. IAEA Fusion Energy Conference, IAEA-CN-123/EX2-2, Gandhinagar, India (2018).} Fourier analysis of the ECE $T_e$ response to modulated ECH separates diffusion and convection in the electron power balance, revealing an inward core electron thermal pinch, forming an internal transport barrier (ITB) in $T_e$ as the ECH is moved on-axis. The pinch is being explored using GENE simulations (now with the first exact gyrokinetic collision operator\footnote{Q. Pan, D. R. Ernst, and P. Crandall, Phys. Plasmas {\bf 27}, 042307 (2020).}). TEM turbulence dominates, driving significant magnetic flutter transport. Even without the ITB, ion channel confinement improves in the core and pedestal as the fraction of off-axis electron heating increases. The pedestal $E_r$ well broadens and deepens, while the intensities of low and intermediate wavenumber density fluctuations respond oppositely. Wide Pedestal QH-Mode has been separately demonstrated with zero net injected NBI torque throughout. We have measured the effective intrinsic torque profile as a function of ECH power fraction (0\%, 32\%, 52\%), while simultaneously measuring electron thermal transport. The intrinsic torque density balances that from edge beam orbit loss to produce near-zero total torque density across the profile. The edge beam orbit loss torque diminishes as the fraction of ECH power increases, yet confinement improves. [Preview Abstract] |
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