Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Plasma Physics
Volume 67, Number 15
Monday–Friday, October 17–21, 2022; Spokane, Washington
Session JO05: Edge and Pedestal PhysicsLive Streamed
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Chair: Roberto Maurizio, Oak Ridge Associated Universities / General Atomics Room: Ballroom 111 B |
Tuesday, October 18, 2022 2:00PM - 2:12PM |
JO05.00001: Theory of Pedestal Micro-turbulence with RMP-induced stochasticity Mingyun Cao, Patrick H Diamond In this work, we present a detailed analytic theory of an electrostatic resistive interchange mode in an extrinsic, static, and ambient stochastic magnetic field [1]. Unlike previous works pursing and developing the hyper-resistivity idea [2], this work addresses issues such as effect of stochasticity on mode structure and maintains the quasi-neutrality of the system at all orders. It is found that the beat of small-scale magnetic perturbations and large-scale cell drives small-scale convective cells, i.e., micro-turbulence, which makes the dynamics of this model intrinsically multi-scale and accounts for the appearance of small-scale structures in simulations by Beyer et al [3]. The micro-turbulence can react on the large-scale cell via an effective turbulent viscosity and turbulent diffusivity (computed by closure), as well as electrostatic scattering, thus forming a feedback loop. We find that velocity fluctuations ‘lock on’ to the stochastic field and explicitly calculate the correlation between electrostatic turbulence and ambient magnetic perturbations. Recent experimental results reported by Choi et al [4] indicate that the effect of stochasticity on pedestal turbulence is to reduce its Jensen-Shannon complexity and predictability—i.e. the distribution of turbulence becomes more random compared with the natural ELM free case [4]. This nontrivial correlation could be a possible cause for this phenomenon. In addition, stochastic magnetic perturbations produce a magnetic braking effect, which resembles the nonlinear force identified by Rutherford [5]. Thus, the net effect of stochastic magnetic field is to reduce the amplification of vorticity. |
Tuesday, October 18, 2022 2:12PM - 2:24PM |
JO05.00002: The isotope effect on pedestal structure: fueling is less impactful than transport Ryan A Chaban, Saskia Mordijck, Tom Osborne, Matthias Knolker, Kathreen E Thome, Aaron M Rosenthal We present results consistent with the picture that increased pedestal pressure with isotope mass is a result of transport rather than fueling using a database of stationary ELM-y hydrogen and deuterium discharges on DIII-D. The isotope effect is an empirically observed decrease in heat and particle transport with increasing hydrogenic isotope mass. The change in transport manifests partially in the electron channel where hydrogen plasmas have decreased ηe=Lne/LTe primarily through increased mean LTe at similar mean Lne. The effect of increased particle transport must be disentangled from the effect of increased fueling and changing divertor conditions since hydrogen neutrals penetrate further and physically sputter less carbon. Fueling increases the ratio neSEP/nePED of reducing the pedestal stability particularly on the ballooning side, yet ELM frequency increases with ηe faster for hydrogen than deuterium. Additionally, while neSEP/nePED is similar for both isotopes, ηe increases independent of species implying the fueling is not the primary driver of the isotope effect and we investigate this using new Lyman-α emission main ion source measurements from the LLAMA diagnostic. This work will inform models of pedestal structure for reactor-relevant hydrogen startup in machines such as ITER and SPARC. |
Tuesday, October 18, 2022 2:24PM - 2:36PM |
JO05.00003: A New Approach to SOL Broadening by Turbulence Spreading. Patrick H Diamond The need to escape from the pessimistic scalings of the HD SOL width model has |
Tuesday, October 18, 2022 2:36PM - 2:48PM Author not Attending |
JO05.00004: Theoretical insight into H-mode poloidal pedestal asymmetries Silvia Espinosa Stronger impurity density in-out poloidal asymmetries than predicted by the most comprehensive neoclassical models have been measured in several tokamaks around the world during the last decade. This calls into question the reduction of turbulence by sheared radial electric fields in H-mode tokamak pedestals. However, these pioneering theories neglect the impurity diamagnetic drift, or fail to retain it self-consistently, while recent measurements indicate that it can be of the same order as the ExB drift. This research presents a self-consistent theoretical model retaining the impurity diamagnetic flow and the two-dimensional features it implies due to its associated non-negligible radial flow divergence. It successfully explains collisionally the experimental impurity density, temperature and radial electric field in-out asymmetries; thus making them consistent with H-mode pedestal turbulence reduction. |
Tuesday, October 18, 2022 2:48PM - 3:00PM |
JO05.00005: Filament structures, intermittent fluctuations and broad average profiles at the boundary of magnetically confined plasma Juan M Losada A stochastic model has been developed in order to describe the dynamics of intermittent fluctuations due to radial motion of blob-like structures in the scrape-off layer (SOL) of magnetically confined plasma. Uncorrelated pulses move radially outwards with a random distribution of amplitudes, sizes and velocities. The pulses have a fixed shape and an exponentially decaying amplitude due to parallel drainage towards the divertor plates. In its simplest form, the model leads to exponentially decaying average radial profiles. |
Tuesday, October 18, 2022 3:00PM - 3:24PM |
JO05.00006: Tracking Blobs with Machine Learning in the Turbulent Edge Plasma of a Tokamak Woonghee Han, Randall A Pietersen, Rafael Villamor-Lora, Matthew Beveridge, Nicola Offeddu, Sajidah Ahmed, Gregor Decristoforo, Theodore Golfinopoulos, Christian Theiler, James L Terry, Earl S Marmar, Iddo Drori The analysis of turbulence in plasmas is fundamental in fusion research. Experimental studies are challenging due to the diverse processes that drive the high-speed dynamics of turbulent phenomena. This work presents a novel application of motion tracking to identify and track turbulent filaments in fusion plasmas, called blobs, in high-frequency video data obtained from Gas Puff Imaging diagnostics. We compare four baseline methods trained on synthetic data and then test on synthetic and real data obtained from plasmas in the Tokamak à Configuration Variable. From the size of the blobs estimated from each of the baseline methods employed, we identify the regime of the blob dynamics as described in Myra et al. [1] which agrees with state-of-the-art conditional averaging methods. As an on-going analysis, we use our blob-tracking models to compare 1) a low- and a high-density plasma, as well as 2) a negative and a positive triangularity plasma, extending our study in [2], in order to find any differences in underlying characteristics of edge fluctuations. |
Tuesday, October 18, 2022 3:24PM - 3:36PM |
JO05.00007: A stochastic model of edge-localised modes in magnetically confined plasmas Eun-Jin Kim, Rainer Hollerbach Magnetically confined plasmas are far from equilibrium and pose considerable challenges in statistical analysis. We discuss a non-perturbative statistical method, namely a time-dependent density function (PDF) approach that is potentially useful for analysing time-varying, large, or non-Gaussian fluctuations and bursty events associated with instabilities in the Low-to-High Confinement Mode (L-H) transition and the H-mode. Specifically, we present a stochastic Langevin model of edge-localised modes (ELMs) by including stochastic noise terms in a previous ODE ELM model. We calculate exact time-dependent PDFs by numerically solving the Fokker-Planck equation and characterise time-varying statistical properties of ELMs for different energy fluxes and noise amplitudes. The stochastic noise is shown to introduce phase-mixing and plays a significant role in mitigating extreme bursts of large ELMs. Furthermore, based on time-dependent PDFs, we provide a path-dependent information geometric theory of the ELM dynamics and demonstrate its utility in capturing self-regulatory relaxation oscillations, bursts, and a sudden change in the system. |
Tuesday, October 18, 2022 3:36PM - 3:48PM |
JO05.00008: Influence of up-down asymmetry in plasma shape on the plasma response to RMPs and it's implication on ELM control Shuai Gu, Carlos A Paz-Soldan, Yueqiang Liu, Youwen Sun, Jong-Kyu Park, SeongMoo Yang, SangKyeun Kim, Yongkyoon In, YoungMu Jeon, Huihui Wang, Qun Ma, Pengcheng Xie The 3D plasma response provides a candidate explanation for the inability to access RMP-ELM suppression at DN plasma shapes in tokamaks, namely that the resonant coupling is reduced in such shapes. Recent RMP-ELM control experiments in KSTAR found that ELMs can be suppressed at LSN shape, while no suppression is obtained in DN shapes when dRsep<0.5 cm. To understand the influence of up-down asymmetry in plasma shape on RMP-ELM control, the 3D plasma response is investigated in both experiment and modeling by systematically morphing the plasma shape, whilst keeping other equilibrium quantities largely unchanged. It reveals that the edge resonant component of plasma response is strongly suppressed at DN shapes. This is validated through targeted comparison with experiments at KSTAR and EAST. KSTAR simulations show that edge resonance decreases as plasma shape approach DN, while EAST magnetic measurements are used to validate simulations. However, the linear MHD model of plasma response still has limitations in modeling ELM suppression access, as it cannot explain the inability of ELM suppression at USN shapes in KSTAR, nor predict the critical dRsep. These findings indicate that the plasma shape should be taken into consideration to maximize access to RMP-ELM control in future devices. |
Tuesday, October 18, 2022 3:48PM - 4:00PM |
JO05.00009: Impact of outer divertor leg closure on the low field side main chamber particle fluxes in DIII-D Kirtan M Davda, E.A. Unterberg, Peter C Stangeby, Morgan W Shafer, Livia Casali, Jonathan G Watkins, Jeffrey L Herfindal, Huiqian Wang A Tokamak Divertor Closure Parameter (TDCP) is considered over a range of magnetic & baffle structure of the DIII-D divertor, where higher the TDCP value, lower the closure. Higher TDCP leads to increased magnitude of flux in the main chamber wall (Iwall). Estimates of Iwall are experimentally determined via Window Frame Analysis (WFA). As TDCP increases, recycled neutrals ionize near the midplane. This along with the neutral recycling at the midplane that induces main chamber recycling (MCR) impact the Iwall value. WFA is used to calculate fwall, the ratio of Iwall to total ion flux at divertor target, Idiv. In this study, Iwall is estimated using three diagnostics, viz., Dα detectors, neutral pressure gauges & Langmuir probes, as these provide an accurate radially resolved flux results inside windowpane region, assuming flux across the outer wall is uniform. Results show lower fwall for lower TDCP (i.e., higher closure) at divertor in both L-mode & H-mode. Comparative studies depict USN floor & USN SAS divertor has the highest closure. |
Tuesday, October 18, 2022 4:00PM - 4:12PM |
JO05.00010: Evidence on the Role of Main-Chamber Neutrals on Density Shoulder Broadening Tsui Cedric, Jose A Boedo, Olivier Fevrier, Artur Perek, Holger Reimerdes, Basil P Duval, Sophie Gorno, Umar A Sheikh, Christian Theiler, Marcelo Baquero-Ruiz, Dominik Brida, Georg Harrer, Nicola Vianello, Nick Walkden, Mirko Wensing Main-chamber neutral density, in addition to enhanced radial transport, plays a significant role in the broadening of the density shoulder based on evidence from TCV. Density profile broadening does not occur when baffles are used to reduce the flux of neutrals into the main-chamber, even in detached conditions where high divertor collisionality is achieved without the use of impurity seeding. Radial turbulent flux remains low in the cases when density shoulder broadening is prevented. Another source for main-chamber neutrals – recycling at the inner wall – was modulated by changing the plasma-wall gap, and was also found to affect the amplitude of the density shoulder and the turbulent transport. |
Tuesday, October 18, 2022 4:12PM - 4:24PM |
JO05.00011: Kinetic effects in tokamak edge plasmas - nonlocal parallel transport and plasma-atomic reactions Dominic Power, Stefan Mijin, Fulvio Militello, Robert J Kingham In tokamak edge plasmas, accurate modelling of transport towards plasma-facing components is critical to the successful operation of future devices. These plasmas are often assumed to be collisional, but there are two important (and related) areas in which this assumption may be violated. First, steep temperature gradients parallel to the magnetic field means electron heat transport may be dominated by fast, low-collisionality particles and so becomes ‘non-local’. Second, enhanced high-energy tails of electron velocity distributions close to the walls, where most plasma-atomic interactions take place, may modify reaction rates and therefore affect the particle, momentum and power balance. |
Tuesday, October 18, 2022 4:24PM - 4:36PM |
JO05.00012: Impact of the separatrix plasma on core L mode performances in the WEST full W environment Clarisse Bourdelle, Jorge Morales, Jean-François Artaud, Tenessee Radenac, Ondrej Grover WEST database analysis shows a correlation of the recycled neutral source around the separatrix with core performances. This observation questions the causality chain between particle source and turbulent transport up to the core in L mode, high recycling plasmas, an unavoidable phase of all scenarios. |
Tuesday, October 18, 2022 4:36PM - 4:48PM |
JO05.00013: Plasma Performance and Operational Space with an RMP-ELM Suppressed Edge Carlos A Paz-Soldan, Nils Leuthold, Priyansh Lunia, Shuai Gu, Minwoo Kim, Wolfgang Suttrop, Youwen Sun, Pengcheng Xie, Nikolas C Logan, Jong-Kyu Park The operational space and plasma performance of plasmas with RMP-ELM suppressed edges are surveyed by comparing AUG, DIII-D, EAST, and KSTAR operating points. RMP-ELM suppression is achieved over a range of plasma currents (Ip=0.4-1.7MA), toroidal fields (Bt=1.3-2.5T), and RMP mode number (1-4). Consistent operational windows in edge safety factor are found across devices, while windows in plasma shaping parameters are distinct. Accessed pedestal parameters reveal a quantitatively similar pedestal density limit for RMP-ELM suppression in all devices of just over 3x1019 m-3. This is surprising given the wide variance of many engineering parameters and edge collisionalities (varying from <0.1 to >10). Wide ranges in input power, confinement time, and stored energy exist, with the achieved triple product found to scale like the product of Ip, Bt, and radius. Preliminary energy confinement scaling laws for RMP-ELM suppressed plasmas are presented and compared with expectations from established scalings. Improved statistical regression is found if plasma rotation is included. Results identify common physics for RMP-ELM suppression and highlight the need to pursue this no-ELM regime at higher magnetic field or different plasma size. |
Tuesday, October 18, 2022 4:48PM - 5:00PM |
JO05.00014: Edge turbulence studies across confinement regime transitions at ASDEX Upgrade Rachel Bielajew, Garrard D Conway, Tim Happel, Amanda E Hubbard, Ulrike Plank, Pablo Rodriguez-Fernandez, Davide Silvagni, Branka Vanovac, Christian Yoo, Anne E White Edge turbulence is thought to play an important role in the transition between the low confinement operating regime, L-mode, and high confinement operating regimes such as H-mode. ELM-free high confinement regimes such as I-mode are promising for future reactor operation, but open questions remain about the role of turbulence in regulating transport in I-mode. There are also open questions about changes in edge turbulence leading up to the I-mode and H-mode transitions, which affect access to I-mode, and the relationship with favorable and unfavorable ion ▽B drift configuration. The Correlation Electron Cyclotron Emission (CECE) diagnostic measures long-wavelength electron temperature fluctuations at ASDEX Upgrade with high temporal and spatial resolution. This work presents outer core and pedestal (ρpol=0.85-1.0) electron temperature fluctuation measurements in a variety of experiments with L-modes, I-modes, and H-modes in different magnetic configurations. These measurements provide important information on scaling and transitions between confinement regimes. Gyrokinetic simulations of these plasmas with the CGYRO code provide further details about the nature of the turbulence across confinement regime transitions. |
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