Disruptive neoclassical tearing mode seeding in DIII-D with implications for ITER

Haye, R.J. La; Chrystal, C.; Strait, E. J.; Callen, J. D.; Hegna, C. C.; Howell, Eric; Okabayashi, M.; Wilcox, R.S.

doi:10.1088/1741-4326/ac351f

Cited by 21 publications

(21 citation statements)

References 28 publications

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“…Experimental equilibrium reconstructions are used to model NTM dynamics in realistic conditions relevant to DIII-D, ITER, and future burning plasma tokamaks. Simulations use a kinetic reconstruction of an experimental DIII-D ITER Baseline Scenario (IBS) discharge [29,30]. The reconstruction is of an ELMing H-mode discharge with ITER similar shape, β n ∼ 1.8, and q 95 ∼ 3.4.…”

Section: A Experimental Equilibriummentioning

confidence: 99%

Growing Neoclassical Tearing Modes Seeded via Transient-Induced-Multimode Interactions

Howell,

King,

Callen

et al. 2021

Preprint

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Nonlinear extended MHD simulations demonstrating seeding of neoclassical tearing modes (NTMs) via MHD-transient-induced multimode interactions are presented. Simulations of NTMs are enabled by two recent NIMROD code developments: the implementation of heuristic neoclassical stresses and the application of transient magnetic perturbations (MPs) at the boundary. NTMs are driven unstable by the inherently pressure driven kinetic bootstrap current, which arises due to collisional viscosity between passing and trapped electrons. These simulations use heuristic closures that model the neoclassical electron and ion stresses. NTM growth requires a seed island, which is generated by a transiently applied MP in simulations. The capability is demonstrated using kineticbased reconstructions with flow of a DIII-D ITER Baseline Scenario discharge [R.J. La Haye, et al., Proceedings IAEA FEC 2020]. The applied MP seeds a 2/1 NTM that grows in two phases: a slow growth phase followed by a faster robust growth phase like that observed experimentally. Additionally, an evolving sequence of higher order core modes are excited at first. Power transfer analysis shows that nonlinear interactions between the core modes and the 2/1 helps drive the initial slow growth. Once the induced 2/1 magnetic island reaches a critical width, the NTM transitions to faster robust growth which is well described by the nonlinear modified Rutherford equation. This work highlights the role of nonlinear mode coupling in seeding NTMs.

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Section: A Experimental Equilibriummentioning

confidence: 99%

Growing Neoclassical Tearing Modes Seeded via Transient-Induced-Multimode Interactions

Howell,

King,

Callen

et al. 2021

Preprint

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“…2018; La Haye et al. 2022) and its database has unusually many examples from which to learn statistically. An important caveat with the underlying database is that magnetics-only equilibrium reconstructions need to be used to get excellent database coverage.…”

Section: Introductionmentioning

confidence: 99%

“…Some of these ideas have been applied in disruption prediction research (Tinguely et al 2019). The DIII-D tokamak has a strong track-record of research on TM stability (La Haye et al 2000;Buttery et al 2008;Turco & Luce 2010;Turco et al 2018;La Haye et al 2022) and its database has unusually many examples from which to learn statistically. An important caveat with the underlying database is that magnetics-only equilibrium reconstructions need to be used to get excellent database coverage.…”

Section: Introductionmentioning

confidence: 99%

Database-wide hazard modelling of the onset of DIII-D tearing modes with field features

2022

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The rate of onset (hazard) of tearing modes is modelled probabilistically using statistical learning algorithms. Axisymmetric energy-density equilibrium fields are taken as raw high-dimensional input features which are reduced with principal component analysis. Signal processing of non-axisymmetric magnetics fluctuation array data provides the target information from which to learn. Model selection, visualization and calibration assessment procedures are detailed. The analysis is deployed at large scale across the DIII-D tokamak database. Standard model selection criteria suggest that the energy-density post-processed feature is a better choice for modelling the onset rate compared to the non-processed equilibrium reconstruction solution. Two example applications of the learned rate function are demonstrated: (i) proximity-to-onset discharge monitoring and (ii) database analysis showing an (expected) observational global trend that the general hazard increases as a plasma performance metric increases. An important connection between the hazard function and its use as a conditional probability generator is reviewed in the Appendix.

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“…While tolerable in most present devices, the energy deposited by ELMs on the first wall of a tokamak device is expected to scale with the edge plasma pressure p ped , which could easily lead to melting of the wall material in ITER or other high-pressure, reactor-scale devices [3,4]. In addition to the heat flux problem, recent work has found that large magnetic perturbations induced by ELMs may trigger neoclassical tearing modes that lead to disruptions in tokamak plasmas where the equilibrium condition had otherwise been stable [5,6].…”

Section: Introductionmentioning

confidence: 99%

Pellet triggering of edge localized modes in low collisionality pedestals at DIII-D

Wilcox¹,

Baylor²,

Bortolon³

et al. 2021

Nucl. Fusion

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Edge localized modes (ELMs) are triggered using deuterium pellets injected into plasmas with ITER-relevant low collisionality pedestals, and the resulting peak ELM energy fluence is reduced by approximately 25-50% relative to natural ELMs destabilized at similar pedestal pressures. Cryogenically frozen deuterium pellets are injected from the low-field side of the DIII-D tokamak at frequencies lower than the natural ELM frequency, and heat flux is measured by infrared cameras. Ideal MHD pedestal stability calculations show that without pellet injection, these low collisionality pedestals were limited by their current density (peeling-limited) rather than their pressure gradient (ballooning-limited). ELM triggering success correlates strongly with pellet mass, consistent with the theory that a large pressure perturbation is required to trigger an ELM in low collisionality discharges that are far from the ballooning stability boundary. For sufficiently large pellets, both instantaneous and time-integrated ELM energy deposition measured by infrared cameras is reduced with respect to naturally occurring ELMs at the inner strike point, which is the position where it is largest for natural ELMs. Energy fluence at the outer strike point is less effected. Cameras observing both heat flux and D-alpha emission often find significant toroidally asymmetric striations in the outboard far scrape-off layer resulting from ELMs that are triggered by pellets. Toroidal asymmetries at the inner strike point are similar between natural and pellet-triggered ELMs, suggesting that the reduction in peak heat flux and total fluence at that location is robust for the conditions reported here.

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Disruptive neoclassical tearing mode seeding in DIII-D with implications for ITER

Cited by 21 publications

References 28 publications

Growing Neoclassical Tearing Modes Seeded via Transient-Induced-Multimode Interactions

Growing Neoclassical Tearing Modes Seeded via Transient-Induced-Multimode Interactions

Database-wide hazard modelling of the onset of DIII-D tearing modes with field features

Pellet triggering of edge localized modes in low collisionality pedestals at DIII-D

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