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 NI02: MFE III: Integrated Modeling and MHDInvited Live
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Chair: Francois Waelbroeck, University of Texas - Austin Room: Ballroom C |
Wednesday, November 10, 2021 9:30AM - 10:00AM |
NI02.00001: Predicting Performance and Stability of Tokamak Plasmas Using Flexible, Integrated Modeling Invited Speaker: Brendan C Lyons Future tokamaks (e.g., EXCITE, FPP) require predictive, integrated models to optimize performance while avoiding transients like disruptions. The STEP (Stability, Transport, Equilibrium, and Pedestal) workflow has been developed in OMFIT to predict stable equilibria self-consistently with core-transport and pedestal calculations. STEP couples theory-based codes to integrate a variety of physics, including MHD stability (DCON, GATO), transport (ONETWO, NEO, STRAHL, TGYRO), equilibrium (EFIT, CHEASE), pedestal formation (EPED), and current-drive, heating, and fueling (CHEF). STEP interfaces the input/output of each code with a centralized ITER-IMAS data structure, allowing codes to be run in arbitrary order and enabling open-loop, feedback, and optimization workflows. This paradigm simplifies the integration of new codes, making STEP highly extensible. Core-pedestal calculations with STEP have been successfully validated against the equilibria and profiles of individual DIII-D discharges and across >500 discharges of the H98,y2 database, with a mean error in confinement time from experiment <18% across three orders of magnitude. STEP has also reproduced results in more exotic DIII-D scenarios, including a U-shaped dependence for stored energy on positive and negative triangularities and an assessment of ideal-MHD stability in negative-central-shear plasmas. Predictive ITER modeling with STEP has shown that pellet fueling enhances fusion gain, with Q≈13 found in the baseline and Q≈9 in the advanced-inductive scenario. Finally, STEP is used to make predictions for the next-step EXCITE tokamak, including a high-pressure (~400 kPa), 80%-bootstrap fraction scenario. In the near-term, STEP calculations will allow for the optimization of heating and current drive to maximize plasma pressure while maintaining MHD stability. |
Wednesday, November 10, 2021 10:00AM - 10:30AM |
NI02.00002: Integrated Modeling of Advanced Tokamaks for Tearing Mode Avoidance on DIII-D Invited Speaker: Kyungjin Kim A newly developed tearing mode (TM) onset model has been integrated to the theory-based IPS-FASTRAN scenario modeling, allowing a “predict first” TM stable solution for non-inductive, high βN>4 operation with the planned DIII-D heating and current drive upgrades. Understanding the physics of the onset and evolution of TM in tokamak plasmas is crucial for high-performance steady-state operations. For TM stability analysis, a classical tearing stability index Δ' is calculated with experimental equilibria using PEST3 and resistive DCON. The onset condition of tearing stability, Δ'>Δ'c>0, has been studied with an analytic model of the tearing stability threshold Δ'c based on the nonlinear neoclassical tearing mode theory, showing qualitatively reasonable agreement against various DIII-D advanced tokamak (AT) discharges including steady-state hybrid, elevated qmin, and high li. A database of DIII-D AT discharges indicates that the onset of (m,n)=(2,1) TM is more sensitive to the global MHD equilibrium than to the local profile gradients at the q=2 rational surface. A stability diagram for the TM onset is suggested by the difference between Δ' and Δ'c with a parametric variation of the current density and pressure profiles around a given MHD equilibrium to evaluate its proximity to the TM onset. A reduced model of Δ'c is constructed from nonlinear 3-D MHD simulations using NIMROD for more accurate prediction of the TM onset in the IPS-FASTRAN scenario modeling. A systematic optimization of the current and pressure profiles is done with a massive ensemble of the IPS-FASTRAN simulations enabled by High Performance Computing at scale, identifying a trade-off between tearing onset and ideal MHD stability limit to achieve the stable βN>4 in a fully non-inductive condition. |
Wednesday, November 10, 2021 10:30AM - 11:00AM |
NI02.00003: Simulations of NTM Seeding via MHD Transients Invited Speaker: Eric C Howell Simulations demonstrating seeding of neoclassical tearing modes (NTMs) via MHD-transient-induced nonlinear multimode interactions are presented. NTMs are the leading physics cause of disruptions in tokamaks. Yet there are many open questions regarding their evolution, e.g., “How are NTMs generated from MHD activity?”, “How does a NTM lock and then trigger a disruption?”, etc. Many of these questions are inherently nonlinear, and simulations are needed to help address these issues in experimentally relevant equilibria. NIMROD simulations of NTMs are enabled by two recent 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 bootstrap current, which arises due to collisional viscosity between passing and trapped electrons. This current is inherently kinetic. These simulations use heuristic closures that model the neoclassical electron and ion stresses. NTM growth requires a seed island, and NIMROD simulations apply a transient MP to generate this seed. The capability is demonstrated using kinetic-based reconstructions with flow of a DIII-D ITER Baseline Scenario discharge [R.J. La Haye, J.D. Callen, et al., Proceedings IAEA FEC 2020]. An applied transient 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. 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. |
Wednesday, November 10, 2021 11:00AM - 11:30AM |
NI02.00004: Early internal detection of MHD by Faraday-effect polarimetry in high-$q_{min}$ DIII-D plasmas and correlation with ideal-wall beta limit Invited Speaker: Mihir D Pandya Faraday-effect polarimetry via the Radial Interferometer Polarimeter (RIP) detects internal magnetic fluctuations in high-$q_{min}$ DIII-D plasmas up to 300 ms before the magnetic sensing coils. The first mode detected by the coils often has $n$ = 2 or $n$ = 1, but the first mode detected by RIP has $n$ = 3, consistent with stability calculations using the DCON code. These plasmas, with $q_{min} > 1.4$, are designed to achieve high $\beta_N$, but $\beta_N$ is commonly limited by tearing modes, the onset of which is difficult to predict. The RIP diagnostic measures the line integral of magnetic fluctuations across the center of the plasma. The detected $n$ = 3 mode appears at the lowest-$m/n$ rational surface available for $n$ = 3, with $m$ = 6, 7, or 8, depending on $q_{min}$. The DCON code calculates the ideal-wall kink-mode $\beta_N$ limits for modes with $n$ = 1 - 3, and these limits are used as a proxy for linear tearing mode stability. For these plasmas, the $\beta_N$ limit for $n$ = 3 is generally the lowest. Hence, as $\beta_N$ increases, DCON predicts the onset of an $n$ = 3 mode first, and sampling different shots, RIP detects n = 3 instability with $\beta_N \sim 50 - 75\%$ of the $n$ = 3 ideal-wall limit. The 6/3 mode, resonant at the $q$ = 2 surface, occurs initially without a 4/2 or 2/1 mode, consistent with the DCON calculation, but the 4/2 and 2/1 modes do appear later in time, and their amplitudes increase as the 6/3 mode amplitude decreases. In other cases, the appearance of an $n$ = 2 mode coincides with the disappearance of $n$ = 3. The narrow eigenfunctions of higher-$n$, core-resonant modes render them challenging to detect with sensing coils. With its unique ability to probe the core, RIP has improved understanding of MHD stability and evolution, and its early detection of MHD will allow application of tearing control tools before modes can grow to large amplitude. |
Wednesday, November 10, 2021 11:30AM - 12:00PM |
NI02.00005: Frequency chirping of neoclassical tearing modes by energetic ions Invited Speaker: Huishan Cai The strong interaction between energetic particles and neoclassical tearing modes (NTMs) have been observed in tokamaks. Energetic ions can affect NTM, while NTM would induce the redistribution and loss of energetic ions. The frequency chirping of NTMs has been observed in TFTR, ASDEX-U, HL-2A and DIII-D. Accompanying it, the losses of energetic ions have also been measured. However, the physics is still unclear. Here, the mechanism of fast frequency chirping of NTMs is studied. On one hand, the resonance between tearing modes and trapped energetic ions can excite a fishbone-like mode. When exceeds to a critical value, tearing mode would transit to fishbone-like mode[1]. The result is consistent with HL-2A experimental results. On the other hand, the resonance can provide an additional torque to change the evolution of frequency of NTMs[2]. Whether enhancing or damping effect on the frequency depends on the direction of island propagation. If island propagates in the ion diamagnetic drift direction, the frequency would be enhanced dramatically and quickly. As the frequency increases, the resonance intensity increases since the population of resonance energetic ions increases. Then, the torque is enhanced. So the frequency is enhanced quickly. The predicted chirping time (few ms) and island propagation direction (ion diamagnetic drift direction) are well consistent with DIII-D experimental results. If island propagates in the electron diamagnetic drift direction, the frequency is damped to a lower value. Hence, it can be predicted that the resonance effect can affect the locking dynamics of island, and may avoid the island locking and the explosive growth of island to cause disruption. |
Wednesday, November 10, 2021 12:00PM - 12:30PM |
NI02.00006: Predictive applications of the QuaLiKiz neural network within integrated modelling for JET scenarios Invited Speaker: Aaron Ho Recent neural network (NN) surrogates of the reduced gyrokinetic turbulent transport model, QuaLiKiz [Bourdelle PPCF 2016], have accelerated its evaluation from 10 s per radial point to 1 ms. The resulting model, named QLKNN, achieves this while also being accurate enough to match previous tokamak modelling simulations [Ho PoP 2021]. This enables the rapid iteration of transport solvers, such as JINTRAC [Romanelli PFR 2014], both for post-discharge analysis and predictive modelling. |
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