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 NO07: Magnetic Confinement: Low-Aspect Ratio TokamaksLive
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Chair: Phil Snyder, GA |
Wednesday, November 11, 2020 9:30AM - 9:42AM Live |
NO07.00001: Characterization of T$_{\mathrm{e}}$ and n$_{\mathrm{e\thinspace }}$Profiles of Discharges Driven Purely by Helicity Injection in the \textsc{Pegasus} Toroidal Experiment G.M. Bodner, M.W. Bongard, R.J. Fonck, M.D. Nornberg, J.A. Reusch, N.J. Richner, C. Rodriguez Sanchez, C.E. Schaefer Understanding the electron confinement and transport in plasmas driven purely by local helicity injection (LHI) is critical to the demonstration of high-performance discharges. Given the proper operating conditions, purely LHI-driven discharges can feature peaked $T_{e}$ profiles with $T_{e,0}\sim 150$ eV. Ohmic discharges in \textsc{Pegasus} at the same field level, $B_{T}\sim 0.15$ T exhibit similar $T_{e}$ profiles albeit with higher $n_{e}$. At lower levels of $B_{T}$, LHI discharges feature hollow $T_{e}$ profiles that increase in \textless $T_{e}$\textgreater $\thinspace $ as the effective loop voltage, $V_{LHI}$, is increased. The increase in \textless $T_{e}$\textgreater $\thinspace $ scales with $V_{LHI}$ rather than the injector electrode voltage, $V_{inj}$, contrary to predictions from open field line theory. The hollowing of the $T_{e}$ profile is hypothesized to be a combination of low $\eta j^{2}$ heating power due to the hollow current profile and low-Z impurity radiation losses. Approximations of $Z_{eff}$ in LHI discharges from voltage balance assuming purely Spitzer and neoclassical resistivity are $\sim 3$ and $\sim 1$, respectively. Thomson scattering and magnetic probe measurements indicate a pressure-free region between the kinetic and magnetic boundaries, possibly indicative of separate Ohmic and stochastic confinement regions. Overall scaling of $I_{p}$ with $V_{LHI\thinspace }$ appears to be consistent with linear Ohmic confinement scaling assuming auxiliary ion and electron heating from magnetic reconnection. [Preview Abstract] |
Wednesday, November 11, 2020 9:42AM - 9:54AM Live |
NO07.00002: The New Pegasus-III Experiment and Plans for RF Heating and Current Drive S.J. Diem, M.W. Bongard, R.J. Fonck, J.A. Goetz, B.A. Kujak-Ford, B.T. Lewicki, M.D. Nornberg, A.C. Palmer, J.A. Reusch, A.C. Sontag, G.R. Winz Solenoid-free startup techniques such as helicity injection (HI) and radiofrequency (RF) wave injection offer the potential to simplify the cost and complexity of reactor-class devices by reducing the technical requirements of, or possibly the need for, a central solenoid. \textsc{P}\textsc{egasus}-III is the next generation of the \textsc{Pegasus} experiments, with increased $B_{T}$ to 0.6 T and extended pulse duration (\textless 100 ms). It is a solenoid-free, low aspect ratio ST that will serve as a dedicated US platform for comparative non-solenoidal tokamak startup studies. It will be equipped with a new local helicity injection (LHI) system capable of $I_{p}<0.3$ MA, a coaxial helicity injection (CHI) system, and an 8 GHz klystron-based system for sustained electron Bernstein wave (EBW) heating and current drive. While the RF system will be initially employed for heating HI-initiated plasmas, EBW modeling indicates that up to 50 kA of current can be driven near $\rho \sim 0.3$ near the fundamental EC resonance. RF on \textsc{P}\textsc{egasus}-III will provide a key enabling reactor relevant technology to directly test proposed plasma startup and ramp-up scenarios envisioned for NSTX-U, investigating methods to synergistically improve the target plasma for consequent bootstrap and NBI current sustainment. [Preview Abstract] |
Wednesday, November 11, 2020 9:54AM - 10:06AM Live |
NO07.00003: NSTX-U research supporting the development of a steady-state Compact Fusion Power Plant D.J. Battaglia, S.M. Kaye, W. Guttenfelder, R. Maingi Steady-state tokamak Compact Fusion Power Plant (CFPP) designs target enhanced thermal confinement (H$_{\mathrm{98y,2}}$\textgreater 1.5) and large bootstrap current fraction (f$_{\mathrm{BS}}\ge $ 0.5) concurrently with low disruptivity and suitable divertor power handling and exhaust. NSTX-U will advance the physics basis and technology solutions required for an Advanced Tokamak CFPP by producing scenarios at large non-inductive current fraction (f$_{\mathrm{NI}}=$ 60 -- 100{\%}) with strong boundary shaping ($\kappa $\textgreater 2.5, $\delta $\textgreater 0.7), f$_{\mathrm{BS}}=$ 60 -- 90{\%}, $\beta_{\mathrm{N}}=$ 4 -- 6 and $\beta_{\mathrm{T}}=$ 5 -- 25{\%} uniquely accessed at small aspect ratio (A\textless 2). The technical capabilities of NSTX-U are directed at exploring the unique transport and stability properties at high $\beta $ and the lowest collisionality ($\nu_{\mathrm{e}}$*\textless 0.1) of any spherical tokamak. This includes investigating if the strong favorable scaling of confinement with collisionality in regimes dominated by electron thermal transport persists at lower $\nu_{\mathrm{e}}$*. The compact nature of NSTX-U, coupled with high heating power leads to high power exhaust levels that enable the evaluation of integrated tests of reactor-relevant divertor solutions, such as liquid lithium PFCs, in order to qualify these potentially transformative solutions for a CFPP. [Preview Abstract] |
Wednesday, November 11, 2020 10:06AM - 10:18AM Live |
NO07.00004: Overview of Progress on the NSTX-U Recovery Project Stefan Gerhardt, John Galayda, Leslie Hill The NSTX-U Recovery Project has exited the design phase and is now in the fabrication {\&} installation phase. The Project received CDE-3B approval signifying completion of final design in June 2020; when combined with the CDE-3A approval from September 2019, full authorization has now been granted to complete the Project. Improvements to the test cell nuclear shielding, including a new labyrinth, are completed. Six inner-PF coils are nearing completion at Sigmaphi in France. The centerstack casing, a large weldment that provides the inner vacuum boundary and supports numerous coils and tiles, is well on the way to completion, and parts for the inner-PF coils supports are being fabricated. Mechanical reinforcements for the in-vessel stabilizing plates and helium distribution lines are being installed. All plasma facing components on the center column and in the divertors are being fabricated, with special features (castellation, fish-scaling) in the high heat flux regions of the machine. The vertical field coils are being realigned to ensure low error fields. A new access control systems, compliant with IEC 61508/61511 is being installed. Progress on the Project, and the plans for commissioning, will be presented. [Preview Abstract] |
Wednesday, November 11, 2020 10:18AM - 10:30AM Live |
NO07.00005: Plan for UTK Thermography and Spectroscopic Diagnostics Evaluation of NSTX-U Spherical Torus Plasma Brian Wirth, Kaifu Gan This presentation describes recently installed spectroscopy, and newly proposed infrared thermography, diagnostics in support of the NSTX Upgrade physics mission; and the plans to utilize this diagnostic information to contribute to the understanding of impurity transport in the scrape off layer (SOL) and the role of lithium versus boron wall conditioning on impurity transport and thermal heat flux spatial and temporal profiles in the upper and lower divertor of NSTX-U. The recently installed diagnostics provide a view of the lower region of the central stack and the upper divertor with high-resolution UV-VIS-NIR spectroscopy. ~Additionally, we plan to install new infrared cameras to provide thermography in three locations, namely the outer strike points of the upper and lower divertor and a wide-angle view of the~most of the first wall.~~The~scientific objectives can be summarized as providing experimental validation of impurity transport through the SOL, in addition to evaluating boron versus lithium wall conditioning and the flow of boron and lithium ions, and chemical species, through the SOL, and to assess i) heat flux width measurements, ii) the role of wall conditioning on the heat flux magnitude and spatial distributions, in addition to spectroscopic imaging of the transport of impurities through the scrape-of-layer, iii) striated heat flux patterns within the divertor and PFCs; and iv) assessing the up-down power balance in NSTX-U. [Preview Abstract] |
Wednesday, November 11, 2020 10:30AM - 10:42AM Live |
NO07.00006: Progress in predicting pedestal transport in NSTX H-modes Walter Guttenfelder, DJ Battaglia, A Diallo, SM Kaye, R Maingi, JM Canik, EA Belli, J Candy The kinetic ballooning mode (KBM) transport constraint used within pedestal structure models like EPED reproduces the pedestal width scaling in many tokamaks. However, it does not reproduce the strong scaling ($\Delta \psi _{\mathrm{N,ped}}$\textasciitilde $\beta_{\mathrm{\theta }}^{\mathrm{1.05}})$ observed in NSTX H-modes. Previous gyrokinetic analyses in the pedestal of NSTX H-modes predict that many theoretical micro-instabilities may play a role in determining pedestal gradients and structure. Analysis has continued to further characterize the linear thresholds and nonlinear transport of these various mechanisms that will be required to form the basis of a predictive model. The cases investigated, spanning a range of pedestal width ($\Delta \psi _{\mathrm{N,ped}}=$0.05-0.3$+)$, are all found to be very near local KBM thresholds across the entire pedestal. Furthermore, ETG instability is also unstable in the outer half region of the pedestal where experimental values of $\eta_{\mathrm{e,exp}}$ are larger than predicted ETG thresholds, $\eta_{\mathrm{e,ETG,crit}}\approx $1.4-1.6. Nonlinear simulations predict that ETG can contribute relevant levels of electron heat flux in these regions, but are unlikely to account for all of the transport. In some pedestals, microtearing modes are also found to be unstable at relatively lower wavenumbers (k$_{\mathrm{\theta }}\rho_{\mathrm{s}}$\textless 0.1), with a broad spectrum of TEM present at increasing k$_{\mathrm{\theta }}\rho_{\mathrm{s}}$. Non-linear simulations of these ion-scale mechanisms are commencing to determine whether they predict significant transport. This work supported by the U.S. Department of Energy under DE-AC02-09CH11466, DE-FC02-04ER54698 and DE-AC02-05CH11231. [Preview Abstract] |
Wednesday, November 11, 2020 10:42AM - 10:54AM Live |
NO07.00007: 3D HHFW full wave simulations for NSTX-U with realistic antenna geometry and SOL plasma Nicola Bertelli, S. Shiraiwa, G.J. Kramer, C. Lau, E.-H. Kim, M. Ono 3D full wave simulations of HHFW that include the scrape-off layer (SOL) and realistic antenna geometry are presented for NSTX-U. NSTX-U will operate with toroidal magnetic fields up to 1 T, up to 10 MW of neutral beam injection (NBI) and up to 6 MW of HHFW for heating and current drive. We focus on the HHFW propagation in NSTX-U for different antenna conditions, plasma scenarios, and edge plasma parameters. In order to perform all of this, we employ the Petra-M code, which is a newly developed state-of-the-art generic electromagnetic simulation tool for modeling RF wave propagation based on MFEM, open source scalable C++ finite element method library. Scans of the antenna phasing and toroidal magnetic field strength are presented to investigate the interaction of the fast waves with the SOL plasma and the core fast wave propagation. The impact of different SOL electron density profiles on the HHFW antenna loading is investigated and an evaluation of induced current on the antenna box is presented. First results are shown from a coupling of the 3D RF solver with the full-orbit following particle code SPIRAL and its impact of the 3D wave fields on the fast ion population from NBI beams in NSTX-U. [Preview Abstract] |
Wednesday, November 11, 2020 10:54AM - 11:06AM Live |
NO07.00008: ELM-free divertor peak heat flux reduction induced by small ELMs in NSTX. Kaifu Gan, Rajesh Maingi, Travis Gray, Brian Wirth It is well known that the edge localized modes (ELMs) will increase the divertor peak heat flux. However, the ELM-free divertor peak heat flux has been observed to decrease with small ELMs under high neutral beam injection (NBI) power (\textgreater 3.5 MW) in NSTX. These small ELMs are observed in certain discharges, resulting in \textless 50{\%} transient increase in deposited power on divertor. At small ELMs peak, the integral power decay width ($\lambda_{\mathrm{int}})$ increases by up to 200{\%} relative to the ELM-free heat flux profile. The broaden heat flux footprint make the divertor peak heat flux at small ELMs peak is just 50{\%} of the divertor peak heat flux at ELM-free. The $\lambda_{\mathrm{int}}$ at the inter-small ELMs is twice as large as the $\lambda_{\mathrm{int}}$ at the ELM-free. Some small ELMs was observed to decrease the divertor peak heat flux at inter small ELMs. In contract, small ELMs with low NBI power (\textless 2 MW) and \textless 50{\%} transient increase in deposited power on divertor were also observed. These small ELMs do not increase the $\lambda_{\mathrm{int}}$ and the ELM-free divertor peak heat flux increases with these small ELMs. This work was supported by the U.S. DOE, contract number DE-SC0008309. [Preview Abstract] |
Wednesday, November 11, 2020 11:06AM - 11:18AM Live |
NO07.00009: Scenario trajectory planning for NSTX-U using machine-learning-accelerated models and genetic optimization Mark Boyer, Jason Chadwick, Stan Kaye Between-shots and real-time actuator trajectory planning will be critical to achieving reliable high performance scenarios in present-day tokamaks, ITER, and beyond. A requirement for this effort is the availability of models that are accurate enough for useful decision making and fast enough for optimization algorithms to meet between-shots and real-time deadlines. While integrated modeling codes are progressing toward the accuracy and completeness needed for these applications, they are too computationally intensive for this purpose. To address this, a novel accelerated simulation capability has been developed for NSTX-U by applying machine learning techniques to both empirical data and TRANSP simulations, enabling profile and equilibrium predictions at real-time relevant time scales. The approach includes machine learning surrogates for high-fidelity TRANSP modules that accelerate calculations by orders of magnitude while maintaining fidelity. For quantities not accurately modeled by TRANSP modules, machine learning is applied to an experimental database to create empirical models. Initial results trajectory optimization using the learning-based model are presented, including a strategy for mitigating the effect of leaving the validity range of the model. [Preview Abstract] |
Wednesday, November 11, 2020 11:18AM - 11:30AM Live |
NO07.00010: Kinetic Equilibrium Reconstructions of Plasmas in the MAST Database and Preparation for Reconstruction of the First Plasmas in MAST Upgrade J.W. Berkery, S.A. Sabbagh, L. Kogan, D. Ryan, J.M. Bialek, L. Guazzotto, Y. Jiang, D.J. Battaglia, S. Gibson Reconstructions of plasma equilibria using kinetic profiles are necessary for stability and disruption analysis of the MAST database, as well as for the upgrade to the device, MAST-U. The VALEN code is used to determine effective resistances in the 3D vessel structures, which are used with nearby loop voltage measurements for estimated currents in the structures during EFIT reconstruction. Kinetic equilibrium reconstructions using all available magnetic sensors, Thomson scattering measurements of electron temperature and density, charge exchange recombination spectroscopy measurements of ion temperature, and internal magnetic field pitch angle measurements from a motional Stark effect diagnostic are performed for a large database of MAST discharges. Excellent convergence errors (\textasciitilde 10$^{\mathrm{-8}})$ are obtained. Initial inclusion of rotation in the equilibria is performed with the FLOW code. The necessary changes to conducting structure in VALEN, and diagnostic setup in EFIT have been completed for MAST-U, enabling kinetic reconstructions to commence from the first plasma discharges of the upgraded device. Stability projections for MAST-U indicate a substantial gap between the no-wall and with-wall beta limits enabled by passive stabilization plates. [Preview Abstract] |
Wednesday, November 11, 2020 11:30AM - 11:42AM Live |
NO07.00011: Pedestal Stability Analysis on MAST Equilibria in Preparation For MAST-U Matthias Knolker, Thomas Osborne, Samuli Saarelma, Stuart Henderson, Orso Menegini, Lucy Kogan, Philip Snyder In preparation for the upcoming MAST-U campaign, pedestal stability of spherical tokamaks is revisited by investigating the edge limiting modes of MAST discharges [1,2]. Linear stability analysis with the ELITE code [3] has been executed showing that the discharges are constrained by medium range peeling-ballooning modes, ranging from n$=$25 to n$=$30. The input parameters into the ELITE code are adapted to match the current profile of spherical tokamaks, where due to the steep q profile at the edge a larger number of poloidal harmonics is excited for each toroidal mode. Pedestal electron temperature ranges from 100-180 eV. As general stability criterion a normalization of the mode growth rate to the Alfven frequency with a critical threshold of 0.1 is found to be consistent. Ion diamagnetic effects appear to make a large stabilizing contribution in low aspect ratio tokamaks. The results demonstrate the usability of the ELITE code on spherical tokamak equilibria and encourage future work predicting MAST-U pedestals based on synthetic equilibria. ((S Saarelma et al 2007 PPCF 49 312) A Kirk et al 2009 PPCF 51 0650163) P Snyder et al 2002, PoP 9, 2037 [Preview Abstract] |
Wednesday, November 11, 2020 11:42AM - 11:54AM Live |
NO07.00012: Plasma radiation transport effects in MAST-U advanced divertors V. A. Soukhanovskii, A. I. Khrabry, H. A. Scott, T. D. Rognlien, D. Moulton, J. Harrison Modest effects of deuterium line radiation trapping in MAST Upgrade tokamak Super-X and snowflake (SF) divertor plasmas are found using SOLPS-EIRENE and UEDGE code divertor plasma modelling, and the CRETIN code radiation transport and collisional-radiative modelling. Lyman series line radiation trapping in the divertor increases deuterium ionization rate, and reduces volumetric rates for recombination and radiation. This may potentially increase divertor detachment density threshold and diminish the advanced divertor geometry benefits. The CRETIN and UEDGE modeling of the detached Super-X and SF divertors shows that 1) divertor plasma properties are modified insignificantly ($\leq 10$ \%) when Lyman opacity effects are included; 2) the deuterium Lyman radiation trapping is non-negligible hence challenging its experimental detection; 3) Carbon C II, C III, and C IV resonance line radiation transport in the vacuum ultraviolet region under most divertor scenarios can be neglected. [Preview Abstract] |
Wednesday, November 11, 2020 11:54AM - 12:06PM Live |
NO07.00013: UEDGE Modeling of Snowflake and Standard Divertors in MAST-U Tokamak Alexander Khrabry, Vsevolod Soukhanovskii, Thomas Rognlien, Maxim Umansky, David Moulton, James Harrison In a snowflake (SF) divertor, two magnetic field nulls are placed close to each other creating four strike points (SPs) cf. two in a standard (ST) divertor. In preparation to MAST-U experiments, ST and SF divertor configurations with various relative locations of the nulls were modeled using a two-dimensional multi-fluid code UEDGE. A full plasma transport model with charge-state-resolved sputtered carbon impurities and enhanced transport (mixing) in the null region has been implemented in the code. Computational grids with high resolution of the two-null SF region have been created. The model has been verified by comparing to previous SOLPS/EIRENE solutions for ST divertor using transport coefficients for MAST tokamak. The modeling results show that: 1) heat and ion flux profiles at primary SPs are substantially broadened and peak values are reduced in SF configurations w.r.t. ST divertor; 2) secondary SPs receive up to 20{\%} of heat and particle fluxes (w.r.t. the outer strike point); 3) SF divertors approach the outer and inner SP detachment conditions at lower upstream density w.r.t. ST divertor; 4) these effects become more prominent with the SF mixing increase. [Preview Abstract] |
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