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 GO03: Low-aspect Ratio Tokamaks: PEGASUS, NSTX-U, MAST-U, ST-40, and OthersLive Streamed
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Chair: James Yang, Princeton Plasma Physics Laboratory Room: Ballroom 100 C |
Tuesday, October 18, 2022 9:30AM - 9:42AM |
GO03.00001: Ohmic Coupling and Sustainment of LHI-Produced Plasmas in the Pegasus Experiment Christopher Pierren, Grant M Bodner, Michael W Bongard, Stephanie J Diem, Raymond J Fonck, Mark D Nornberg, Nathan J Richner, Cuauhtemoc Rodriguez Sanchez, Carolyn E Schaefer, Justin D Weberski Reducing or eliminating the Ohmic solenoid (OH) is likely necessary for the ST path and may benefit tokamaks in general, motivating the development of solenoid-free startup techniques. Viable methods will: initiate plasmas with readily conservable poloidal flux / Ip; are compatible with sustainment current drive (CD) techniques; and have attractive MHD stability. Local helicity injection (LHI), being developed on the Pegasus ST, is a promising solenoid-free startup technique. Experiments that tested the viability of LHI startup plasmas using Ohmic drive for sustainment are presented here. Ip in LHI-initiated plasmas was coupled to and further grown under pure Ohmic drive, realizing OH flux savings and conserving poloidal flux across the handoff. Magnetic fluctuations associated with LHI reconnection activity decayed to OH-like levels within several ms after LHI termination, suggesting a rapid healing of flux surfaces. The typically hollow LHI current profiles appear to be MHD favorable, evidenced by a delayed onset of low m, n = 1 tearing modes with respect to purely Ohmic comparison discharges. |
Tuesday, October 18, 2022 9:42AM - 9:54AM |
GO03.00002: The New Pegasus-III Experiment Steffi J Diem, Michael W Bongard, Michael T Borchardt, Raymond J Fonck, John A Goetz, Benjamin A Kujak-Ford, Benjamin T Lewicki, Mark D Nornberg, Joshua A Reusch, Aaron C Sontag, Gregory R Winz Minimizing or eliminating the need for induction from a central solenoid during startup, ramp-up and sustainment of a tokamak plasma is a critical challenge in magnetic fusion. Solenoid-free startup techniques such as helicity and RF wave injection offer the potential to simplify large-scale devices by reducing the requirements of the solenoid. The Pegasus-III Experiment is a new solenoid-free ST (A > 1.22, Ip < 0.3 MA, BT < 0.6 T, pulse length ~ 100 ms) focused on studying innovative non-solenoidal tokamak startup techniques. Pegasus-III will be equipped with a new local helicity injection (LHI) system capable of Ip < 0.3 MA, sustained and transient coaxial helicity injection (CHI) systems, and a 28 GHz RF system for electron Bernstein wave and electron cyclotron heating. Initial experiments will establish high-Ip LHI scenarios, followed by deployment and tests of transient CHI, modest sustained CHI and low-power RF studies. Pegasus-III will provide key enabling fusion power plant relevant technology to directly test proposed plasma startup and ramp-up scenarios envisioned for larger scale ST devices and investigate methods to synergistically improve the target plasma for consequent bootstrap and NBI current sustainment. |
Tuesday, October 18, 2022 9:54AM - 10:06AM |
GO03.00003: Gyrokinetic investigation of pedestal transport on NSTX Joseph M Schmidt In the NSTX-U spherical tokamak, the addition of lithium coatings sprayed onto several plasma facing components improves confinement and minimizes edge localized modes (ELMs). We use the gyrokinetic code, GENE, to study the effect of lithium conditioning on pedestal instabilities and transport. Lithiated discharges have broader, ELM-free, pedestals. We investigate the roles of ETG, ITG/TEM, MTM, and KBM. A comprehensive study of their stability and proximity to threshold will be undertaken. Among other things, we will consider the particle transport in relation to the changes in particle sources produced by the lithium conditioning. With this analysis a fuller picture can be constructed of the effects of lithium coatings on pedestal transport, which may contribute to the development of optimized, ELM-free reactor scenarios. |
Tuesday, October 18, 2022 10:06AM - 10:18AM |
GO03.00004: Design method for optimized feedforward+feedback shape control Josiah T Wai, Mark D Boyer, Will Wehner, Anders Welander, Egemen Kolemen Despite the wealth of experience in plasma shape control, difficulties still persist in pushing the plasma to operational limits and scenarios. We present a design methodology for obtaining feedforward trajectories for a given scenario and combining this with robust feedback control. Feedforward is calculated from the target shape evolution and serves the purpose of identifying a 'global' solution for the full-shot trajectory, balancing priorities throughout the shot, and also compensates for plasma resistivity and current profile evolution. Feedback control is used to compensate for 'local' disturbances and inevitable modeling errors. We use Model Predictive Control (MPC) for the feedback method, and shape transitions are guided by the time-dependent MPC weights. By comparison, many operational control systems use different controllers for each shape type (diverted vs limited etc). Results are validated with the Gsevolve flight simulator and NSTX-U data and performance improvements are demonstrated such as eliminating undesired upper-single-null / lower-single-null switching and operation closer to hardware limits. |
Tuesday, October 18, 2022 10:18AM - 10:30AM |
GO03.00005: The Gyrokinetic Critical Pedestal Constraint Jason F Parisi, Walter Guttenfelder, Andreas Kleiner We present a new framework that augments existing pedestal models by using gyrokinetic simulations to determine a stability boundary analogous to the ballooning critical pedestal (BCP) constraint in an EPED-like approach [1]. By incorporating the critical linear temperature and density gradients for dominant micro instabilities, we find a new pedestal pressure gradient constraint — the Gyrokinetic Critical Pedestal (GCP) constraint — for NSTX discharges. Local, linear gyrokinetic stability analysis is performed in CGYRO [2] by varying the experimental equilibrium self-consistently, which is then used to predict pedestal width and height. This self-consistent equilibrium calculation rescales temperature and density gradients starting from the experimental point. Since our model distinguishes between density and temperature profiles, we characterise how stability thresholds — and therefore pedestal evolution — are affected by varying temperature and density profiles. This analysis is performed at multiple time intervals in the inter-ELM buildup, which shows how turbulent transport evolves during the pedestal ELM cycle. |
Tuesday, October 18, 2022 10:30AM - 10:42AM |
GO03.00006: Equilibrium operational space and stability limits in early operation of MAST-U Jack Berkery, Steven A Sabbagh, Lucy Kogan, Sam Gibson, David Ryan, Veronika Zamkovska, Jalal Butt, James R Harrison The MAST-U spherical tokamak has recently completed its first campaign of physics operation. MAST-U has operated with Ohmic, or one or two neutral beams for heating, at 400-800 kA plasma current, in conventional or “SuperX” divertor configurations. Equilibrium reconstructions with magnetics only or additionally with motional Stark effect diagnostic measurement of the magnetic pitch angle provide key plasma physics parameters vs. time for each discharge. The disruption event characterization and forecasting (DECAF) code produces diagrams which show where the prevalence of operation occurred as well as the limits in various operational spaces. When compared to stability limits, the operation of MAST-U so far has generally stayed out of low q, low density instability regions, and below the high density Greenwald limit, high beta global stability limits, and high elongation vertical stability limit. MAST-U has reached βN/li = 3.3 so far, well below the ideal no-wall limit, which was projected to be about βN/li = 7 [1]. MAST-U still has the potential to reach higher elongation, which could benefit the plasma performance. [1] Berkery J W et al 2020 Plasma Phys. Control. Fusion 62 085007. |
Tuesday, October 18, 2022 10:42AM - 10:54AM |
GO03.00007: Development of plasma magnetic equilibrium control for the MAST upgrade tokamak Himank Anand, David A Humphreys, Graham J McArdle, Martin Kochan, Nicholas W Eidietis, Jayson L Barr, Anders Welander, Elizabeth Cho, Zichuan A Xing, Andrey Lvovskiy, Ivo Carvalho Active real-time estimation and control of the magnetic equilibrium is required for the MAST Upgrade (MAST-U) tokamak for exploring extended- and expanded-leg divertor geometries. Assessment of the proposed local expansion method for MAST-U for strike point position estimation has been successfully simulated and modelled on the DIII-D tokamak with a long-legged divertor plasma configuration and has also been tested experimentally on MAST-U. The local expansion method has been further extended for estimating additional magnetic equilibrium parameters (inner gap, X-point and strike point position) and implemented on the MAST-U control system. TokSys closed-loop simulations with the axisymmetric linear and non-linear plasma models, have been performed for the assessment, identification, and verification of the algorithm implementation and controller performance for the various axisymmetric control variables. Demonstration of full magnetic equilibrium control with modified local expansion method is planned for the upcoming MAST-U 2022 experimental campaign. |
Tuesday, October 18, 2022 10:54AM - 11:06AM |
GO03.00008: Multi-reservoir particle balance analysis of RMP discharges at MAST Kurt Flesch, Ian Waters, James R Harrison, Heinke G Frerichs, Andrew Kirk, Oliver Schmitz A marked decrease in particle confinement time with a coinciding increase of neutral fueling to the plasma was found in L- and H-mode discharges at MAST when resonant magnetic perturbations (RMPs) were applied, which together indicate that transport is enhanced. To investigate this result that came from a 0-D particle balance, a multi-pronged approach was developed using experimental data and plasma modeling to understand the effect of neutral fueling and exhaust on the plasma pump-out. A time dependent multi-reservoir particle balance model with atomic, molecular, plasma, and wall reservoirs was developed to resolve further the specific role neutral particles play on fueling the plasma during pump-out. EMC3-EIRENE simulations estimate that molecular dissociation makes up approximately one quarter of ions fueling the plasma, which is reproduced in the multi-reservoir model along with a slight decrease of that fraction when RMPs are applied. The result was shown to be sensitive to the absorption mechanism of atoms vs. molecules at the wall, which implies a necessary enhancement to the multi-reservoir model. |
Tuesday, October 18, 2022 11:06AM - 11:18AM |
GO03.00009: First snowflake divertor experiments in MAST-U. Vlad Soukhanovskii, G. Cunningham, J. R Harrison, F. Federici, P. Ryan First snowflake (SF) divertor experiments demonstrated steady-state snowflake-plus divertor configurations in 400 kA ohmic L-mode plasmas. The experimental equilibria confirmed the expected SF magnetic geometry properties, namely the increased connection lengths and strike point flux expansion due to the second poloidal field (PF) null (cf. the standard divertor with one null). The infra-red video bolometer diagnostic showed the radiated power peaking around the PF nulls. The divertor ion fluxes measured by the target Langmuir probes showed increased ion flux in the secondary SF strike point region, suggesting that some particle and heat redistribution might have taken place in the convective SF zone. Comparisons with previously reported simulation results obtained with the UEDGE multi-fluid transport code will be discussed. A unique aspect of the SF implementation in MAST-U is the up-down divertor symmetry that could potentially lead to sharing heat and particle exhaust among eight divertor strike points. |
Tuesday, October 18, 2022 11:18AM - 11:30AM |
GO03.00010: 2D Spatial Structures of Fishbone instabilities inferred from Beam Emission Spectroscopy in the Mega Amp Spherical Tokamak Upgrade Henry Hingyin Wong, Clive A Michael, Anthony R Field, Rory Scannell, Daniel Dunai, Alsu Sladkomedova, Neal A Crocker, Ken G McClements, Troy Carter Structures of fast particle-driven internal kink modes (“fishbones”, FBs) are obtained using beam emission spectroscopy (BES) in the Mega Amp Spherical Tokamak Upgrade (MAST-U). FBs are sometimes observed to cause redistribution and loss of beam-injected ions in tokamaks. These processes reduce the effectiveness of beam heating and current drive, and the losses pose a threat to plasma-facing components. The contribution of FBs to plasma fluctuations is isolated using cross-correlation analysis of measurements from Mirnov coils and a BES diagnostic. Preliminary analysis of BES data shows that the radial structures of FBs are peaked at or near the magnetic axis as they were in MAST. The upgraded BES with a larger 2D poloidal view enabled measurements of poloidal wavenumbers (k??) and the poloidal spatial scales of the FBs. These measurements can be used for validation of nonlinear theories of FBs in the future. The work also shows that 2D BES could be an alternative to 2D electron cyclotron emission imaging (ECEI) for measuring 2D spatial structures of core instabilities in tokamaks with low magnetic fields that prohibit the implementation of ECEI diagnostics. |
Tuesday, October 18, 2022 11:30AM - 11:42AM |
GO03.00011: ST40: Advancing the Physics Basis of Spherical Tokamak Reactors Michele Romanelli, Peter Buxton, steven McNamara, Alsu Sladkomedova, Jari Varjie, Chris Marsden, matteo moscheni, salomon janhunen, michail anastopoulos, James Bland, marco sertoli, aleksei dnestrovskii, Mikhail Gryaznevich, sergei medvedev, Stanley M Kaye, Ahmed Diallo, Choong-Seeok Chang, Michael Barnes, Yujia Zhang, Sergei Sharapov The spherical tokamak ST40 [1] built and operated by Tokamak Energy Ltd, has recently demonstrated record ion temperatures above 100M K by operating at 0.5MA plasma current, 1.9 T magnetic field and 1.8MW neutral beam heating power [2]. On the way to the 100M K milestone, ST40 has also produced a wealth of data that allowed for further advances in the physics basis of spherical tokamaks. The large population of suprathermal ions from NBI proved to have an impact on both MHD and microturbulence stabilization. Gyrokinetic analysis has shown that energetic ions reduce turbulent ion transport in ST40, although remaining above neoclassical levels. Moreover, the coupling between turbulence and MHD is shown to lead to improved sawtooth control. The interplay between beam-driven Alfvénic modes of sweeping frequency (chirping modes) and H-L and L-H transitions in the global confinement of plasmas has been investigated together with the effect of beta and the ST geometry on toroidal Alfvén instabilities and ITG turbulence. Injection of Ar and Ne has led to improved confinement, confirming previous theoretical results [3]. Transport analysis has allowed the assessment of transport models including TGLF, Bohm gyro-Bohm, CDBM and CPTM. All models show predictions of the ion temperature close to the experimental values, however they fail to reproduce the scaling of confinement with toroidal magnetic field observed in STs [4]. The impact of the ST configuration on the width of the SOL is being studied with the support of the XGC1 global gyrokinetic code. |
Tuesday, October 18, 2022 11:42AM - 11:54AM |
GO03.00012: Merging Formation of High-Beta Spherical Tokamak with Absolute Minimum-B in TS-6 merging experiment Yasushi Ono, Haruaki Tanaka, Shun Kamiya, Haruka Yamaguchi, Jungkyun Kim, Yunhan Cai, Ryo Someya, Shinjiro Takeda, Hiroshi Tanabe, Shunsuke Usami, Ritoku Horiuchi, Chio Z Cheng The high-power reconnection heating of merging spherical tokamak (ST) plasmas has been developed by TS-3, TS-4U, UTST, TS-6, MAST and ST-40 experiments and PIC simulations by NIFS. It allows us to realize direct access to burning high-beta ST plasma often with absolute minimum-B without using any additional heating like neutral beam injection (NBI). Those merging experiments confirmed the promising scaling of ion heating energy increasing with square of reconnecting magnetic field ~ poloidal magnetic field, provided that the current sheet is compressed to the order of ion gyroradius. We found that the reconnection heating forms interesting high-beta ST plasmas with hollow ion temperature profiles. This high-beta ST often has an absolute minimum-B profile in the second stability regime for ballooning instabilities. It is also noted that the toroidal field does not affect ion heating energy as high as 40-50% of poloidal magnetic energy of merging ST plasmas when the merging ST plasmas have safety factor q>1. The toroidal magnetic field does not affect the global hollow ion temperature profile of the produced new ST plasma but peaks its electron temperature profile locally at the O-point. |
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