Bulletin of the American Physical Society
60th Annual Meeting of the APS Division of Plasma Physics
Volume 63, Number 11
Monday–Friday, November 5–9, 2018; Portland, Oregon
Session GO5: SPARC, C-Mod, and High Field Tokamaks |
Hide Abstracts |
Chair: Carlos Paz-Soldan, General Atomics Room: OCC B113-114 |
Tuesday, November 6, 2018 9:30AM - 9:42AM |
GO5.00001: SPARC and the high-field path Robert Mumgaard, SPARC Team The SPARC tokamak and accompanying R&D program is based around the high-field approach to near-term fusion power in a privately funded milestone-based risk-retirement program. Building on Alcator C-Mod results, SPARC is a pre-conceptual design to use high-temperature superconductors (HTS) in a compact (R0=1.65), high-field (B0=12 T) pulsed tokamak to demonstrate net-gain from fusion energy for the first time. SPARC builds on the well-established tokamak physics basis for ITER as well as the previous series of high-field copper tokamak designs (Ignitor, CIT, BPX, and FIRE). Whereas the high-field copper designs were seen as technological dead-ends due to the large recirculating power, HTS—with its high critical magnetic field—now provides a technological pathway towards compact, tokamak-based fusion power plants such as that envisioned in the ARC design. The technological, physics, and organizational approach for the high-field path keyed on SPARC and HTS magnets will be discussed and placed in context as part of the wider fusion R&D effort. |
Tuesday, November 6, 2018 9:42AM - 9:54AM |
GO5.00002: Performance Projections For SPARC Martin J Greenwald, Dan Brunner, Zachary S Hartwig, James Henderson Irby, Brian LaBombard, Yijun Lin, Earl S Marmar, Robert T Mumgaard, Brandon N Sorbom, Anne Elisabeth White, Dennis G Whyte, Steve Wukitch SPARC, the fourth in the series of compact high-field tokamaks at MIT will be a D-T burning experiment based on emerging HTS magnet technology. The projected SPARC performance, using a pre-conceptual design point, rests on conservative plasma physics – essentially the ITER baseline scenario. In terms of dimensionless plasma parameters, SPARC is closer to the database median than ITER. In fact, a number of shots exist in the ITER confinement database that simultaneously match all of the plasma and geometric dimensionless parameters, increasing confidence in the projections. Under the baseline assumptions for confinement and access to H-mode, SPARC would achieve Q ~ 3.6 and generate up to 100 MW of fusion power. Using different confinement assumptions, we find that SPARC has robust access to Q > 2 even at H98 of 0.9, that is, one standard deviation below the scaling mean. It would reach Q = 1 in L-mode (H89 = 1) and Q ~ 2.6 in a modestly improved L-mode (H89 = 1.4). There is a long tail on its high end performance, with Q = 5 in reach with H98 of 1.1 (one standard deviation above the scaling mean) and perhaps higher in I-mode. Advanced operation with hybrid modes or ITBs could be explored in this device, but are not required for the mission. |
Tuesday, November 6, 2018 9:54AM - 10:06AM |
GO5.00003: Initial Scoping of the SPARC Divertor D.G. Whyte, D. Brunner, B. LaBombard, SPARC Team Although the divertor heat flux is expected to be intense in SPARC, initial scoping shows that it should be manageable with a combination of modest radiation fractions and strike point sweeping on an inertially-cooled target. The small size, and therefore short current relaxation time, of the SPARC core plasma allows the core to reach steady state in under 10 seconds. This has two important benefits for the divertor: (1) The divertor does not need to be actively cooled during the pulse, greatly reducing the demands on first-wall component engineering. (2) The total plasma fluence on the divertor is greatly reduced in comparison to ITER, easing erosion lifetime concerns. Using the empirical inverse poloidal magnetic field heat flux width scaling and various assumed radiation fractions, thermal simulations are performed of the divertor target with active strike point sweeping. Results indicate that only a modest radiation fraction (~50%) is needed to maintain the divertor surface temperature to acceptable levels. |
Tuesday, November 6, 2018 10:06AM - 10:18AM |
GO5.00004: ICRF Heating for SPARC Yijun Lin, Stephen James Wukitch, Paul Thaddeus Bonoli, Andrew Seltzman, John Christopher Wright ICRF is the proposed auxiliary heating method (30 MW at 120 MHz) for the SPARC experiment based on its successful use on Alcator C-Mod and elsewhere. ICRF heating for SPARC will have the fortunate combination: modest antenna loading for good power coupling and high single-pass-absorption for effective heating. The antenna loading, mainly set by the density profile near the plasma edge, is similar to that on Alcator C-Mod and much higher than that on ITER. The single-pass-absorption of the fast waves, strongly affected by the machine size, is close to that on ITER and higher than that on Alcator C-Mod. Additionally, the k|| spectrum of the launched RF waves is a critical element influencing both coupling and absorption; the antenna will be optimized to launch k|| spectra for good performance. To minimize the impact of impurities at high RF power, the latest techniques in antenna design and operation, such as field alignment of the antenna and image current minimization, will be considered in the analysis and design. To cope with ELMs and other load-varying events, ferrite tuners will be used in transmission lines for real-time matching. The latest result from analysis and modeling will be presented. |
Tuesday, November 6, 2018 10:18AM - 10:30AM |
GO5.00005: Advantages of DT operation in the compact, high-field tokamaks Brandon N Sorbom, Daniel Brunner, Martin J Greenwald, Zachary S Hartwig, James Henderson Irby, Earl S Marmar, Joseph Minervini, Robert Mumgaard, Dennis G Whyte, Anne Elisabeth White The proposed SPARC device is a compact (R ~ 1.6 m), high-field (B ~ 12 T) tokamak designed to achieve Q > 2 and demonstrate fusion break-even with a high power output ~100 MW. In order to achieve this mission, SPARC will conduct deuterium-tritium plasma operations, becoming the third tokamak (after JET and TFTR) to operate with tritium fuel. SPARC’s small size leads to a relatively short current relaxation time, which enables the physics equilibrium of net-gain plasmas in short (~10 s) pulses. Such short pulses reduce by orders of magnitude two critical operational challenges: the total neutron fluence to device components and the on-site and in-vessel tritium inventories. In particular, the significantly reduced tritium inventories predicted for SPARC (~10g) are on par with JET and TFTR during their tritium phases. Thus, SPARC intentionally by design takes advantage of the large body of knowledge generated during the operation of both devices, placing SPARC much closer in terms of nuclear challenges to previous DT fusion devices and providing demonstrated and relevant precedents in permitting, device operations, nuclear systems, health physics and safety, and decommissioning and disposal. |
Tuesday, November 6, 2018 10:30AM - 10:42AM |
GO5.00006: Dependence of Alfvén eigenmode stability on device magnetic field strength and consequences for next-generation tokamaks Elizabeth A Tolman, Nuno F Loureiro, Paulo Rodrigues, Jerry W Hughes, Earl S Marmar Recently-proposed tokamak concepts use magnetic fields up to 12 T [1,2], far higher than in conventional devices, to reduce size and cost. Theoretical and computational study of trends in plasma behavior with increasing field strength is critical to such proposed devices. We consider trends in Alfvén eigenmode (AE) stability. Energetic particles, including alphas from D-T fusion, can destabilize AEs, possibly causing loss of alpha heat and damage to the device. AEs are sensitive to device magnetic field via the field dependence of resonances, alpha particle beta, and alpha orbit width. We describe the origin and effect of these dependences analytically and using numerical techniques reported in [3]. We suggest high-field machines may partially cut off AE resonances, reducing growth rates of AEs and the energy of alphas interacting with them. High-field burning plasma regimes have non-negligible alpha particle beta and AE drive, but high electron density and field strength reduces this beta relative to low-field machines with similar power densities. [1] Sorbom et al., Fus. Eng. and Des. 100 (2015): 378-405. [2] Greenwald et al., PSFC Rep. RR-18-2 (2018). [3] Rodrigues et al., Nucl. Fusion 55.8 (2015): 083003. |
Tuesday, November 6, 2018 10:42AM - 10:54AM |
GO5.00007: Critical edge ion heat flux for L-H transition: parameter dependence and projections to high field tokamaks Jerry W. Hughes, Matthias Schmidtmayr, Francois Ryter, Elizabeth A Tolman, Norman M. Cao, Alexander J Creely, Martin J Greenwald, Nathan T Howard, Amanda E Hubbard, Yijun Lin, Abhilash Mathews, Matthew L Reinke, John Edward Rice, Elisabeth Wolfrum, Stephen James Wukitch, the Alcator C-Mod Team, the ASDEX Upgrade Team L-H transition experiments on Alcator C-Mod with toroidal field BT of 4.0—7.8T strengthen the basis for projecting power requirements for future fusion devices, including burning plasmas such as ITER and SPARC. The favored ITPA scaling law for H-mode power threshold Pth captures the approximate dependence of Pth on BT and density n, though uncertainty remains. C-Mod transport and power balance analysis confirms and extends a prior result from ASDEX Upgrade (AUG): a critical surface-integrated ion heat flux per particle Qi/n is needed to enable the L-H transition. Analysis indicates that Qi at the L-H transition increases linearly with both n and BT, trends not necessarily reflected in experimental Pth due to the changing balance of edge electron and ion heat fluxes, which depends in turn on the auxiliary heating scheme and electron-ion equilibration. Data drawn from both C-Mod and AUG yield an expression for the edge ion heat flux at the L-H transition, Qi/S∼n1.07BT 0.76, where S is the plasma surface area. The result is consistent with a critical shear in edge E×B being necessary for H-mode access. |
Tuesday, November 6, 2018 10:54AM - 11:06AM |
GO5.00008: Current Quench Avoidance using Lower Hybrid Current Drive in the Alcator C-Mod Tokamak Matthew Reinke, Steve Scott, Robert S Granetz, Cristina Rea, Jerry W Hughes, Kevin J Montes, Stephen James Wukitch Experiments from Alcator C-Mod demonstrate that following a full thermal collapse due to a large impurity influx, a current quench (CQ) can be avoided using lower hybrid (LH) current drive. This existence proof shows a CQ does not necessarily follow a thermal quench (TQ) when the helicity is maintained externally via RF, opening new optimization strategies for disruption avoidance. In a 400 kA, ne,bar~ 0.5x1020 m-3, 5.4 T L-mode plasma, 700 kW of 4.6 GHz LH power is used to approach fully non-inductive operation, dropping Vloop from 0.8 V to ~0.1 V. A high-Z injection increases the PRAD above the input power, causing a cooling wave to propagate inward with temperatures dropping over ~100 ms to those resembling a post-TQ plasma. While the Ip and ne,bar remain fixed within ~10%, on-axis Te drops from 3 keV to 10’s of eV, and Te becomes peaked off-axis at r/a ~ 0.7 with values as low as 170 eV. As the impurity density decays, the plasma re-heats, the RF turns off and the plasma reverts to its pre-injection state over ~500 ms. During this event, Vloop reaches up to 1.5 V, corresponding to a max E/Ecrit ~ 9. Similar plasmas exhibit the high-Z cooling behavior, but MHD-induced loss of fast electrons and termination of LH power results in a transition to a standard CQ in < 10 ms. |
Tuesday, November 6, 2018 11:06AM - 11:18AM |
GO5.00009: A Machine Learning-based Real Time Disruption Predictor on DIII-D Cristina Rea, Keith Erickson, Robert S Granetz, Robert Johnson, Nicholas Eidietis, Kevin J Montes, Roy Alexander Tinguely Machine Learning-based disruption predictors have shown different performances on DIII-D and Alcator C-Mod. Nevertheless, it is important to develop predictors to avoid disruptions without empirical tuning in future devices, like ITER or SPARC. A new disruption prediction algorithm called DPRF (Disruption Prediction via Random Forests) is now embedded in the DIII-D plasma control system; it predicts impending disruptions with >100 ms warning time, and has a low false alarm rate. DPRF real-time disruptivity warning was exploited during an ITER baseline scenario DIII-D discharge to ramp down Ip and actively avoid an impending disruption. DPRF was trained on >5k disruptive and non-disruptive discharges, during flattop Ip and independent of their cause. DPRF average computation time is ~300 us, and input signals are mainly dimensionless or cast in dimensionless form, which facilitates the algorithm’s portability across different devices. DPRF’s novelty is the accessible interpretability of its predictions: by identifying the causes underlying disruption events, a better understanding of disruption dynamics is achieved, and a clear path toward the design of disruption avoidance strategies can be provided. |
Tuesday, November 6, 2018 11:18AM - 11:30AM |
GO5.00010: Mode Structure, Coherence, and Locality of Edge Modes on Alcator C-Mod Theodore Golfinopoulos, Brian LaBombard, Amanda E Hubbard, Jerry W Hughes, Dennis G Whyte, Robert S Granetz, Seung Gyou Baek, Evan M Davis, Paul C Ennever, Martin J Greenwald, James Henderson Irby, Earl S Marmar, Miklos Porkolab, John Edward Rice, Stephen M Wolfe, Stephen James Wukitch Persistent edge fluctuations are implicated as the impurity exhaust mechanism that sustains steady-state, non-ELMing confinement regimes. These modes appear in a limited frequency band, and a global character with long coherence length is often assumed. Here, we examine the coherence time and length scales for two edge fluctuations: the Quasi-Coherent Mode (QCM, f~30-200 kHz, 5<n<30, kθρs<0.1) of EDA H-mode, and the Weakly Coherent Mode of I-mode (WCM, f~100-500 kHz, 5<n<25, kθρs<0.1). Using data from Mirnov coils, phase contrast imaging, two-color interferometry, and reflectometry, we find that both modes have relatively short coherence times of several wave periods, and similarly short coherence lengths of several wavelengths. This suggests a wavelet interpretation of the fluctuation, wherein short, uncorrelated bursts of characteristic wavelets appear at the edge, and is reminiscent of WCM behavior observed on AUG, as well as "blobby" scrape-off-layer turbulence. The intermittent and localized nature of these bursts permits spatial asymmetries in the power spectra. This can manifest itself in a surprising way: the QCM peak frequency can occasionally vary with toroidal angle for a portion of a discharge. |
Tuesday, November 6, 2018 11:30AM - 11:42AM |
GO5.00011: Assessment of Methods to Infer Fluctuation Induced Transport with a Mirror Langmuir Mach Probe Brian LaBombard, Adam Q Kuang, William McCarthy, Daniel Brunner In principle, Langmuir probes can infer cross-field particle and energy transport arising from plasma turbulence. However, several challenges arise: (1) density, temperature and electric field must be measured or estimated at the same point in space; (2) electron temperature and floating potential fluctuations must be recorded to infer potential fluctuations; and (3) plasma interactions with the probe body (e.g. flows parallel and perpendicular to B) can affect local densities and potentials. A Mirror Langmuir Probe (MLP) bias technique can address challenges (1) and (2). Nevertheless, fluctuation-induced fluxes obtained by traditional analysis are found to exhibit significant scatter, which may be due to (3). Here we explore two new analysis techniques with the aim of mitigating the effects of (3). The first one attempts to correct for influence of plasma flows by applying a Mach probe analysis. The second one exploits the tendency for the plasma fluctuations to have a well-defined poloidal phase velocity in certain regions of the plasma, allowing transport to be deduced from a single MLP electrode. These two methods are compared against traditional techniques, taking advantage of the wealth of data collected on Alcator C-Mod. |
Tuesday, November 6, 2018 11:42AM - 11:54AM |
GO5.00012: The role of polarization currents in disconnecting blobs from the divertor target in Alcator C-Mod Adam Q Kuang, Brian LaBombard, Dan Brunner Cross-field transport in the scrape-off layer (SOL) is dominated by coherent field-aligned structures of high density and temperature known as blobs. Theoretical work on blob dynamics indicates that cross-field νExB propagation is driven by the level of blob polarization which is a function of the current closure mechanism within the blob: they move slower when electrically connected to the sheath in the divertor than when the current closure is dominated by current paths above the X-point. Multi-machine experiments have suggested that increased radial particle transport and the formation of a SOL density shoulder is due to an increased divertor collisionality. However, magnetic shear enhanced polarization currents that affect the current closure above the X-point were not considered in these analyses. At Alcator C-Mod we find that the SOL profiles and fluctuation statistics are independent of changes in divertor collisionality, suggesting that blobs are always disconnected from the divertor target sheath and that the current must close by some other mechanism. C-Mod data indicates that the shear enhanced polarization current is a significant contributor and is sufficient to fulfill this role. |
Tuesday, November 6, 2018 11:54AM - 12:06PM |
GO5.00013: Observation and Quasilinear Modeling of Rotation Reversal Hysteresis in Alcator C-Mod Plasmas Norman M. Cao, John Edward Rice, Patrick Henry Diamond, Anne Elisabeth White, Seung Gyou Baek, Mark A Chilenski, Eric M Edlund, Paul C Ennever, Jerry W Hughes, James Henderson Irby, Matt Reinke, Pablo Rodriguez Fernandez, Alcator C-Mod Team Analysis and modeling of a new set of rotation reversal hysteresis experiments unambiguously show that changes in turbulence are responsible for the intrinsic rotation reversal and the Linear to Saturated Ohmic Confinement (LOC/SOC) transition on Alcator C-Mod. Plasmas on each side of the transition exhibit different toroidal rotation profiles and different turbulent fluctuations, despite having profiles of density and temperature that are indistinguishable within measurement uncertainty – suggesting a bifurcation process. This process is modelled using a linear mode quasilinear transport approximation, with quasilinear weights calculated through linear gyrokinetic simulation. The deactivation of subdominant (in linear growth rate) ion-scale ITG and intermediate-scale TEM-like instabilities is identified as the only possible change in turbulence across the reversal which is consistent with the experimentally measured profiles and the inferred heat and particle fluxes. Thus, this work suggests a path for understanding the LOC/SOC transition and rotation reversal hysteresis through the dynamics of linearly subdominant, rather than dominant, modes and by changes in relative mode saturation levels. |
Tuesday, November 6, 2018 12:06PM - 12:18PM |
GO5.00014: Up/Down Impurity Density Asymmetries in C-Mod Plasmas John Edward Rice, Matt Reinke, Norman M. Cao, Jerry W Hughes, Joanna Ashbourn, Darin R Ernst, Amanda E Hubbard, Jim Irby
|
Tuesday, November 6, 2018 12:18PM - 12:30PM |
GO5.00015: Multi-Machine, Multi-Constraint Validation of TGLF on Alcator C-Mod and ASDEX Upgrade Alexander J Creely, Garrard D Conway, Simon J Freethy, Tobias Goerler, Nathan T Howard, Pablo Rodriguez Fernandez, Anne Elisabeth White, the ASDEX Upgrade Team The turbulent transport code TGLF [Staebler PoP 2016] is validated on 11 plasma discharges on Alcator C-Mod and ASDEX Upgrade. Traditional transport validation studies focus on a single plasma discharge, due to the computational resources required. Increasingly accurate reduced models such as TGLF and optimization frameworks such as VITALS [Rodriguez-Fernandez FST 2018], however, enable another approach to validation. This study employs heat fluxes, electron temperature fluctuations from CECE [Creely RSI 2018], and perturbative diffusivity [Creely NF 2016] to validate TGLF on 11 discharges on two machines. This study is motivated by recent results suggesting that multi-scale gyrokinetics is required to accurately model some plasmas, while ion-scale models are sufficient for others [Howard PoP 2016, Creely NF Sub.]. To that end, TGLF is validated in both ion- and multi-scale configurations. Multi-scale simulations agree with experiment in all cases. The ratio of the high-k to low-k peak linear growth rates correlates with the importance of multi-scale effects. |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2025 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700