| 1. |
- Fenstermacher, M.E., et al.
(författare)
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DIII-D research advancing the physics basis for optimizing the tokamak approach to fusion energy
- 2022
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 62:4
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Tidskriftsartikel (refereegranskat)abstract
- DIII-D physics research addresses critical challenges for the operation of ITER and the next generation of fusion energy devices. This is done through a focus on innovations to provide solutions for high performance long pulse operation, coupled with fundamental plasma physics understanding and model validation, to drive scenario development by integrating high performance core and boundary plasmas. Substantial increases in off-axis current drive efficiency from an innovative top launch system for EC power, and in pressure broadening for Alfven eigenmode control from a co-/counter-I p steerable off-axis neutral beam, all improve the prospects for optimization of future long pulse/steady state high performance tokamak operation. Fundamental studies into the modes that drive the evolution of the pedestal pressure profile and electron vs ion heat flux validate predictive models of pedestal recovery after ELMs. Understanding the physics mechanisms of ELM control and density pumpout by 3D magnetic perturbation fields leads to confident predictions for ITER and future devices. Validated modeling of high-Z shattered pellet injection for disruption mitigation, runaway electron dissipation, and techniques for disruption prediction and avoidance including machine learning, give confidence in handling disruptivity for future devices. For the non-nuclear phase of ITER, two actuators are identified to lower the L-H threshold power in hydrogen plasmas. With this physics understanding and suite of capabilities, a high poloidal beta optimized-core scenario with an internal transport barrier that projects nearly to Q = 10 in ITER at ∼8 MA was coupled to a detached divertor, and a near super H-mode optimized-pedestal scenario with co-I p beam injection was coupled to a radiative divertor. The hybrid core scenario was achieved directly, without the need for anomalous current diffusion, using off-axis current drive actuators. Also, a controller to assess proximity to stability limits and regulate β N in the ITER baseline scenario, based on plasma response to probing 3D fields, was demonstrated. Finally, innovative tokamak operation using a negative triangularity shape showed many attractive features for future pilot plant operation.
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| 2. |
- Creely, A. J., et al.
(författare)
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Overview of the SPARC tokamak
- 2020
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Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 86:5
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Tidskriftsartikel (refereegranskat)abstract
- The SPARC tokamak is a critical next step towards commercial fusion energy. SPARC is designed as a high-field (B-0 = 12.2 T), compact (R-0 = 1.85 m, a = 0.57 m), superconducting, D-T tokamak with the goal of producing fusion gain Q > 2 from a magnetically confined fusion plasma for the first time. Currently under design, SPARC will continue the high-field path of the Alcator series of tokamaks, utilizing new magnets based on rare earth barium copper oxide high-temperature superconductors to achieve high performance in a compact device. The goal of Q > 2 is achievable with conservative physics assumptions (H-98,H- y2 = 0.7) and, with the nominal assumption of H-98,H- y2 = 1, SPARC is projected to attain Q approximate to 11 and P-fusion approximate to 140 MW. SPARC will therefore constitute a unique platform for burning plasma physics research with high density (< n(e)> approximate to 3 x 10(20) m(-3)), high temperature (< Te > approximate to 7 keV) and high power density (P-fusion/V-plasma approximate to 7 MWm(-3)) relevant to fusion power plants. SPARC's place in the path to commercial fusion energy, its parameters and the current status of SPARC design work are presented. This work also describes the basis for global performance projections and summarizes some of the physics analysis that is presented in greater detail in the companion articles of this collection.
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| 3. |
- Tinguely, R. A., et al.
(författare)
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Experimental and synthetic measurements of polarized synchrotron emission from runaway electrons in Alcator C-Mod
- 2019
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 59:9
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Tidskriftsartikel (refereegranskat)abstract
- © 2019 IAEA, Vienna. This paper presents the first experimental analysis of polarized synchrotron emission from relativistic runaway electrons (REs) in a tokamak plasma. Importantly, we show that the polarization information of synchrotron radiation can be used to diagnose spatially-localized RE pitch angle distributions. Synchrotron-producing REs were generated during low density, Ohmic, diverted plasma discharges in the Alcator C-Mod tokamak. The ten-channel motional Stark effect diagnostic was used to measure spatial profiles of the polarization angle pol and the fraction fpol of detected light that was linearly-polarized. Spatial transitions in pol of 90-from horizontal to vertical polarization and vice versa-are observed in experimental data and are well-explained by the gyro-motion of REs and high directionality of synchrotron radiation. Polarized synchrotron emission is modeled with the synthetic diagnostic Soft; its output Green's (or detector response) functions reveal a critical RE pitch angle at which pol flips by 90° and fpol is minimal. Using Soft, we determine the dominant RE pitch angle which reproduces measured pol and fpol values. The spatiotemporal evolutions of pol and fpol are explored in detail for one C-Mod discharge. For channels viewing REs near the magnetic axis and flux surfaces q = 1 and 4/3, disagreements between synthetic and experimental signals suggest that the sawtooth instability may be influencing RE dynamics. Furthermore, other sources of pitch angle scattering, not considered in this analysis, could help explain discrepancies between simulation and experiment.
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| 4. |
- Izzo, V. A., et al.
(författare)
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Runaway electron deconfinement in SPARC and DIII-D by a passive 3D coil
- 2022
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 62:9
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Tidskriftsartikel (refereegranskat)abstract
- The operation of a 3D coil-passively driven by the current quench (CQ) loop voltage-for the deconfinement of runaway electrons (REs) is modeled for disruption scenarios in the SPARC and DIII-D tokamaks. Nonlinear magnetohydrodynamic (MHD) modeling is carried out with the NIMROD code including time-dependent magnetic field boundary conditions to simulate the effect of the coil. Further modeling in some cases uses the ASCOT5 code to calculate advection and diffusion coefficients for REs based on the NIMROD-calculated fields, and the DREAM code to compute the runaway evolution in the presence of these transport coefficients. Compared with similar modeling in Tinguely et al (2021 Nucl. Fusion 61 124003), considerably more conservative assumptions are made with the ASCOT5 results, zeroing low levels of transport, particularly in regions in which closed flux surfaces have reformed. Of three coil geometries considered in SPARC, only the n = 1 coil is found to have sufficient resonant components to suppress the runaway current growth. Without the new conservative transport assumptions, full suppression of the RE current is maintained when the thermal quench MHD is included in the simulation or when the RE current is limited to 250kA, but when transport in closed flux regions is fully suppressed, these scenarios allow RE beams on the order of 1-2 MA to appear. Additional modeling is performed to consider the effects of the close ideal wall. In DIII-D, the CQ is modeled for both limited and diverted equilibrium shapes. In the limited shape, the onset of stochasticity is found to be insensitive to the coil current amplitude and governed largely by the evolution of the safety-factor profile. In both devices, prediction of the q-profile evolution is seen to be critical to predicting the later time effects of the coil.
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| 5. |
- Tinguely, R. A., et al.
(författare)
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Modeling the complete prevention of disruption-generated runaway electron beam formation with a passive 3D coil in SPARC
- 2021
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 61:12
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Tidskriftsartikel (refereegranskat)abstract
- The potential formation of multi-mega-ampere beams of relativistic 'runaway' electrons (REs) during sudden terminations of tokamak plasmas poses a significant challenge to the tokamak's development as a fusion energy source. Here, we use state-of-the-art modeling of disruption magnetohydrodynamics coupled with a self-consistent evolution of RE generation and transport to show that a non-axisymmetric in-vessel coil will passively prevent RE beam formation during disruptions in the SPARC tokamak, a compact, high-field, high-current device capable of achieving a fusion gain Q > 2 in deuterium-tritium plasmas.
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| 6. |
- Tinguely, R. A., et al.
(författare)
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On the minimum transport required to passively suppress runaway electrons in SPARC disruptions
- 2023
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 65:3
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Tidskriftsartikel (refereegranskat)abstract
- In Izzo et al (2022 Nucl. Fusion 62 096029), state-of-the-art modeling of thermal and current quench (CQ) magnetohydrodynamics (MHD) coupled with a self-consistent evolution of runaway electron (RE) generation and transport showed that a non-axisymmetric (n = 1) in-vessel coil could passively prevent RE beam formation during disruptions in SPARC, a compact high-field tokamak projected to achieve a fusion gain Q > 2 in DT plasmas. However, such suppression requires finite transport of REs within magnetic islands and re-healed flux surfaces; conservatively assuming zero transport in these regions leads to an upper bound of RE current ∼ 1 M A compared to ∼ 8.7 M A of pre-disruption plasma current. Further investigation finds that core-localized electrons, within r / a < 0.3 and with kinetic energies ∼ 0.2 - 15 M e V , contribute most to the RE plateau formation. Yet only a relatively small amount of transport, i.e. a diffusion coefficient ∼ 18 m 2 s − 1 , is needed in the core to fully mitigate these REs. Properly accounting for (a) the CQ electric field’s effect on RE transport in islands and (b) the contribution of significant RE currents to disruption MHD may help achieve this.
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| 7. |
- Tinguely, R. A., et al.
(författare)
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Measurements of runaway electron synchrotron spectra at high magnetic fields in Alcator C-Mod
- 2018
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 58:7
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Tidskriftsartikel (refereegranskat)abstract
- In the Alcator C-Mod tokamak, runaway electron (RE) experiments have been performed during low density, ?attop plasma discharges at three magnetic felds: 2.7, 5.4, and 7.8 T, the last being the highest feld to-date at which REs have been generated and measured in a tokamak. Time-evolving synchrotron radiation spectra were measured in the visible wavelength range (λ∼300-1000 nm) by two absolutely-calibrated spectrometers viewing co- and counter-plasma current directions. In this paper, a test particle model is implemented to predict momentum-space and density evolutions of REs on the magnetic axis and q = 1, 3/2, and 2 surfaces. Drift orbits and subsequent loss of confnement are also incorporated into the evolution. These spatiotemporal results are input into the new synthetic diagnostic SOFT (Hoppe et al 2018 Nucl. Fusion 58 026032) which reproduces experimentally-measured spectra. For these discharges, it is inferred that synchrotron radiation dominates collisional friction as a power loss mechanism and that RE energies decrease as magnetic feld is increased. Additionally, the threshold electric feld for RE generation, as determined by hard x-ray and photo-neutron measurements, is compared to current theoretical predictions.
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| 8. |
- Tinguely, R. A., et al.
(författare)
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Spatiotemporal evolution of runaway electrons from synchrotron images in Alcator C-Mod
- 2018
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 60:12
-
Tidskriftsartikel (refereegranskat)abstract
- In the Alcator C-Mod tokamak, relativistic runaway electron (RE) generation can occur during the flattop current phase of low density, diverted plasma discharges. Due to the high toroidal magnetic field (B-0 = 5.4 T), RE synchrotron radiation is measured by a wide-view camera in the visible wavelength range (lambda approximate to 400-900 nm). In this paper, a statistical analysis of over one thousand camera images is performed to investigate the plasma conditions under which synchrotron emission is observed in C-Mod. In addition, the spatiotemporal evolution of REs during one particular discharge is explored in detail via a thorough analysis of the distortion-corrected synchrotron images. To accurately predict RE energies, the kinetic solver CODE (Landreman et al 2014 Comput. Phys. Commun. 185 847-855) is used to evolve the electron momentum-space distribution at six locations throughout the plasma: the magnetic axis and flux surfaces q = 1, 4/3, 3/2, 2, and 3. These results, along with the experimentally-measured magnetic topology and camera geometry, are input into the synthetic diagnostic SOFT (Hoppe et al 2018 Nucl. Fusion 58 026032) to simulate synchrotron emission and detection. Interesting spatial structure near the surface q = 2 is found to coincide with the onset of a locked mode and increased MHD activity. Furthermore, the RE density profile evolution is fit by comparing experimental to synthetic images, providing important insight into RE spatiotemporal dynamics.
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