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 UO03: Magnetohydrodynamics and Stability, Heating and Current Drive, Energetic ParticlesLive Streamed
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Chair: Ksenia Aleynikova, IPP Max Planck, Greifswald Room: Ballroom 100 C |
Thursday, October 20, 2022 2:00PM - 2:12PM |
UO03.00001: MHD origin of the density limit Gianluca Spizzo, Marco Veranda, Nicholas Vivenzi, Matteo Agostini, Daniele Bonfiglio, Susanna Cappello, Lorella Carraro, Maria Ester Puiatti, Paolo Scarin, David Terranova, Marco Valisa, Matteo Zuin The density limit constraints the parameter space of toroidally confined fusion plasmas. In this work we explain the physical mechanism responsible for the large power input required to sustain a reversed-field pinch (RFP) discharge over a threshold in the Greenwald parameter n/nG (nG = IP / πa2 with IP the plasma current). In fact, in the RFX-mod RFP, by increasing n/nG, first a transition from the high-confinement, helical state (QSH) to the low-confinement, chaotic multiple-helicity state (MH) is observed: by increasing density further, at n/nG ≈ 0.5 a poloidally symmetric (m=0), toroidally localized annulus of high density and high radiation forms, in all respect similar to the tokamak MARFE. For larger n/nG, operating the machine becomes difficult, and very few discharges with n/nG ≈ 1 are obtained, though no disruptions are seen. The RFP MARFE is caused by a convective cell which accumulates density in an annulus, whose size is proportional to the edge resonant m/n=0/1 magnetic island. |
Thursday, October 20, 2022 2:12PM - 2:24PM |
UO03.00002: Understanding the roots of tearing mode onset and growth in DIII-D Nathan J Richner, Laszlo Bardoczi A new database study of >2600 DIII-D H-mode discharges assesses the dominant drive of 2,1 tearing modes (TMs), their parameter sensitivities, and the relative occurrence of different seeding mechanisms. As TMs degrade confinement and can lead to disruptions, resolving TM onset physics is crucial for projecting stabilization and avoidance strategies to ITER and beyond. Across the analyzed dataset, 2,1 amplitude grows linearly in the majority of shots, implying that neoclassical stability dominates the mode evolution. The observed onset sensitivities to βN, qmin, and shape parameters also show qualitative agreement with neoclassical TM theory. The onset time distribution follows Poisson point statistics, which is consistent with a random trigger plus a threshold for robust growth. The relative importance of various seed mechanisms such as sawteeth, ELMs, and 3-mode coupling is quantified across reactor-relevant scenarios, and suggests different scenario-dependent approaches to avoiding deleterious 2,1 activity. The results of this work can also help inform the development of scenarios that are passively stable to disruptive 2,1 islands. Work supported by US DOE under DE-FC02-04ER54698. |
Thursday, October 20, 2022 2:24PM - 2:36PM |
UO03.00003: H-mode inhibition: Using shape to destabilize infinite-n ballooning modes in reactor conditions Andrew O Nelson, Carlos A Paz-Soldan, Samuli Saarelma By preventing the growth of pedestal gradients, edge instability to high toroidal mode number (n) ballooning modes can inhibit access to H-mode regimes in tokamak plasmas. In this work, we explore the potential for robust L-mode reactor operation through this mechanism by modeling infinite-n ballooning stability as a function of internal profiles and equilibrium shape using a combination of the CHEASE and BALOO codes. Negative triangularity is found to be a primary lever on preventing access to the 2nd stability region for high-n modes, though the critical triangularity necessary for L-mode operation is observed to depend in a complicated way on the equilibrium aspect ratio, elongation and squareness. In order to stabilize high-n ballooning modes, the local shear over the entire bad curvature region must be sufficiently negative to overcome curvature destabilization on the low field side, which is not possible below a critical triangularity. Extreme low or high squareness can also be employed to close access to the 2nd stable region. Scalings of the ballooning-limited pedestal height are provided as a function of plasma and machine parameters to aid future scenario design. These results suggest that negative triangularity reactors should maintain L-mode-like operation. |
Thursday, October 20, 2022 2:36PM - 2:48PM |
UO03.00004: Thermal ion kinetic effect and Landau damping in fishbone modes Chang Liu, Stephen C Jardin, Nikolai N Gorelenkov, James J Yang, Mario L Podesta In this work we extend the kinetic-MHD simulation approach for macroscopic instabilities in plasmas to include the kinetic effects of both thermal ions and energetic ions. The new coupling scheme includes synchronization of density and parallel velocity between thermal ions and MHD, in addition to pressure coupling, to ensure the quasineutrality condition and avoid numerical errors. The new approach has been implemented in the kinetic-MHD code M3D-C1-K, and was used to study the thermal ion kinetic effects and Landau damping in fishbone modes in both DIII-D and NSTX. It is found that the thermal ion kinetic effects can cause an increase of the frequencies of the non-resonant n = 1 fishbone modes driven by energetic particles for qmin > 1, and Landau damping can provide signficant stabilization effects. A nonlinear simulation for n = 1 fishbone mode in NSTX is also performed, and the perturbation on magnetic flux surfaces and the transport of energetic particles are calculated. |
Thursday, October 20, 2022 2:48PM - 3:00PM |
UO03.00005: Nonlinear magnetohydrodynamic modeling of current-drive-induced sawtooth-like crashes in the W7-X stellarator Yao Zhou, Ksenia Aleynikova, Nathaniel M Ferraro Sawtooth-like core electron temperature crashes have been observed in W7-X experiments with electron cyclotron current drive applied. We present a nonlinear magnetohydrodynamic simulation of this phenomenon using the newly developed stellarator modeling capability of the M3D-C1 code. The near-axis current drive gives rise to two ι=1 resonances in the rotational transform so that two consecutive (1,1) internal kink modes are seen in the simulation. A small-amplitude crash at the inner resonance takes place first, which may correspond to the sawtooth precursors observed in the experiments. A bigger crash at the outer resonance then follows, which shows semi-quantitative agreements with experimental measurements on metrics such as the crash amplitude and the inversion radius of the temperature change. These results validate the stellarator modeling capability of M3D-C1 and illustrate the mechanism of the current-drive-induced sawtooth-like crashes in W7-X. |
Thursday, October 20, 2022 3:00PM - 3:12PM |
UO03.00006: Development of a conceptual ion cyclotron resonance heating (ICRH) system and its antenna design for CMFX. Debjyoti Basu, Julio J Martinell, Mauricio O Chacon, Carlos A Romero-Talamás, Brian L Beaudoin, CMFX Team The Centrifugal Mirror Fusion Experiment (CMFX) is a new experiment to centrifugally confine and stabilize plasmas confined in a magnetic mirror configuration. Radially sheared plasma rotation in the azimuthal direction (Vθ) will provide the initial plasma heating and instability suppression. There is plan to develop an ion cyclotron resonance heating (ICRH) system which can be used as possible heating mechanism in later stages to further increase plasma temperature. Analytical and simulation studies are required for magnetic field configurations with two species minority and hybrid heating and single species second harmonics heating frequencies in the density regime n=1×1018 - 5×1019 m-3 with parabolic type density profile and a magnetic field profile with peak value B0=0.5T. Initially, it is aimed to increase fuel ion temperature up to 20% with ICRH prototype system from its initial background plasma temperature 0.5keV. Here, `D' and `He' will be considered the minority heating species. An array of two loops and dipole straps are considered for ICRH antennas, and simulation studies are carried out with COMSOL Multiphysics. All analytical and simulation results will be presented. |
Thursday, October 20, 2022 3:12PM - 3:24PM |
UO03.00007: Characterization of RF-enhanced potentials with varying antenna power ratio on Alcator C-Mod Raymond Diab, Yijun Lin, Stephen J Wukitch, Seung Gyou Baek, Adam Q Kuang, Earl S Marmar, James L Terry Impurity contamination caused by RF-enhanced potentials on antenna structures has long been detrimental to high-power operation of ICRF systems in tokamaks with metallic PFCs. With the C-Mod 4-strap field-aligned antenna, impurity generation was minimized with an appropriate power ratio between the inner and outer straps. In this work, we present a detailed characterization of RF-enhanced potentials during the antenna power ratio modulation experiments on C-Mod. With 1 MW of RF power and appropriate power ratio, the Gas Puff Imaging diagnostic, magnetically connected to the lower corner of the antenna, measures no change in poloidal velocity compared to the case with no RF. At the same time, the floating potential of a swept Langmuir probe, magnetically connected to the upper corner of the antenna, goes back to its background value. This suggests, for the first time, that near-field RF rectification effects can be cancelled with appropriate antenna power ratio. When deviating from the optimal power ratio, both the amplitude and width of enhanced potential structures are seen to increase. Moreover, with 1.5 MW of RF power, RF-enhanced potentials are minimized but not cancelled, suggesting the existence of a threshold residual induced current for rectified potentials to appear. |
Thursday, October 20, 2022 3:24PM - 3:36PM |
UO03.00008: DIII-D Experimental Program on the Physics and Technology of Efficient RF Off-Axis Current Drive Robert I Pinsker, X. Chen, C. C Petty, B. Van Compernolle, C. P Moeller, M. Porkolab, S. J Wukitch, A. Seltzman DIII-D is engaged in an experimental effort to evaluate three technologies for efficient rf non-inductive current drive (CD) in the mid-radius region (ρ~0.5 – 0.85). CD in this region is needed for a steady-state tokamak reactor; its efficiency is a key economic factor. At DIII-D, current drive with nearly-vertically-launched Electron Cyclotron waves ("Top launch") has yielded twice the CD efficiency of the usual outside launch, in agreement with modeling. A pair of new top launchers is being tested in 2022 with up to 2.5 MW. The fast wave in the lower hybrid range of frequencies ("helicon"), was tested in DIII-D in 2021 at up to 0.3 MW coupled at 0.48 GHz with a comb-line traveling wave antenna; present efforts include upgrading the antenna feedthroughs and improving the transmission lines, which should permit doubling of the coupled power in 2022 and measurement of absorption and CD efficiencies. An innovative compact launcher for High-Field-Side launch of slow waves at 4.6 GHz is in final fabrication at MIT; installation in DIII-D is slated for FY23. A 2 MW power system with eight 4.6 GHz klystrons was installed and is being tested; the complete system will be ready for experiments in FY24. |
Thursday, October 20, 2022 3:36PM - 3:48PM |
UO03.00009: Operational space for Lower Hybrid scenarios in the full tungsten environment of WEST Jorge Morales, Valeria Ostuni, Clarisse Bourdelle, Jean-Francois Artaud, Remi Dumont, Annika Ekedahl, Nicolas Fedorczak, Dominique Guilhem, Patrick Maget, Pierre Manas, Philippe Moreau WEST (tungsten –W– Environment in Steady-state Tokamak) is a device specialized for long pulse operation in a tungsten environment. Its main purpose is to develop reactor compatible long pulse scenarios and to study plasma-wall interactions in a metallic environment [1, 2, 3]. In WEST, we use routinely Lower Hybrid Current Drive (LHCD) systems for plasma heating and current drive. During plasma operation, we need to consider three main limits to achieve robust steady-state LHCD scenarios: (i) the power reflected back to the antennas needs to be below a specified threshold, (ii) fast electron ripple losses must be limited, (iii) power density needs to be large enough to increase central electron temperature (Te0) above 3 keV and avoid radiative collapses [4]. This value is required because at this temperature we are beyond the tungsten radiation peak (located at 1.5 keV). Above 3 keV the tungsten-cooling factor is lower and decreases slowly with Te. |
Thursday, October 20, 2022 3:48PM - 4:00PM |
UO03.00010: Future Pathways of Additive Manufactured RF Launcher Design Andrew Seltzman, Stephen J Wukitch A high field side lower hybrid multijunction launcher at 4.6 GHz and n||=-2.7 constructed for DIII-D is expected to improve current drive efficiency compared to low-field-side launch scenarios in similar plasma discharges. Additive Manufacture (AM) enabled designs that reduce circulating power with a traveling wave power divider and aperture impedance matching. Laser powder bed fusion enables novel launcher designs that cannot be achieved by conventional manufacturing. GRCop-84 and GRCop-42 copper alloys resist annealing at 900°C while maintaining high thermal conductivity and strength superior to Glidcop and CuCrZr. Hot isostatic pressing consolidated all remaining internal voids increasing >99.8% density to 100%. Chemical or chemical-mechanical polishing refined internal surface roughness to 0.3µm Ra, better than extruded copper waveguide. Brazing, E-beam and laser welding to join launcher and waveguide segments are compared. Poloidal power dividers were printed monolithically with pentagonal waveguides modified for AM that self-support overhanging elements with a chamfered roof while maintain RF tuning properties and low return loss. Launcher designs that may be monolithically printed are explored. |
Thursday, October 20, 2022 4:00PM - 4:12PM |
UO03.00011: Negative triangularity plasma shapes and the Alfvén slow-magnetosonic wave coupling Gerrit J Kramer, Chio Z Cheng, Michael Van Zeeland, Kathreen E Thome The coupling between the Alfvén and slow magnetosonic waves can be modified by the plasma shaping. This can have a profound impact on the available number of eigenmodes, their stability, and potential impact on tokamak performance. Alfvén waves form a continuous spectrum with gaps when different poloidal harmonics have the same frequency. In finite pressure plasmas, slow magnetosonic waves can couple to the Alfvén waves and modify the continuous spectrum, especially at the lower frequencies as was shown in [1] using ideal MHD modeling. The coupling is governed by the geodesic magnetic curvature which depends on the shape of the magnetic field. The possibilities to manipulate the low frequency eigenmode spectrum, from zero up to the TAE frequency, are explored by varying the ellipticity and triangularity of the plasma shape. Simulations indicate that both ellipticity and triangularity change the eigenmode spectrum but changes in triangularity are more effective. For a DIII-D like plasma it was found that at moderate negative triangularity (-0.4) the number of potential eigenmodes was minimal. These simulation results can be tested experimentally during the upcoming negative triangularity experiments in DIII-D. |
Thursday, October 20, 2022 4:12PM - 4:24PM |
UO03.00012: Effects of Fast Particles on MHD and Drift waves Jan Weiland, Tariq Rafiq, Christopher Wilson, Eugenio Schuster The purpose of the present work is to include the effects of fast particles in our present fluid model for tokamak transport. This means that the model will no longer be reactive to frequencies approaching the precession frequency of fast particles. In order to obtain a smooth transition, we also include resonance broadening as suggested in Ref. [1]. The theories of fast particles as usually implemented in MHD model and the fluid description of drift waves have been combined, including the effect of resonance broadening as given by Ghantous et al. [1]. As it is easily seen from theories of Fishbone instabilities [2], the evaluation of the kinetic resonance gives a factor as the ratio of mode frequency to precession frequency in front of the resonance. This means that the fast particle resonance can be ignored at the typical drift wave frequencies. As we find, resonance broadening can be seen as having waves trapped in the velocity distribution. Just as particle trapping in waves, the trapping of waves reduces the resonance. In particle trapping, we need a source in velocity space like an external neutral beam or nuclear reactions to maintain the resonance. The same is true for wave trapping. This is why Ghantous et al. [1] find that resonance broadening reduces the effect of the resonance. We have verified this effect in a two-fluid description of drift wave transport that includes fast particles. |
Thursday, October 20, 2022 4:24PM - 4:36PM |
UO03.00013: Observation of fast-ion driven Alfvén-eigenmodes in JET and their effect on turbulence and thermal transport. Juan Ruiz Ruiz, Jeronimo Garcia Olaya, Felix I Parra, Michael Barnes, Michael Hardman, Samuele Mazzi, Yevgen Kazakov, Jon C Hillesheim, Carine Giroud Recent experiments using the 3-ion ICRH heating scheme [Kazakov NF 2015] have been successful at generating substantial populations of MeV range fast ions in the deep core of JET, mimicking the effect of fusion-born alpha particles in future burning plasmas. These fast ions are capable of destabilizing a wide range of Alfvén modes as observed using magnetics, reflectometer and Doppler backscattering measurements. Previous nonlinear gyrokinetic simulations have shown that turbulence existing at the ion-gyroradius scale can be stabilized [Mazzi Nat. Phys. 2022, PPCF 2022], producing close to neoclassical levels of ion heat flux. This results in a thermal transport regime dominated by the electron heat flux. We report on the transport and gyrokinetic modelling using GS2 in conditions when Alfvén eigenmodes are both stable and unstable, as observed from magnetics and Doppler backscattering fluctuation measurements. We probe the origins of the anomalous electron thermal transport in the presence of MeV range fast ions and unstable Alfvén eigenmodes. The implications of these scenarios to burning plasmas will be discussed. |
Thursday, October 20, 2022 4:36PM - 4:48PM |
UO03.00014: Recipe of helium neutral beam injection for studying helium transport in fusion plasma Yutaka Fujiwara, Kenichi Nagaoka, Momoru Sato, Gen Motojima, Naoyuki Suzuki, Hiromi Kato, Ryuichi Sakamoto, Daisuke Nagata, Shuji Kamio, Masaki Osakabe, Katsuyoshi Tsumori, Haruhisa Nakano, Katsunori Ikeda, Mikiro Yoshinuma, Tatsuya Kobayashi, Katsumi Ida In order to accelerate the understanding of helium transport physics, the helium neutral beam experiment started in Large Helical Device (LHD) from the FY2021 experiment. We have modified a pumping system and Neutral Beam (NB) injection system to study helium transport in a typical fusion experimental device. A positive-ion-source-based NB for hydrogen and deuterium beam has been modified to inject helium NB into a plasma. Also, a pumping system was enhanced to increase the helium pumping rate for the vacuum vessel. In the NB, the gas supply system was modified to puff helium and argon gas, enabling a highly focused 78 keV helium-NB to inject into the plasma. In the pumping system, the turbo molecular pumps were added to increase the helium pumping speed, and a factor of approximately 1.5 successfully increased. It was observed that the arc current of the NB ion source decreased during the helium-NB operation. The cause was discovered to be the production of helium bubbles on the surface of tungsten filaments, and the recovery way using argon discharge was demonstrated. In the helium transport experiment using the helium-NB at LHD, measurements of energetic-helium-ion and bulk-helium-ion were successfully performed, and their distributions were observed. |
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