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 TI02: Invited: Magnetic Fusion: DisruptionsLive
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Chair: Nick Eidietis, GA |
Thursday, November 12, 2020 9:30AM - 10:00AM Live |
TI02.00001: Spatially-dependent simulation of runaway electron mitigation experiments on DIII-D Invited Speaker: Matthew Beidler New simulations with the Kinetic Orbit Runaway electron (RE) Code KORC [1] show RE deconfinement losses to the wall are the primary current dissipation mechanism in DIII-D experiments with high-Z impurity injection, and not collisional slowing down. The majority of simulations also exhibit an increase in the RE beam energy due to acceleration by the induced toroidal electric field, even while the RE beam current is decreasing. These findings corroborate recent experimental observations investigating the use of high-Z impurity injection to mitigate post-disruption, RE beams on ITER. KORC incorporates time-sequenced, experimental reconstructions of the magnetic and electric fields, and line integrated electron density to construct spatiotemporal models of electron and partially-ionized impurity transport in the companion plasma. Simulation results indicate current profile changes due to increased pitch angle scattering of REs by injected impurities lower the rotational transform and lead to the deconfinement of REs. Comparisons of experimental current evolution and KORC results demonstrate the importance of including Coulomb collisions with partially-ionized impurity physics, initial RE energy, pitch angle, and spatial distributions, and spatiotemporal electron and partially-ionized impurity transport. This research provides an initial quantification of the efficacy of RE mitigation via injected impurities, and identification of the critical role played by loss of confinement as compared to the relatively slow collisional damping. [1] Carbajal et al., \textit{Phys. Plasmas} \textbf{24}, 042512 (2017) [Preview Abstract] |
Thursday, November 12, 2020 10:00AM - 10:30AM Live |
TI02.00002: The physics of how a minority runaway electron population can dominate the charge state balance and radiative cooling of a post thermal quench plasma Invited Speaker: Nathan Garland A mitigated tokamak disruption, as currently envisioned for ITER, will have a post thermal quench plasma that is cold and has a large population of high-atomic-number impurities. Runaway electrons in this case, even if they carry the full current, will be of minute density. Standard plasma kinetics tell us that the collisional effect diminishes as the relative speed between two colliding particles increases, but relativistic runaway electrons, at a speed near light speed, can dominate the charge balance in a cold plasma with significant high-Z impurities, despite a density that is 2-4 orders of magnitude smaller than that of background thermal electrons. The underlying cause is found to be the relativistic enhancement of the cross sections for both collisional ionization and excitation [1], a QED effect known since the seminal work of Møller, Breit, and Bethe. Collisional excitation is found to have a particularly subtle role here, for both radiative cooling and charge state balance. We illustrate this subtle physics through a collisional-radiative (CR) model that builds upon the popular FLYCHK code, and elucidate the impact on runaway dynamics itself [1]. We explore both steady-state and time-dependent CR evolutions, and outline the implications for accommodating these effects into plasma modeling. The impact of different atomic species and electron distributions is presented. By including the QED effects and with the help of uncertainty quantification, we demonstrate an improved predictive capability and a path forward for in-situ CR modeling of fusion plasma simulations. \\ \\ $[1]$ Garland et al. Physics of Plasmas 27, 040702 (2020) [Preview Abstract] |
Thursday, November 12, 2020 10:30AM - 11:00AM Live |
TI02.00003: MHD modeling of dispersive shell-pellet injection as an alternative disruption-mitigation technique Invited Speaker: Valerie Izzo Simulations of dispersive shell-pellet (DSP) injection with the 3D MHD code NIMROD show outer flux surfaces maintained as core thermal energy is radiated, followed by loss of edge confinement during the second stage of a two-stage current redistribution, producing a current spike and a rapid loss of runaway electron (RE) test-particles. The DSP technique has been demonstrated on DIII-D [1], and is designed to produce high impurity assimilation, giving high radiated-energy fraction without the massive high-Z material injection---required for shattered pellet injection, currently planned for the ITER disruption mitigation system---that can exacerbate RE production. In the simulations, an ablated carbon shell quantity similar to DIII-D experiments leaves flux surfaces intact until the payload delivery. Dilution cooling by added carbon shell electrons drops the core temperature by \textgreater 1keV, but without any significant loss of stored thermal energy by radiation or conduction. After payload delivery, the region of magnetic stochasticity expands from the core outward, and the edge remains warmer than the core. The end of the thermal quench (TQ) is characterized by large amplitude MHD fluctuations simultaneous with an increase in total plasma current ("I$_{\mathrm{p}}$ spike"), and a fast loss of remaining RE test-particles. This late time MHD and I$_{\mathrm{p}}$ spike is associated with the loss of a negative current layer formed due to ``flux trapping'' in the first stage of current redistribution, as described by Wesson [2]. The technique may therefore have advantages for RE deconfinement. Results of DIII-D modeling will be considered in light of ITER DMS requirements. [1] E.M. Hollmann, et al, PRL 122, 065001 (2019). [2] J.A. Wesson, D.J. Ward, M.N. Rosenbluth, \textit{Nucl. Fusion} \textbf{30, }1011 (1990). [Preview Abstract] |
Thursday, November 12, 2020 11:00AM - 11:30AM Live |
TI02.00004: 3D Radiation Analysis Following Shattered Pellet Injection in JET and Progress Towards Understanding the Radiation Shortfall Invited Speaker: Ryan Sweeney Shattered-pellet-injection (SPI) shutdowns of high-performance ITER discharges must radiate large fractions of the plasma thermal energy uniformly to prevent damage. Analysis of the radiation in JET SPI experiments indicates that as the ratio of the plasma thermal energy to the poloidal magnetic energy increases, the assumed-axisymmetric radiation efficiency $\langle f_{\marthrm{rad}}\rangle$ decreases, consistent with massive-gas-injection experiments. However, an asymmetry in the radiation is found that increases as $\langle f_{\marthrm{rad}}\rangle$ decreases. Measurements from four toroidally displaced bolometers are consistent with a helical radiation source. The negative correlation of the asymmetry with $\langle f_{\marthrm{rad}}\rangle$ might result from a systematic error that underestimates the radiated energy when the asymmetry is large. Assuming the post-SPI thermal quench radiation is field aligned, the bolometers constrain a helical structure that passes near the injected neon plume. By tracing this structure to two toroidally displaced bolometer fans, errors in the local radiated energy are reduced by tens of percent, though the toroidal distribution is largely unconstrained. Assuming a Gaussian centered about the injection produces favorable toroidal peaking factors less than two but increases the radiation efficiency only marginally. More peaked distributions would further increase the radiation efficiency, reducing conducted heat loads at the expense of radiation peaking. The JOREK code qualitatively reproduces the temporal evolution of the bolometer fans and will be compared with the experiment. [Preview Abstract] |
Thursday, November 12, 2020 11:30AM - 12:00PM Live |
TI02.00005: MHD Transient Seeding of Disruptive Neoclassical Tearing Modes Invited Speaker: James Callen New toroidal theory coupled with analysis of DIII-D discharges have identified critical physics and parameters for which MHD transients (ELMs and sawteeth) seed growing neoclassical tearing modes (NTMs) that lead to locked modes and disruptions in tokamaks. It has been recognized since their experimental identification (Chang, Callen et al, PRL 1995) that NTMs require a transient excitation to seed robust algebraic temporal growth. A recent slab model (Beidler et al, PoP 2018) used NIMROD calculations to show externally imposed MHD transients in a flowing plasma in a sheared magnetic field can induce magnetic reconnection and growth of a nonlinear tearing mode at a resonant surface. A toroidal extension of this model explores how ELMs and sawteeth can seed growing m/n$=$2/1 NTMs in recent well-diagnosed ITER-baseline-scenario (IBS)-type DIII-D discharges. This new model involves: equilibrium and transient poloidal and toroidal plasma flows in a tokamak, magnetic reconnection induced by a MHD pulse, nonlinear modified Rutherford equation (MRE) for magnetic island growth (or decay), bootstrap current drive, NTM mode frequency dependence of the stabilizing ion polarization current, and mode frequency evolution in the toroidal geometry. Key conditions for robust NTM growth are benchmarked with data from multiple DIII-D discharges; they include a large enough bootstrap current drive and resonant delta B induced by the MHD transient to reduce the NTM mode frequency to open the usually stabilizing polarization current "gate." In ITER, order of magnitude smaller MHD transients are predicted to seed 2/1 NTMs. This work provides criteria for transient-MHD-induced excitation and robust growth of 2/1 NTMs, e.g., for real-time monitoring. [Preview Abstract] |
Thursday, November 12, 2020 12:00PM - 12:30PM Live |
TI02.00006: Non-ideal stability and control of ITER baseline demonstration discharges Invited Speaker: Jeremy Hanson DIII-D experiments and simulations provide new insights into the origins of disruption inducing mode locking events, showing how they correlate with increased plasma response to magnetic probing. Simulations show that changes in ideal and resistive stability impact the response in the ITER baseline regime, well below the pressure limit of the external kink mode. The dependencies of the response measurements on the plasma normalized internal inductance $\ell_\mathrm{i}$ and beta $\beta_\mathrm{N}$ are qualitatively consistent with ideal MHD, although in most cases the amplitude of the measurements exceeds predictions, indicating that the experimental discharges are less stable than expected. This result is surprising in light of similar comparisons made previously in strongly rotating discharges, wherein ideal MHD predicted poorer stability than implied by measurements, and better agreement was obtained with simulations including drift-kinetic modifications to the stability [1]. New resistive MHD simulations show improved compatibility with the measurements, and the closest agreement is obtained by including the experimental plasma rotation in the simulations. Although the input neutral beam (NB) torque is near zero, the simulations show that this level of rotation leads to significant screening of the pitch-resonant field component at the $q=2$ surface. Furthermore, we have demonstrated an elegant response control scheme, based on direct measurements and NB power feedback in the ITER-like discharges, without relying on computationally intensive real-time stability simulations or machine learning. These results provide a foundation and control technique to anticipate and optimize stability in low rotation reactor regimes like the ITER baseline. [1] F. Turco et al., Phys. Plasmas 22, 022503 (2015). [Preview Abstract] |
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