Bulletin of the American Physical Society
58th Annual Meeting of the APS Division of Plasma Physics
Volume 61, Number 18
Monday–Friday, October 31–November 4 2016; San Jose, California
Session YI2: MFE: Disruptions, MHD, & RFPsInvited
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Chair: Robert Granetz, Massachusetts Institute of Technology Plasma Science & Fusion Center Room: 210 CDGH |
Friday, November 4, 2016 9:30AM - 10:00AM |
YI2.00001: Poloidal radiation asymmetries during disruption mitigation by massive gas injection on the DIII-D tokamak Invited Speaker: N.W. Eidietis Measurements of poloidal asymmetry in the radiated power during thermal quench (TQ) mitigation by massive gas injection (MGI) on DIII-D show poloidal peaking in the radiated heat flux at the wall generally consistent with 3D resistive MHD modeling, that indicates a large n=1 tearing mode causes these asymmetries. Radiation asymmetries are a concern to ITER because they can cause localized melting of the first wall even if globally the mitigation successfully radiates 100$\%$ of the plasma thermal energy. Toroidal radiation asymmetries have been well-studied, but until now the equally important poloidal asymmetries were not well constrained. Radiation emissivity profiles are reconstructed by tomographic inversion of AXUV photodiode arrays, from which the peaking measurements are derived. The poloidal peaking measurements are compared to NIMROD 3D resistive MHD simulations. Qualitatively, the measured and modeled peaking evolve similarly. In both cases, peaking during the TQ changes little with toroidal phase, consistent with predictions of n=1 MHD during the TQ producing the asymmetry. Quantitatively, the measured TQ peaking amplitudes are comparable to but consistently higher than the modeled values. This is a result of the measured radiation exhibiting high emissivity lobes at larger minor radius (and outside the separatrix) than the modeled cases, which may indicate incomplete treatment of the plasma-neutral interaction at the plasma edge in the model. This work, combined with previous measurement and modeling and toroidal radiation asymmetries, provides a basis for constraining localized mitigation radiation heat flux in ITER. [Preview Abstract] |
Friday, November 4, 2016 10:00AM - 10:30AM |
YI2.00002: Parallel Impurity Spreading During Massive Gas Injection. Invited Speaker: V.A. Izzo Extended-MHD simulations of disruption mitigation in DIII-D demonstrate that both pre-existing islands (locked-modes) and plasma rotation can significantly influence toroidal spreading of impurities following massive gas injection (MGI). Given the importance of successful disruption mitigation in ITER and the large disparity in device parameters, empirical demonstrations of disruption mitigation strategies in present tokamaks are insufficient to inspire unreserved confidence for ITER. Here, MHD simulations elucidate how impurities injected as a localized jet spread toroidally and poloidally. Simulations with large pre-existing islands at the q$=$2 surface reveal that the magnetic topology strongly influences the rate of impurity spreading parallel to the field lines. Parallel spreading is largely driven by rapid parallel heat conduction, and is much faster at low order rational surfaces, where a short parallel connection length leads to faster thermal equilibration. Consequently, the presence of large islands, which alter the connection length, can slow impurity transport; but the simulations also show that the appearance of a 4/2 harmonic of the 2/1 mode, which breaks up the large islands, can increase the rate of spreading. This effect is seen both for simulations with spontaneously growing and directly imposed 4/2 modes. Given the prevalence of locked-modes as a cause of disruptions, understanding the effect of large islands is of particular importance. Simulations with and without islands also show that rotation can alter impurity spreading, even reversing the predominant direction of spreading, which is toward the high-field-side in the absence of rotation. Given expected differences in rotation for ITER vs. DIII-D, rotation effects are another important consideration when extrapolating experimental results. [Preview Abstract] |
Friday, November 4, 2016 10:30AM - 11:00AM |
YI2.00003: Radiation effects on the runaway electron avalanche Invited Speaker: Chang Liu Runaway electrons are a critical area of research into tokamak disruptions. A thermal quench on ITER can result in avalanche production of a large amount of runaway electrons and a transfer of the plasma current to be carried by runaway electrons. The potential damage caused by the highly energetic electron beam poses a significant challenge for ITER to achieve its mission. It is therefore extremely important to have a quantitative understanding of the avalanche process, including (1) the critical energy for an electron to run away to relativistic energy and (2) the avalanche growth rate dependence on electric field, which is related to the poloidal flux change required for an e-fold in current. It is found that the radiative energy loss of runaway electrons plays an important role in determining these two quantities. In this talk we discuss three kinds of radiation from runaway electrons, synchrotron radiation, Cerenkov radiation, and electron cyclotron emission (ECE) radiation. Synchrotron radiation, which mainly comes from the cyclotron motion of highly relativistic runaway electrons, dominates the energy loss of runaway electrons in the high-energy regime. The Cerenkov radiation from runaway electrons gives an additional correction to the Coulomb logarithm in the collision operator, which changes the avalanche growth rate. The ECE emission [1] mainly comes from electrons in the energy range $1.2<\gamma<3$, and gives an important approach to diagnose the runaway electron distribution in momentum and pitch angle. To study the runaway electron dynamics in momentum space including all the radiation and scattering effects, we use a novel tool, the adjoint method [2] to obtain both the runaway probability and the expected slowing-down time. The method is then combined with kinetic simulations to calculate the avalanche threshold and growth rate.\\ $^1$C. Paz-Soldan et al., Nucl. Fusion 56, 056010 (2016).\\ $^2$C. Liu, D.P. Brennan, A. Bhattacharjee, and A.H. Boozer, Phys. Plasmas 23, 010702 (2016). [Preview Abstract] |
Friday, November 4, 2016 11:00AM - 11:30AM |
YI2.00004: Impact of toroidal and poloidal mode spectra on the control of non-axisymmetric fields in tokamaks Invited Speaker: Matthew J. Lanctot In several tokamaks, non-axisymmetric magnetic field studies show applied n=2 fields can lead to disruptive n=1 locked modes, suggesting nonlinear mode coupling. A multimode plasma response to n=2 fields can be observed in H-mode plasmas, in contrast to the single-mode response found in Ohmic plasmas. These effects highlight a role for n$\textgreater$1 error field correction in disruption avoidance, and identify additional degrees of freedom for 3D field optimization at high plasma pressure. In COMPASS, EAST, and DIII-D Ohmic plasmas, n=2 magnetic reconnection thresholds in otherwise stable discharges are readily accessed at edge safety factors q$\sim$3 and low density. Similar to previous studies, the thresholds are correlated with the “overlap” field for the dominant linear ideal MHD plasma mode calculated with the IPEC code. The overlap field measures the plasma-mediated coupling of the external field to the resonant field. Remarkably, the critical overlap fields are similar for n=1 and 2 fields with m$\textgreater$nq fields dominating the drive for resonant fields. Complementary experiments in RFX-Mod show fields with m$\textless$nq have negligible impact. In H-mode plasmas, applied n=2 fields in DIII-D elicit transport responses with differing poloidal spectrum dependences, including a reduction in toroidal angular momentum that is not fully recoverable using fields that imperfectly match the applied field. These results have motivated an international effort to document n=2 error field thresholds in order to establish control requirements for ITER. This work highlights unique requirements for n$\textgreater$1 control, including the need for multiple rows of coils to control selected plasma parameters for specific functions (e.g., rotation control or ELM suppression). Optimal multi-harmonic (n=1 and n=2) error field control may be achieved using control algorithms that continuously respond to time-varying 3D field sources and plasma parameters. [Preview Abstract] |
Friday, November 4, 2016 11:30AM - 12:00PM |
YI2.00005: Resistive Wall Mode Stability Forecasting in NSTX and NSTX-U Invited Speaker: Jack Berkery Disruption prevention in tokamak fusion plasmas requires accurate identification and prediction of global MHD instabilities. We examine, in the NSTX device and its upgrade NSTX-U, characterization and forecasting of resistive wall modes (RWMs), which are crucial components of disruption event chains. The kinetic RWM growth rate is solved by the MISK code through a dispersion relation combining ideal and kinetic mode energy functionals, $\delta W$ and $\delta W_{K}$. A model for the ideal $n \quad =$ 1 no-wall $\delta W$ term, depending on parameters measurable in real-time, has been recently developed by using the DCON code on more than 5,000 NSTX equilibria. When applied to NSTX-U discharges at higher aspect ratio, the model accurately predicts the $n \quad =$ 1 no-wall limit calculated by DCON through the aspect ratio dependence of the model. Full MISK calculations of $\delta W_{K}$ cannot be performed in real time, but a simplified model based on physics insight from MISK takes a form that depends on ExB frequency, collisionality, and energetic particle fraction. The model will examine when the plasma toroidal rotation profile falls into weaker RWM stability regions based upon this kinetic modification to ideal theory, which contains broad stabilizing resonances via mode-particle interaction. This approach enables, for the first time, the ability to anticipate a growing RWM rather than reacting to one. The reduced model results are tested on a database of NSTX discharges with unstable RWMs. For each discharge, a newly-written disruption event characterization code (DECAF) finds the chain of events leading to a disruption by applying criteria that define each of the physical events. With a simple threshold test of mode amplitude an RWM event was found in each case, and 59{\%} were within 20 wall times of the disruption. The earlier RWM warnings are not false positives; they caused significant, transient decreases in $\beta_{N}$. [Preview Abstract] |
Friday, November 4, 2016 12:00PM - 12:30PM |
YI2.00006: Density-Gradient-Driven trapped-electron-modes in improved-confinement RFP plasmas Invited Speaker: James Duff Short wavelength density fluctuations in improved-confinement MST plasmas exhibit multiple features characteristic of the trapped-electron-mode (TEM), strong evidence that drift wave turbulence emerges in RFP plasmas when transport associated with MHD tearing is reduced. Core transport in the RFP is normally governed by magnetic stochasticity stemming from long wavelength tearing modes that arise from current profile peaking. Using inductive control, the tearing modes are reduced and global confinement is increased to values expected for a comparable tokamak plasma. The improved confinement is associated with a large increase in the pressure gradient that can destabilize drift waves. The measured density fluctuations have frequencies \textgreater 50 kHz, wavenumbers k\textunderscore phi*rho\textunderscore s\textless 0.14, and propagate in the electron drift direction. Their spectral emergence coincides with a sharp decrease in fluctuations associated with global tearing modes. Their amplitude increases with the local density gradient, and they exhibit a density-gradient threshold at R/L\textunderscore n\textasciitilde 15, higher than in tokamak plasmas by \textasciitilde R/a. the GENE code, modified for RFP equilibria, predicts the onset of microinstability for these strong-gradient plasma conditions. The density-gradient-driven TEM is the dominant instability in the region where the measured density fluctuations are largest, and the experimental threshold-gradient is close to the predicted critical gradient for linear stability. While nonlinear analysis shows a large Dimits shift associated with predicted strong zonal flows, the inclusion of residual magnetic fluctuations causes a collapse of the zonal flows and an increase in the predicted transport to a level close to the experimentally measured heat flux. Similar circumstances could occur in the edge region of tokamak plasmas when resonant magnetic perturbations are applied for the control of ELMs. [Preview Abstract] |
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