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 GP19: Poster Session: Magnetic Confinement: DIII-D Tokamak I (9:30am -12:30pm)On Demand
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GP19.00001: New regime for high-beta hybrid using off-axis electron cyclotron current drive on DIII-D C.C. Petty, J.R. Ferron, T.H. Osborne, K.E. Thome, M.A. Van Zeeland, C.T. Holcomb, F. Turco The DIII-D tokamak has developed a new regime for high-beta hybrid plasmas where the broad current profile is achieved with off-axis electron cyclotron current drive (ECCD) rather than anomalous poloidal magnetic flux pumping. The high-beta hybrid regime without sawteeth is a candidate for the Q$=$5 steady-state scenario on ITER, but the anomalous flux pumping mechanism that maintains q$_{\mathrm{min}}$\textgreater 1 is not yet understood. Experiments on DIII-D have found that high performance with $\beta _{\mathrm{N}}=$3.7 and H$_{\mathrm{98y2}}=$1.6 is maintained at high-density (above cutoff density for on-axis ECCD) when 3.4 MW of ECCD is moved from $\rho $\textless 0.2 to $\rho =$0.5. In this new hybrid regime, the change in ECCD profile from on-axis to off-axis is predicted from TRANSP simulations of the neoclassical poloidal flux evolution to increase q$_{\mathrm{min}}$ by 0.5 to a value well above 1, in agreement with MSE-constrained equilibrium reconstructions and consistent with the disappearance of the fishbone instability, showing there is no evidence for anomalous flux pumping. Off-axis ECCD allows higher density operation without encountering the density cutoff, which increases the confinement time. About half of the confinement improvement is due to 30{\%} lower electron thermal transport; the remaining improvement is due to reduced beam ion transport that correlates with weaker Alfven eigenmode activity. [Preview Abstract] |
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GP19.00002: Divertor heat flux profiles in negative triangularity DIII-D discharges F. Scotti, C.J. Lasnier, J. Lore, A. Marinoni, M.E. Austin, S.L. Allen L-mode discharges with negative upper triangularity ($\delta_u$) shape in DIII-D exhibited wider scrape-off layer (SOL) power fall off lengths ($\lambda_q$) compared to H-mode discharges with similar shaping. Lower single null diverted discharges with negative upper triangularity and near zero lower triangularity were developed in DIII-D at plasma current $I_p$=900kA with neutral beam ($P_{NBI}$=4-13MW) and electron cyclotron heating ($P_{ECH}$=1.5 MW). SOL power fall-off lengths were derived from upstream Thomson scattering profiles and divertor infrared thermography. The inter-ELM $\lambda_q$ measured in a negative upper triangularity ($\delta_u\sim$-0.2) H-mode discharge was comparable with those measured in H-mode discharges with positive $\delta_u$. The SOL power fall off length in L-mode discharges with $\delta_u \sim$-0.4 was up to 50$\%$ larger than in the similarly-shaped H-mode plasma. $\lambda_q$ in negative triangularity L-mode discharges was reduced when compared to L-mode plasmas in positive triangularity at lower injected power and to multi-machine L-mode scaling. 2D transport modeling with the SOLPS-ITER code is underway to estimate the change in edge transport coefficients between negative triangularity L and H-mode discharges and their effect on divertor plasma. [Preview Abstract] |
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GP19.00003: LH-transition Prevented by Ballooning Instability in Negative Triangularity DIII-D Discharges Samuli Saarelma, Max Austin, Alessandro Marinoni, Matthias Knolker, Carlos Paz-Soldan, Lothar Schmitz, Philip Snyder The LH-transition is suppressed in DIII-D when the top triangularity is made sufficiently negative and plasma instead stays in L-mode well above the LH power threshold in positive triangularity shape [1]. The ideal MHD stability analysis finds that there is a drastic change in stability limit for the local infinite-n ballooning modes when the top triangularity is made negative enough. The lowered ballooning stability limit prevents the edge pressure gradient from increasing, thus preventing the LH-transition and the pedestal formation. The modelling also shows that if the top triangularity is strongly negative the bottom triangularity does not affect the stability boundary as long as it stays less negative than the top triangularity. The predictive analysis using the EPED model [2] shows that even if H-mode were achieved in a negative triangularity discharge, it would have very low pedestal pressure and would not benefit from the core-edge synergy that is driven by the stabilizing effect of the increasing Shafranov-shift on finite-n peeling-ballooning modes seen in plasmas with positive triangularity. [1] M. Austin et al., Phys. Rev. Lett. 122, 115001 (2019) [2] Snyder P.B. et al Phys. Plasmas 16, 056118 (2009) [Preview Abstract] |
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GP19.00004: Electron Temperature Turbulence and Poloidal Turbulent Flow in Negative Triangularity Plasmas on DIII-D G. Wang, K. Barada, R. Hong, T.L. Rhodes, W.A. Peebles, M.E. Austin, A. Marinoni The reactor-relevant negative triangularity (-$\delta )$ shape recently achieved in the DIII-D Tokamak is potentially a good candidate for future fusion reactors. It generally has an energy confinement similar to H-mode plasmas in conventional positive triangularity ($+\delta )$ shape but without ELMs. This work presents electron temperature turbulence and poloidal turbulent flow in -$\delta $ plasmas on DIII-D for understanding their transport and confinement properties. In an Inner Wall Limiter (IWL) configuration with an L-mode edge, the core electron temperature turbulence level in -$\delta $ shape is similar to or slightly less than a $+\delta $ L-mode in the IWL configuration, but near the separatrix the -$\delta $ shape has much lower (\textasciitilde 40{\%}) turbulence. The poloidal turbulent flow for IWL plasmas (both -$\delta $ and $+\delta )$ shows no strong velocity shear contrary to conventional $+\delta $ H-mode plasmas. However, in an L-mode diverted configuration a slight well in the edge poloidal velocity is observed in the -$\delta $ shape, and the electron temperature turbulence level is lower from the edge to the core compared to the IWL case. Interestingly, as the plasma goes into an ``H-mode like'' condition, the edge velocity well becomes much deeper, while electron temperature fluctuation level is not reduced. [Preview Abstract] |
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GP19.00005: Effect of neutral beam injection on sawtooth stability in DIII-D negative triangularity plasmas D. Liu, W. W. Heidbrink, Y. Q. Liu, M. A. Van Zeeland, L. N. Zhou, M. E. Austin, A. Marinoni Recent energetic particle experiments in DIII-D show that sawtooth stability can be strongly affected by injected neutral beam geometry in negative triangularity plasmas. It was observed that when the central safety factor q0 drops below unity, sawteeth are destabilized in negative triangularity plasmas with co-current neutral beam injection (NBI), while they are stabilized in negative triangularity plasmas with similar q0 but counter-current NBI. A significant variation in sawtooth period and sawtooth precursor was also observed depending on the plasma triangularity and injection beam geometry and power. The sawtooth induced fast-ion transport in these cases is relatively weak, as there is no measurable neutron reduction at each sawtooth crash. Non-perturbative and perturbative simulations with the magnetohydrodynamic-kinetic hybrid stability code MARS-K are being carried out to investigate the kinetic effect of energetic particles with different distribution on the n$=$1 kink mode stability as well as the effect of plasma shape and toroidal rotation/shear. The simulation results will be compared with experimental observations with the goals of validating the sawtooth theory and utilizing NBI and electron cyclotron waves to control sawtooth. [Preview Abstract] |
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GP19.00006: Mitigation of Confinement Degradation near Te~Ti with Neon Injection on DIII-D George McKee, Dinh Truong, Zheng Yan, Colin Chrystal, Kathreen Thome Injection of Neon gas into NBI and ECH heated plasmas with nearly equilibrated ion and electron temperatures increases global energy confinement by nearly 25\% while simultaneously doubling the radiated power and reducing turbulence. The energy confinement time in plasmas heated with 3 MW of NBI ($T_{i}>T_{e}$) is reduced by up to 40\% when 3 MW of ECH are injected ($T_{i} \sim T_{e}$), which is thought to result from reduced critical gradients to ITG and TEM turbulence near equilibrated temperatures. Interestingly, density fluctuations undergo very little change with ECH, while thermal transport increases significantly; new measurements with UF-CHERS demonstrated that ion temperature fluctuations increase at higher $T_{e}/T_{i}$, explaining the increased transport. Injecting Neon gas into these NBI+ECH heated discharges results in a significant increase in core ion temperature, confinement time, and radiated power, while reducing intrinsic carbon density; consistently, low-wavenumber density turbulence is found to decrease by approximately 20\%, as measured with BES near $\rho=0.75$. These results show that the physics behind the Radiative-Enhanced (RI)-Mode is effective near $T_{e}/T_{i}\sim 1$ but that $T_{e}/T_{i}$ and Ne injection alter turbulence through different mechanisms. [Preview Abstract] |
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GP19.00007: Comparison of DBS and CER measurements of the $E \times B$ rotation in the DIII-D Tokamak Quinn Pratt, Terry Rhodes, Colin Chrystal, Troy Carter Measurements of the $E \times B$ angular velocity, $\omega_{E\times B}$, are made with both Charge Exchange Recombination Spectroscopy (CER) and Doppler Back Scattering (DBS) diagnostics. DBS measures scattered mm-wave radiation from electron density fluctuations\footnote{Review of Scientific Instruments 80, 083507 (2009)} while CER measures emission from impurity ions\footnote{Review of Scientific Instruments 75, 3481 (2004)}. Radial profiles of $\omega_{E\times B}$ are compared at different phases of a counter-current Neutral Beam Injection pulse. Good agreement is found between DBS and CER, despite measuring different physical processes. The high time resolution of DBS captures dynamics of the poloidal rotation profile as the applied torque varies. Comparing and contrasting $\omega_{E\times B}$ measurements is important due to the key role this value, and its shear, play in turbulence and transport theory. Work supported by USDOE Grants DE-SC0019352, DE-FG02-08ER54984, and DE-FC02-04ER54698. [Preview Abstract] |
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GP19.00008: Investigating the Interactions between Intrinsic Rotation and Turbulence through Multifield Fluctuation Correlation Analysis Xijie Qin, George Mckee, Zheng Yan, Matt Kriete, Dinh Truong, Benedikt Geiger Intrinsic rotation is predicted to be driven by long-wavelength turbulence, $k \sim \rho_i$, while $E \times B$ shear that results from intrinsic and externally driven rotation can suppress turbulence and enhance turbulence decorrelation. To investigate those two competing mechanisms, experiments were performed to measure turbulence and the phase relationships between various fluctuating fields: density fluctuations ($\~{n}$) from Beam Emission Spectroscopy (BES), ion temperature fluctuation ($\~{T}_i$) and toroidal velocity fluctuations ($\~{v}_{\phi}$) from Ultra-Fast Charge Exchange Recombination Spectroscopy (UF-CHERS). In plasmas heated by NBI or NBI+ECH on DIII-D, toroidal rotation, $E \times B$ shear, and momentum transport changed significantly as $T_e/T_i$ increased with ECH injection. The correlation and phase relationship between $\~{v}_{\phi}$ and $\~{n}$ are examined and compared at varying $T_e/T_i$. The effects of $E \times B$ shear on turbulence are investigated by obtaining a 2D $E \times B$ flow-field through velocimetry analysis and calculating turbulence correlation length and decorrelation times through inter-channels correlation analysis. These results will elucidate the role of turbulence in driving intrinsic rotation. [Preview Abstract] |
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GP19.00009: Statistical Electron Transport Analysis during T$_{\mathrm{e}}$ Crash and Recovery Events Bingzhe Zhao Large electron temperature crash and recovery events are observed in the outer core region in some DIII-D hybrid mode discharges with modulated NBI. Some characteristics of these events include an ELM-like T$_{\mathrm{e}}$ recovery curve and stationary density. A statistical analysis process is developed in order to investigate different transport properties in this scenario and potentially other scenarios with similar characteristics. The underlying physics model is the Braginskii energy conservation equation for electrons, which provides a connection between T$_{\mathrm{e}}$, and temporal and spatial differentiations of T$_{\mathrm{e}}$. Local transport properties can be statistically determined by fitting all T$_{\mathrm{e-}}$ related data, which is given by the ECE system, into the physics model. The work includes benchmarking of this analysis process with modulated electron cyclotron heating (MECH) discharges, which have similar characteristics in terms of electron temperature and density history, and the transport properties of MECH discharges are better understood in comparison with hybrid discharges. Events from MNBI hybrid discharges are analyzed with a similar process, and the results are compared with global power balance. . [Preview Abstract] |
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GP19.00010: Structure Measurement of Global Alfven Eigenmodes via Fast Sweep Frequency-modulated Profile Reflectometry in DIII-D L. Zeng, N. A. Croker, W. A. Peebles, T. L. Rhodes Measurement of fast-ion driven global Alfvén eigenmodes (GAE, $\omega < \omega_{ci}$) is achieved via fast sweep, frequency modulated density profile reflectometry in beam-heated L-mode DIII-D plasmas. These modes are of interest because they may cause electron energy transport. The radial distribution of the reflectometer phase (or time delay) perturbations induced by GAE modes with frequency ~5 MHz is measured over the range $0.2 < \rho < 0.6$ with the peak occurring at $\rho $~ 0.4. The phase perturbations are proportional to the mode generated cutoff location displacements allowing the local $n_{e}$ fluctuation profile generated by the GAE to be determined. Comparison of the determined mode structure will be made with independent fixed frequency reflectometer measurements as well as theoretical expectations. This new measurement can broaden the capability of profile reflectometry and advance development of AE spectroscopy as a tool for non-invasive diagnosis of fast-ions in DIII-D and burning plasmas such as ITER. [Preview Abstract] |
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GP19.00011: Chirping Modes Destabilized by Thermal Ions and Their Impacts on High Ion Temperature Plasmas in DIII-D Tokamak Xiaodi Du In a plasma having reactor-relevant ion-temperature of \textasciitilde 20 keV, it is observed that thermal ions play crucial roles in destabilizing the n$=$1, low-frequency Magnetohydrodynamic instabilities with bursting and chirping characters. These characters are commonly viewed as a signature of energetic particle driven modes. Each bursting event leads to a decrease of the central bulk ion temperature by \textasciitilde 1 keV and limits the maximum achievable ion temperature. The instabilities are excited, when bulk Ti exceeds certain threshold and the stability of the mode is not sensitive to the change of the neutral beam combinations. The internal mode structure moves, expands and shrinks radially in a short bursting time window of \textasciitilde 1 ms, exhibiting a non-perturbative feature which is previously reported for EP-driven instabilities [1,2]. Doppler backscattering measurements of density fluctuation reveals that, during each burst, the multiple perturbation harmonics appears sequentially from \textasciitilde 50 kHz to \textasciitilde 1 MHz as an ascending order in frequency and then disappears sequentially as a descending order. The resonance conditions, especially the possible resonant interactions between the multi-frequency coherent perturbations and thermal ions will be discussed. [Preview Abstract] |
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GP19.00012: Novel internal measurements and analysis of ion cyclotron frequency range fast-ion driven modes in DIII-D NA Crocker, GH DeGrandchamp, SX Tang, KE Thome, JB Lestz, EV Belova, AI Zalzali, RO Dendy, WA Peebles, K Barada, R Hong, TL Rhodes, G Wang, L Zeng, WW Heidbrin Novel measurements and analysis of fast-ion driven modes in the ion cyclotron frequency range in DIII-D are presented, including local internal density fluctuations ($\~{n}$) obtained via an array of Doppler Backscattering systems. Two types of modes are excited simultaneously by co-current neutral injection via the more tangential of the two beamlines: 1) Alfvén eigenmodes (AE) at $f\sim0.4f_{ci}$, identified as global AEs (GAE) driven by Landau resonance with the fast ions, and 2) modes at low harmonics of $f_{ci}$, identified as collective ion cyclotron emission (ICE). Multiple aspects of theory for the underlying instabilities are validated. For instance, the GAE observations are compared with analytic theory and simulation via the HYM hybrid MHD code. Also, observations of ICE $\~{n}$ in both core and edge test prevalent theories for spatial localization of ICE and are compared to simulations via the EPOCH PIC code, which uses a locally uniform approximation. [Preview Abstract] |
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GP19.00013: Dependence of Magnetic Perturbation Induced Fast Ion Losses on Perturbation Spectrum and Plasma Response in the ASDEX Upgrade and DIII-D Tokamaks K. Gage, J. Galdon-Quiroga, B. Tal, G. Birkenmeier, M. Willensdorfer, G. Suarez Lopez, L. Sanchis-Sanchez, J. Gonzalez-Martin, P. Cano-Megias, M. Nocente, O. Putignano, W.W. Heidbrink, X. Chen, M. Van Zeeland, J. Hanson The impact of externally applied magnetic perturbations (MPs) on fast-ion losses has been investigated using the light ion beam probe (LIBP) technique[1] in the ASDEX Upgrade (AUG) and DIII-D tokamaks. This allows for experimental inference of the orbit displacement for first orbit losses. In AUG, the displacement against a series of applied spectra has been studied by varying the phase between the upper and lower MP coils. Experimentally, a minimum in the orbit displacement was found for a phase of 50\textdegree . This is offset from the minimum of the plasma boundary displacement, suggesting losses could be disentangled from ELM mitigation. Experiments at DIII-D focus on the impact of the plasma response over large number of shots, where the plasma response was varied by a scan in the plasma $\beta $. In this case, rigid rotation of the MPs was applied in an n$=$1 configuration. Measurements show an increase in displacement with the plasma response. [1] X.Chen \textit{et al}, Rev Sci Instrum \textbf{85}, 11E701 (2014) [Preview Abstract] |
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GP19.00014: Species mix, magnetic field strength, and distribution function dependence of instabilities near the ion cyclotron range of frequencies Genevieve DeGrandchamp, William Heidbrink, Kathreen Thome, Michael Van Zeeland, Cami Collins, Xiaodi Du, Steve Vincena, Shawn Tang, Neal Crocker, Mark Koepke, Samuel Nogami Frontier Science experiments on the DIII-D tokamak explored energetic ion-driven instabilities in the magnetosphere by studying related phenomena in tokamak plasmas: high-frequency Alfv\'{e}n eigenmodes and ion cyclotron emission (ICE). The instabilities' dependence on plasma ion species mix, magnetic field strength, and energetic ion species and their phase space distribution was explored. Ten different beam-ion distributions were studied by varying: species (H$^{\mathrm{+}}$/D$^{\mathrm{+}})$, direction (co- vs. counter-current), energy (81/55 kV), and radial location (on- vs. off-axis) of the energetic ions at birth. The background plasma was D$^{\mathrm{+}}$ with H$^{\mathrm{+}}$ and $_{\mathrm{3}}$He$^{\mathrm{++}}$ in different mixtures throughout the experiment. Prominent ICE harmonics from co-injecting beams changed with increasing H$^{\mathrm{+}}$, whereas ICE from counter-injecting beams is not similarly affected. The instabilities were measured with toroidal magnetic loops digitized at 200 MSamples/s. Additional loops were recently installed and enable more detailed measurements which are used to characterize modes observed in pure D$^{\mathrm{+}}$ shots recreated from the Frontier experiment. [Preview Abstract] |
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GP19.00015: Analysis of Alfven Eigenmode Radial Phase Variation and Energy Flow in DIII-D Discharges Erik C. Hansen, W.W. Heidbrink, M.E. Austin, G.J. Kramer, M.A. Van Zeeland Radial variations in the phase of fast-ion driven Alfven eigenmodes are often observed [Phys. Rev. Lett. \textbf{106 }(2011) 075003]. Recent theoretical work [Nucl. Fusion \textbf{59} (2019) 094001] showed that these radially spiraling, poloidally sheared mode structures occur when the drive and damping of the instability occurs at different spatial positions. Inspired by that work, in this study radial profiles from an electron cyclotron emission diagnostic are analyzed for a large database of Alfven eigenmode instabilities that include BAAE, BAE, RSAE, TAE, and EAE activity. The dependence of the radial phase profile on plasma parameters and instability type is discussed and compared with theoretical predictions of the expected locations of energy transfer. [Preview Abstract] |
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GP19.00016: Validation of TGLF-EP+Alpha model of Alfv\'en Eigenmode induced transport with the Imaging FIDA diagnostic on DIII-D Claudio Marini, Cami Collins, Eric Bass, William Heidbrink, Craig Petty, Michael Van Zeeland, Daniel Lin, Luke Stagner A reduced critical-gradient model for transport of energetic particles (EPs) by Alfv\'en Eigenmodes (AEs), constituted by the combination of TGLF-EP and Alpha codes, is tested against DIII-D L-mode discharges, where the EP population is measured with a new high resolution Imaging Fast Ion D-Alpha (I-FIDA) diagnostic. The outstanding spatial resolution of the I-FIDA diagnostic enables detailed comparisons to the transported EP profile and critical gradient threshold predicted by the model. The I-FIDA system measures a portion of the fast-ion population, generating 2D images with $\leq$2 mm resolution of the co-passing EPs in the energy range E$\simeq$40-80 keV with integration time $\geq 5$ ms. A beam modulation technique is employed to extract the active part of the FIDA signal, limiting the profile measurement rate to $\leq 50$ Hz. To compare measurements to the model, the EP diffusion coefficient profile computed by TGLF-EP+Alpha is input to the TRANSP (NUBEAM) code to compute the fast-ion distribution, which is processed by the FIDASIM code to generate synthetic diagnostic signal. Trends in the critical gradient threshold predicted by the model will be tested experimentally in L-mode discharges for a range of parameters, including magnetic shear and EP profiles and fraction. [Preview Abstract] |
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GP19.00017: Electromagnetic Torque Measurements in DIII-D through Internal/External Magnetic Field Decomposition E.J. Strait, S. Munaretto, R.M. Sweeney, N.C. Logan, A.O. Nelson In a tokamak, spatially resolved 2-axis measurements at the vacuum vessel wall enable a non-axisymmetric helical field to be decomposed into contributions from the plasma and from external sources such as induced wall currents or non-axisymmetric coils [1]. This is a consequence of the more general principle that a vector magnetic field on a closed surface can be separated into contributions from currents internal and external to the surface [2]. A model of the sources is not required. One implication is that the electromagnetic torque on the plasma can be determined directly from conventional magnetic sensors at the wall, equivalent to the better-known Maxwell stress approach [3]. Unlike the Maxwell stress, the present method also enables separate estimates of the internal and external fields responsible for the torque. Examples from DIII-D data show the potential of this technique to illuminate the dynamics of tearing mode locking, and to test theoretical models of the torques from intrinsic and applied external fields. [1] R.M. Sweeney, PoP 2019. [2] A.H. Boozer, NF 2015. [3] N.C. Logan, PPCF 2010. [Preview Abstract] |
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GP19.00018: Multi-n Resistive Wall Modes Controlled at Non-Zero Amplitude Alexander Battey, Jeremy Hanson, Jim Bialek, Gerald Navratil A new model-based feedback algorithm has the ability to simultaneously control n$=$1 and n$=$2 resistive wall modes in DIII-D. Higher-n RWMs are sometimes observed following successful stabilization of the n$=$1 mode, motivating the development of multi-n control. The algorithm is based on the VALEN RWM stability model and has been implemented on real-time GPU hardware. In order to know when multiple n-number feedback is necessary, the marginal stability points of the n$=$1 and 2 modes need to be better understood. Historically, MHD spectroscopy has been used to validate stability models through comparisons of the plasma response to applied, open-loop perturbations. Our new closed-loop technique allows for RWMs to be controlled at nonzero amplitudes, thereby enabling the response to be studied while maintaining stability. VALEN simulations demonstrate a link between the closed-loop response and plasma stability, with the response amplitude increasing as the open-loop marginal point is approached and exceeded. The robust application of multi-n control has the potential to allow for greater fusion-power density by enabling safe operation at elevated values of normalized beta. [Preview Abstract] |
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GP19.00019: Physics Understanding with NIMROD Simulation of Slowly Rotating 3D Field for Locked Mode Avoidance Michio Okabayashi, Dylan P. Brennan, Shizuo Inoue, Edward J. Strait, Nikolas Logan, Robert J. La Haye, Linda Sugiyama Nonlinear, resistive NIMROD simulations show stable edge tearing layers are critical to the observed core locking avoidance when applying a slowly rotating 3D external field. DIIID discharges avoiding locking this way have been observed to exhibit an edge localized tearing layer synchronized with the 3D field. The amplitude of the observed tearing layer suggests that the perturbed current density must be comparable to the equilibrium current density. In contrast, at q$=$2 and 3 the response to the 3D external field is minimal. A hypothesis is that the stable Hmode edge tearing layers reduce the influence of both the static, intrinsic error field (EF) and the rotating applied 3D field on the core tearing layers (S. Inoue PPCF 2018, IAEA 2018). Initial NIMROD simulations have been carried out based on experimental profiles. The results suggest the observed effect can be explained by taking into account the velocity of the rotating external 3D field relative to the EF and the mode moving in the plasma frame and show the relative drift velocity must be large enough to induce the screening effect to the mode from both magnetic fields. [Preview Abstract] |
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GP19.00020: Upgrades and new applications of the DIII-D magnetic diagnostics system Stefano Munaretto, E J Strait The complexity of the magnetic diagnostic system of DIII-D increased over the years in order to provide complete and reliable information about the plasma and to adapt to the evolving research focus. Magnetic measurements are used to provide fundamental information about the plasma, such as its position and shape, the presence and nature of MHD instabilities and other toroidal asymmetries, both in real time and for post discharge analysis. These quantities vary by several orders of magnitude in terms of amplitude and of the scale of temporal evolution. A new solution, based on terminal strips and plug-in connectors, allows for a clean and flexible signal processing system while maintaining low noise and drift that enables the detection of magnetic fields ranging from Gauss to Tesla. The detection of the weak and quasi-static fields requires a large set of sensors. We present here how this system has been upgraded to maintain its original capabilities after new hardware installations in the machine and how its use has been expanded for real time MHD spectroscopy and real time distinction of magnetic fields coming from the plasma and from eddy currents in the wall, both of which enhance disruption prediction capabilities at DIII-D. Work supported by US DOE under DE-FC02-04ER54698. [Preview Abstract] |
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GP19.00021: Neoclassical Toroidal Viscosity Torque Profile Prediction Via Deep Learning Mitchell Clement, Nikolas Logan, Mark Boyer Using neoclassical toroidal viscosity (NTV) torque, at the plasma edge, can be vital in optimizing pedestal performance by controlling the rotation profile and/or alignment of the radial electric field zero-crossing with a rational surface to facilitate RMP ELM suppression. The offset rotation provided by NTV to spin the core could help provide tearing stability in various scenarios. The Generalized Perturbed Equilibrium Code (GPEC) package can be used to calculate the plasma stability and NTV torque profile generated by 3D magnetic fields. These calculations, however, involve complex integrations over space and energy distributions, which takes time to compute. GPECnet is a densely connected neural network that has been trained on GPEC data, to predict NTV torque and the least stable plasma $\delta $W in real-time. Initially, GPECnet has been trained solely on data representative of the wide pedestal quiescent H-mode scenario, in which neutral beams are often balanced and toroidal rotation is low across the plasma profile. This work provides the foundation for active control of the rotation shear using a combination of beams and 3D fields for robust and high performance QH mode operation. [Preview Abstract] |
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GP19.00022: Plasma response modeling of magnetic island bifurcations. Dmitriy Orlov, Todd Evans, Laszlo Bardoczi, Wen Wu, Eric Howell In this work, we present plasma response simulation of the magnetic island bifurcation in NSTX-U and DIII-D discharges. In tokamaks, naturally occurring MHD islands, such as tearing modes, neoclassical tearing modes, locked modes, and externally driven static magnetic islands produced by RMPs from field-errors and 3D control or EFC coils are known to have significant effects on the confinement of energy, particle and momentum. The initial linear two-fluid M3D-C1 simulations were performed for NSTX-U H-mode plasmas predicting this new class of magnetic island bifurcations. Recently, magnetic island heteroclinic bifurcations were empirically observed in the DIII-D core tokamak plasma for the first time. We expand our understanding of heteroclinic bifurcations in NSTX-U and DIII-D using linear NIMROD simulations in order to determine how this process affects the NTM stability, growth rates, and locking that results in disruptions during discharges with various normalized poloidal pressure and aspect ratios. [Preview Abstract] |
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GP19.00023: Edge MHD stability in the DIII-D inductive high beta poloidal scenario D.B. Weisberg, A.M. Garofalo, E.J. Strait, T. Osborne, S. Ding, X.Z. Gong The inductive evolution of the DIII-D high beta poloidal scenario to high fusion gain factor at constant normalized beta is observed to be associated with a fast external kink mode which shifts the internal transport barrier (ITB) location, enhances confinement, and can terminate in a resistive wall mode (RWM) instability. Progress has been made in understanding the onset of this MHD mode, which occurs after the plasma rotation outside of the ITB spins down, reducing its stabilizing effect on the RWM. This MHD event is consistently observed to trigger a global re-organization that shifts the ITB radially inward, increases the energy confinement time by 50\%, and spins up both the core and edge rotation. Analysis of RWM feedback shows that the I-coil response is strongly affected by the ELM activity, indicating coupling between edge localized modes and the RWM. Stability analysis of the pedestal reveals that while the large type-I compound ELMs seen in high q95 discharges are limited by coupled peeling-ballooning modes, the small high frequency ELMs seen in lower q95 discharges are exclusively limited by ballooning modes, and it is shown that RWM onset may be triggered by sudden transition back to a type-I ELM regime. Proposed techniques for mode avoidance are also discussed. [Preview Abstract] |
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GP19.00024: Hyper-dimensional time-series data analysis with reservoir computing networks to predict plasma profiles in tokamak Azarakhsh Jalalvand, Joseph Abbate, Rory Conlin, Egemen Kolemen, Geert Verdoolaege A Reservoir Computing Network (RCN) is a special type of recurrent neural network, in which the input and the recurrent connections are randomly generated and only the output weights are trained. Besides the ability to process temporal information, key features of RCN include ease of training and robustness against noise. RCNs have been shown to be very effective for a variety of tasks, e.g., in the analysis of hyperdimensional data and time-evolving chaotic systems. The aim of this work is to extend the application of RCN to the field of tokamak profile control and disruption prediction. We investigate the potential of such neural networks for achieving competitive performance in predicting plasma profiles on confinement time scales using experimental data from DIII-D. We consider the prediction of five profiles, namely, electron density, electron temperature, Ion rotation, safety factor, and plasma pressure. The preliminary experiments demonstrate that the RCN-based plasma profile predictor has similar performance to state-of-the-art (deep) convolutional neural networks and long short-term memory (LSTM) models but with significantly easier and faster training procedure. [Preview Abstract] |
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GP19.00025: High energy super-H plasmas mitigated with single and dual Shattered Pellet Injection J. L. Herfindal, D. Shiraki, L. R. Barylor, I. Bykov, E. M. Hollmann, R. A. Moyer, N. W. Eidietis Mitigation of high-energy super-H mode disruptions in DIII-D through Shattered Pellet Injection (SPI) may result in multiple radiation flashes, depending on pellet composition. The experiment used pellets of varying size and composition (changing Ne/D2 ratio) to examine effects that excess D2 may have on the mitigation. The majority of shutdowns with pellets composed of both Ne and D2 resulted in two radiation flashes -- one when the pellet hits the edge and another when the core thermal energy drops at the end of the thermal quench. Mitigation of super-H mode plasma disruptions using pellets composed only of pure Ne follow an outside-in mitigation with a single radiation flash, similar to typical DIII-D H-mode SPI mitigated discharges. Additional experiments using multiple shattered pellets at different toroidal locations show an increase in current quench duration for H and super-H mode discharges compared to single pellet injection with similar Ne and D2 quantities. A comparison of pre-thermal quench radiation profiles due to high-Z impurity injection by SPI and dual SPI into H and super H-mode plasmas will be presented. [Preview Abstract] |
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GP19.00026: Experimental study of Alfv\'enic instabilities driven by runaway electrons during the current quench in DIII-D A. Lvovskiy, C. Paz-Soldan, N.W. Eidietis, A. Dal Molin, G. Degrandchamp, X.D. Du, E.M. Hollmann, C. Liu, M. Nocente, D. Shiraki Suppressed formation of post-disruption runaway electron (RE) beams in DIII-D correlates with increased RE loss and presence of MHz-range kinetic instabilities driven by REs during the current quench. The frequency of these instabilities decreases with decreasing toroidal magnetic field while the failure rate of the RE beam formation increases. The magnetic structure of proposed compressional Alfv\'en waves is accessed using an upgraded set of high-frequency magnetic antennas. Analysis of the RE energy reconstructed from hard X-ray bremsstrahlung measurements shows that the instabilities are driven by REs with energy of a few MeV. The energy of REs, thus the presence of the instabilities, can be controlled via actuation of plasma and impurity injection parameters. It is found that hot (about 10 keV) pre-disruption plasma leads to formation of RE beams with higher RE current but lower maximum RE energy (sub-MeV) and no observable kinetic instabilities. An opposite phenomena is observed for cold (about 1--2 keV) pre-disruption plasmas. Argon massive gas injection (MGI) in amounts greater than 150 torr$\cdot$l is found to reduce the energy of REs and increase the rate of RE beam formation. No such effect is observed for D$_2$ and Ne MGI. [Preview Abstract] |
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GP19.00027: Internal measurement of magnetic fluctuations in runaway plateau plasmas in DIII-D M. D. Pandya, B. E. Chapman, J. Chen, D. L. Brower, W. X. Ding, N. W. Eidietis, E. M. Hollmann, A. Lvovskiy, K. J. McCollam, C. Paz-Soldan, J. S. Sarff, E. J. Strait, R. Yoneda A band of low-frequency ($f$ \textless 20 kHz) magnetic fluctuations is measured in post-disruption runaway-electron-plateau plasmas, which at low current (\textasciitilde 180kA) and large edge safety factor (q \textasciitilde 10) are calculated to be linearly stable to MHD. The measurements are made with the Radial Interferometer Polarimeter (RIP). Consisting of three horizontal chords, one at the equatorial midplane and the other two at z $= \quad +$/- 13.5 cm, RIP measures the line integral of the equilibrium and fluctuating density and magnetic field. During the plateau, the RIP-measured magnetic fluctuation amplitude is about 25 G (delta-b/B \textasciitilde 0.1{\%}). With a vertical sweep of the plasma across the RIP chords, the magnetic fluctuation profile is found to be flat. The origin of these fluctuations is not yet known. The magnetic and density fluctuations measured by RIP have finite coherence and cross-phase, the latter of which may help identify the source of the fluctuations. MHD stability calculations for the runaway plateau have for the most part been based on equilibrium reconstructions lacking internal constraints. However, RIP is now being added as a constraint in EFIT, which could contribute to improved estimates of stability. [Preview Abstract] |
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GP19.00028: Actuator Management via Nonlinear Real-time Optimization Andres Pajares, Eugenio Schuster, Michael Walker, David Humphreys An actuator management algorithm based on nonlinear, real-time optimization techniques has been designed, tested in simulations, and is currently being implemented within the DIII-D Plasma Control System (PCS). Tokamaks are highly complex devices in which a multitude of control tasks must be carried out by many shared actuators. This motivates the development of actuator management algorithms within present and future PCS designs. The primary goal of an actuator manager is to calculate in real time actuator requests that can fulfill as many control objectives as possible despite physical saturation limits and potential actuator failures. In this work, an actuator management algorithm has been designed based on an Augmented Lagrangian optimization method. Different control objectives can be embedded in this scheme both as terms within a cost function that needs to be minimized (e.g., control effort) and as constraints (e.g., absolute or rate limits for power, torque, etc.). Its performance has been tested in DIII-D simulations using the Control-Oriented Transport Simulator (COTSIM). Moreover, it is currently being implemented within the DIII-D PCS as a further step towards developing a higher level of integration for the present architecture. [Preview Abstract] |
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GP19.00029: Control-Oriented Current-Profile Response Modeling Using Neural Network Accelerated Versions of TGLF and NUBEAM for DIII-D S. Morosohk, T. Rafiq, E. Schuster, O. Meneghini, M.D. Boyer Model-based control methods for robust current-profile regulation rely on control-oriented as opposed to physics-oriented models. These typically use a number of limiting assumptions to be able to achieve the necessary calculation speeds. The evolution of the current profile depends both on the plasma resistivity, which is primarily a function of the temperature, and on the deposition characteristics of the heating and current drive sources. Recent work [1, 2] has shown success with the use of neural networks to recreate the neutral-beam heat and current depositions computed by the Monte Carlo module NUBEAM and the heat and particle fluxes computed by the quasilinear transport model TGLF. These neural network models can be run in CPU-microseconds, enabling the possibility of real-time prediction for profile control purposes. In this work, the electron heat flux profile computed by the TGLF neural network and the neutral-beam heat and current depositions computed by the NUBEAM neural network are integrated into the magnetic diffusion and electron heat transport equations within COTSIM (Control-Oriented Transport Simulator). This eliminates the need for empirical correlations for the electron temperature and the neutral-beam heat and current depositions. [1] S. Morosohk, M.D. Boyer and E. Schuster, APS-DPP 2018. [2] O. Meneghini et al. 2017 Nuclear Fusion 57 086034. [Preview Abstract] |
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GP19.00030: Real-time Equilibrium Reconstruction with Kinetic Constraints on DIII-D \newline Ricardo Shousha, John Ferron, Zichuan Xing, Oak Nelson, Keith Erickson, Egemen Kolemen Since not all quantities of interest in tokamak plasmas can be measured directly, real-time equilibrium reconstruction codes (rtEFIT [1]) are used. However, plasma internal profiles remain largely unconstrained when only data from magnetics are used. Therefore, it is crucial to include data from diagnostics that constrain profiles. Recently, real-time diagnostics that provide electron- and ion temperature- and density profiles have become available. An algorithm has been developed that fits pressure profiles using a modified hyperbolic tangent with linear core function. The contributions are summed and used to constrain the internal profiles. This algorithm was successfully tested during experiments. For the first time, the pressure pedestal was clearly identifiable and the reconstructed bootstrap current peak was consistent with expectations. The new version can generate a polynomial fit to the ratio of electron and ion temperature. This eliminates the risk of misalignment of the electron- and ion pressure pedestal. Furthermore, the algorithm can compute a fast ion pressure profile based on the electron pressure and injected beam power. \newline [1]: Ferron, J.R., et al, Nucl. Fus. 38(1998)1055 [Preview Abstract] |
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