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221. Effects of Axial Boundary Conductivity on a Free Stewartson-Shercliff Layer

Author(s):
Caspary, Kyle J.; Choi, Dahan; Ebrahimi, Fatima; Gilson, Erik P.; Goodman, Jeremy; Ji, Hantao
Abstract:
The effects of axial boundary conductivity on the formation and stability of a magnetized free Stewartson-Shercliff layer (SSL) in a short Taylor-Couette device are reported. As the axial field increases with insulating endcaps, hydrodynamic Kelvin-Helmholtz-type instabilities set in at the SSLs of the conducting fluid, resulting in a much reduced flow shear. With conducting endcaps, SSLs respond to an axial field weaker by the square root of the conductivity ratio of endcaps to fluid. Flow shear continuously builds up as the axial field increases despite the local violation of the Rayleigh criterion, leading to a large number of hydrodynamically unstable modes. Numerical simulations of both the mean flow and the instabilities are in agreement with the experimental results.
Type:
Dataset
Issue Date:
2018

222. Electromagnetic Particle Injector for Fast Time Response Disruption Mitigation in Tokamaks

Author(s):
Raman, R.; Lay, W.-S.; Jarboe, T.R.; Menard, J.E.; Ono, M.
Abstract:
A novel, rapid time-response, disruption mitigation system referred to as the Electromagnetic Particle Injector (EPI) is described. This method can accurately deliver the radiative payload to the plasma center on a <10 ms time scale, much faster, and deeper, than what can be achieved using conventional methods. The EPI system accelerates a sabot electromagnetically. The sabot is a metallic capsule that can be accelerated to desired velocities by an electromagnetic impeller. At the end of its acceleration, within 2 ms, the sabot will release a radiative payload, which is composed of low-z granules, or a shell pellet containing smaller pellets. The primary advantage of the EPI concept over gas propelled systems is its potential to meet short warning time scales, while accurately delivering the required particle size and materials at the velocities needed for achieving the required penetration depth in high power ITER-scale discharges for thermal and runaway current disruption mitigation. The present experimental tests from a prototype system have demonstrated the acceleration of a 3.2 g sabot to over 150 m/s within 1.5 ms, consistent with the calculations, giving some degree of confidence that larger ITER-scale injector can be developed.
Type:
Dataset
Issue Date:
November 2018

223. Elemental and topographical imaging of microscopic variations in deposition on NSTX-U and DIII-D samples

Author(s):
Skinner, C.H.; Chrobak, C.P.; Kaita, R.; Koel, B.E.
Abstract:
Tokamak plasma facing components have surface roughness that can cause microscopic spatial variations in erosion and deposition and hence influence material migration, erosion lifetime, dust and tritium accumulation, and plasma contamination. However high spatial resolution measurements of deposition on the scale of the surface roughness have been lacking to date. We will present elemental images of graphite samples from NSTX-U and DIII-D DiMES experiments performed with a Scanning Auger Microprobe at sub-micron resolution that show strong microscopic variations in deposition and correlate this with 3D topographical maps of surface irregularities. The NSTX-U samples were boronized and exposed to deuterium plasmas and the DiMES samples had localized Al and W films and were exposed to dedicated helium plasmas. Topographical maps of the samples were performed with a 3D confocal optical microscope and compared to the elemental deposition pattern. The results revealed localized deposition concentrated in areas shadowed from the ion flux, incident in a direction calculated (for the DiMES case) by taking account of the magnetic pre-sheath.
Type:
Dataset
Issue Date:
October 2018

224. Experimental demonstration of hydraulic jump control in liquid metal channel flow using Lorentz force

Author(s):
Fisher, Adam; Kolemen, Egemen; Hvasta, Mike
Abstract:
In this paper, hydraulic jump control using electromagnetic force in a liquid metal flow is presented. The control methods used give insight into the hydraulic jump behavior in the presence of magnetic fields and electrical currents. Flowing liquid metals is a proposed solution to heat flux challenges posed in fusion reactors, specifically the tokamak. Unfortunately, thin, fast-flowing liquid metal divertor concepts for fusion reactors are susceptible to hydraulic jumps that drastically reduce the liquid metal flow speed, leading to potential problems such as excessive evaporation, unsteady power removal, and possible plasma disruption. Highly electrically conductive flows within the magnetic fields do not exhibit traditional hydraulic jump behavior. There is very little research investigating the use of externally injected electrical currents and magnetic fields to control liquid metal hydraulic jumps. By using externally injected electrical currents and a magnetic field, a Lorentz force (also referred to as j × B force) may be generated to control the liquid metal jump behavior. In this work, a free-surface liquid metal—GaInSn eutectic or “galinstan”—flow through an electrically insulating rectangular duct was investigated. It was shown that applying a Lorentz force has a repeatable and predictable impact on the hydraulic jump, which can be used for liquid metal control within next-generation fusion reactors.
Type:
Dataset
Issue Date:
2018

225. Fast animal pose estimation using deep neural networks

Author(s):
Pereira, Talmo D.; Aldarondo, Diego E.; Willmore, Lindsay; Kislin, Mikhail; Wang, Samuel S.-H.; Murthy, Mala; Shaevitz, Joshua W.
Abstract:
Recent work quantifying postural dynamics has attempted to define the repertoire of behaviors performed by an animal. However, a major drawback to these techniques has been their reliance on dimensionality reduction of images which destroys information about which parts of the body are used in each behavior. To address this issue, we introduce a deep learning-based method for pose estimation, LEAP (LEAP Estimates Animal Pose). LEAP automatically predicts the positions of animal body parts using a deep convolutional neural network with as little as 10 frames of labeled data for training. This framework consists of a graphical interface for interactive labeling of body parts and software for training the network and fast prediction on new data (1 hr to train, 185 Hz predictions). We validate LEAP using videos of freely behaving fruit flies (Drosophila melanogaster) and track 32 distinct points on the body to fully describe the pose of the head, body, wings, and legs with an error rate of <3% of the animal's body length. We recapitulate a number of reported findings on insect gait dynamics and show LEAP's applicability as the first step in unsupervised behavioral classification. Finally, we extend the method to more challenging imaging situations (pairs of flies moving on a mesh-like background) and movies from freely moving mice (Mus musculus) where we track the full conformation of the head, body, and limbs.
Type:
Dataset
Issue Date:
30 May 2018

228. Modeling of a Laser-Induced Rydberg Spectroscopy diagnostic for Direct Measurement of the Local Electric Field in the Edge Region of NSTX/NSTX-U

Author(s):
Reymond, L.; Diallo, A.; Vekselman, V.
Abstract:
We discuss a novel diagnostic allowing direct measurements of the local electric field in the edge region in NSTX/NSTX-U. This laser based diagnostic's principle consists of depleting the naturally populated $n=3$ level to a Rydberg state --sensitive to electric fields-- that will result in a suppression of part of the $D_{\alpha}$ emission. We refer to this approach as Laser-Induced Rydberg Spectroscopy (LIRyS). It is shown that the local electric field can be measured through the Stark induced resonances observed as dips in the $D_\alpha$ emission. Using forward-modeling of simulated absorption spectra, we show precisions reaching \SI{\pm 2}{\kilo\volt\per\meter} in regions with a local electric field of \SI{15}{\kilo\volt\per\meter}.
Type:
Dataset
Issue Date:
July 2018

229. On the scattering correction of fast-ion D-alpha signal on NSTX-U

Author(s):
Hao, G.Z; Heidbrink, W.W.; Liu, D.; Stagner, L.; Podesta, M.; Bortolon, A.
Abstract:
Analysis of fast-ion D-alpha (FIDA) data on National Spherical Torus Experiment-Upgrade (NSTX-U) shows that the cold Dα line contaminates the FIDA baseline. The scattered light is comparable to the FIDA emission. A scattering correction is required to extract the FIDA signal. Two methods that relate the scattered light contamination to the intensity of the cold Dα line are employed. One method uses laboratory measurements with a calibration lamp; the other method uses data acquired during plasma operation and singular value decomposition analysis. After correction, both the FIDA spectra and spatial profile are in better agreement with theoretical predictions.
Type:
Dataset
Issue Date:
June 2018

230. Plasma boundary shape control and real-time equilibrium reconstruction on NSTX-U

Author(s):
Boyer, M.; Battaglia, D.; Mueller, D.; Eidietis, N.; Erickson, K.; Ferron, J.; Gates, D.; Gerhardt, S.; Johnson, R.; Kolemen, E.; Menard, J.; Myers, C.; Sabbagh, S.; Scotti, F.; Vail, P.
Abstract:
The upgrade to the National Spherical Torus eXperiment (NSTX-U) included two main improvements: a larger center-stack, enabling higher toroidal field and longer pulse duration, and the addition of three new tangentially aimed neutral beam sources, which increase available heating and current drive, and allow for flexibility in shaping power, torque, current, and particle deposition profiles. To best use these new capabilities and meet the high-performance operational goals of NSTX-U, major upgrades to the NSTX-U Control System (NCS) hardware and software have been made. Several control algorithms, including those used for real-time equilibrium reconstruction and shape control, have been upgraded to improve and extend plasma control capabilities. As part of the commissioning phase of first plasma operations, the shape control system was tuned to control the boundary in both inner-wall limited and diverted discharges. It has been used to accurately track the requested evolution of the boundary (including the size of the inner gap between the plasma and central solenoid, which is a challenge for the ST configuration), X-point locations, and strike point locations, enabling repeatable discharge evolutions for scenario development and diagnostic commissioning.
Type:
Dataset
Issue Date:
March 2018

231. Quantitative imaging of carbon dimer precursor for nanomaterial synthesis in the carbon arc

Author(s):
Vekselman, V.; Khrabry, A.; Kaganovich, I.; Stratton, B.; Selinsky, R. S.; Raitses, Y.
Abstract:
Delineating the dominant processes responsible for nanomaterial synthesis in a plasma environment requires measurements of the precursor species contributing to the growth of nanostructures. We performed comprehensive measurements of spatial and temporal profiles of carbon dimers in sub-atmospheric-pressure carbon arc by laser-induced fluorescence. Measured spatial profiles of carbon dimers coincide with the growth region of carbon nanotubes (Fang et al 2016 Carbon 107 273-80) and vary depending on the arc operation mode, which is determined by the discharge current and the ablation rate of the graphite anode. The carbon dimer density profile exhibits large spatial and time variations due to motion of the arc core. A comparison of the experimental data with the 2D simulation results of self-consistent arc modeling shows a good agreement. The model predicts well the main processes determining spatial profiles of carbon dimers.
Type:
Dataset
Issue Date:
January 2018

232. Reductions in Retrieval Competition Predict the Benefit of Repeated Testing

Author(s):
Rafidi, Nicole S; Hulbert, Justin C; Brooks, Paula P; Norman, Kenneth A
Abstract:
Repeated testing (as opposed to repeated study) leads to improved long-term memory retention, but the mechanism underlying this improvement remains controversial. In this work, we test the hypothesis that retrieval practice benefits subsequent recall by reducing competition from related memories. This hypothesis implies that the degree of reduction in competition between retrieval practice attempts should predict subsequent memory for the practiced items. To test this prediction, we collected electroencephalography (EEG) data across two sessions. In the first session, participants practiced selectively retrieving exemplars from superordinate semantic categories (high competition), as well as retrieving the names of the superordinate categories from exemplars (low competition). In the second session, participants repeatedly studied and were then tested on Swahili-English vocabulary. One week after session two, participants were again tested on the vocabulary. We trained a within-subject classifier on the data from session one to distinguish high and low competition states. We then used this classifier to measure competition across multiple retrieval practice attempts in the second session. The degree to which competition decreased for a given vocabulary word predicted whether that item was subsequently remembered in the third session. These results are consistent with the hypothesis that repeated testing improves retention by reducing competition.
Type:
Dataset
Issue Date:
April 2018

233. Regarding the optimization of O1-mode ECRH and the feasibility of EBW startup on NSTX-U

Author(s):
Lopez, N; Poli, F
Abstract:
Recently published scenarios for fully non-inductive startup and operation on the National Spherical Torus eXperiment Upgrade (NSTX-U) (Menard et al 2012 Nucl. Fusion 52 083015) show Electron Cyclotron Resonance Heating (ECRH) as an important component in preparing a target plasma for efficient High Harmonic Fast Wave and Neutral Beam heating. The modeling of the propagation and absorption of EC waves in the evolving plasma is required to define the most effective window of operation, and to optimize the launcher geometry for maximal heating and current drive during this window. Here, we extend a previous optimization of O1-mode ECRH on NSTX-U to account for the full time-dependent performance of the ECRH using simulations performed with TRANSP. We find that the evolution of the density profile has a prominent role in the optimization by defining the time window of operation, which in certain cases may be a more important metric to compare launcher performance than the average power absorption. This feature cannot be captured by analysis on static profiles, and should be accounted for when optimizing ECRH on any device that operates near the cutoff density. Additionally, the utility of the electron Bernstein wave (EBW) in driving current and generating closed flux surfaces in the early startup phase has been demonstrated on a number of devices. Using standalone GENRAY simulations, we find that efficient EBW current drive is possible on NSTX-U if the injection angle is shifted below the midplane and aimed towards the top half of the vacuum vessel. However, collisional damping of the EBW is projected to be significant, in some cases accounting for up to 97% of the absorbed EBW power
Type:
Dataset
Issue Date:
June 2018

234. Regional hydroclimatic variability due to contemporary deforestation in southern Amazonia and associated boundary layer characteristics

Author(s):
Khanna, Jaya; Medvigy, David; Fisch, Gilberto; Neves, Theomar Trindade de Araújo Tiburtino
Abstract:
Amazonian deforestation causes systematic changes in regional dry season precipitation. Some of these changes at contemporary large scales (a few hundreds of kilometers) of deforestation have been associated with a ‘dynamical mesoscale circulation’, induced by the replacement of rough forest with smooth pasture. In terms of decadal averages, this dynamical mechanism yields increased precipitation in downwind regions and decreased precipitation in upwind regions of deforested areas. Daily, seasonal, and interannual variations in this phenomenon may exist, but have not yet been identified or explained. This study uses observations and numerical simulations to develop relationships between the dynamical mechanism and the local- and continental-scale atmospheric conditions across a range of time scales. It is found that the strength of the dynamical mechanism is primarily controlled by the regional-scale thermal and dynamical conditions of the boundary layer, and not by the continental- and global-scale atmospheric state. Lifting condensation level and wind speed within the boundary layer have large and positive correlations with the strength of the dynamical mechanism. The strength of these relationships depends on time scale and is strongest over the seasonal cycle. Overall, the dynamical mechanism is found to be strongest during times when the atmosphere is relatively stable. Hence, for contemporary large scales of deforestation this phenomenon is found to be the prevalent convective triggering mechanism during the dry and parts of transition seasons (especially during the dry-to-wet transition), significantly affecting the hydroclimate during this period.
Type:
Dataset and Software
Issue Date:
2018

235. Regulation of Harvester Ant Foraging as a Closed-Loop Excitable System

Abstract:
Ant colonies regulate activity in response to changing conditions without using centralized control. Harvester ant colonies forage in the desert for seeds, and their regulation of foraging manages a tradeoff between spending and obtaining water. Foragers lose water while outside in the dry air, but the colony obtains water by metabolizing the fats in the seeds they eat. Previous work shows that the rate at which an outgoing forager leaves the nest depends on its recent experience of brief antennal contact with returning foragers that carry a seed. We examine how this process can yield foraging rates that are robust to uncertainty and responsive to temperature and humidity across minutes to hour-long timescales. To explore possible mechanisms, we develop a low-dimensional analytical model with a small number of parameters that captures observed foraging behavior. The model uses excitability dynamics to represent response to interactions inside the nest and a random delay distribution to represent foraging time outside the nest. We show how feedback of outgoing foragers returning to the nest stabilizes the incoming and outgoing foraging rates to a common value determined by the ``volatility’’ of available foragers. The model exhibits a critical volatility above which there is sustained foraging at a constant rate and below which there is cessation of foraging. To explain how the foraging rates of colonies adjust to temperature and humidity, we propose a mechanism that relies on foragers modifying their volatility after they leave the nest and get exposed to the environment. Our study highlights the importance of feedback in the regulation of foraging activity and points to modulation of volatility as a key to explaining differences in foraging activity in response to conditions and across colonies. Our results present opportunities for generalization to other contexts and systems with excitability and feedback across multiple timescales.
Type:
collection and Dataset

236. Scenario Development During Commissioning Operations on the National Spherical Torus Experiment Upgrade

Author(s):
Battaglia, D.J.; Boyer, M.D.; Gerhardt, S.; Mueller, D.; Myers, C.E.; Guttenfelder, W.; Menard, J.E.; Sabbagh, S.A.; Scotti, F.; Bedoya, F.; Bell, R.E.; Berkery, J.W.; Diallo, A.; Ferraro, N.; Jaworski, M.A.; Kaye, S.M.; LeBlanc, B.P.; Ono, M.; Park, J.-K.; Podesta, M.; Raman, R.; Soukhanovskii, V.A.
Abstract:
The National Spherical Torus Experiment Upgrade (NSTX-U) will advance the physics basis required for achieving steady-state, high-beta, and high-confinement conditions in a tokamak by accessing high toroidal field (1 T) and plasma current (1.0 - 2.0 MA) in a low aspect ratio geometry (A = 1.6 - 1.8) with flexible auxiliary heating systems (12 MW NBI, 6 MW HHFW). This paper describes progress in the development of L- and H-mode discharge scenarios and the commissioning of operational tools in the first ten weeks of operation that enable the scientific mission of NSTX-U. Vacuum field calculations completed prior to operations supported the rapid development and optimization of inductive breakdown at different values of ohmic solenoid current. The toroidal magnetic field (B_T0 = 0.65 T) exceeded the maximum values achieved on NSTX and novel long-pulse L-mode discharges with regular sawtooth activity exceeded the longest pulses produced on NSTX (tpulse > 1.8s). The increased flux of the central solenoid facilitated the development of stationary L-mode discharges over a range of density and plasma current (Ip). H-mode discharges achieved similar levels of stored energy, confinement (H98y,2 > 1) and stability (beta_N/beta_N-nowall > 1) compared to NSTX discharges for Ip < 1 MA. High-performance H-mode scenarios require an L-H transition early in the Ip ramp-up phase in order to obtain low internal inductance (li) throughout the discharge, which is conducive to maintaining vertical stability at high elongation (kappa > 2.2) and achieving long periods of MHD quiescent operations. The rapid progress in developing L- and H-mode scenarios in support of the scientific program was enabled by advances in real-time plasma control, efficient error field identification and correction, effective conditioning of the graphite wall and excellent diagnostic availability.
Type:
Dataset
Issue Date:
April 2018

237. Two-dimensional full-wave simulations of waves in space and tokamak plasmas

Author(s):
Kim, E.-W.; Bertelli, N.; Johnson, J.R.; Valeo, E.; Hosea, J.; Perkins, R.
Abstract:
We illustrate the capabilities of a recently developed two-dimensional full wave code (FW2D) in space and tokamak plasmas by adopting various values of density, magnetic field configuration and strength as well as boundary shape. As example, we first showed fast compressional wave propagation in the inner magnetosphere is dramatically modified by a plasmaspheric plume at Earth's magnetosphere. The results show that wave energy is trapped in the plume showing a leaky eigenmode-like structure with plume, which is similar to the detected magnetosonic waves. We also performed simulations of high harmonic fast waves in the scrape-off layer (SOL) plasmas of the National Spherical Torus eXperiment (NSTX)/NSTX-Upgrade. Comparison the results with previous full-wave simulations show that although the FW2D code uses a cold plasma approximation, the electric field and the fraction of the power losses in the SOL plasmas show excellent consistency and agreement with the previous full wave simulations performed by the AORSA code.
Type:
Dataset
Issue Date:
October 2018

238. Whistler wave generation by anisotropic tail electrons during asymmetric magnetic reconnection in space and laboratory

Author(s):
Yoo, Jongsoo; Jara-almonte, J.; Yerger, Evan; Wang, Shan; Qian, Tony; Le, Ari; Ji, Hantao; Yamada, Masaaki; Fox, William; Kim, Eun-Hwa; Chen, Li-Jen; Gershman, Daniel
Abstract:
Whistler wave generation near the magnetospheric separatrix during reconnection at the dayside magnetopause is studied with data from the Magnetospheric Multiscale (MMS) mission. The dispersion relation of the whistler mode is measured for the first time near the reconnection region in space, which shows that whistler waves propagate nearly parallel to the magnetic field line. A linear analysis indicates that the whistler waves are generated by temperature anisotropy in the electron tail population. This is caused by loss of electrons with a high velocity parallel to the magnetic field to the exhaust region. There is a positive correlation between activities of whistler waves and the lower-hybrid drift instability (LHDI) both in laboratory and space, indicating the enhanced transport by LHDI may be responsible for the loss of electrons with a high parallel velocity.
Type:
Dataset
Issue Date:
August 2018

239. A Reduced Resistive Wall Mode Kinetic Stability Model for Disruption Forecasting

Author(s):
Berkery, J.W.; Sabbagh, S.A.; Bell, R.E.; Gerhardt, S.P.; LeBlanc, B.P.
Abstract:
Kinetic modification of ideal stability theory from stabilizing resonances of mode-particle interaction has had success in explaining resistive wall mode (RWM) stability limits in tokamaks. With the goal of real-time stability forecasting, a reduced kinetic stability model has been implemented in the new Disruption Event Characterization and Forecasting (DECAF) code, which has been written to analyze disruptions in tokamaks. The reduced model incorporates parameterized models for ideal limits on beta, a ratio of plasma pressure to magnetic pressure, which are shown to be in good agreement with DCON code calculations. Increased beta between these ideal limits causes a shift in the unstable region of delta W_K space, where delta W_K is the change in potential energy due to kinetic effects that is solved for by the reduced model, such that it is possible for plasmas to be unstable at intermediate beta but stable at higher beta. Gaussian functions for delta W_K are defined as functions of E cross B frequency and collisionality, with parameters reflecting the experience of the National Spherical Torus Experiment (NSTX). The reduced model was tested on a database of discharges from NSTX and experimentally stable and unstable discharges were separated noticeably on a stability map in E cross B frequency, collisionality space. The reduced model only failed to predict an unstable RWM in 15.6% of cases with an experimentally unstable RWM and performed well on predicting stability for experimentally stable discharges as well.
Type:
Dataset
Issue Date:
May 2017

240. A Riccati Solution for the Ideal MHD Plasma Response with Applications to Real-time Stability Control

Author(s):
Glasser, A.; Kolemen, E.; Glasser, A.H.
Abstract:
To effectuate near real-time feedback control of ideal MHD instabilities in a tokamak geometry, a rapid solution for stability analysis is a prerequisite. Toward this end, we reformulate the δW stability method with a Hamilton-Jacobi theory, elucidating analytical and numerical features of the generic tokamak ideal MHD stability problem. The plasma response matrix is demonstrated to be the solution of an ideal MHD matrix Riccati differential equation (MRDE). Since Riccati equations are prevalent in the control theory literature, such a shift in perspective brings to bear a range of numerical methods that are well-suited to the robust, fast solution of control problems. We discuss the usefulness of Riccati techniques in solving the stiff ODEs often encountered in ideal MHD stability analyses-—for example, in tokamak edge and stellarator physics. We then demonstrate the applicability of such methods to an existing 2D ideal MHD stability code—DCON—enabling its parallel operation in near real-time. Output is shown to match with high accuracy, and with wall-clock time ≪ 1s. Such speed may help enable active feedback ideal MHD stability control, especially in tokamak plasmas whose ideal MHD equilibria evolve with inductive timescale τ > 1s-—as in ITER.
Type:
Dataset
Issue Date:
March 2018