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1. Observation of quasi-coherent edge fluctuations in Ohmic plasmas on NSTX

Author(s):
Banerjee, S.; A. Diallo; S.J. Zweben
Abstract:
A quasi-coherent mode with frequency f = 40 kHz is observed in Ohmic plasmas in NSTX with the gas puff imaging diagnostic (GPI). This mode is located predominantly just inside the separatrix, with a maximum fluctuation amplitude similar to that of the broadband turbulence in the same frequency range. The quasi-coherent mode has a poloidal wavelength 16 cm and a poloidal velocity 49 km/s in the electron diamagnetic direction, which are similar to the characteristics expected from a linear drift-wave like mode in the edge.
Type:
Dataset
Issue Date:
April 2016

2. Hydrogen Retention in Lithium on Metallic Walls from “In Vacuo” Analysis in LTX and Implications for High-Z Plasma-Facing Components in NSTX-U

Author(s):
Kaita, R.; Lucia, M.; Allain, J. P.; Bedoya, F.; Capece, A.; Jaworski, M.; Koel, B. E.; Majeski, R.; Roszell, J.; Schmitt, J.; Scotti, F.; Skinner, C. H.; Soukhanovskii, V.
Abstract:
The application of lithium to plasma-facing components (PFCs) has long been used as a technique for wall conditioning in magnetic confinement devices to improve plasma performance. Determining the characteristics of PFCs at the time of exposure to the plasma, however, is difficult because they can only be analyzed after venting the vacuum vessel and removing them at the end of an operational period. The Materials Analysis and Particle Probe (MAPP) addresses this problem by enabling PFC samples to be exposed to plasmas, and then withdrawn into an analysis chamber without breaking vacuum. The MAPP system was used to introduce samples that matched the metallic PFCs of the Lithium Tokamak Experiment (LTX). Lithium that was subsequently evaporated onto the walls also covered the MAPP samples, which were then subject to LTX discharges. In vacuo extraction and analysis of the samples indicated that lithium oxide formed on the PFCs, but improved plasma performance persisted in LTX. The reduced recycling this suggests is consistent with separate surface science experiments that demonstrated deuterium retention in the presence of lithium oxide films. Since oxygen decreases the thermal stability of the deuterium in the film, the release of deuterium was observed below the lithium deuteride dissociation temperature. This may explain what occurred when lithium was applied to the surface of the NSTX Liquid Lithium Divertor (LLD). The LLD had segments with individual heaters, and the deuterium-alpha emission was clearly lower in the cooler regions. The plan for NSTX-U is to replace the graphite tiles with high-Z PFCs, and apply lithium to their surfaces with lithium evaporation. Experiments with lithium coatings on such PFCs suggest that deuterium could still be retained if lithium compounds form, but limiting their surface temperatures may be necessary.
Type:
Dataset
Issue Date:
2016

3. Accessing Real-Life Episodic Information from Minutes versus Hours Earlier Modulates Hippocampal and High-Order Cortical Dynamics

Author(s):
Chen, Janice; Honey, Christopher; Simony, Erez; Arcaro, Michael; Norman, Kenneth; Hasson, Uri
Abstract:
It is well known that formation of new episodic memories depends on hippocampus, but in real-life settings (e.g., conversation), hippocampal amnesics can utilize information from several minutes earlier. What neural systems outside hippocampus might support this minutes-long retention? In this study, subjects viewed an audiovisual movie continuously for 25 min; another group viewed the movie in 2 parts separated by a 1-day delay. Understanding Part 2 depended on retrieving information from Part 1, and thus hippocampus was required in the day-delay condition. But is hippocampus equally recruited to access the same information from minutes earlier? We show that accessing memories from a few minutes prior elicited less interaction between hippocampus and default mode network (DMN) cortical regions than accessing day-old memories of identical events, suggesting that recent information was available with less reliance on hippocampal retrieval. Moreover, the 2 groups evinced reliable but distinct DMN activity timecourses, reflecting differences in information carried in these regions when Part 1 was recent versus distant. The timecourses converged after 4 min, suggesting a time frame over which the continuous-viewing group may have relied less on hippocampal retrieval. We propose that cortical default mode regions can intrinsically retain real-life episodic information for several minutes.
Type:
Dataset
Issue Date:
3 August 2015

4. Blob Structure and Motion in the Edge and SOL of NSTX

Author(s):
Zweben, S.J.; J.R. Myra; W.M. Davis; D.A. D'Ippolito; T.K. Gray; S.M. Kaye; B.P. LeBlanc; R.J. Maqueda; D.A. Russell; D.P. Stotler
Abstract:
Blob analysis dataset
Type:
Dataset
Issue Date:
January 2016

5. Quasi-linear gyrokinetic predictions of the Coriolis momentum pinch in NSTX

Author(s):
Guttenfelder W.; S.M. Kaye; Y. Ren; W. Solomon; R.E. Bell; J. Candy; S.P. Gerhardt; B.P. LeBlanc; H. Yuh
Abstract:
This paper presents quasi-linear gyrokinetic predictions of the Coriolis momentum pinch for low aspect-ratio NSTX H-modes where previous experimental measurements were focused. Local, linear calculations predict that in the region of interest (just outside the mid-radius) of these relatively high-beta plasmas, profiles are most unstable to microtearing modes that are only effective in transporting electron energy. However, sub-dominant electromagnetic and electrostatic ballooning modes are also unstable, which are effective at transporting energy, particles and momentum. The quasi-linear prediction of transport from these weaker ballooning modes, assuming they contribute transport in addition to that from microtearing modes in a nonlinear turbulent state, leads to a very small or outward convection of momentum, inconsistent with the experimentally measured inward pinch, and opposite to predictions in conventional aspect ratio tokamaks. Additional predictions of a low beta L-mode plasma, unstable to more traditional electrostatic ion temperature gradient-trapped electron mode instability, show that the Coriolis pinch is inward but remains relatively weak and insensitive to many parameter variations. The weak or outward pinch predicted in NSTX plasmas appears to be at least partially correlated to changes in the parallel mode structure that occur at finite beta and low aspect ratio, as discussed in previous theories. The only conditions identified where a stronger inward pinch is predicted occur either in the purely electrostatic limit or if the aspect ratio is increased. As the Coriolis pinch cannot explain the measured momentum pinch, additional theoretical momentum transport mechanisms are discussed that may be potentially important.
Type:
Dataset
Issue Date:
April 2016

6. Time dependent analysis of visible helium line-ratios for electron temperature and density diagnostic using synthetic simulations on NSTX-U

Author(s):
Munoz Burgos, J.M.; Barbui, T.; Schmitz, O.; Stutman, D.; Tritz, K.
Abstract:
Helium line-ratios for electron temperature (Te) and density (ne) plasma diagnostic in the Scrape-Off-Layer (SOL) and Edge regions of tokamaks are widely used. Due to their intensities and proximity of wavelengths, the singlet 667.8 and 728.1 nm, and triplet 706.5 nm visible lines have been typically preferred. Time- dependency of the triplet line (706.5 nm) has been previously analyzed in detail by including transient effects on line-ratios during gas-puff diagnostic applications. In this work, several line-ratio combinations within each of the two spin systems are analyzed with the purpose of eliminating transient effects to extend the application of this powerful diagnostic to high temporal resolution characterization of plasmas. The analysis is done using synthetic emission modeling and diagnostic for low electron density NSTX SOL plasma conditions for several visible lines. This analysis employs both quasi-static equilibrium and time-dependent models in order to evaluate transient effects of the atomic population levels that may affect the derived electron temperatures and densities as a helium gas-puff penetrates the plasma. Ratios between the most intense lines are usually preferred due to their higher signal to noise ratio. The analysis of a wider range of spectral lines will help to extend this powerful diagnostic to experiments where the wavelength range of the measured spectra may be constrained either by limitations of the spectrometer, or by other conflicting lines from different ions.
Type:
Dataset
Issue Date:
November 2016

7. Far-infrared tangential interferometer/polarimeter design and installation for NSTX-U

Author(s):
Scott, E.R.; Barchfeld, R.; Riemenschneider, P.; Domier, C.W.; Muscatello, C.M.; Sohrabi, M.; Kaita, R.; Ren, Y.; Luhmann Jr., N.C.
Abstract:
The Far-infrared Tangential Interferometer/Polarimeter (FIReTIP) system has been refurbished and is being reinstalled on the National Spherical Torus Experiment-Upgrade (NSTX-U) to supply real-time line-integrated core electron density measurements for use in the NSTX-U plasma control system (PCS) to facilitate real-time density feedback control of the NSTX-U plasma. Inclusion of a visible light heterodyne interferometer in the FIReTIP system allows for real-time vibration compensation due to movement of an internally mounted retroreflector and the FIReTIP front-end optics. Real-time signal correction is achieved through use of a National Instruments CompactRIO field-programmable gate array.
Type:
Dataset
Issue Date:
November 2016

8. Three New Extreme Ultraviolet Spectrometers on NSTX-U for Impurity Monitoring

Author(s):
Weller, M.E.; Beiersdorfer, P.; Soukhanovskii, V.; Magee, E.W.; Scotti, F.
Abstract:
Three extreme ultraviolet (EUV) spectrometers have been mounted on the National Spherical Torus Experiment-Upgrade (NSTX-U). All three are flat-field grazing-incidence spectrometers and are dubbed X-ray and Extreme Ultraviolet Spectrometer (8 ñ 70 ≈), Long-Wavelength Extreme Ultraviolet Spectrometer (190 ñ 440 ≈), and Metal Monitor and Lithium Spectrometer Assembly (MonaLisa, 50 ñ 220 ≈). XEUS and LoWEUS were previously implemented on NSTX to monitor impurities from low- to high-Z sources and to study impurity transport while MonaLisa is new and provides the system increased spectral coverage. The spectrometers will also be a critical diagnostic on the planned laser blow-off (LBO) system for NSTX-U, which will be used for impurity edge and core ion transport studies, edge-transport code development, and benchmarking atomic physics codes.
Type:
Dataset
Issue Date:
November 2016

9. Effect of progressively increasing lithium conditioning on edge transport and stability in high triangularity NSTX H-modes

Author(s):
Maingi, R.; Canik, J.M.; Bell, R.E.; Boyle, D.P.; Diallo, A.; Kaita, R.; Kaye, S.M.; LeBlanc, B.P.; Sabbagh, S.A.; Scotti, F.; Soukhanovskii, V.A.
Abstract:
A sequence of H-mode discharges with increasing levels of pre-discharge lithium evaporation (�dose�) was conducted in high triangularity and elongation boundary shape in NSTX. Energy confinement increased, and recycling decreased with increasing lithium dose, similar to a previous lithium dose scan in medium triangularity and elongation plasmas. Data-constrained SOLPS interpretive modeling quantified the edge transport change: the electron particle diffusivity decreased by 10-30x. The electron thermal diffusivity decreased by 4x just inside the top of the pedestal, but increased by up to 5x very near the separatrix. These results provide a baseline expectation for lithium benefits in NSTX-U, which is optimized for a boundary shape similar to the one in this experiment.
Type:
Dataset
Issue Date:
August 2016

10. Initial operation of the NSTX-Upgrade real-time velocity diagnostic

Author(s):
Podesta, M; Bell, R.E.
Abstract:
A real-time velocity (RTV) diagnostic based on active charge-exchange recombination spectroscopy is now operational on the National Spherical Torus Experiment-Upgrade (NSTX-U) spherical torus (Menard et al 2012 Nucl. Fusion 52 083015). The system has been designed to supply plasma velocity data in real time to the NSTX-U plasma control system, as required for the implementation of toroidal rotation control. Measurements are available from four radii at a maximum sampling frequency of 5 kHz. Post-discharge analysis of RTV data provides additional information on ion temperature, toroidal velocity and density of carbon impurities. Examples of physics studies enabled by RTV measurements from initial operations of NSTX-U are discussed.
Type:
Dataset
Issue Date:
November 2016

12. Resonance in Fast-Wave Amplitude in the Periphery of Cylindrical Plasmas and Application to Edge Losses of Wave Heating Power in Tokamaks

Author(s):
Perkins, R.J.; Hosea, J.C.; Bertelli, N.; Taylor, G.; Wilson, J.R.
Abstract:
Heating magnetically confined plasmas using waves in the ion-cyclotron range of frequencies typically requires coupling these waves over a steep density gradient. This process has produced an unexpected and deleterious phenomenon on the National Spherical Torus eXperiment (NSTX): a prompt loss of wave power along magnetic field lines in front of the antenna to the divertor. Understanding this loss may be key to achieving effective heating and expanding the operational space of NSTX-Upgrade. Here, we propose that a new type of mode, which conducts a significant fraction of the total wave power in the low-density peripheral plasma, is driving these losses. We demonstrate the existence of such modes, which are distinct from surface modes and coaxial modes, in a cylindrical cold-plasma model when a half wavelength structure fits into the region outside the core plasma. The latter condition generalizes the previous hypothesis regarding the occurence of the edge losses and may explain why full-wave simulations predict these losses in some cases but not others. If valid, this condition implies that outer gap control is a potential strategy for mitigating the losses in NSTX-Upgrade in addition to raising the magnetic field or influencing the edge density.
Type:
Dataset
Issue Date:
July 2016

13. Mitigation of Alfven activity by 3D magnetic perturbations on NSTX

Author(s):
Kramer, G.J; Bortolon, A.; Ferraro, N.M.; Spong, D.A.; Crocker, N.A.; Darrow, D.S.; Fredrickson, E.D.; Kubota, S.; Park, J.-K.; Podesta, M.; Heidbrink, W.W.
Abstract:
Observations on the National Spherical Torus eXperiment (NSTX) indicate that externally applied non-axisymmetric magnetic perturbations (MP) can reduce the amplitude of Toroidal Alfven Eigenmodes (TAE) and Global Alfven Eigenmodes (GAE) in response to pulsed n=3 non-resonant fields. From full-orbit following Monte Carlo simulations with the 1- and 2-fluid resistive MHD plasma response to the magnetic perturbation included, it was found that in response to MP pulses the fast-ion losses increased and the fast-ion drive for the GAEs was reduced. The MP did not affect the fast-ion drive for the TAEs significantly but the Alfven continuum at the plasma edge was found to be altered due to the toroidal symmetry breaking which leads to coupling of different toroidal harmonics. The TAE gap was reduced at the edge creating enhanced continuum damping of the global TAEs, which is consistent with the observations. The results suggest that optimized non-axisymmetric MP might be exploited to control and mitigate Alfven instabilities by tailoring the fast-ion distribution function and/or continuum structure.
Type:
Dataset
Issue Date:
August 2016

15. Fusion Nuclear Science Facilities and Pilot Plants Based on the Spherical Tokamak

Author(s):
Menard, J.E.; Brown, T.; El-Guebaly, L.; Boyer, M.; Canik, J.; Colling, B.; Raman, R.; Wang, Z.; Zhai, Y.; Buxton, P.; Covele, B.; D'Angelo, C.; Davis, A.; Gerhardt, S.; Gryaznevich, M.; Harb, M.; Hender, T.C.; Kaye, S.; Kingham, D.; Kotschenreuther, M.; Mahajan, S.; Maingi, R.; Marriott, E.; Meier, E.T.; Mynsberge, L.; Neumeyer, C.; Ono, M.; Park, J.-K.; Sabbagh, S.A.; Soukhanovskii, V.; Valanju, P.; Woolley, R.
Abstract:
A Fusion Nuclear Science Facility (FNSF) could play an important role in the development of fusion energy by providing the nuclear environment needed to develop fusion materials and components. The spherical torus/tokamak (ST) is a leading candidate for an FNSF due to its potentially high neutron wall loading and modular configuration. A key consideration for the choice of FNSF configuration is the range of achievable missions as a function of device size. Possible missions include: providing high neutron wall loading and fluence, demonstrating tritium self-sufficiency, and demonstrating electrical self-sufficiency. All of these missions must also be compatible with a viable divertor, first-wall, and blanket solution. ST-FNSF configurations have been developed simultaneously incorporating for the first time: (1) a blanket system capable of tritium breeding ratio TBR approximately 1, (2) a poloidal field coil set supporting high elongation and triangularity for a range of internal inductance and normalized beta values consistent with NSTX/NSTX-U previous/planned operation, (3) a long-legged divertor analogous to the MAST-U divertor which substantially reduces projected peak divertor heat-flux and has all outboard poloidal field coils outside the vacuum chamber and superconducting to reduce power consumption, and (4) a vertical maintenance scheme in which blanket structures and the centerstack can be removed independently. Progress in these ST-FNSF missions vs. configuration studies including dependence on plasma major radius R0 for a range 1m to 2.2m are described. In particular, it is found the threshold major radius for TBR = 1 is R0 greater than or equal to 1.7m, and a smaller R0=1m ST device has TBR approximately 0.9 which is below unity but substantially reduces T consumption relative to not breeding. Calculations of neutral beam heating and current drive for non-inductive ramp-up and sustainment are described. An A=2, R0=3m device incorporating high-temperature superconductor toroidal field coil magnets capable of high neutron fluence and both tritium and electrical self-sufficiency is also presented following systematic aspect ratio studies.
Type:
Dataset
Issue Date:
October 2016

16. Linear gyrokinetic simulations of microinstabilities within the pedestal region of H-mode NSTX discharges in a highly shaped geometry

Author(s):
Coury, M.; Guttenfelder, W.; Mikkelsen, D.; Canik, J.; Canal, G.; Diallo, A.; Kaye, S.; Kramer, G.; Maingi, R.
Abstract:
Linear (local) gyrokinetic predictions of edge microinstabilities in highly shaped, lithiated and non-lihiated NSTX discharges are reported using the gyrokinetic code GS2. Microtearing modes dominate the non-lithiated pedestal top. The stabilization of these modes at the lithiated pedestal top enables the electron temperature pedestal to extend further inwards, as observed experimentally. Kinetic ballooning modes are found to be unstable mainly at the mid-pedestal of both types of discharges, with unstable trapped electron modes nearer the separatrix region. At electron wavelengths, ETG modes are found to be unstable from mid-pedestal outwards for eta(e,exp)~2.2, with higher growth rates for the lithiated discharge. Near the separatrix, the critical temperature gradient for driving ETG modes is reduced in the presence of lithium, reflecting the reduction of the lithiated density gradients observed experimentally. A preliminary linear study in the edge of non-lithiated discharges shows that the equilibrium shaping alters the electrostatic modes stability, found more unstable at high plasma shaping.
Type:
Dataset
Issue Date:
June 2016

17. Exploration of magnetic perturbation effects on advanced divertor configurations in NSTX-U

Author(s):
Frerichs, H.; Waters, I.; Schmitz, O.; Canal, G.P.; Evans, T.E.; Feng, Y.; Soukhanovskii, V.A.
Abstract:
The control of divertor heat loads - both steady state and transient - remains a key challenge for the successful operation of ITER and FNSF. Magnetic perturbations provide a promising technique to control ELMs (transients), but understanding their detailed impact is difficult due to their symmetry breaking nature. One approach for reducing steady state heat loads are so called 'advanced divertors' which aim at optimizing the magnetic field configuration: the snowflake and the (super-)X-divertor. It is likely that both concepts - magnetic perturbations and advanced divertors - will have to work together, and we explore their interaction based on the NSTX-U setup. An overview of different divertor configurations under the impact of magnetic perturbations is presented, and the resulting impact on plasma edge transport is investigated with the EMC3-EIRENE code. Variations in size of the magnetic footprint of the perturbed separatrix are found, which is related to the level of flux expansion on the divertor target. Non-axisymmetric peaking of the heat flux related to the perturbed separatrix is found at the outer strike point, but only in locations where flux expansion is not too large.
Type:
Dataset
Issue Date:
June 2016

18. Phase coherence of parametric-decay modes during high-harmonic fast-wave heating in the National Spherical Torus Experiment

Author(s):
Carlsson, J.; Wilson, J.R.; Hosea, J.; Greenough, N.; Perkins, R.
Abstract:
Third-order spectral analysis, in particular the auto bicoherence, was applied to probe signals from high-harmonic fast-wave heating experiments in the National Spherical Torus Experiment. Strong evidence was found for parametric decay of the 30 MHz radio-frequency (RF) pump wave, with a low-frequency daughter wave at 2.7 MHz, the local majority-ion cyclotron frequency. The primary decay modes have auto bicoherence values around 0.85, very close to the theoretical value of one, which corresponds to total phase coherence with the pump wave. The threshold RF pump power for onset of parametric decay was found to be between 200 kW and 400 kW.
Type:
Dataset
Issue Date:
June 2016

19. Validation and benchmarking of two particle-in-cell codes for a glow discharge

Author(s):
Carlsson, J.; Khrabrov, A.; Kaganovich, I.; Sommerer, T.; Keating, D.
Abstract:
The two particle-in-cell codes EDIPIC and LSP are benchmarked and validated for a parallel-plate glow discharge in helium, in which the axial electric field had been carefully measured, primarily to investigate and improve the fidelity of their collision models. The scattering anisotropy of electron-impact ionization, as well as the value of the secondary-electron emission yield, are not well known in this case. The experimental uncertainty for the emission yield corresponds to a factor of two variation in the cathode current. If the emission yield is tuned to make the cathode current computed by each code match the experiment, the computed electric fields are in excellent agreement with each other, and within about 10% of the experimental value. The non-monotonic variation of the width of the cathode fa ll with the applied voltage seen in the experiment is reproduced by both codes. The electron temperature in the negative glow is within experimental error bars for both codes, but the density of slow trapped electrons is underestimated. A more detailed code comparison don e for several synthetic cases of electron-beam injection into helium gas shows that the codes are in excellent agreement for ionization rate, as well as for elastic and excitation collisions with isotropic scattering pattern. The remaining significant discrepancies between the two codes are due to differences in their electron binary-collision models, and for anisotropic scattering due to elastic and excitation collisions.
Type:
Dataset
Issue Date:
2017

20. Dynamic reconfiguration of the default mode network during narrative comprehension

Author(s):
Simony, Erez; Honey, Christopher; Chen, Janice; Lositsky, Olga; Yeshurun, Yaara; Wiesel, Ami; Hasson, Uri
Abstract:
Does the default mode network (DMN) reconfigure to encode information about the changing environment? This question has proven difficult, because patterns of functional connectivity reflect a mixture of stimulus-induced neural processes, intrinsic neural processes and non-neuronal noise. Here we introduce inter-subject functional correlation (ISFC), which isolates stimulus-dependent inter-regional correlations between brains exposed to the same stimulus. During fMRI, we had subjects listen to a real-life auditory narrative and to temporally scrambled versions of the narrative. We used ISFC to isolate correlation patterns within the DMN that were locked to the processing of each narrative segment and specific to its meaning within the narrative context. The momentary configurations of DMN ISFC were highly replicable across groups. Moreover, DMN coupling strength predicted memory of narrative segments. Thus, ISFC opens new avenues for linking brain network dynamics to stimulus features and behaviour.
Type:
Dataset
Issue Date:
18 July 2016

21. Neural pattern change during encoding of a narrative predicts retrospective duration estimates

Author(s):
Lositsky, Olga; Chen, Janice; Toker, Daniel; Honey, Christopher; Hasson, Uri; Norman, Kenneth
Abstract:
What mechanisms support our ability to estimate durations on the order of minutes? Behavioral studies in humans have shown that changes in contextual features lead to overestimation of past durations. Based on evidence that the medial temporal lobes and prefrontal cortex represent contextual features, we related the degree of fMRI pattern change in these regions with people's subsequent duration estimates. After listening to a radio story in the scanner, participants were asked how much time had elapsed between pairs of clips from the story. Our ROI analysis found that the neural pattern distance between two clips at encoding was correlated with duration estimates in the right entorhinal cortex and right pars orbitalis. Moreover, a whole-brain searchlight analysis revealed a cluster spanning the right anterior temporal lobe. Our findings provide convergent support for the hypothesis that retrospective time judgments are driven by 'drift' in contextual representations supported by these regions.
Type:
Dataset
Issue Date:
12 March 2016

23. Sherlock Movie Watching Dataset

Author(s):
Chen, Janice
Abstract:
Our daily lives revolve around sharing experiences and memories with others. When different people recount the same events, how similar are their underlying neural representations? In this study, participants viewed a fifty-minute audio-visual movie, then verbally described the events while undergoing functional MRI. These descriptions were completely unguided and highly detailed, lasting for up to forty minutes. As each person spoke, event-specific spatial patterns were reinstated (movie-vs.-recall correlation) in default network, medial temporal, and high-level visual areas; moreover, individual event patterns were highly discriminable and similar between people during recollection (recall-vs.-recall similarity), suggesting the existence of spatially organized memory representations. In posterior medial cortex, medial prefrontal cortex, and angular gyrus, activity patterns during recall were more similar between people than to patterns elicited by the movie, indicating systematic reshaping of percept into memory across individuals. These results reveal striking similarity in how neural activity underlying real-life memories is organized and transformed in the brains of different people as they speak spontaneously about past events.
Type:
Dataset
Issue Date:
26 October 2016

24. Saturation of Alfven modes in tokamaks

Author(s):
White, R; Gorelenkov, N.; Gorelenkova, M.; Podesta, M.; Ethier, S.; Chen, Y.
Abstract:
Growth of Alfven modes driven unstable by a distribution of high energy particles up to saturation is investigated with a guiding center code, using numerical eigenfunctions produced by linear theory and a numerical high energy particle distribution, in order to make detailed comparison with experiment and with models for saturation amplitudes and the modification of beam profiles. Two innovations are introduced. First, a very noise free means of obtaining the mode-particle energy and momentum transfer is introduced, and secondly, a spline representation of the actual beam particle distribution is used.
Type:
Dataset
Issue Date:
November 2016

26. Phase space effects on fast ion distribution function modeling in tokamaks

Author(s):
Podesta, M.; M. Gorelenkova; E.D. Fredrickson; N.N. Gorelenkov
Abstract:
Integrated simulations of tokamak discharges typically rely on classical physics to model energetic particle (EP) dynamics. However, there are numerous cases in which energetic particles can suffer additional transport that is not classical in nature. Examples include transport by applied 3D magnetic perturbations and, more notably, by plasma instabilities. Focusing on the effects of instabilities, ad-hoc models can empirically reproduce increased transport, but the choice of transport coefficients is usually somehow arbitrary. New approaches based on physics-based reduced models are being developed to address those issues in a simplified way, while retaining a more correct treatment of resonant wave-particle interactions. The kick model implemented in the tokamak transport code TRANSP is an example of such reduced models. It includes modifications of the EP distribution by instabilities in real and velocity space, retaining correlations between transport in energy and space typical of resonant EP transport. The relevance of EP phase space modifications by instabilities is first discussed in terms of predicted fast ion distribution. Results are compared with those from a simple, ad-hoc diffusive model. It is then shown that the phase-space resolved model can also provide additional insight into important issues such as internal consistency of the simulations and mode stability through the analysis of the power exchanged between energetic particles and the instabilities.
Type:
Dataset
Issue Date:
April 2016

27. Large-volume flux closure during plasmoid-mediated reconnection in coaxial helicity injection

Author(s):
Ebrahimi, F.; Raman, R.
Abstract:
A large-volume flux closure during transient coaxial helicity injection (CHI) in NSTX-U is demonstrated through resistive magnetohydrodynamics (MHD) simulations. Several major improvements, including the improved positioning of the divertor poloidal field coils, are projected to improve the CHI start-up phase in NSTX-U. Simulations in the NSTX-U configuration with constant in time coil currents show that with strong flux shaping the injected open field lines (injector flux) rapidly reconnect and form large volume of closed flux surfaces. This is achieved by driving parallel current in the injector flux coil and oppositely directed currents in the flux shaping coils to form a narrow injector flux footprint and push the injector flux into the vessel. As the helicity and plasma are injected into the device, the oppositely directed field lines in the injector region are forced to reconnect through a local Sweet–Parker type reconnection, or to spontaneously reconnect when the elongated current sheet becomes MHD unstable to form plasmoids. In these simulations for the first time, it is found that the closed flux is over 70% of the initial injector flux used to initiate the discharge. These results could work well for the application of transient CHI in devices that employ super conducting coils to generate and sustain the plasma equilibrium.
Type:
Dataset
Issue Date:
April 2016

29. Development of miniaturized, spectroscopically assisted Penning gauges for fractional helium and hydrogen neutral pressure measurements

Author(s):
Flesch, K.; Kremeyer, T.; Schmitz, O.; Soukhanovskii, V.; Wenzel, U.
Abstract:
Direct measurements of the helium (He) fractional neutral pressure in the neutral gas around fusion devices is challenging because of the small mass difference between the abundant D2 molecules and the He atoms which will be produced by D-T fusion. To study He exhaust, Penning gauges were used to measure total neutral pressure assisted by spectroscopy to resolve the D and He partial pressures. In this contribution, initial results are shown from developing this technique into a miniaturized configuration for direct in-situ measurements in the divertor of fusion devices. The configuration is based off a gauge originally designed for the National Spherical Tokamak Experiment-Upgrade (NSTX-U). The goal of this new miniaturized design it to reduce the space required by the gauge on the device and use of the inherent magnetic field of the machine rather than permanent magnets inside the gauge, enabling it to be adapted into a system that can be extended directly into the divertor region. The feasibility test of the method for NSTX-U and the Wendelstein 7-X (W7-X) stellarator are surveyed. For W7-X, a commercial Penning Gauge has been installed on an outboard vacuum flange as a generic feasibility test in the neutral gas environment of a stellarator. At an integration time of 25s, helium lines can be seen down to 10^-5 mbar and H-alpha lines down to 10^-6 mbar. Successful measurement of the total as well as the fractional neutral pressures of He and H has been shown. A first prototype of the miniature Penning gauge has been tested in Madison and shows a near linear power law scaling between current and pressure: I = C*P^n with n = 1.0 - 1.2. Pressure measurements were achieved starting at 10^-3 mbar and down to 10^-6 mbar. A modular gauge is being assembled, which allows easy interchangeability of the anode to test new anode geometries, in order to improve optical access and increase spectroscopic sensitivity. This shall enable an increase of the time resolution of the spectroscopically assisted fractional neutral pressure measurements to up to 1kHz.
Type:
Dataset
Issue Date:
November 2016

30. Laboratory study of low-beta forces in arched, line-tied magnetic flux ropes

Author(s):
Myers, Clayton; Yamada, Masaaki; Ji, Hantao; Yoo, Jongsoo; Jara-Almonte, Jonathan; Fox, William
Abstract:
The loss-of-equilibrium is a solar eruption mechanism whereby a sudden breakdown of the magnetohydrodynamic force balance in the Sun's corona ejects a massive burst of particles and energy into the heliosphere. Predicting a loss-of-equilibrium, which has more recently been formulated as the torus instability, relies on a detailed understanding of the various forces that hold the pre-eruption magnetic flux rope in equilibrium. Traditionally, idealized analytical force expressions are used to derive simplified eruption criteria that can be compared to solar observations and modeling. What is missing, however, is a validation that these idealized analytical force expressions can be applied to the line-tied, low-aspect-ratio conditions of the corona. In this paper, we address this shortcoming by using a laboratory experiment to study the forces that act on long-lived, arched, line-tied magnetic flux ropes. Three key force terms are evaluated over a wide range of experimental conditions: (1) the upward hoop force; (2) the downward strapping force; and (3) the downward toroidal field tension force. First, the laboratory force measurements show that, on average, the three aforementioned force terms cancel to produce a balanced line-tied equilibrium. This finding validates the laboratory force measurement techniques developed here, which were recently used to identify a dynamic toroidal field tension force that can prevent flux rope eruptions [Myers et al., Nature 528, 526 (2015)]. The verification of magnetic force balance also confirms the low-beta assumption that the plasma thermal pressure is negligible in these experiments. Next, the measured force terms are directly compared to their corresponding analytical expressions. While the measured and analytical forces are found to be well correlated, the low-aspect-ratio, line-tied conditions in the experiment are found to both reduce the measured hoop force and increase the measured tension force with respect to analytical expectations. These two co-directed effects combine to generate laboratory flux rope equilibria at lower altitudes than are predicted analytically. Such considerations are expected to modify the loss-of-equilibrium eruption criteria for analogous flux ropes in the solar corona.
Type:
Dataset
Issue Date:
November 2016

31. Quasi-static and dynamic magnetic tension forces in arched, line-tied magnetic flux ropes

Author(s):
Myers, Clayton; Yamada, Masaaki; Ji, Hantao; Yoo, Jongsoo; Jara-Almonte, Jonathan; Fox, William
Abstract:
Solar eruptions are often driven by magnetohydrodynamic instabilities such as the torus and kink instabilities that act on line-tied magnetic flux ropes. Recent laboratory experiments designed to study these eruptive instabilities have demonstrated the key role of both dynamic (Myers et al 2015 Nature 528, 526) and quasi-static (Myers et al 2016 Phys. Plasmas, in press) magnetic tension forces in contributing to the equilibrium and stability of line-tied magnetic flux ropes. In this paper, we synthesize these laboratory results and explore the relationship between the dynamic and quasi-static tension forces. While the quasi-static tension force is found to contribute to the flux rope equilibrium in a number of regimes, the dynamic tension force is substantial mostly in the so-called failed torus regime where magnetic self-organization events prevent the flux rope from erupting.
Type:
Dataset
Issue Date:
December 2016

32. Compact and multi-view solid state neutral particle analyzer arrays on National Spherical Torus Experiment-Upgrade

Author(s):
Liu, D.; Heidbrink, W.W.; Tritz, K.; Fredrickson, E.D.; Hao, G.Z.; Zhu, Y.B.
Abstract:
A compact and multi-view Solid State Neutral Particle Analyzer (SSNPA) diagnostic based on silicon photodiode arrays has been successfully tested on the National Spherical Torus Experiment-Upgrade (NSTX-U). The SSNPA diagnostic provides spatially, temporally, and pitch-angle resolved measurements of fast-ion distribution by detecting fast neutral flux resulting from charge exchange (CX) reactions. The system consists of three 16-channel subsystems: t-SSNPA viewing the plasma mid-radius and neutral beam (NB) line #2 tangentially, r-SSNPA viewing the plasma core and NB line #1 radially and p-SSNPA with no intersection with any NB lines. Due to the setup geometry, the active CX signals of t-SSNPA and r-SSNPA are mainly sensitive to passing and trapped particles respectively. In addition, both t-SSNPA and r-SSNPA utilize three vertically stacked arrays with different filter thickness to obtain coarse energy information. The experimental data show that all channels are operational. The signal to noise ratio is typically larger than 10 and the main noise is x-ray induced signal. The active and passive CX signals are clearly observed on t-SSNPA and r-SSNPA during NB modulation. The SSNPA data also indicate significant losses of passing particles during sawteeth, while trapped particles are weakly affected. Fluctuations up to 120 kHz, have been observed on SSNPA, and they are strongly correlated with magnetohydrodynamics (MHD) instabilities.
Type:
Dataset
Issue Date:
November 2016

33. Nonlinear fishbone dynamics in spherical tokamaks

Author(s):
Wang, F.; Fu, G.Y.; Shen, W.
Abstract:
Linear and nonlinear kinetic-MHD hybrid simulations have been carried out to investigate linear stability and nonlinear dynamics of beam-driven fishbone instability in spherical tokamak plasmas. Realistic NSTX parameters with finite toroidal rotation were used. The results show that the fishbone is driven by both trapped and passing particles. The instability drive of passing particles is comparable to that of trapped particles in the linear regime. The effects of rotation are destabilizing and a new region of instability appears at higher qmin (>1.5) values, qmin being the minimum of safety factor profile. In the nonlinear regime, the mode saturates due to flattening of beam ion distribution, and this persists after initial saturation while mode frequency chirps down in such a way that the resonant trapped particles move out radially and keep in resonance with the mode. Correspondingly, the flattening region of beam ion distribution expands radially outward. A substantial fraction of initially non- resonant trapped particles become resonant around the time of mode saturation and keep in resonance with the mode as frequency chirps down. On the other hand, the fraction of resonant passing particles is significantly smaller than that of trapped particles. Our analysis shows that trapped particles provide the main drive to the mode in the nonlinear regime.
Type:
Dataset
Issue Date:
January 2017

34. Remote-sensing gas measurements with coherent Rayleigh-Brillouin scattering

Author(s):
Gerakis, A.; Shneider, M. N.; Stratton, B. C.
Abstract:
We measure the coherent Rayleigh-Brillouin scattering (CRBS) signal integral as a function of the recorded gas pressure in He, Co2, SF6, and air, and we confirm the already established quadratic dependence of the signal on the gas density. We propose the use of CRBS as an effective diagnostic for the remote measurement of gas’ density (pressure) and temperature, as well as polarizability, for gases of known composition.
Type:
Dataset
Issue Date:
2016

35. Real-time Radiative Divertor Feedback Control Development for the NSTX-U Tokamak using a Vacuum Ultraviolet Spectrometer

Author(s):
Soukhanovskii, V.A.; Kaita, R.; Stratton, B.
Abstract:
A radiative divertor technique is planned for the NSTX-U tokamak to prevent excessive erosion and thermal damage of divertor plasma-facing components in H-mode plasma discharges with auxiliary heating up to 12 MW. In the radiative (partially detached) divertor, extrinsically seeded deuterium or impurity gases are used to increase plasma volumetric power and momentum losses. A real-time feedback control of the gas seeding rate is planned for discharges of up to 5 s duration. The outer divertor leg plasma electron temperature Te estimated spectroscopically in real time will be used as a control parameter. A vacuum ultraviolet spectrometer McPherson Model 251 with a fast charged-coupled device detector is developed for temperature monitoring between 5 and 30 eV, based on the delta n=0;1 line intensity ratios of carbon, nitrogen or neon ions lines in the spectral range 300 to 1600 A. A collisional-radiative model-based line intensity ratio will be used for relative calibration. A real-time Te-dependent signal within a characteristic divertor detachment equilibration time of ~ 10-15 ms is expected.
Type:
Dataset
Issue Date:
November 2016

37. Understanding the dynamics and energetics of magnetic reconnection in a laboratory plasma: Review of recent progress on selected fronts

Author(s):
Yamada, M.; Yoo, J.; Myers, C. E.
Abstract:
Magnetic reconnection is a fundamental process at work in laboratory, space and astrophysical plasmas, in which magnetic field lines change their topology and convert magnetic energy to plasma particles by acceleration and heating. One of the most important problems in reconnection research has been to understand why reconnection occurs so much faster than predicted by MHD theory. Following the recent pedagogical review of this subject [M. Yamada, R. Kulsrud, and H. Ji, Rev. Mod. Phys. {\bf 82}, 603 (2010)], this paper presents a review of more recent discoveries and findings in the research of fast magnetic reconnection in laboratory, space, and astrophysical plasmas. In spite of the huge difference in physical scales, we find remarkable commonality between the characteristics of the magnetic reconnection in laboratory and space plasmas. In this paper, we will focus especially on the energy flow, a key feature of the reconnection process. In particular the experimental results on the energy conversion and partitioning in a laboratory reconnection layer [M. Yamada {\it et al.}, Nat. Commu. {\bf 5}, 4474 (2014)] are discussed and compared with quantitative estimates based on two-fluid analysis. In the Magnetic Reconnection Experiment (MRX), we find that energy deposition to electrons is localized near the X-point and is mostly from the electric field component perpendicular to the magnetic field. The mechanisms of ion acceleration and heating are also identified and a systematic and quantitative study on the inventory of converted energy within a reconnection layer with a well-defined but variable boundary. The measured energy partition in a reconnection region of similar effective size ($L \approx$ 3 ion skin depths) of the Earth's magneto-tail [J. Eastwood {\it et al.}, Phys. Rev. Lett. {\bf 110}, 225001 (2013)] is notably consistent with our laboratory results. Finally, to study the global aspects of magnetic reconnection, we have carried out a laboratory experiment on the stability criteria for solar flare eruptions, including {\textquotedblleft}storage and release{\textquotedblright} mechanisms of magnetic energy. We show that toroidal magnetic flux generated by magnetic relaxation (reconnection) processes generates a new stabilizing force which prevents plasma eruption. This result has lead us to discovery of a new stabilizing force for solar flares [C. E. Myers {\it et al.}, Nature {\bf 528}, 526 (2015)]
Type:
Dataset
Issue Date:
May 2016

38. Theory based scaling of edge turbulence and implications for the scrape-off layer width

Author(s):
Myra, J.R.; Russell, D.A.; Zweben, S.J.
Abstract:
Turbulence and plasma parameter data from the National Spherical Torus Experiment NSTX [M. Ono, S.M. Kaye, Y.-K.M. Peng, G. Barnes et al., Nucl. Fusion 40, 557 (2000)] is examined and interpreted based on various theoretical estimates. In particular, quantities of interest for assessing the role of turbulent transport on the midplane scrape-off layer heat flux width are assessed. Because most turbulence quantities exhibit large scatter and little scaling within a given operation mode, this paper focuses on length and time scales and dimensionless parameters between operational modes including Ohmic, low (L), and high (H) modes using a large NSTX edge turbulence database [S.J. Zweben, W.M. Davis, S.M. Kaye, J.R. Myra et al., Nucl. Fusion 55, 093035 (2015)]. These are compared with theoretical estimates for drift and interchange rates, profile modification saturation levels, a resistive ballooning condition, and dimensionless parameters characterizing L and high H mode conditions. It is argued that the underlying instability physics governing edge turbulence in different operational modes is in fact similar, and is consistent with curvature-driven drift ballooning. Saturation physics, however, is dependent on the operational mode. Five dimensionless parameters for drift-interchange turbulence are obtained and employed to assess the important of turbulence in setting the scrape-off layer heat flux width lambda_q and its scaling. An explicit proportionality of the width lambda_q to safety factor and major radius (qR) is obtained under these conditions. Quantitative estimates and reduced model numerical simulations suggest that the turbulence mechanism is not negligible in determining lambda_q in NSTX, at least for high plasma current discharges.
Type:
Dataset
Issue Date:
November 2016

39. Massive Gas Injection Valve Development for NSTX-U

Author(s):
Raman, R.; Plunkett, G.J.; Way, W.-S.
Abstract:
NSTX-U research will offer new insight by studying gas assimilation efficiencies for MGI injection from different poloidal locations using identical gas injection systems. In support of this activity, an electromagnetic MGI valve has been built and tested. The valve operates by repelling two conductive disks due to eddy currents induced on them by a rapidly changing magnetic field created by a pancake disk solenoid positioned beneath the circular disk attached to a piston. The current is driven in opposite directions in the two solenoids, which creates a cancelling torque when the valve is operated in an ambient magnetic field, as would be required in a tokamak installation. The valve does not use ferromagnetic materials. Results from the operation of the valve, including tests conducted in 1 T external magnetic fields, are described. The pressure rise in the test chamber is measured directly using a fast time response baratron gauge. At a plenum pressure of just 1.38 MPa (~200 psig), the valve injects 27 Pa.m^3 (~200 Torr.L) of nitrogen with a pressure rise time of 3 ms.
Type:
Dataset
Issue Date:
May 2016

40. Diagnostics for molybdenum and tungsten erosion and transport in NSTX-U

Author(s):
Scotti, F.; Soukhanovskii, V.; Weller, M.
Abstract:
A comprehensive set of spectroscopic diagnostics is planned in the National Spherical Torus Experi- ment Upgrade to connect measurements of molybdenum and tungsten divertor sources to scrape-o↵ layer (SOL) and core impurity transport, supporting the installation of high-Z plasma facing compo- nents which is scheduled to begin with a row of molybdenum tiles. Imaging with narrow-bandpass interference filters and high-resolution spectroscopy will be coupled to estimate divertor impurity influxes. Vacuum ultraviolet and extreme ultraviolet spectrometers will allow connecting high-Z sources to SOL transport and core impurity content. The high-Z diagnostics suite complements the existing measurements for low-Z impurities (carbon and lithium), critical for the characterization of sputtering of high-Z materials.
Type:
Dataset
Issue Date:
November 2016

41. Parallel electron force balance and the L-H transition

Author(s):
Stoltzfus-Dueck, T.
Abstract:
In one popular description of the L-H transition, energy transfer to the mean flows directly depletes turbulence fluctuation energy, resulting in suppression of the turbulence and a corresponding transport bifurcation. However, electron parallel force balance couples nonzonal velocity fluctuations with electron pressure fluctuations on rapid timescales, comparable with the electron transit time. For this reason, energy in the nonzonal velocity stays in a fairly fixed ratio to the free energy in electron density fluctuations, at least for frequency scales much slower than electron transit. In order for direct depletion of the energy in turbulent fluctuations to cause the L-H transition, energy transfer via Reynolds stress must therefore drain enough energy to significantly reduce the sum of the free energy in nonzonal velocities and electron pressure fluctuations. At low k, the electron thermal free energy is much larger than the energy in nonzonal velocities, posing a stark challenge for this model of the L-H transition.
Type:
Dataset
Issue Date:
May 2016

42. Supplementary Model Output to "Climate, soil organic layer, and nitrogen jointly drive forest development after fire in the North American boreal zone"

Author(s):
Trugman, Anna
Abstract:
This dataset contains all the model output used to generate the figures and data reported in the article "Climate, soil organic layer, and nitrogen jointly drive forest development after fire in the North American boreal zone". The data was generated during spring 2015 using the a modified version of the Ecosystem Demography model version 2, provided as a supplement accompanying the article. The data was generated using the computational resources supported by the PICSciE OIT High Performance Computing Center and Visualization Laboratory at Princeton University. The dataset contains a pdf Readme file which explains in detail how the data can be used. Users are recommended to go through this file before using the data.
Type:
Dataset
Issue Date:
2016

43. Collisional dependence of Alfven mode saturation in tokamaks

Author(s):
Zhou, M.; White, R.
Abstract:
Saturation of \alfven modes driven unstable by a distribution of high energy particles as a function of collisionality is investigated with a guiding center code, using numerical eigenfunctions produced by linear theory and numerical high energy particle distributions. The most important resonance is found and it is shown that when the resonance domain is bounded, not allowing particles to collisionlessly escape, the saturation amplitude is given by the balance of the resonance mixing time with the time for nearby particles to collisionally diffuse across the resonance width. Saturation amplitudes are in agreement with theoretical predictions as long as the mode amplitude is not so large that it produces stochastic loss from the resonance domain.
Type:
Dataset
Issue Date:
December 2016

44. Fermi-LAT Example Data and Template

Abstract:
A subset of the Fermi-LAT public data for use with NPTFit: https://github.com/bsafdi/NPTFit The data here is for use with the Jupyter example notebooks provided with the main code. Details of the files provided are given below. All files are provided as numpy arrays binned as nside=128 HEALPix maps. For the full public data, see: http://fermi.gsfc.nasa.gov/ssc/data/access/
Type:
Dataset

46. Graphite: Dielectric Tensor and Cross Sections for Spheres and Spheroids

Abstract:
Dielectric tensor for crystalline graphite from X-ray to microwave frequencies, as discussed in the paper "Graphite Revisited" (Draine 2016, Astrophysical Journal, in press). Cross sections for absorption and scattering by graphite spheres and spheroids are also tabulated, as well as Planck-averaged cross sections for absorption and scattering of radiation with a Planck spectrum.
Type:
Dataset