Vekselman, V.; Khrabry, A.; Kaganovich, I.; Stratton, B.; Selinsky, R. S.; Raitses, Y.
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
Rafidi, Nicole S; Hulbert, Justin C; Brooks, Paula P; Norman, Kenneth A
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.
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
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.
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.
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.
Kim, E.-W.; Bertelli, N.; Johnson, J.R.; Valeo, E.; Hosea, J.; Perkins, R.
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.
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
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.
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.
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.