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Shen, B.

Publications and source records attributed to Shen, B..

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Scenario adaptive disruption prediction study for next generation burning-plasma tokamaks

Next generation High Performance (HP) tokamaks risk damage from unmitigated disruptions at high current and power. Achieving reliable disruption prediction for a device's HP operation based on its Low Performance (LP) data is a key to its success. In this letter, through explorative data analysis and dedicated numerical experiments on multiple existing tokamaks, we demonstrate how the operational regimes of tokamaks can affect the power of a trained disruption predictor. First, our results suggest data-driven disruption predictors trained on abundant LP discharges work poorly on the HP regime of the same tokamak, which is a consequence of the distinct distributions of the tightly correlated signals related to disruptions in these two regimes. Second, we find that matching operational parameters among tokamaks strongly improves cross-machine accuracy which implies our model learns from the underlying scalings of dimensionless physics parameters like q 95 , β p and confirms the importance of these parameters in disruption physics and cross machine domain matching from the data-driven perspective. Finally, our results show in the absence of HP data from the target devices, the best predictivity of the HP regime for the target machine can be achieved by combining LP data from the target with HP data from other machines. Furthermore, these results provide a possible disruption predictor development strategy for next generation tokamaks, such as ITER and SPARC, and highlight the importance of developing baseline scenario discharges of future tokamaks on existing machines to collect more relevant disruptive data.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Tracking Ultrafast Photocurrents in the Weyl Semimetal TaAs Using THz Emission Spectroscopy

We investigate here polarization-dependent ultrafast photocurrents in the Weyl semimetal TaAs using terahertz (THz) emission spectroscopy. Our results reveal that highly directional, transient photocurrents are generated along the noncentrosymmetric c axis regardless of incident light polarization, while helicity-dependent photocurrents are excited within the ab plane. This is consistent with earlier static photocurrent experiments, and demonstrates on the basis of both the physical constraints imposed by symmetry and the temporal dynamics intrinsic to current generation and decay that optically induced photocurrents in TaAs are inherent to the underlying crystal symmetry of the transition metal monopnictide family of Weyl semimetals.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Machine learning for disruption warnings on Alcator C-Mod, DIII-D, and EAST

In this paper, we report on disruption prediction using a shallow machinelearning method known as Random Forests, trained on large databases containingonly plasma parameters that are available in real-time on Alcator C-Mod, DIII-D, and EAST. The database for each tokamak contains parameters sampled ~10 6 times throughout ~10 4 discharges (disruptive and non-disruptive) over the last 4years of operation. It is found that a number of parameters (e.g.P rad /P input ,l i ,n/n G ,B n=1 /B 0 ) exhibit changes in aggregate as a disruption is approached on one ormore of these tokamaks. However, for each machine, the most useful parameters,as well as the details of their precursor behaviors, are markedly different. When the prediction problem is framed using a binary classification scheme to discriminate between time slices "close to disruption" and "far from disruption," it is found that the prediction algorithms differ substantially in performance among the three machines ona time slice-by-time slice basis, but have similar disruption detection rates (~80-90%) on a shot-by-shot basis after appropriate optimisation. This could have important implications for disruption prediction and avoidance on ITER, for which developmentof a training database of disruptions may be infeasible. The algorithm's output is interpretable using a method that identifies the most strongly contributing inputsignals, which may have implications for avoiding disruptive scenarios. Lastly, to further support its real-time capability, successful applications in inter-shot and real-time environments on EAST and DIII-D are also discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Study of Critical Current and n-Values of 2G HTS Tapes: Their Magnetic Field-Angular Dependence

Since many applications of YBCO tapes operate in external magnetic fields, it is necessary to investigate the magneto-angular dependence of critical current and n-values in coated conductors. In this paper, five commercial YBCO tapes with different microstructures produced by three different manufacturers are chosen. The selected samples have a width of 2.0, 4.0, 4.8, 6.0 or 12 mm, with copper, brass or stainless steel laminations. The critical current density dependence J{sub c}(B, θ) and n-values characteristics n(B, θ) of the tapes are comprehensively measured under various magnetic fields and orientations. Afterwards, the obtained experimental data sets are successfully fitted using a novel multi-objective model which considers the material anisotropy. By using this approach, a fitting function I{sub c}(B, θ) can always be obtained to accurately describe the experimental data, regardless of the fabrication and width differences of the superconducting tapes. Moreover, our experiment shows that when subject to different external magnetic fields, the angular dependence of n-values characteristics is directly correlated with the corresponding critical current profiles. Our results are helpful to predict the critical current of electromagnetically interacting 2G HTS wires, thereby improving the design and performance of the devices made from YBCO tapes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Active Quenching Technique for YBCO Tapes: Quench Acceleration and Protection

The application of resistive-type superconducting fault current limiters (RSFCLs) in electrical networks is very attractive due to their relative compactness, light weight, and good performance. However, this technology still has drawbacks: asymmetrical quench, uncertain limiting velocity, passive action and incapability of enlarging capacity. Here, we present an active quenching technique which can potentially solve these problems. The quenching process is triggered using high-frequency (HF) AC fields, which are generated by two coupled copper coils attached on both sides of the YBCO sample. Our experiments show that this approach is effective in guaranteeing uniform quench and therefore is expected to be able to significantly extend the service life of the device. Moreover, the quench speed can be considerably increased by the HF field. We find that the performance of acceleration is positively correlated to the transport current, intensity and frequency of the AC field. In addition, a DC magnetic field is added around the sample holder simultaneously with the AC field, to study the field-angular-frequency dependence of the quench time t(B{sub AC}f{sub AC}B{sub DC}𝜃{sub DC}). Experimental results prove that the DC magnetic field can cooperate with the HF AC field to accelerate quench, which means better performance can be produced with lower costs with the two fields acting together. In all, this technique showed outstanding performance regarding quench acceleration and tape protection. We believe the HF-assisted quenching technology has a promising future in current-limiting devices and hope our findings could be helpful for its potential applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

An efficient technique for magnetic equilibrium reconstruction with q profile constraints and its application on the EAST tokamak

An efficient technique for building plasma equilibria with preset or constrained q profiles by solving the nonlinear Grad–Shafranov equation iteratively is proposed. Here, the technique uses a set of dynamic virtual poloidal field probes in the plasma core region. Numerical examples are shown for monotonic q profiles and negative central magnetic shear equilibria which are obtained with this technique. As a proof of principle, the technique is applied on the EAST tokamak, to constrain the plasma equilibrium reconstruction with the experimental q = 1 surface information from the ECE diagnostic in addition to the external magnetic measurements.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Ideal magnetohydrodynamic theory for localized interchange modes in toroidal anisotropic plasmas

Ideal magnetohydrodynamic theory for localized interchange modes is developed for toroidal plasmas with anisotropic pressure. The work extends the existing theories of Johnson and Hastie [Phys. Fluids 31, 1609 (1988)], etc., to the low n mode case, where n is the toroidal mode number. Also, the plasma compressibility is included, so that the coupling of the parallel motion to perpendicular one, i.e., the so-called apparent mass effect, is investigated in the anisotropic pressure case. Lastly, the singular layer equation is obtained, and the generalized Mercier's criterion is derived.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Expansion of the tetragonal magnetic phase with pressure in the iron arsenide superconductor Ba 1 - x K x Fe 2 As 2

In the temperature-concentration phase diagram of most iron-based superconductors, antiferromagnetic order is gradually suppressed to zero at a critical point, and a dome of superconductivity forms around that point. The nature of the magnetic phase and its fluctuations is of fundamental importance for elucidating the pairing mechanism. In Ba 1–x K x Fe 2 As 2 and Ba 1–x Na x Fe 2 As 2 , it has recently become clear that the usual stripelike magnetic phase, of orthorhombic symmetry, gives way to a second magnetic phase, of tetragonal symmetry, near the critical point, in the range from x = 0.24 to x = 0.28 for Ba 1–x K x Fe 2 As 2 . In a prior study, an unidentified phase was discovered for x < 0.24 but under applied pressure, whose onset was detected as a sharp anomaly in the resistivity. Here we report measurements of the electrical resistivity of Ba 1–x K x Fe 2 As 2 under applied hydrostatic pressures up to 2.75 GPa, for x = 0.22, 0.24, and 0.28. The critical pressure above which the unidentified phase appears is seen to decrease with increasing x and vanish at x = 0.24, thereby linking the pressure-induced phase to the tetragonal magnetic phase observed at ambient pressure. In the temperature-concentration phase diagram of Ba 1–x K x Fe 2 As 2 , we find that pressure greatly expands the tetragonal magnetic phase, while the stripelike phase shrinks. As a result, this reveals that pressure may be a powerful tuning parameter with which to explore the interplay between magnetism and superconductivity in this material.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Electronic and optical properties of novel carbon allotropes

The vibrational properties, electronic structures and optical properties of novel carbon allotropes, such as monolayer penta-graphene (PG), double-layer PG and T12-carbon, were studied by first-principles calculations. Results of phonon calculations demonstrate that these exotic carbon allotropes are dynamically stable. The bulk T12 phase is an indirect-gap semiconductor having a quasiparticle (QP) bandgap of ~5.19 eV. When the bulk material transforms to a two-dimensional (2D) phase, the monolayer and double-layer PG become quasi-direct gap semiconductors with smaller QP bandgaps of ~4.48 eV and ~3.67 eV, respectively. Furthermore, the partial charge density analysis indicates that the 2D phases retain part of the electronic characteristics of the T12 phase. The linear photon energy-dependent dielectric functions and related optical properties including refractive index, extinction coefficient, absorption spectrum, reflectivity, and energy-loss spectrum were also computed and discussed. Additionally, the chemical stability of monolayer PG and the electronic and optical properties of double-side hydrogenated monolayer PG were also investigated. Furthermore, the results obtained from our calculations are beneficial to practical applications of these exotic carbon allotropes in optoelectronics and electronics.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Enhancing superconducting critical current by randomness

The key ingredient of high critical currents in a type-II superconductor is defect sites that pin vortices. Here, we demonstrate that a random pinscape, an overlooked pinning system in nanopatterned superconductors, can lead to a substantially larger critical current enhancement at high magnetic fields than an ordered array of vortex pin sites. We reveal that the better performance of a random pinscape is due to the variation of the local density of its pinning sites, which mitigates the motion of vortices. This is confirmed by achieving even higher enhancement of the critical current through a conformally mapped random pinscape, where the distribution of the local density of pinning sites is further enlarged. Our findings highlight the potential of random pinscapes in enhancing the superconducting critical currents of applied superconductors in which random pin sites of nanoscale defects emerging in the materials synthesis process or through ex-situ irradiation are the only practical choice for large-scale production. Our results may also stimulate research on effects of a random pinscape in other complementary systems such as colloidal crystals, Bose-Einstein condensates, and Luttinger liquids.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Dynamics and diffusion mechanism of low-density liquid silicon

A first-order phase transition from a high-density liquid to a low-density liquid has been proposed to explain the various thermodynamic anomies of water. It also has been proposed that such liquid–liquid phase transition would exist in supercooled silicon. Computer simulation studies show that, across the transition, the diffusivity drops roughly 2 orders of magnitude, and the structures exhibit considerable tetrahedral ordering. The resulting phase is a highly viscous, low-density liquid silicon. Investigations on the atomic diffusion of such a novel form of liquid silicon are of high interest. Here we report such diffusion results from molecular dynamics simulations using the classical Stillinger–Weber (SW) potential of silicon. We show that the atomic diffusion of the low-density liquid is highly correlated with local tetrahedral geometries. We also show that atoms diffuse through hopping processes within short ranges, which gradually accumulate to an overall random motion for long ranges as in normal liquids. There is a close relationship between dynamical heterogeneity and hopping process. We point out that the above diffusion mechanism is closely related to the strong directional bonding nature of the distorted tetrahedral network. Here, our work offers new insights into the complex behavior of the highly viscous low density liquid silicon, suggesting similar diffusion behaviors in other tetrahedral coordinated liquids that exhibit liquid–liquid phase transition such as carbon and germanium.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Halo current diagnostic system of experimental advanced superconducting tokamak

The design, calibration, and installation of disruption halo current sensors for the Experimental Advanced Superconducting Tokamak are described in this article. All the sensors are Rogowski coils that surround conducting structures, and all the signals are analog integrated. Coils with two different cross-section sizes have been fabricated, and their mutual inductances are calibrated. Sensors have been installed to measure halo currents in several different parts of both the upper divertor (tungsten) and lower divertor (graphite) at several toroidal locations. Initial measurements from disruptions show that the halo current diagnostics are working well.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Critical fields and vortex pinning in overdoped Ba 0.2 K 0.8 Fe 2 As 2

We determine the upper and lower critical fields, the penetration depth and the vortex pinning characteristics of single crystals of overdoped Ba 0.2 K 0.8 Fe 2 As 2 with T c ~ 10 K. We find that bulk vortex pinning is weak and vortex dynamics to be dominated by the geometrical surface barrier. The temperature dependence of the lower critical field, H c1 , displays a distinctive upturn at low temperatures, which is suggestive of two distinct superconducting gaps. Furthermore, the penetration depth, λ, varies linearly with temperature below 4 K indicative of line nodes in the superconducting gap. As a result, these observations can be well described in a model based on a multi-band nodal superconducting gap.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Antiferromagnetic and nematic phase transitions in BaF e 2 ( A s 1 - x P x ) 2 studied by ac microcalorimetry and SQUID magnetometry

In this work, we study the antiferromagnetic (AFM) and structural phase transitions in single-crystal BaF e 2 ( A s 1 – x P x ) 2 ( x = 0 , 0.3 ) at temperatures T N and T S , respectively, by high-resolution ac microcalorimetry and SQUID magnetometry. The specific heat measurements of both as-grown and annealed BaF e 2 A s 2 display a sharp peak at the AFM-structural transitions. A kink in the entropy of annealed BaF e 2 A s 2 gives evidence for splitting of the two transitions by approximately 0.5 K. No additional features could be identified in the specific heat of both BaF e 2 A s 2 and BaF e 2 ( A s 0.7 P 0.3 ) 2 in the temperature regions around T * > T S where torque measurements had revealed the “true” nematic phase transition, even though the Ginzburg-Landau model used to fit the magnetic torque data indicates that the expected thermal anomaly should be easily observable with our experimental resolution.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗