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At least 1,027 records · Page 57
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Coexistence of unconventional spin Hall effect and antisymmetric planar Hall effect in IrO 2

Crystal symmetry plays an important role in the Hall effects. Unconventional spin Hall effect (USHE), characterized by Dresselhaus and out-of-plane spins, has been observed in materials with low crystal symmetry. Recently, antisymmetric planar Hall effect (APHE) was discovered in rutile RuO 2 and IrO 2 (101) thin films, which also exhibit low crystal symmetry. In this study, we report the observation of both USHE and APHE in IrO 2 (111) films, using spin-torque ferromagnetic resonance and harmonic Hall measurements, respectively. Notably, the unconventional spin-torque efficiency from Dresselhaus spin was more than double that of a previous report. Additionally, the temperature dependence of APHE suggests that it arises from the Lorentz force, constrained by crystal symmetry. Here, symmetry analysis supports the coexistence of USHE and APHE and demonstrates that both originate from the crystal symmetry of IrO 2 (111), paving the way for a deeper understanding of Hall effects and related physical phenomena.

Crystal structure↗
DOE OSTI2022

Magnetoquantum oscillations in the specific heat of a topological Kondo insulator

Surprisingly, magnetoquantum oscillations (MQOs) characteristic of a metal with a Fermi surface have been observed in measurements of the topological Kondo insulator SmB 6 . As these MQO have only been observed in measurements of magnetic torque (dHvA) and not in measurements of magnetoresistance (SdH), a debate has arisen as to whether the MQO are an extrinsic effect arising from rare-earth impurities, defects, and/or aluminum inclusions or an intrinsic effect revealing the existence of charge-neutral excitations. In this work, we report the first observation of MQO in the low-temperature specific heat of SmB 6 . The observed frequencies and their angular dependence for these flux-grown samples are consistent with previous results based on magnetic torque for SmB 6 but the inferred effective masses are significantly larger than previously reported. Such oscillations can only be observed if the MQO are of bulk thermodynamic origin; the measured magnetic-field dependent oscillation amplitude and effective mass allow us to rule out suggestions of an extrinsic, aluminum inclusion-based origin for the MQO.

topological materials↗
DOE OSTI2022

Unified theory of spin and charge in a ferromagnet

Here, we derive a unified theory of spin and charge degrees of freedom in a ferromagnet. The spin-transfer torque and spin electromotive force are examined from the coarse-grained perspective of collective coordinates. The resulting equations of motion reflect a balance of conservative, gyroscopic (Berry-phase), and dissipative forces. We then expand the space of collective coordinates by adding the electric charge. The adiabatic spin-transfer torque and spin electromotive force (emf) turn out to be a gyroscopic force; their nonadiabatic counterparts are a dissipative force.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Experimental evidence of electron-cyclotron current drive-based neoclassical tearing mode suppression threshold reduction during mode locking on DIII-D

The repetitive locking-unlocking process of a large $m/n = 2/1$ neoclassical tearing mode (NTM) is studied on the DIII-D tokamak. Cyclical periods of partial suppression are achieved when β p and the expected bootstrap current $J_{\mathrm{BS}}$ drop upon back-transitions to L-mode. Diminished L-mode bootstrap current is required to explain decreased island size as modeled by the generalized Rutherford equation. At EC power levels too low to fully suppress the NTM, partial suppression is enough to reduce the torque on the mode to the point where viscous drag from the background plasma overcomes the electromagnetic torque and restores fast plasma rotation and H-mode. Additional current drive is shown to further reduce the size of the mode until full suppression is achieved. These results suggest that the suppression of locked modes may be aided by the lower bootstrap current in L-mode plasmas, possibly improving chances of disruption avoidance in ITER.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Deconvolving the roles of E × B shear and pedestal structure in the energy confinement quality of super H-mode experiements

Analysis of “super H-mode” experiments on DIII-D has put forward that high plasma toroidal rotation, not high pedestal, plays the essential role in achieving energy confinement quality H 98y2 >>1. Recently, super H-mode experiments with variable input torque have confirmed that high rotation shear discharges have very high levels of H 98y2 (>1.5), independent of the pedestal height, and that high pedestal discharges with low rotation shear have levels of H 98y2 only slightly above 1 (≤1.2). Although some increase in stored energy with higher pedestal occurs, the energy confinement quality mainly depends on the toroidal rotation shear, which varies according to different levels of injected neutral beam torque per particle. Here, quasi-linear gyrofluid modeling achieves a good match of the experiment when including the E×B shear; without including plasma rotation, the modeling predicts a confinement quality consistent with the empirical observation of H 98y2 ~1.2 at low rotation. Nonlinear gyrokinetic transport modeling shows that the effect of E×B turbulence stabilization is far larger than other mechanisms, such as the so-called hot-ion stabilization (T i /T e ) effect. Consistent with these experimental and modeling results are previous simulations of the ITER Baseline Scenario using a super H-mode pedestal solution, which showed the potential to exceed the Q=10 target if the pedestal density could be increased above the Greenwald limit. A close look at these simulations reveals that the predicted energy confinement quality is below 1 even at the highest pedestal pressure. The improvement in Q at higher pedestal density is due to the improved fusion power generation at the higher core density associated with higher pedestal density, not to an improved energy confinement quality.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

DIII-D research to provide solutions for ITER and fusion energy

The DIII-D tokamak has elucidated crucial physics and developed projectable solutions for ITER and fusion power plants in the key areas of core performance, boundary heat and particle transport, and integrated scenario operation, with closing the core-edge integration knowledge gap being the overarching mission. New experimental validation of high-fidelity, multi-channel, non-linear gyrokinetic turbulent transport models for ITER provides strong confidence it will achieve Q ≥ 10 operation. Experiments identify options for easing H-mode access in hydrogen, and give new insight into the isotopic dependence of transport and confinement. Analysis of 2,1 islands in unoptimized low-torque IBS demonstration discharges suggests their onset time occurs randomly in the constant β phase, most often triggered by non-linear 3-wave coupling, thus identifying an NTM seeding mechanism to avoid. Pure deuterium SPI for disruption mitigation is shown to provide favorable slow cooling, but poor core assimilation, suggesting paths for improved SPI on ITER. At the boundary, measured neutral density and ionization source fluxes are strongly poloidally asymmetric, implying a 2D treatment is needed to model pedestal fuelling. Detailed measurements of pedestal and SOL quantities and impurity charge state radiation in detached divertors has validated edge fluid modelling and new self-consistent 'pedestal-to-divertor' integrated modeling that can be used to optimize reactors. New feedback adaptive ELM control minimizes confinement reduction, and RMP ELM suppression with sustained high core performance was obtained for the first time with the outer strike point in a W-coated, compact and unpumped small-angle slot divertor. Advances have been made in integrated operational scenarios for ITER and power plants. Wide pedestal intrinsically ELM-free QH-modes are produced with more reactor-relevant conditions, Low torque IBS with W-equivalent radiators can exhibit predator-prey oscillations in Te and radiation which need control. High-β P scenarios with q min > 2, q 95 –7.9, β N > 4, β T –3.3% and H 98y2 > 1.5 are sustained with high density ($\overline{n}$ = 7E19 m -3 , f G –1) for 6 τ E , improving confidence in steady-state tokamak reactors. Diverted NT plasmas achieve high core performance with a non-ELMing edge, offering a possible highly attractive core-edge integration solution for reactors.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Error field detection and correction studies towards ITER operation

In magnetic fusion devices, error field (EF) sources, spurious magnetic field perturbations, need to be identified and corrected for safe and stable (disruption-free) tokamak operation. Within Work Package Tokamak Exploitation RT04, a series of studies have been carried out to test the portability of the novel non-disruptive method, designed and tested in DIII-D (Paz-Soldan et al 2022 Nucl. Fusion62 126007), and to perform an assessment of model-based EF control strategies towards their applicability in ITER. In this paper, the lessons learned, the physical mechanism behind the magnetic island healing, which relies on enhanced viscous torque that acts against the static electro-magnetic torque, and the main control achievements are reported, together with the first design of the asynchronous EF correction current/density controller for ITER.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

First tungsten radiation studies in DIII-D’s ITER baseline demonstration discharges

Abstract ITER Baseline Scenario plasmas were studied in DIII-D using krypton and xenon gases as a proxy for the tungsten that will be present in ITER. These impurities were chosen for having the same radiative loss rate Lz as tungsten would exhibit in the hotter ITER core. Results show that the scenario with these core radiators spans the range of impurity concentration and W radiated fraction expected for ITER, and up to 50% higher values, explored at zero injected torque, as well as 1 Nm and full co-torque injection with T ∼ 3 Nm. Stationary discharges with duration >2–4 τ R are achieved with f rad ⩾ 30% leading to a reduction in confinement of ∼10%, and a comparison with real metal radiators in the same range of f rad shows that the higher Lz at the lower temperatures in these plasmas yields too pessimistic results on the survivability and performance of this scenario in ITER. Simulations of ITER power balance including W radiation show that with concentration up to three times higher than in the DIII-D plasmas the scenario can be stationary, remaining at acceptable core radiated fraction values.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Quantification of locked mode instability triggered by a change in confinement

Abstract This work presents the first analysis of the disruptive locked mode (LM) triggered by the dynamics of a confinement change. It shows that, under certain conditions, the LM threshold during the transient is significantly lower than expected from steady states. We investigate the sensitivity to a controlled n = 1 error field (EF) activated prior to the L-H transition in the COMPASS tokamak, at q 95 ∼ 3, β N ∼ 1, and using EF coils on the high-field side of the vessel. A threshold for EF penetration subsequent to the L-H transition is identified, which shows no significant trend with density or applied torque, and is an apparent consequence of the reduced intrinsic rotation of the 2/1 mode during this transient phase. This finding challenges the assumption made in theoretical and empirical works that natural mode rotation can be predicted by global plasma parameters and urges against using any parametric EF penetration scaling derived from steady-state experiments to define the EF correction strategy in the entire discharge. Furthermore, even at EFs below the identified penetration threshold, disruptive locking of sawtooth-seeded 2/1 tearing modes is observed after about 30% of L-H transitions without external torque.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2026

Density limit in peeling-limited pedestals at and above the Greenwald value in DIII-D high poloidal beta plasmas

High pressure, peeling limited pedestals with pedestal normalized beta β N,ped >2 and pedestal top density n ped at or above the Greenwald density nG have been achieved in DIII-D high poloidal beta plasmas, with high global normalized beta β N >3 and energy confinement H 98 ~1.2-1.7. Higher β N allows higher pedestal density above the Greenwald value and higher pedestal pressure, even with a low injected torque. MHD modeling confirms that the experimental profiles lie near the peeling-mode unstable boundary with high normalized pressure gradient βMHD and high edge current density. Experimental analysis and stability calculations indicate that the high poloidal beta with strong Shafranov shift, high βMHD and weak/negative magnetic shear improves the pedestal stability by decoupling the peeling and ballooning modes and stabilizing the ballooning modes, thus facilitating access to the second stable region of peeling-ballooning mode. The access to the second stable peeling-ballooning stability region opens Super-H-like channels without extremely strong shaping or strong torque injection. The high-pressure peeling pedestal allows the pedestal density to go beyond the Greenwald limit with strong ExB shear maintained: pedestal pressure increases with pedestal density even when n ped >n G , until reaching the ideal MHD instability boundary, where giant ELMs occur. The giant ELMs are dominated by a strong n=1 component and cause a large reduction of the edge pressure, but a negligible change of the core pressure, consistent with kink/peeling-mode induced instability. The pedestal recovers from the collapse and typically sustains a high baseline density, around the Greenwald limit, during the whole discharge duration. Experiments also found that internal transport barriers and n ped ~0.9nG, peeling limited pedestals could be simultaneously achieved in high β N plasmas, while an internal feedback between ITB strength and pedestal performance is found.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

A digital twin based on OpenFAST linearizations for real-time load and fatigue estimation of land-based turbines

Monitoring a wind turbine requires intensive instrumentation, which would be too cost-prohibitive to deploy to an entire wind plant. This work presents a technique that uses readily available measurements to estimate signals that would otherwise require additional instrumentation. This study presents a digital twin concept with a focus on estimating wind speed, thrust, torque, tower-top position, and loads in the tower using supervisory control and data acquisition (SCADA) measurements. The model combines a linear state-space model obtained using OpenFAST linearizations, a wind speed estimator, and a Kalman filter algorithm that integrates measurements with the state model to perform state estimations. The measurements are: top acceleration, generator torque, pitch, and rotational speed. The article extends previous work that derived the linear state-space model using a different method. The new implementation, based on OpenFAST linearization capability, allows for a systematic extension of the method to more states, inputs, outputs, and to the offshore environment. Results from the two methods are compared, and the validation is made with additional measurements using the GE 1.5-MW turbine located at the National Renewable Energy Laboratory test site. Real-time damage equivalent loads of the tower bottom moment are estimated with an average accuracy of approximately 10%. Overall, the results from this proof of concept are encouraging, and further application of the model will be considered.

17 WIND ENERGY↗
DOE OSTI2022

Lifetime fatigue response due to wake steering on a pair of utility-scale wind turbines

Quantifying the impacts on turbine loads during wind farm control is an important consideration in assessing power production benefits. Wake steering controls aim to improve total wind farm performance by coordinating the control actions of individual turbines, wherein an upstream turbine is intentionally yawed at an offset angle from the measured wind direction. Consequently, this redirects its wake for improved power production and potentially reduces fatigue loads of the downstream turbines. This paper studies the lifetime fatigue loads associated with wake steering by using utility-scale wind turbine experimental data to conduct an analysis on a pair of wind turbines. This study was part of a large field experiment in which a group of five GE 1.5-MW SLE CWE turbines were selected as targets for conducting wake steering research. A standard loads instrumentation package and data acquisition system were installed on two turbines within the cluster to measure turbine fundamental loads. The time-series databases were used to calculate loads statistics as well as short-term and lifetime damage equivalent loads. Fatigue calculations followed the guidance in Annex H of the International Electrotechnical Commission standard 61400-1, Edition 4. Lifetime fatigue calculation results are presented in this analysis; three methods of assessing lifetime fatigue were used to determine percent difference for blade root moments, main shaft moments, main shaft torque, and tower top torque. For all three fatigue treatments, some components’ lifetime fatigue increases for the controlled turbine; however, the downwind turbine experienced a reduction in lifetime fatigue and combined effect for the turbine pair results in a reduction of fatigue when wake steering controls are applied.

17 WIND ENERGY↗
DOE OSTI2021

Ferromagnetic gyroscopes for tests of fundamental physics

Abstract A ferromagnetic gyroscope (FG) is a ferromagnet whose angular momentum is dominated by electron spin polarization and that will process under the action of an external torque, such as that due to a magnetic field. Here we model and analyze FG dynamics and sensitivity, focusing on practical schemes for experimental realization. In the case of a freely floating FG, we model the transition from dynamics dominated by libration in relatively high externally applied magnetic fields, to those dominated by precession at relatively low applied fields. Measurement of the libration frequency enables in situ determination of the magnetic field and a technique to reduce the field below the threshold for which precession dominates the FG dynamics. We note that evidence of gyroscopic behavior is present even at magnetic fields much larger than the threshold field below which precession dominates. We also model the dynamics of an FG levitated above a type-I superconductor via the Meissner effect, and find that for FGs with dimensions larger than about 100 nm the observed precession frequency is reduced compared to that of a freely floating FG. This is due to an effect akin to negative feedback that arises from the distortion of the field from the FG by the superconductor. Finally we assess the sensitivity of an FG levitated above a type-I superconductor to exotic spin-dependent interactions under practical experimental conditions, demonstrating the potential of FGs for tests of fundamental physics.

Physics↗
DOE OSTI2025

Quantum Oscillations in the Heat Capacity of Kondo Insulator YbB 12

Here, we observe magnetic quantum oscillations in the heat capacity of the Kondo insulator YbB 12 . The frequency of these oscillations, 𝐹 = 700 T, agrees with that from magnetoresistance and torque magnetometry experiments for 𝜇 0 ⁢𝐻 >35 T in the Kondo insulating phase. Remarkably, the quantum-oscillation amplitudes in the heat capacity are substantial, with Δ⁢$\tilde{C}/T$ ≈ 0.5 mJ mol −1 K −2 at 0.8 K, accounting for 13% of the known linear heat-capacity coefficient 𝛾. Double-peak structures of quantum-oscillation amplitudes due to the distribution function of fermions were identified and used to determine the value of the effective mass from the heat capacity, which agrees well with that from torque magnetometry. These observations support bulk charge-neutral fermions contributing to the quantum oscillations in YbB 12 .

36 MATERIALS SCIENCE↗
DOE OSTI2020

Crystal orientation dependence of interfacial magnetic anisotropy at heavy-metal/magnetic-garnet interfaces

Ferrimagnetic insulators capped with a heavy metal are becoming an increasingly interesting materials system in spintronics due to their unique ability for electrical manipulation and detection of magnetic states and spin textures via spin-orbit torques. The ability to engineer magnetic anisotropy is a powerful tool for tuning the recently discovered phenomena in these bilayers such as electrical switching or the stabilization of topological magnetic textures. Here, we observe large shifts in the magnetic anisotropy in Tm 3 Fe 5 O 13 and Y 3 Fe 5 O 12 thin films due to heavy-metal capping layers, which strongly depends on the orientation of the substrate and therefore the orientation of the epitaxial films. This work suggests large Rashba spin-orbit coupling at the metal/ferrimagneticinsulator interface, which can be engineered in spintronic devices that utilize spin-orbit torques for electrical control of the magnetization in magnetic insulators.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Nontrivial Fermi surface topology of the kagome superconductor CsV 3 Sb 5 probed by de Haas–van Alphen oscillations

Here, we have investigated the normal state Fermi-surface properties of the kagome superconductor CsV 3 Sb 5 using torque magnetometry with applied fields (H) up to 35 T. The torque signal shows clear de Haas-van Alphen (dHvA) oscillations above 15 T. The oscillations are smooth and consist of seven distinct frequencies with values from similar to ~18 T to 2135 T. The presence of higher frequencies in CsV 3 Sb 5 is further confirmed by carrying out additional measurements using the tunnel diode oscillator technique. All frequencies measured at different tilt angles (θ) of the field direction with respect to the c axis show a 1/cosθ dependence, implying that the Fermi surfaces corresponding to these frequencies are two dimensional (2D). The absence of dHvA oscillations at θ = 90° further supports the presence of 2D Fermi surfaces. The Berry phase (φ) determined from the Landau level fan diagram for all frequencies is similar to ~ 0.4. This value is close to the theoretical value of φ = 0.5 for a nontrivial system, which strongly supports the nontrivial topology of the Fermi surfaces of these frequencies. Several quantities characterizing the Fermi surface are calculated employing the Lifshitz-Kosevich theory. These findings are crucial for exploring the interplay between nontrivial band topology, charge-density wave, and unconventional superconductivity of CsV 3 Sb 5 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Magnetoelastic coupling anisotropy in the Kitaev material α-RuCl 3

Here we show that the Kitaev material α-RuCl 3 is among the most prominent candidates to host a quantum spin-liquid state endowed with fractionalized excitations. Recent experimental and theoretical investigations have separately revealed the importance of both the magnetoelastic coupling and the magnetic anisotropy in dependence of the applied magnetic-field direction. In this combined theoretical and experimental research, we investigate the anisotropic magnetic and magnetoelastic properties for magnetic fields applied along the main crystallographic axes as well as for fields canted out of the honeycomb plane. We found that the magnetostriction anisotropy is unusually large compared to the anisotropy of the magnetization, which is related to the strong magnetoelastic $\tilde{Γ}'$-type coupling in our ab-initio derived model. We observed large, nonsymmetric magnetic anisotropy for magnetic fields canted out of the honeycomb ab plane in opposite directions, namely, toward the +c* or -c* axes, respectively. The observed directional anisotropy is explained by considering the relative orientation of the magnetic field with respect to the coaligned RuCl 6 octahedra. Magnetostriction measurements in canted fields support this nonsymmetric magnetic anisotropy; however, these experiments are affected by magnetic torque effects. Comparison of theoretical predictions with experimental findings allow us to recognize the significant contribution of torque effects in experimental setups where α-RuCl 3 is placed in canted magnetic fields.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Effect of the Coulomb force on fission fragment angular momenta

Nuclear fission produces fragments endowed with typically half a dozen units of angular momentum. After scission has occurred, the fragments are still interacting via the Coulomb force which exerts a torque on deformed fragments, thereby accelerating their rotations. Here, the essential features of this effect can be understood by simple perturbative considerations from which the effect on the fragment spin distributions can be obtained. Complete dynamical calculations of the coupled evolution of the spins validate the perturbative treatment, which ignores the second-order terms, showing that the fragment spins evolve essentially independently in the presence of the Coulomb torques.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗