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At least 1,603 records · Page 89

All sources NASA NTRS DOE OSTI

DOE OSTI2018

Observation of Efficient Lower Hybrid Current Drive at High Density in Diverted Plasmas on the Alcator C-Mod Tokamak

Here, efficient lower hybrid current drive (LHCD) is demonstrated at densities up to $\bar{n}_e$ ≈ 1.5 x 10 20 m –3 in diverted plasmas on the Alcator C-Mod tokamak by operating at increased plasma current - and therefore reduced Greenwald density fraction. This density exceeds the nominal “LH density limit” at $\bar{n}_e$ ≈ 1.0 x 10 20 m –3 reported previously, above which an anomalous loss of current drive efficiency was observed. The recovery of current drive efficiency to a level consistent with engineering scalings is correlated with a reduction in density shoulders and turbulence levels in the far scrape-off layer. Concurrently, rf wave interaction with the edge and/or scrape-off-layer plasma is reduced, as indicated by a minimal broadening of the wave frequency spectrum measured at the plasma edge. These results have important implications for sustaining steady-state tokamak operation and indicate a pathway forward for implementing efficient LHCD in a reactor.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2020

Modeling and Harmonic Stability of MMC-HVDC With Passive Circulating Current Filters

A modular multilevel converter (MMC) is an emerging converter technology widely used in wind power applications using high-voltage direct current (HVDC) transmission, and it has been largely investigated in medium-voltage solar harvesting as well as electric motor drives. Different from studies dealing with active circulating current control loops, this paper focuses on impedance modeling and harmonic stability studies for MMCs with passive circulating current filters (PCCFs). A power stage circuit including a PCCF is transformed and remodeled to obtain small-signal formulations. This transformation leads to additional high-order matrix computing complexity due to the added impedance subnetwork matrix from the PCCF as well as relevant crossed-frequency impacts. To simplify the computational burden, the proposed model aims to solve one arm equation instead of all six MMC arm equations. To achieve this result, additional challenges occur when the MMC is connected to a renewable energy source instead of the grid voltage, where low-level zero-sequence components might exist in the neutral point of three phases, especially in the case of no integrated advanced modular voltage balancing control, which prevents computational simplification. To overcome this challenge, further impedance matrix adjustments are conducted in this paper to theoretically suppress the impact of zero-sequence harmonics when obtaining small-signal impedance, taking into account the frequency coupling effect. Finally, simulations are carried out to validate the developed impedance model under different operating scenarios. Harmonic stability case studies of MMCs with PCCFs connected to renewable energy current sources are also presented, where frequency-domain stability analyses based on Bode diagrams are compared to time-domain simulation waveforms and FFTs, which validate the effectiveness of the proposed impedance model.

24 POWER TRANSMISSION AND DISTRIBUTION↗

DOE OSTI2017

Investigation of HTS Twisted Stacked-Tape Cable (TSTC) Conductor for High-Field, High-Current Fusion Magnets

Here, the critical currents of a twisted single-tape were measured at 4.2 K in fields up to 17 T. It was confirmed that the critical current of a twisted-tape was similar to that of a flat tape in c-axis fields. Based on the single-tape critical-currents, a 40-tape (4-mm width) twisted stacked-tape cable (TSTC) conductor was experimentally evaluated at fields up to 17 T at 4.2 K. It was found that the Lorentz load degradation of the TSTC conductor was negligible at the end of a cyclic test. The TSTC cabling method has been considered to be suitable for developing a high-field high-current REBCO conductor for magnet applications as well as power transmission cables. A method to fabricate the inner legs of a D-shape toroidal field coil using a TSTC conductor has been discussed. This method allows the conductor to properly resist the transverse Lorentz load and mitigate ac loss issues by adopting a parallel-HTS-tape flat cable configuration where necessary.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2021

Critical Current Distributions of Recent Bi-2212 Round Wires

Bi 2 Sr 2 CaCu 2 O 8+x (Bi-2212) is the only high-field, high-temperature superconductor (HTS) capable of reaching a critical current density J c (16 T, 4.2 K) of 6500 A·mm –2 in the highly desirable round wire (RW) form. However, state-of-the-art Bi-2212 conductors still have a critical current density (J c ) to depairing current density (J d ) ratio around 20 to 30 times lower than that of state-of-the-art Nb–Ti or REBCO. Previously, we have shown that recent improvements in Bi-2212 RW J c are due to improved connectivity associated with optimization of the heat treatment process, and most recently due to a transition to a finer and more uniform powder manufactured by Engi-Mat. One quantitative measure of connectivity may be the critical current (I c ) distribution, since the local I c in a wire can vary along the length due to variable vortex-microstructure interactions and to factors such as filament shape variations, grain-to-grain connectivity variations and blocking secondary phase distributions. Modeling the experimental V-I transition measured on a low resistance shunt as a complex sum of voltage contributions of individual filament and wire sub-sections allows a numerical extraction of the I c distribution from the d 2 V/dI 2 treatment of the V-I curves. Here we compare ~0.1 m length I c distributions of Bi-2212 RWs with recent state-of-the-art very high-J c Engi-Mat powder and lower J c and older Nexans granulate powder. We do find that the I c spread for Bi-2212 wires is about twice the relative standard of high-J c Nb–Ti well below H irr . We do not yet see any obvious contribution of the Bi-2212 anisotropy to the I c distribution and are rather encouraged that these Bi-2212 round wires show relative I c distributions not too far from high-J c Nb–Ti wires.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2022

Simulation for the fault current limiting operation of REBCO CORC superconducting cables with different core materials

With high power density and low loss, CORC superconducting cables composed of RE- Ba 2 Cu 3 O 7–δ (REBCO) coated conductors are of great interest for power transmission applications. They can also effectively be designed to serve as fault current limiters, thanks to their sharp superconducting-to-normal transition. Coupled electromagnetic-thermal finite element (FE) simulations implemented in COMSOL Multiphysics were developed and validated against the published results for the fault current limiting performance of two CORC cables of REBCO coasted conductors wound on either copper or stainless steel cores. For improved accuracy, temperature dependencies of electrical and thermal properties of all component materials were considered in the simulations. The simulations were performed in the cross-sections of the cables for every individual layer of each REBCO tape to deliver comprehensive understanding of evolution of the current distribution and temperature rise in those layers. Furthermore, the studies can suggest approaches to optimize cable design for fault current limiting (FCL) applications by assessing the role of individual components. In the paper, possible computational errors, challenges and improvements are also discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2016

Load-Dependent Soft-Switching Method of Half-Bridge Current Doubler for High-Voltage Point-of-Load Converter in Data Center Power Supplies

With the increased cloud computing and digital information storage, the energy requirement of data centers keeps increasing. A high-voltage point of load (HV POL) with an input series output parallel structure is proposed to convert 400 to 1 VDC within a single stage to increase the power conversion efficiency. The symmetrical controlled half-bridge current doubler is selected as the converter topology in the HV POL. A load-dependent soft-switching method has been proposed with an auxiliary circuit that includes inductor, diode, and MOSFETs so that the hard-switching issue of typical symmetrical controlled half-bridge converters is resolved. The operation principles of the proposed soft-switching half-bridge current doubler have been analyzed in detail. Then, the necessity of adjusting the timing with the loading in the proposed method is analyzed based on losses, and a controller is designed to realize the load-dependent operation. A lossless RCD current sensing method is used to sense the output inductor current value in the proposed load-dependent operation. In conclusion, experimental efficiency of a hardware prototype is provided to show that the proposed method can increase the converter's efficiency in both heavy- and light-load conditions.

24 POWER TRANSMISSION AND DISTRIBUTION↗

DOE OSTI2019

Testing and Assessment of EMFs and Touch Currents from 25-kW IPT System for Medium-Duty EVs

Inductive power transfer (IPT) technology has gained massive attention for electric vehicle charging due to the associated reliability, convenience, and safety. Nevertheless, the loose coupling between the transmitter and receiver generates intensive electromagnetic fields (EMFs) around the system, which may influence living objects' safety if the typical limits are exceeded. Moreover, these fields induce voltages in the vehicle body, which may lead to touch current passing through the human body when it gets in contact with two metal parts or one part and the ground. Following the international standards and recommendations, this study presents comprehensive test guidelines for the near-EMFs and touch currents from a commercial 25-kW wireless charger from Momentum Dynamics, which was installed in an electric shuttle operating at the National Renewable Energy Laboratory (NREL) campus. The paper investigates the measuring devices, test setup, test points and standard limits. The impact of the misalignments on the measurements is considered during testing. EMFs are measured in areas around and inside the shuttle and near the power supply and compared with the reference levels defined by the international standards, including ICNIRP 2010, IEEE C95.1 and ACGIH TLV 2017. Touch currents are evaluated among different metal parts and ground in comparison with the standard limits. The results show that both EMFs and touch currents comply with the standard limits. Moreover, thermal inspection for the system after installation was provided. Finally, a 5-day of operation data for the wireless charger are collected and presented.

33 ADVANCED PROPULSION SYSTEMS↗

DOE OSTI2014

Distribution of ion current density on a rotating spherical cap substrate during ion-assisted deposition

The uniformity of ion density is critical for applications relying on the ion assisted deposition technique for the fabrication of the high quality thin films. The authors propose and describe here a method allowing one to calculate the ion density distribution on spherical substrate holders under stationary and rotating conditions for different positions of the ion source. The ion beam shape was approximated by a cos{sup n} function, and the ion current density was represented by a function inversely proportional to the distance from the ion source in accordance with our experimental results. As an example, a calculation of the current density distribution on the spherical cap substrate was performed for a broad beam ion source operated with an anode current of 3 A. The authors propose an approach for process optimization with respect to the ion source position and its inclination, in terms of uniformity and absolute value of the ion current density.

42 ENGINEERING↗

DOE OSTI2014

Magnetic resonance visualization of conductive structures by sequence-triggered direct currents and spin-echo phase imaging

Purpose: Instrument visualization in interventional magnetic resonance imaging (MRI) is commonly performed via susceptibility artifacts. Unfortunately, this approach suffers from limited conspicuity in inhomogeneous tissue and disturbed spatial encoding. Also, susceptibility artifacts are controllable only by sequence parameters. This work presents the basics of a new visualization method overcoming such problems by applying sequence-triggered direct current (DC) pulses in spin-echo (SE) imaging. SE phase images allow for background free current path localization. Methods: Application of a sequence-triggered DC pulse in SE imaging, e.g., during a time period between radiofrequency excitation and refocusing, results in transient field inhomogeneities. Dependent on the additional z-magnetic field from the DC, a phase offset results despite the refocusing pulse. False spatial encoding is avoided by DC application during periods when read-out or slice-encoding gradients are inactive. A water phantom containing a brass conductor (water equivalent susceptibility) and a titanium needle (serving as susceptibility source) was used to demonstrate the feasibility. Artifact dependence on current strength and orientation was examined. Results: Without DC, the brass conductor was only visible due to its water displacement. The titanium needle showed typical susceptibility artifacts. Applying triggered DC pulses, the phase offset of spins near the conductor appeared. Because SE phase images are homogenous also in regions of persistent field inhomogeneities, the position of the conductor could be determined with high reliability. Artifact characteristic could be easily controlled by amperage leaving sequence parameters unchanged. For an angle of 30° between current and static field visualization was still possible. Conclusions: SE phase images display the position of a conductor carrying pulsed DC free from artifacts caused by persistent field inhomogeneities. Magnitude and phase images are acquired simultaneously under the same conditions without the use of extra measurement time. The presented technique offers many advantages for precise instrument localization in interventional MRI.

62 RADIOLOGY AND NUCLEAR MEDICINE↗

DOE OSTI2013

Noninductive plasma generation and current drive in the Globus-M spherical tokamak

Experimental results on the generation and maintenance of the toroidal current in the Globus-M spherical tokamak by using waves in the lower hybrid frequency range without applying an inductive vortex electric field are presented. For this purpose, the original ridge guide antennas forming a field distribution similar to that produced by multiwaveguide grills were used. The high-frequency field (900 MHz) was used for both plasma generation and current drive. The magnitude of the generated current reached 21 kA, and its direction depended on the direction of the vertical magnetic field. Analysis of the experimental results indicates that the major fraction of the current is carried by the suprathermal electron beam.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2013

Quasi-adiabatic dynamics of ions in a bifurcated current sheet

The study is devoted to ion dynamics in bifurcated current sheets with a two-peak current-density distribution observed in the Earth's magnetotail and solar wind. The ion motion is described by a Hamiltonian system with two degrees of freedom. The presence of a small parameter {kappa} characterizing the ratio between the amplitudes of the normal and tangential magnetic field components allows one to separate variables into fast and slow ones and introduce the quasi-adiabatic invariant of motion I{sub z}. Conservation of this invariant makes it possible to analytically describe the dynamics of charged particles. Deviations of the particle dynamics from the quasi-adiabatic one, which are caused by the nonconservation of the quasi-adiabatic invariant, are investigated. The jump of the invariant {Delta}I{sub z} is shown to depend on the small parameter according to the power-law {Delta}I{sub z} {approx} {kappa}{sup h}, where the exponent h varies between unity and 3/4, depending on the level of current sheet bifurcation. The obtained dependence of {Delta}I{sub z} on {kappa} coincides with analytic expressions in the limiting cases of nonbifurcated and completely bifurcated current sheets.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2013

The analysis of leakage current in MIS Au/SiO{sub 2}/n-GaAs at room temperature

The aim of this study is to determine the reverse-bias leakage current conduction mechanisms in Au/SiO{sub 2}/n-GaAs metal-insulator-semiconductor type Schottky contacts. Reverse-bias current-voltage measurements (I–V) were performed at room temperature. The using of leakage current values in SiO{sub 2} at electric fields of 1.46–3.53 MV/cm, ln(J/E) vs. <##> } E $ /sqrt E $ √E </##> graph showed good linearity. Rom this plot, dielectric constant of SiO{sub 2} was calculated as 3.7 and this value is perfect agreement with 3.9 which is value of SiO{sub 2} dielectric constant. This indicates, Poole-Frenkel type emission mechanism is dominant in this field region. On the other hand, electric fields between 0.06–0.73 and 0.79–1.45 MV/cm, dominant leakage current mechanisms were found as ohmic type conduction and space charge limited conduction, respectively.

36 MATERIALS SCIENCE↗

DOE OSTI2016

Isothermal current–voltage characteristics of high-voltage 4H-SiC junction barrier Schottky rectifiers

The forward-pulse isothermal current–voltage characteristics of 4H-SiC junction barrier Schottky rectifiers (JBSs) with a nominal blocking voltage of 1700 V are measured in the temperature range from–80 to +90°C (193–363 K) up to current densities j of ~5600 A/cm{sup 2} at–80°C and 3000 A/cm{sup 2} at +90°C. In these measurements, the overheating of the structures relative to the ambient temperature, ΔT, did not exceed several degrees. At higher current densities, the effective injection of minority carriers (holes) into the base of the structures is observed, which is accompanied by the appearance of an S-type differential resistance. The pulsed isothermal current–voltage characteristics are also measured at a temperature of 77 K.

36 MATERIALS SCIENCE↗

DOE OSTI2015

Aquantis C-Plane Ocean Current Turbine Project

The Aquantis 2.5 MW Ocean Current Generation Device technology developed by Dehlsen Associates, LLC (DA) is a derivation of wind power generating technology (a means of harnessing a slow moving fluid) adapted to the ocean environment. The Aquantis Project provides an opportunity for accelerated technological development and early commercialization, since it involves the joining of two mature disciplines: ocean engineering and wind turbine design. The Aquantis Current Plane (C-Plane) technology is an ocean current turbine designed to extract kinetic energy from a current flow. The technology is capable of achieving competitively priced, continuous, base-load, and reliable power generation from a source of renewable energy not before possible in this scale or form.

16 TIDAL AND WAVE POWER↗

DOE OSTI2013

Letter Report on 500 nA Pulsed Current from Field Ionization Source

We recently produced a milestone 500 nA of pulsed current using 40 Ir field ionizer electrodes in our ion source. In conclusion, we have produced the milestone pulsed current of 500 nA using 40 electrochemically etched iridium tips in a field ionization source. The pulsed current output is repeatable and scales as expected with gas fill pressure and bias voltage. We expect these current will be sufficient to produce neutral yields of 1 ∙ 10 7 DT n/s.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

NASA NTRS1962

Estimation of Critical Temperature for Surface Ion Currents from Electron Emission Data

A method of calculating a relation for the critical surface temperature for ion current emission is presented. The method is based on the S-shaped electron emission curves for surfaces in the presence of ionizable vapors and upon the assumptions of thermodynamic equilibrium involved in the Saha-Langmuir relation. A comparison of the critical temperatures so calculated with the relation obtained from actual ion emission data on the cesium-tungsten system shows good agreement over a wide current density range. Critical temperatures for cesium ion current densities of 0.21 and 1.9 amperes per square centimeter calculated by this method are presented for surfaces of rhenium, molybdenum, tantalum, and niobium. Tentative relations of the form log j equals A (sub p) plus (B (sub p) over T) are presented for these same systems, where j is the ion current density, A (sub p) and B (sub p) are constants, and T is the temperature.

ELECTRON EMISSION↗

DOE OSTI2025

Per-Phase Control for CHB Converters with Negative-Sequence Current and DC Voltage Balancing Control

This paper presents a per-phase controller for a grid-connected Cascaded H-Bridge (CHB) converter featuring negative sequence current regulation and DC bus voltage balancing capabilities. Unlike conventional three-phase controllers, each phase of the CHB can be separately and controlled to regulate the DC voltage and reactive power flow. per-phase negative sequence current control strategy is incorporated into the proposed controller to ensure compliance with IEEE Standard 2800-2022, which requires the absorption of negative-sequence reactive current during grid voltage disturbances. Furthermore, a feed-forward control-based DC bus voltage balancing method is employed, which eliminates the need for the integrator commonly used in traditional approaches. The performance of the proposed controller is validated under various unbalanced grid scenarios and DC bus voltage imbalances through electromagnetic transient (EMT) simulation using a three-phase, three-level grid-connected CHB–Dual Active Bridge (DAB) converter testbed developed on MATLAB/Simulink. Additionally, the performance of the negative-sequence current regulation is investigated through simulation under different unbalanced grid voltage scenarios.

negative sequence current↗

DOE OSTI2011

Lower hybrid current drive at plasma densities required for thermonuclear reactors

Driving current in high-density plasmas is essential for the progress of thermonuclear fusion energy research based on the tokamak concept. The lower hybrid current drive (LHCD) effect, is potentially the most suitable tool for driving current at large plasma radii, consistent with the needs of ITER steady state scenario. Unfortunately, experiments at reactor grade high plasma densities with kinetic profiles approaching those required for ITER, have shown problems in penetration of the LH power into the core plasma. These plasmas represent a basic reference for designing possible methods useful for assessing the LHCD concept in ITER. On the basis of the phenomenology observed during LHCD experiments carried out in different machines, and model of the spectral broadening effect due to parametric instability, an interpretation and possible solution of the related important problem is presented.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗