Current Ramp Research Articles

Laser absorption tomography for ammonia measurement in diesel engine exhaust

Tomographic measurements of ammonia concentration in diesel engine exhaust dosed with diesel exhaust fluid are demonstrated through spatial scanning of a 10.4 μm quantum cascade laser beam. Optical access through the exhaust pipe was facilitated by a polyimide film. The laser beam was translated in 0.5 cm increments and rotated in 6 degree angular increments, resulting in 2130 unique laser beam positions per experiment. The data was then used to reconstruct ammonia concentration images through an inverse Radon transform. The laser was driven by a current ramp at 200 Hz and phase-locked with the doser. Data were collected for a duration of 500 ms at each laser beam position. Each laser current ramp was used to create one image, resulting in a total of 100 images per experiment. The images were compiled into a 500 ms-duration video showing the evolution of the phase-averaged ammonia concentration field, assuming a repeatable periodic concentration field with respect to each doser pulse. The experiments were performed for the exhaust temperature range of 453–574 K, nominal pipe diameters of 28 cm and 33 cm, and for different mixers to observe the ammonia concentration distribution upstream of an SCR catalyst. The spatially averaged ammonia concentration was within 10% of average values simultaneously obtained from an FTIR system.

A Coupled-Inductor-Based Bidirectional Circuit Breaker for DC Microgrid

DC microgrids have attracted more and more attention with its universality, high efficiency, and potential application market. However, dc fault protection is still a challenge to the development of dc microgrids. In this article, a novel bidirectional dc solid-state circuit breaker based on coupled inductor is introduced. It can realize bidirectional energy flow and bidirectional isolation and interruption of short-circuit faults with fewer components. The working principle of the topology and the working process under different conditions are described, and some properties of the novel circuit breaker are explored, including minimum fault current ramp rate and load step analysis, which provide a reference for the selection of component parameters. Finally, the feasibility and performance of the novel bidirectional circuit breaker is evaluated by simulations and experiments.

Vanadium oxide coatings to self-regulate current sharing in high-temperature superconducting cables and magnets

High-temperature superconductors such as REBa2Cu3O7 − δ (REBCO, RE = rare earth) enable high-current cables and high-field magnets. By removing the turn-to-turn insulation in a magnet application, recent experiments demonstrated that REBCO magnets can self-protect against catastrophic damage during a superconducting-to-normal transition (quench), i.e., when the stored magnetic energy rapidly converts to heat. The current can bypass the hot spot during a quench, thereby reducing the localized heat dissipation. The removal of the insulation between turns, however, leads to excessive eddy currents during current ramping, thereby forcing a much-prolonged magnet charging time. To address this issue, we investigate vanadium oxide (VOx) coatings as a temperature-dependent self-switching medium that automatically manages current sharing. VOx coatings (with 1.70 ≤ × ≤ to 2.07) were deposited by reactive cathodic arc deposition, initially on insulating glass to determine the electrical properties, and later on commercial REBCO tapes. The coatings are x-ray amorphous but with a short-range crystalline ordering according to Raman spectrometry. The resistivity of VOx decreased by at least three orders of magnitude when the temperature increased from 80 to 300 K. The coating process is compatible with commercial REBCO tapes as evidenced by the negligible change in the critical current caused by the coating process. The results from current sharing experiments and circuit analysis suggest that the VOx coating can effectively self-regulate current sharing in REBCO magnets, suppress excessive eddy currents, and enable self-protection during quenches.

AC Loss and Ramp Up Related Heating Effects in Superconducting MgB2 Coils

Superconducting coils modeled by using single core MgB2 wires having copper or iron or both in the cross section as sheath materials have been examined numerically by using the magnetic field formulation and heat transfer modules of COMSOL Multiphysics software. Heating effects due to the core ac losses and skin effects in MgB2 coils are revealed for alternating currents in a liquid helium environment. Temperature changes, magnetic flux density profiles, core ac losses for each coil are obtained in one full cycle of applied alternating current. The effect of copper layer thickness on core ac loss behavior of MgB2/Cu coil and transport losses in MgB2/Cu and MgB2/Fe coils as a function of current ramp rate are investigated for an applied direct current up to 300 A.

CORC cable terminations with integrated Hall arrays for quench detection

ReBCO superconducting cables have the potential to enable compact thermonuclear fusion reactors that operate at magnetic fields exceeding 20 T and allow operation at temperatures far exceeding the boiling point of liquid helium, potentially allowing for demountable magnets. Normal zone detection remains a challenge, and while novel quench detection techniques are an active area of research, few are non-invasive, provide real-time quench detection, and have been demonstrated with current ramp rates relevant for fusion reactors. To address this problem, a CORC cable termination is developed with integrated Hall sensors to monitor current redistribution as a proxy for quench detection. The methodology exploits the current sharing and layered topology in CORC cables, and allows quench detection using a localized sensor instead of co-wound voltage wires or optical fibers. Experiments are presented where current redistribution is measured from induced quenches, and in a 0.2 meter CORC sample it is found that the Hall sensors detect normal zone transitions with a similar magnitude and temporal resolution as voltage measurements. To emulate the conditions of dynamic poloidal and central solenoidal fields, experiments are repeated with ramp rates up to 10 kA s−1 that demonstrate the potential to detect normal zone development over a range of experimental parameters.

Effect of fuel isotope mass on q-profile formation in JET hybrid plasmas

The initial current ramp phase of JET hybrid plasmas is used to optimise the target q-profile for main heating to allow access to high β and avoid MHD instabilities. Mixed protium-deuterium experiments, carried out at JET since the installation of the beryllium-tungsten wall, have shown that the q-profile evolution during this Ohmic phase varies systematically with average main ion isotope mass, indicating the need for re-optimisation for future T and mixed D-T experiments. Current diffusion modelling shows that the key factor was a reduction in electron temperature profile peaking as the hydrogenic isotope mass was increased. This was correlated with an increase in plasma radiation by metallic impurities, consistent with the increased sputtering yield by higher mass isotopes during the current ramp phase. Reduced electron temperature peaking can lead to magnetic shear reversal and the appearance of a 2/1 mode, which can lock, causing the JET massive gas injection system to be triggered to avoid an unmitigated disruption. The potential for a further reduction in electron temperature peaking in T and D-T plasmas could, therefore, result in an increased risk of disruptions. To mitigate this risk, electron temperature peaking measurements have been included in the real-time control system to allow this type of disruption to be avoided by central heating, density increase or early pulse termination. These experiments indicate the need for integrated modelling of impurity behaviour, including the plasma core, scrape-off layer and plasma wall interactions, to predict q-profile evolution in the current ramp phase and anticipate the effects of isotope changes.

CORC® wires containing integrated optical fibers for temperature and strain monitoring and voltage wires for reliable quench detection

Safe operation of large superconducting magnets wound from high-temperature superconductors (HTS) requires reliable detection of the onset of a quench. A novel method to integrate optical fibers and voltage wires within the core of multi-tape HTS CORC® wires has been developed that allows real time monitoring of local changes in strain, temperature and of the superconducting state of the magnet windings. The ability to detect highly localized changes in temperature with Rayleigh scattering in the embedded optical fibers provides invaluable information about local heating at hot spots from which a quench may originate. Integrated voltage contacts allow accurate voltage measurements in long CORC® wires without being affected by high current ramp rates or electromagnetic interference. They also allow detection of inductively driven redistribution of current between tapes in CORC® wires that may occur at high current ramp rates. Continuous monitoring of temperature and voltage was used to detect the formation of local hot spots induced by a heater or by operating the CORC® wire above its critical current. The results show that, within the boundary conditions of the experiment and the method by which the optical fibers were integrated into the CORC® wire in this study, the speed and resolution with which hot spots can be detected with optical fibers lagged that of the integrated voltage wires. This study also shows that integrated voltage wires reliably detected the formation of a local hot spot in a 5.1 meter long coiled CORC® wire, down to a hot spot size covering 0.1% of the conductor length and at current ramp rates as high as 2000 A s−1. Voltage measurements thus remain an effective option for quench detection in magnets wound with HTS conductors for which current sharing between tapes allows for operation within the flux flow regime.

Dynamic characteristics of PdCoO2/β-Ga2O3 Schottky junctions

A high-frequency diode is an essential component in electrical circuits providing the current rectification function for AC/DC converters, radio frequency detectors, and automotive inverters. Schottky barrier diodes based on wide-bandgap semiconductors are promising for the high-frequency applications owing to short reverse recovery time that minimizes the energy dissipation during the switching. In this study, we report dynamic characteristics of Schottky junctions composed of a layered oxide metal PdCoO2 and an n-type β-Ga2O3 substrate. Rectifying current–voltage characteristics with reasonably small hysteresis were demonstrated up to a high frequency of 3 MHz in the PdCoO2/β-Ga2O3 Schottky junctions. For the on-state to off-state switching with the current ramp rate of approximately −2 × 1010 A/scm2, the reverse recovery time was as short as 11 ns. The short reverse recovery time was constantly obtained in the operation temperature range of 25–350 °C, showing low-loss switching properties of the PdCoO2/β-Ga2O3 Schottky junctions. The Schottky barrier height of ∼1.78 eV and the ideality factor of ∼1.06 were maintained after the 108-times on–off switching cycles. The fast switching with less energy dissipation and high durability of the PdCoO2/β-Ga2O3 Schottky junctions would be suitable for application in high-frequency power devices operating at high temperature.

Electromagnetic behaviour and thermal stability of a conduction-cooled, no-insulated 2G-HTS coil at intermediate temperatures

The electromagnetic and thermal properties of a double pancake coil made of second generation high temperature superconductor, 2G-HTS, have been studied. The coil was wound with no-insulation between turns (NI coil) and was later impregnated with epoxy resin and glued to a copper support plate. The coil was thermally anchored to the cryocooler cold finger and cooled by conduction. After several thermal cycles no degradation of its superconducting properties was observed.The coil was operated under high vacuum and high currents (up to 400 A in steady conditions) at different temperatures in the range between 5 K and 77 K, with special focus on the analysis above 30 K. The charge and discharge characteristics, and the experimentally measured and numerically estimated critical currents, have been studied. The different loss contributions during current ramp and the thermal contact conductance between different parts of the double pancake coil have been measured. The implications of these two factors on the thermal stability and the behaviour of the whole cryogenic system are discussed.

Energy storage sizing for grid compatibility of intermittent renewable resources: A California case study

High levels of intermittent renewable sources will lead to large swings in demand for other generation resources, increasing the risk of overgeneration. Rooftop solar installations exacerbate the potential issues as well. Energy storage systems can mitigate these problems but need to be properly sized to reach network wide goals.This paper presents a method to estimate the necessary energy capacity and power for storage systems to align intermittent resources with network ramp-rate limitations. Case studies for the California Independent System Operator in 2017 and projecting to the renewable portfolio standard target of 60% in 2030 are presented. The effect of curtailment on storage requirements is also analyzed. Lastly, the impact of behind-the-meter solar installations in the Los Angeles area is estimated. These analyses show that significant amounts of storage and some curtailment will be necessary to reach the RPS targets given current network ramping limits unless other dispatchable renewable resources are deployed. This is the first step to develop grid-level planning tools by estimating storage needs with consideration of the demand, renewable generation characteristics, and curtailment levels.

Microstructural evolution of 3YSZ flash‐sintered with current ramp control

AbstractThe effect of a controlled current ramp during flash sintering (FS) on the densification and microstructural evolution of 3 mol% yttria‐stabilized zirconia was investigated. The samples were flash sintered using a current ramp control with six different current ramp rates and compared with samples sintered by FS without current ramp control. In both cases, maximum electric current densities of 100 and 200 mA mm−2 were used. The microstructure of cylindrical samples was observed, showing grain size heterogeneity between the curved surface and the core for the flash‐sintered (FSed) samples regardless of the maximum current density used. By contrast, the current ramp FSed samples exhibited a homogeneous grain size when the electric current density of 100 mA mm−2 was applied. Thus, controlling a current ramp during FS can be an alternative for avoiding grain size heterogeneity on ceramics sintered by this technique.

Первые наблюдения альфвеновских каскадов на токамаке Глобус-М2 и их применение для анализа минимума запаса устойчивости

On the spherical tokamak Globus-M2 in discharges with toroidal magnetic field B= 0.7 T by means of magnetic probes and a number of other diagnostics during neutral beam injection at the current ramp up stage, magnetic field oscillations in the range of 100-300 kHz having their frequency increasing in time, had been observed. Eigenmodes with wavenumbers $n = 1-3$ (toroidal number) and $m = 2-4$ (poloidal number) were being registered. These oscillations were identified as Alfven cascades. By means of multi-channel fluctuation reflectometer, observed eigenmodes were found to be localized near magnetic shear reversal radius. Application of MHD-spectroscopy technique allowed us to determine safety factor $q_{min}$ temporal evolution and experimental values are in well agreement with modelling results, provided by ASTRA transport code.

Reproducibility of the field homogeneity of a metal-clad no-insulation all-REBCO magnet with a multi-layer ferromagnetic shim

We report that the field homogeneity of a large-scale metal-clad no-insulation all-ReBa2Cu3O7−x magnet is reproducible after occasional operating conditions, in this case the current ramp down/up and magnet warm-up/cool-down conditions. First, we shimmed the magnet by cylindrically arraying ferromagnetic shims on the wall of the room-temperature bore of the magnet to improve the spatial field homogeneity. As a result, the field homogeneity of the magnet was improved from 557.7 to 14.1 ppm in a 10 mm diameter spherical volume. The resultant acquisition of field homogeneity tremendously enhanced from an inhomogeneous state greatly degraded by screening currents qualified us to study the temporal stability of the field homogeneity. We used a magnet-energizing protocol with the field drift minimized to shrink a nuclear magnetic resonance (NMR) lineshape into a much narrower super-imposable lineshape. We demonstrate the technique by NMR experiments, showing excellent reproducibility in every case, presenting strong evidence of recovery to certain time-unvarying field gradients and to a state of homogeneity. These findings suggest a new route for high-resolution high-temperature superconducting NMR and magnetic-resonance-imaging magnet technology.

Temperature Sensing Characteristics and Long Term Stability of Power LEDs Used for Voltage vs. Junction Temperature Measurements and Related Procedure

A detailed study about the direct measurement of junction temperature Ti of off-the-shelf power light emitting diodes (LED) is presented. The linear dependence on temperature of the voltage drop across the device terminals at a constant current is in particular exploited and fully characterized, in the temperature range from T = 35 °C to 135 °C, with tests repeated at one thousand different probe currents. The accurate experimental data, obtained on several LED samples bearing two different part numbers, are reported, showing that they exhibit a high degree of linearity in wide current ranges, a circumstance that allows for a fast and reliable calibration as sensors. The measurement error is also fully characterized in terms of repeatability and stability over time, with measurements repeated after 600, 900, 1200 and 1800 hours of applied electro-thermal stress, demonstrating that the relevant sensor parameters stabilize after a few hundred hours of operation. A full set of parameters is provided for the two device models, allowing the direct use of each LED for the self-monitoring of the junction temperature and ensure compliance with their safe operating area over time. Moreover, a procedure and a simple circuit for the real time measurement of Ti, while the LED is on, are presented. The procedure does not require a stable current source, and relies instead on the application of a sub-threshold current ramp for such a short time that the change in the output light is not perceived by human eyes.

Discharge simulation and volt-second consumption analysis during ramp-up on the CFETR tokamak**Project supported by the National Key Research and Development Program of China (Grant Nos. 2017YFE0300500 and 2017YFE0300501), the National Natural Science Foundation of China (Grant Nos. 11875290 and 11875253), and the Fundamental Research Funds for the Central Universities of China (Grant No. WK3420000004).

The plasma current ramp-up is an important process for tokamak discharge, which directly affects the quality of the plasma and the system resources such as volt-second consumption and plasma current profile. The China Fusion Engineering Test Reactor (CFETR) ramp-up discharge is predicted with the tokamak simulation code (TSC). The main plasma parameters, the plasma configuration evolution and coil current evolution are given out. At the same time, the volt-second consumption during CFETR ramp-up is analyzed for different plasma shaping times and different plasma current ramp rates dIP/dt with/without assisted heating. The results show that the earlier shaping time and the faster plasma current ramp rate with auxiliary heating will enable the volt-second to save 5%–10%. At the same time, the system ability to provide the volt-second is probably 470 V · s. These simulations will give some reference to engineering design for CFETR to some degree.

Impact of Flux Jumps on High-Precision Powering of Nb3Sn Superconducting Magnets

Nb3Sn superconducting magnets represent a technology enabler for future high-energy particle accelerators. A possible impediment, though, comes from flux jumps that, so far, could not be avoided by design unlike for NbTi technology. However, the impact of flux jumps on the magnet powering has not been properly investigated to date. Flux jumps appear during current ramps at relatively low value of current and tend to disappear towards nominal current. They are usually detected as voltage jumps between different magnet coils but they might also produce overall voltage jumps across the magnet electrical terminals. Such jumps might perturb the power converter feedback control loop and therefore potentially jeopardize its precision performance during energy ramps. This work aims at: (i) presenting preliminary experimental test results on some HL-LHC Nb3Sn model and prototype magnets, and (ii) attempting to build a simplified electrical model of the flux jumps, with focus only at its interaction with the power converter feedback control loop. Such a work is a starting point for outlining possible power converters control strategies able to minimize flux jumps impact on high-precision powering of Nb3 Sn superconducting magnets.

Simulation of disruptions triggered by Vertical Displacement Events (VDE) in tokamak and leading edge effect in plasma energy deposition to material surfaces

The paper describes two major non-linear properties of the vertical instability of a tokamak plasma, which has a vertical elongation: (a) inductive excitation of surface (edge) currents stabilizing the instability and converting it into fast equilibrium evolution, and (b) creation of a wetting zone without the normal component of the magnetic field when the plasma has contact with material surfaces. Two major disruption effects for both mitigated and non-mitigated disruptions, important for JET and ITER, were considered: (a) excitation of vertical disruption during the current quench (i.e., abnormal plasma current ramp down) and (b) related to the wetting zone, potential leading edge effect in plasma energy deposition to the in-vessel tiles during disruptions. Our considerations together with a 2-D version of the VDE (Vertical Disruption Event) code are based on a new mathematical model, called Tokamak MHD (TMHD), as a replacement for the conventional model, a model that cannot solve numerical problems related to extreme plasma anisotropy and negligible mass. The code includes a 3-D model of surface currents on a thin conductive wall and has a well-specified algorithm for extension to vertical disruptions that excite asymmetric kink modes.

Finite element analysis of hydrogen retention in ITER plasma facing components using FESTIM

Abstract The behaviour of hydrogen isotopes in ITER monoblocks was studied using the code FESTIM (Finite Element Simulation of Tritium In Materials) which is introduced in this publication. FESTIM has been validated by reproducing experimental data and the Method of Manufactured Solutions was used for analytical verification. Following relevant plasma scenarios, both transient heat transfer and hydrogen isotopes (HIs) diffusion have been simulated in order to assess HIs retention in monoblocks. Relevant materials properties have been used. Each plasma cycle is composed of a current ramp up, a current plateau, a current ramp down and a resting phase before the following shot. 100 cycles are simulated. The total HIs inventory in the tokamak during resting phases reaches 1.8 × 10 − 3 mg whereas during the implantation phases it keeps increasing as a power law of time. Particle flux on the cooling channel of the monoblock is also computed. The breakthrough time is estimated to be t = 1 × 10 5 s which corresponds to 24 cycles. Relevance of 2D modelling has been demonstrated by comparing the total HIs inventory obtained by 2D and 1D simulations. Using 1D simulations, a relative error is observed compared to 2D simulations which can reach -25% during the resting phase. The error during implantation phases keeps increasing.

Current caring capacity of bare REBCO tapes at high ramp rates at 77 K and 4.2 K.

In this paper we present the results of an experimental investigation of single 12 mm REBCO tapes as well as soft-soldered 5-folded REBCO stacks at current ramp rates up to 320 kA/s. Experimental ring-samples made of one or five HTS tapes with joints were manufactured and charged in external magnetic field. The experiments performed at 77 K showed that the current caring capacity of the samples is very weakly affected by transport current ramp rate. No transitions to the normal state were observed in liquid nitrogen at external magnetic field ramp rates up to 8 T/s. The maximum transport currents achieved by the samples were limited by the thermal equilibrium with the coolant. In contrast, at 4.2 K both single and stacked samples demonstrated preliminary quenches caused by the origination of local defects in the superconducting layers of the HTS tapes with resistances of about 1 µOhm at 4.2 K. Reexamination of the damaged samples at 77 K showed that at ramp rates higher than 200 kA/s the currents of the thermal equilibrium with the coolant appeared to be the same before and after the defects appearance.

Modeling of Dynamic Current Distribution in REBCO Insulated Coils Using a Volume Integral Formulation for Protection Purpose

Dynamic current distribution in HTS coils submitted to current ramps induces a varying inductive voltage, which can be problematic for safe quench detection as its order of magnitude may hide the increasing dissipative voltage appearing at the beginning of a transition. In order to estimate the transient voltage component, which depends on the coil geometry as well as the current profile and the coil magnetization state, we developed a model using a Volume Integral formulation based on a generalization of the Partial Element Equivalent Circuit method. The objective is to split the coil's voltage end-to-end value into an inductive and a dissipative component so as to evaluate a safe voltage threshold value to implement in the detection system, enabling early transition detection to prevent the coil from major damages. The model is compared to experimental data and its application to coil's protection is detailed on a small-scale test coil.