Currents In Excess Research Articles

Transient response of 50 KiloAmp Y-Ba-Cu-O rings and ring pairs to pulsed magnetic fields

Shielding current limits and magnetic diffusion characteristics have been measured at 77 K in large-grain YBCO rings with oriented microstructures. The samples are surrounded by a drive coil that can achieve magnetic fields in excess of 1 Tesla and induce currents in excess of 50 kA when driven by a current pulse of a few msec duration. Simultaneous magnetic measurements with a Rogowski coil and a Hall probe allow determination of the induced current in the sample and the field in the center of the sample. These measurements show that field penetration occurs in a complex way that includes delays and transient effects caused by magnetic diffusion and sample heating. Dramatic threshold effects are observed that are probably related to a creep-flow transition coupled with local heating effects. Geometric effects are investigated using a single drive coil and a pair of YBCO rings with various spacings. A test geometry equivalent to a simple penetration-type inductive fault current limiter is used; the experimental results are therefore of interest for design and characterization of these devices.

Progress towards a low-cost commercial coated conductor

ABSTRACTYBCO films prepared from metal trifluoroacetate (TFA) precursors on oxide-buffered textured non-magnetic substrates have achieved performance levels equaling that on oxide buffered textured Ni substrates. Critical current densities of 0.7 MA/cm2 to 1.0 MA/cm2 have been achieved in 0.4 µm thick YBCO films on short-length of CeO2/YSZ/Y2O3/Ni/Ni-13wt%Cr substrates. High-quality epitaxial buffers comprising a Ni layer, Y2O3 seed, YSZ barrier and CeO2 cap layers have been deposited over meter long tapes of deformation textured Ni and Ni-13%Cr using reel-to-reel processes. High-performance TFA-based YBCO films have been deposited on 0.1 m to 0.3 m lengths of these oxide buffered substrates using reel to reel processes. Critical current densities up to 1.0 MA/cm2 have been achieved in 0.4 µm thick YBCO films on CeO2/YSZ/Y2O3/Ni substrates. Using multiple coats of the metal trifluoroacetate precursors, thicker YBCO films have been demonstrated on oxide buffered substrates. Critical currents in excess of 100A/cm-width have been achieved for 1.2 µm -1.6 µm thick YBCO films on short lengths of CeO2/YSZ/Y2O3/Ni substrates.

Self-accelerating chlorine evolution on porous anodes of finite thickness

Operation of a finite-thickness porous electrode under the chlorine evolution conditions is analyzed in the framework of the convective diffusion model. According to calculations, the gas evolution in the anode’s porous coating is a necessary but insufficient condition for the self-acceleration of the electrode process. Despite the gas evolution process in the anode pores, the anodic process on the low-activity electrodes decelerates, which is externally manifested in an increased Tafel slope of the polarization curve as compared with that for a smooth electrode. Self-acceleration of the anodic chlorine evolution takes place only on electrodes with true exchange currents in excess of 10-4 A cm-2. Externally, the self-acceleration effect manifests itself in the emergence of a low-polarizability portion in the high-current region of the polarization curve. Such a different effect of the gas evolution process on the chlorine reaction kinetics at porous electrodes of different catalytic activity is due to an altered balance between the diffusive and convective current constituents in the overall process rate following a change in the exchange current for either electrode.

Performance and pulse shortening effects in a 200-kV PASOTRON/sup TM/ HPM source

The PASOTRON/sup TM/ is a unique, high-power microwave source that uses a long-pulse (/spl les/100 /spl mu/s) plasma-cathode electron-gun and plasma-filled slow-wave structure (SWS) to produce high-energy microwave pulses. The device utilizes no externally-produced magnetic fields; relying on a beam-generated plasma channel in the SWS to transport the beam. Peak powers of up to 5 MW were previously reported in C-band using a rippled-wall waveguide SWS. Scaling experiments indicated that increasing the beam voltage above the 90 kV C-band operation produces significantly higher peak powers. We report results from an L-band PASOTRON/sup TM/ BWO designed to operate at 200 kV. The plasma-cathode E-gun built for this device generated beams with voltages of up to 225 kV and currents in excess of 1 kA for pulse lengths of up to 200 /spl mu/s. The L-band PASOTRON/sup TM/ BWO produced 10-20 MW of peak power in the TM/sub 01/ mode, which was converted in the output to a TE/sub 11/ mode with fixed polarization. The PASOTRON/sup TM/ also directly produced TE-mode radiation in the 5-10 MW power range with a rotating output polarization, the rate of which can be controlled externally. The peak power and poise width was limited by the stability of the plasma channel at high peak powers and excessive plasma generation in the SWS during the long pulse length.

Boron Nitride Emitters

AbstractNew observations are presented on the emission of electrons from n-type boron nitride (BN) cold cathode films. These carbon-doped BN films demonstrate a significant improvement in the electron emission current, on the order of 1 to 3 orders of magnitude, as a function of the extraction field for many different materials and morphologies.These BN cold cathodes have yielded stable DC currents in excess of 4 mA at an extraction field of approximately 30 V/μm.

Photodetector nonlinearity limitations on a high-dynamic range 3 GHz fiber optic link

The performance of a dc to 3 GHz externally modulated link utilizing balanced high-power photodetection is presented. Nonlinearity measurements of high power photodiodes show 1 dB compression currents in excess of 55 mA and an output third-order intercept point of +32 to +34 dBm. These high current photodetectors permit the use of high power lasers as external modulator sources for low noise fiber optic links with only small degradations in the predicted link dynamic range. An externally modulated link with a 240 mW Nd:YAG laser, a dual-output 4-V V, (dc) modulator, and balanced 100 mA total photocurrent yielded a link noise figure from 15.5 to 17.5 dB, a spur-free dynamic range of 119.5 dBHz/sup 2/3/, and a 1-dB compression dynamic range of 168.4 dBHz.

Peak current density and brightness from poly( p-phenylenevinylene) based light-emitting diodes

We report measurements of peak current and brightness from polymer light-emitting diodes based on poly( p-phenylenevinylene). The devices were operated at low duty cycles and supported peak currents in excess of 700 A/cm 2 and produced peak brightnesses of up to 5 Mcd/m2. The values are higher than was previously thought attainable and we discuss them with reference to application in passive matrix displays and polymer laser diodes.

High-brightness photoelectric field-emission cathodes for free-electron laser applications

Field emission from very fine needles in an electric field is characterized by a very small energy spread, less than 1 eV, and emittance of the order of 10 −8π m rad, normalized. In microsecond pulses, total currents in excess of 6 A have been observed, corresponding to normalized brightness of the order of 10 15 A/m 2 str, which is four orders of magnitude higher than conventional sources. Recently, photoemission has been observed from needles operating in fields too small for significant field emission. Total currents as high as 10 A have been observed in nanosecond pulses. The quantum efficiency is of the order of unity. Using e-beams of such high brightness, single-pass FELs can operate at very long and very short wavelengths, with very low total e-beam current. In the FIR, it is possible to construct Cerenkov FELs with enough gain ( e 20) to reach saturation in a single pass using only 10 mA total electron-beam current. In the UV, it is possible to build single-pass Compton-backscatter FELs operating at total current less than 300 mA.

Pulsed currents carried by whistlers. IX. In situ measurements of currents disrupted by plasma erosion

In a magnetized laboratory plasma described in the companion paper [Stenzel and Urrutia, Phys. Plasmas 4, 26 (1997)], a large positive voltage step (V≫kTe/e) is applied to electrodes. The current front propagates in the whistler mode in the parameter regime of electron magnetohydrodynamics. The topology of the current density is that of nested helices. Large transient currents in excess of the electron saturation current can be drawn. A transient radial electric field associated with the current rise, excites a compressional, large amplitude, radially outgoing sound wave, which leaves the current channel depleted of plasma. The current collapses due to the density erosion. Electric field reversal excites a rarefaction wave which leads to a partial density and current recovery. Periodic plasma inflow and outflow cause the current to undergo strong relaxation oscillations at a frequency determined by the electrode diameter and the sound speed. In addition, a broad spectrum of microinstabilities is observed in regions of high current density. For drift velocities approaching the thermal speed, the spectrum extends beyond the ion plasma frequency (ωpi) up to the electron plasma frequency (ωpe). Correlation measurements above ωpi reveal modes propagating along the electron drift at speeds above the sound speed but well below the electron drift speed.

Low-temperature grown GaAs tunnel junctions

A GaAs tunnel junction is formed by molecular beam epitaxy at low substrate temperatures to incorporate excess arsenic, followed by an anneal to precipitate the excess arsenic. This tunnel junction is comparable in resistance and peak current density to tunnel junctions grown stoichiometrically. Owing to the inhomogeneity in this two-phase tunnel junction, there is only a slight indication of a current peak. This lack of a valley in the tunnelling characteristic results in a low voltage drop even for currents in excess of the peak current.

Externally-modulated 3 GHz fibre optic link utilising high current and balanced detection

The performance of a DC to 3 GHz externally modulated link utilising balanced high-power photodetectors is presented. High power photodiodes with 1 dB compression currents in excess of 56 mA are used with a 240 mW NdYAG laser, a dual-output 4 V V/sub x/ modulator, and balanced 100 mA total photocurrent yielding a link gain of 3 to 6 dB and a noise figure of 15.5-17.5 dB.

Acoustically focused oceanographic sampling in the Haro Strait experiment

The tidal fronts of Haro Strait, British Columbia, are associated with currents in excess of 3 kn, with significant horizontal stratification, leading to significant frontal turbulence and upwelling. Volumetric mapping of these fronts using traditional oceanographic sampling techniques is virtually impossible due to the spatial and temporal aliasing produced by the extremely dynamic oceanography. The concept of acoustically focused oceanographic sampling (AFOS) was developed to overcome this problem. AFOS combines the autonomous oceanographic sampling network (AOSN) [Curtin etal ., Oceanography 6(3) (1993)] with acoustic tomography. The network nodes used for communication with and navigation of AUVs is equipped with vertical arrays and tomography sources. In addition an AUV carries a source for adaptive, moving source tomography. The array data are transmitted to shore via a wireless local-area network and processed in real time for estimation of current distribution and frontal geometry. The results are used for adaptively focusing the AUV’s sampling of the front using surface mapping side-scan sonars and acoustic Doppler current profilers, as well as CTDs. [Work supported by ONR.]

New insulated-gate thyristor structure with MOSFET coupled base regions

A new insulated-gate thyristor (IGTH) structure in which the base of n-p-n transistor is coupled to the base of p-n-p transistor through a MOSFET is described for the first time, In the new structure, called base coupled insulated gate thyristor (BC-IGTH), the parasitic lateral p-n-p carrier injection inherent in previously reported thyristor structures such as the MCT, BRT, and IGTH is absent. The absence of parasitic lateral p-n-p carrier injection results in low on state voltage drop and high controllable current capability for this structure. The turn-on process in the new structure is fundamentally different from other MOS-gated thyristor structures in that in the new structure, the higher gain n-p-n transistor is turned-on first, which then provides the base drive for the lower gain p-n-p transistor. Multicellular 800 V devices of the new thyristor structure were fabricated using a double-diffused DMOS process, and were found to give on-state drop of 1.1 V at 200 A/cm/sup 2/, and controllable currents in excess of 100 A/cm/sup 2/ were obtained by forming MOS-gate controlled emitter-to-base resistive shorts.

Processing of thallium-based superconducting tapes for high current density

Optimum thermomechanical processing conditions have been developed to achieve a high current density in powder-in-tube (PIT) thallium-based (Tl,Pb)-1223 and (Tl,Bi)-1223 tapes. Critical currents in excess of 25 A corresponding to a current density of 20 000 A/cm 2 have been achieved in these tapes at 77 K. Heat treatment and intermediate deformation sequences, heating rates, and starting precursor phase assemblages have been examined to optimize the current density. A combination of processing conditions that result in incremental densification, less microcracking and liquid formation in the early and final stages of thermomechanical treatment is found to result in the highest current densities.

Performance of a high current H− radio frequency volume ion source

To evaluate the performance of a high current H− rf-driven volume ion source, the Superconducting Super Collider ion source was modified to accommodate a set of cesium dispensers so that a trace amount of cesium could be introduced into the plasma grid collar. A plasma grid heater element controls the temperature of the cesiated surfaces and the rate of cesium dispensation. With this modification, beam currents in excess of 100 mA and electron to H− current ratios close to one have been observed. In contrast, the uncesiated source provided 30 mA of H− beam with an e/H− ratio ≥25. In both cases the source was operated with a 100 μs beam pulse width at a 10 Hz repetition rate, and an extraction voltage of 35 kV. This cesium enhanced source is a strong candidate for use in high current H− accelerators.

A new Insulated-Gate Thyristor structure with turn-off achieved by controlling the base-resistance

A new Insulated-Gate Thyristor (IGTH) design for achieving high controllable current capability is described. The design consists of square cells with high density of MOS-channels modulating the resistance of the base region of the NPN transistor of the thyristor structure. The diagonal regions between the square cells is used as the turn-on regions while the other regions under the MOS gate between the cell diffusions are connected by P/sup -/ diffusion to obtain a MOS-gate controlled low resistance path for turn-off. The IGTH was fabricated using a double-diffused DMOS process and 1200 V devices with controllable currents in excess of 150 A was obtained. >

Characterization of multifilamentary Nb/sub 3/Sn superconducting wires for use in the 45-T hybrid magnet

The National High Magnetic Field Laboratory (NHMFL), in collaboration with the Francis Bitter National Magnet Laboratory, is constructing a 45-T hybrid magnet system. Here we report the results of studies to characterize candidate Nb/sub 3/Sn superconductor for use in the superconducting outsert coils. We have determined the effects of strain and magnetic field on the critical current, measured ac losses, and measured residual resistivity ratios (RRR). Critical currents in excess of 1000 A mm/sup -2/ non-Cu have been measured in sample wires at 12 T and 4.2 K combined with a hysteresis loss of less than 500 mJ cm/sup -3/ We also present predicted 1.8 K performance, based on empirical models, and present a methodology for calculating sample holder prestrain.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A high current H− radio frequency volume ion source

The superconducting super collider rf-driven volume ion source routinely provided 35 kV, ≳30 mA H− beams with normalized rms emittances (εn-rms) &amp;lt;0.1 π mm mrad. The source was typically operated with a 100 μs beam pulse width at a 10 Hz repetition rate. To enhance H− output, the ion source was modified to accommodate a set of cesium dispensers so that a trace amount of cesium could be introduced into the plasma grid collar. A plasma grid heater element controls the temperature of the cesiated surfaces and the rate of cesium dispensation. Beam currents in excess of 100 mA and electron to H− current ratios close to one have been observed. Emittance measurements at 70 mA suggest a 20% increase over uncesiated emittance values. Improvements in the beam emittance are fully expected when the extraction optics are optimized. This enhanced, very reliable source is a strong candidate for high current H− accelerators.

Induced foot-currents in humans exposed to VHF radio-frequency EM fields

In order to limit the specific absorption rate (SAR) in the ankles of a person exposed to an electric field at frequencies below 100 MHz, induced current limits are prescribed in the 1992 ANSI/IEEE safety standard. The authors have measured the induced currents passing through the feet of nine subjects exposed to vertically polarized electric fields from nearby antennas, transmitting at frequencies between 90 and 104 MHz (in the FM broadcast band). The experimental results are in excellent agreement with the results obtained analytically for frequencies up to 110 MHz. The analytical results were obtained by applying the finite difference time domain (FDTD) method to an anatomically-based model of an average height male (1.75 m) and to a model of the tallest subject in the study (1.91 m). For the mean height of the nine subjects (1.75 m), the measured induced foot current for a unit strength vertically polarized electric field varied with frequency from 4.46 to 3.45 mA/(V/m) for frequencies between 90-104 MHz. For the tallest subject (1.91 m), the corresponding values ranged from 5.42 to 4.45 mA/(V/m), Foot currents in excess of the induced current limits in RF safety guidelines for both the controlled and uncontrolled environments could result even when the vertical component of the incident electric fields comply with the corresponding field limits. It is important, therefore, to not only measure the E- and H-fields, but also the induced currents up to the recommended maximum frequency of 100 MHz, and perhaps up to the upper frequency of the FM broadcast band (108 MHz). >

A new method for net-shape forming of large, single-domain YBa 2Cu 3O 6 + x

A new process has been developed for the net-shape fabrication of bulk, single-domain YBa 2Cu 3O 6 + x (123) specimens. To date, transport critical currents in excess of 10 000 A cm −2 have been obtained. This process, designated the seeded preform (SPF) technique, involves the permeation of peritectic liquid through a preform of pressed Y 2BaCuO 5 (211) powder. The use of a single-crystal seed of Nd 1 + x Ba 2 − x Cu 3O y ensures that only a single 123 domain is nucleated during the subsequent transformation: liquid + 211 → 123. The SPF process enjoys several important advantages over the melt-texturing techniques which are currently available. In particular, it offers the potential of net-shape forming of bulk 123 samples with complex geometries, and circumvents problems such as sagging, and substrate reaction. Also unique to the SPF process is that it produces large, single domain 123, with essentially no encapsulated liquid phase. Finally, relative to conventional melt-texturing, there is a refinement of the size of the 211 inclusions in the final microstructure, and the particles are more uniformly distributed.