Capacitor Discharge Circuit Research Articles

Thermophysical properties of a Ti–44%Al–8%Nb–1%B alloy in the solid and molten states

The families of titanium aluminide intermetallic alloys have attractive high temperature mechanical properties which make them potential candidate materials for a wide range of applications, particularly in the aeronautic and automobile sectors. The development of appropriate manufacturing techniques is an essential stage in the engineering exploitation of these materials, e.g., Induction Skull Melting is one of the techniques which needs to be optimised for the casting of titanium aluminides. Research is underway to develop a computer model of this process but data are required for the key thermophysical properties. Pulse-heating techniques have been used to measure properties for the Ti–44Al–8Nb–1B system. Rectangular samples have been prepared and are resistively heated as part of a fast capacitor discharge circuit. Time-resolved measurements with sub-μs resolution of currents through the specimen were made with a Pearson probe current monitor using the induction principle. Voltages across the specimen were determined with knife-edge contacts and voltage dividers, and radiance temperatures of the sample were measured with a pyrometer. These measurements allow the calculation of specific heat and dependencies between enthalpy, electrical resistivity and temperature of the alloy up into the liquid phase. Data for thermal diffusivity have been obtained by using the Wiedeman–Franz relation. The results are compared with those obtained using DSC and the four-probe method to measure the temperature dependence of the resistivity.

Thermophysical properties of solid and liquid Inconel 718 alloy

Wire-shaped alloy samples are resistively volume heated as part of a fast capacitor discharge circuit. Time resolved measurements with submicrosecond resolution allow the calculation of specific heat and the mutual dependences between enthalpy, electrical resistivity, temperature, and density of the alloy in the solid and liquid phase. Thermal conductivity is estimated from resistivity data by the Wiedemann-Franz law at the end of the solid phase and at the beginning of the liquid phase. These high-speed measurements are compared to results of quasistatic measurements of specific heat obtained by differential scanning calorimetry and expansion measurements by dilatometry. The results are presented and compared with results of other research groups.

Determination of the Critical Point of Gold

Wire-shaped gold specimens are placed in a new, improved high-pressure vessel, which is part of a fast capacitor-discharge circuit and in which static pressures above 600 MPa can be reached with distilled water as the pressure-transmitting medium. The specimens are self-heated resistively by a current pulse. The current through the specimen, the voltage drop across it, and its temperature are recorded as a function of time with submicrosecond resolution. The radial expansion of the specimen is determined with a CCD camera, Experiments are performed at different pressures. When the critical pressure is exceeded, there is no liquid–gas phase transition; hence, no sudden change in the thermal expansion rate is observed. The results for temperature, pressure, and specific volume at the critical point of gold are as follows: T c =7400±1100 K, p c=530±20 MPa, and v c=0.13±0.03 × 10−3m3·kg−1.

The simple spot-welding apparatus

A simple spot-welding apparatus has been developed for the fabrication of stable electrodes used in the measurement of transport properties in condensed matter physics. The apparatus is especially suited for the use of brittle and small sized samples of rare-earth intermetallic compounds (REICs). The spot welding is made by a sharp pulse (150 A and several μs), generated by the precise time generation IC in a capacitor discharge circuit, to avoid breaking the sample. Stable electrodes of six 15 μ Au wires can be made on a REIC sample with length less than 1 mm.

A 42-Tesla pulse transformer with a mechanically high resistant field former

A new transformer produces magnetic field pulses up to 42 Tesla in field volumes of about 0.1 cm/sup 3/. The transformer combines the current pulses, I, of n (n=1...6) individual capacitor discharge circuits into a single current pulse n/spl middot/I. With n=6, peak currents of I/sub max/=16 kA lead to a secondary peak current of n/spl middot/I/sub max/=580 kA. Thus the product of the currents of the individual primary circuits is large, while each single current and the capacitor voltage remains low.

The Electrothermal Enhancement of Propellant Burning by Plasma Injection

AbstractA series of experimental demonstrations of plasma injection into a burning propellant interface is presented. The propellant is a nitro‐guanidine based material in the form of a right cylinder with a bore along its central axis. It is ignited by a hot‐wire system and, when a pre‐set pressure is achieved, a second wire is exploded using a high‐voltage capacitor discharge circuit. The second wire forms a plasma, precisely at, or well away from the burning interface. Comparing the pressure histories from these different geometries allows the effect of increased burning rate to be separated from Joule heating of the product gases. Analysis of these histories shows a pressure increase over that caused by Joule heating of the products.

Investigations of thermophysical properties of liquid metals with a rapid resistive heating technique

Wire-shaped metal samples are resistively pulse heated as part of a capacitor discharge circuit. With heating rates between 10 8 and 10 9 K/s, temperatures up to about 12000 K can be achieved. The measurements allow the determination of heat capacity and the mutual dependencies between enthalpy, electrical resistivity, temperature and volume. They are performed from room temperature up to superheated liquid states far above the normal boiling point. A summary of data of the following elements is given: W, Re, Ta, Mo, Nb, Fe, Co, Ni, Cu, Pb, and In.

The effects of the electric field associated with a laser-induced pulsed discharge on the ablation-generated plumes of YBa2Cu3O7−X

The effects of a pulsed discharge on the luminous plume generated during the excimer laser ablation of YBa2Cu3O7−X are investigated in an oxygen environment. A high-voltage capacitor discharge circuit is used to generate a pulsed discharge. The pulsed discharge is initiated by the laser-induced plume. The electric fields are applied along the trajectory of the plume. The time evolution of the discharge current exhibits a distinct dependence on the polarity and magnitude of the electric field and the oxygen background pressure. When an electric field is applied parallel to the plume trajectory two successive plumes may be observed. The first plume, directly induced by the impact of the ablation pulse upon the target, emits bright light, but is electrically neutral. The second plume (reentrant plume), accompanied by a discharge current (charged plume) is observed a few to 100 μs after the first electrically neutral plume. When the parallel field contains a large radial component, i.e., a perpendicular component to the plume trajectory, the charge neutrality of the first plume is lost and only a single charged plume is observed. When the direction of E is reversed (antiparallel field), only a single charged plume is observed directly induced by the laser ablation. The peak current that accompanies the charged plume exceeds 10 A. The time lag of the charged plumes and the minimum voltage required for the discharge are studied as a function of the ambient oxygen pressure. Time-resolved spectra of the plumes are investigated by using a gated intensified diode-array spectrometer. Two-dimensional images of the evolving plume are taken every 100 ns, using a gated, intensified charge-coupled-device camera. A simple model for the observed plume phenomena is proposed.

Improved thermophysical measurements on solid and liquid tantalum

Wire-shaped tantalum samples are resistively pulse heated as part of a coaxially constructed capacitor discharge circuit. With heating rates of more than 109 K · s−1, temperatures up to about 10,000 K are reached. The tantalum wire is contained, with water as the surrounding medium, in a high-pressure vessel with sapphire windows and a maximum pressure capability of 5 kbar. Time correlated measurements of the current through the wire and the voltage drop across it, as well as surface radiation and wire expansion, were performed to permit the determination of thermophysical properties of the solid and liquid tantalum.

High-pressure, high-temperature thermophysical measurements on molybdenum

Wire-shaped molybdenum samples were resistively pulse heated in a coaxial capacitor discharge circuit. Heating rates greater than 109 K s-1 were attained, and temperatures up to 10000 K reached. Time-correlated measurements were made with submicrosecond resolution of the current through the wire and voltage drop across it, as well as measurements of surface radiation and wire expansion. These quantities allow determination of such thermophysical properties as the heat capacity at constant pressure and the mutual dependencies between temperature, volume, specific enthalpy and electrical resistivity in superheated liquid molybdenum.

Capacitive-Discharge Circuit for Controlled Heating of Anisotropic Graphite Tubes. Part II: Characterization for Graphite Furnace Atomic Absorption

Performance data from a recently described capacitive-discharge heating system are presented. The capacitive-discharge heating circuit, which used a 0.15 F capacitor bank, is shown to provide reproducible heating of a small anisotropic graphite tube (5 mm o.d. × 9 mm long, 1 mm wall thickness) at heating rates of up to 65°C per ms to a final temperature of 2500°C. The performance and resistance of graphite from two manufacturers are compared. The effect of the capacitor bank voltage on the heating rate and maximum achievable temperature is studied. Photographic and optical pyrometric evidence of the circumferential temperature anisotropy is presented. Pyrolytically coated anisotropic graphite tubes were investigated. Preliminary investigations using Pb indicated that sample deposition on the unbroken lamellar graphitic planes provides about 1.5 times the peak-area and peak-height sensitivity obtained from sample deposition across the lamellar planes.

High-pressure, high-temperature thermophysical measurements on tungsten

Abstract Wire shaped tungsten samples (0.25 mm diameter, 40 mm length, 99.97%) are resistively pulse heated as part of a coaxially built up capacitor discharge circuit. With heating rates of more than 109 K/s, temperatures up to 12000 K are reached. The tungsten wire is contained with the surrounding medium water in a high pressure vessel with sapphire windows and a maximum pressure capability of 0.5 GPa. Time correlated measurements of the current through the wire and the voltage drop across it as well as surface radiation and wire expansion can be performed simultaneously and allow the determination of thermophysical properties for the liquid tungsten (see key words). All measuring systems have rise times less than 10 ns.

Capacitive Discharge Circuit for Controlled Heating of Anisotropic Graphite Tubes: Part I

Circuitry suitable for capacitive discharge heating of anisotropic graphite tubes for atomic absorption and atomic fluorescence is detailed and initial calibration data are presented.

Characterization of an Analytical Theta-Pinch Plasma Generated with a Unidirectional Capacitive Discharge

The plasma produced by a high-current capacitive discharge through a graphite fiber bundle is compressed by a magnetic field coaxial with the plasma. The magnetic field is generated by the plasma current in a large coil surrounding the plasma. The field induces an azimuthal (theta) current in the plasma. This current couples with the external magnetic field and produces a radial Lorentz force which reduces the rate of plasma expansion. A diode shunt in the capacitive discharge circuit is used for the generation of a unidirectional discharge current. This arrangement eliminates zero-crossings of the discharge current and thus increases the effectiveness of the magnetic field in controlling the radiative properties of the plasma. Design features of the discharge circuit are presented, as well as a comparison of the plasma properties with oscillatory and unidirectional discharge current waveforms.

A bipolar pulse tester for semiconductor devices

The design and construction of a high voltage pulse generator suitable for bipolar pulse testing of semiconductor devices is described. The apparatus uses field effect transistors as the switching elements in a multistage capacitor discharge circuit based on the Marx generator principle.

Variable pulse width piezoelectric ultrasonic transducer driver

Requirements of ceramic piezoelectric ultrasonic transducer drive circuits are discussed in the light of today's advanced non-destructive testing techniques. A new drive circuit based upon power MOSFET devices, which overcomes many of the shortcomings of capacitor discharge circuits, is described. This driving technique enables transducers of a wide range of resonant frequencies to be driven from a single drive unit. It also enables transducer characteristics to be optimized for particular applications by control of the drive pulse shape.

Longitudinal magnet forces?

The Ampere electrodynamics of metallic conductors and experiments supporting it predict that the interaction of a current-carrying wire with its own magnetic field should produce longitudinal mechanical forces in the conductor, existing in addition to the transverse Lorentz forces. The longitudinal forces should stretch the conductor and have been referred to as Ampere tension. In 1964 it was discovered that a current pulse would break a straight copper wire into many fragments without visible melting. A metallurgical examination of the pieces confirmed that the metal parted in the solid state. The same observation has now been made with aluminum wires. In the latest experiments the wire was bent into a semicircle and arc-connected to a capacitor discharge circuit. The arc connections ruled out rupture by Lorentz hoop tension and indicated that longitudinal forces may also arise in circular magnet windings. Explanations of wire fragmentation by thermal shock, longitudinal stress waves, Lorentz pinch-off, bending stresses, and material defects have been considered and found unconvincing. Computed Ampere tensions would be sufficient to fracture hot wires. The Ampere tension would double the hoop tension normally expected in dipole magnets. This should be borne in mind in the design of large dipole magnets contemplated for MHD power generators and railgun accelerators.

A New Scheme for Subsynchronous Resonance Damping of Torsional Oscillations and Transient Torque - Part II, Performance

Part I, a companion paper, presents the concept and suggests practical hardware for a capacitor discharge circuit (named the NGH Scheme) designed to solve SSR problems. Results of analytical studies using hybrid computer simulations indicate that the NGH Scheme is effective in improving both the transient torque and system stability forms of SSR problems. These studies have shown that a combination of the NGH Scheme and generator excitation control would allow series capacitor compensation in a particular 500 kV line in the Salt River Project system to be extended from 15%, the value limited previously by SSR resonance considerations, to 75%.

A new technique for the measurement of vapour superheat in a vapour-liquid droplet flow

A novel design of probe to measure vapour superheat in a vapour-liquid droplet flow is described. The probe is an exposed junction thermocouple with a fine wire tip which is heated intermittently to remove liquid drops. The tip is heated electrically by including the thermocouple junction in a capacitor discharge circuit. The probe is shown to measure steam superheats successfully in the type of non-equilibrium steam-water droplet flows likely to be generated during water reactor reflood by emergency cooling water. Design improvements to extend the range of application of the probe are outlined.

Overvoltages on EHV transmission lines due to faults and subsequent bypassing of series capacitors

It is shown that overvoltages exceeding 2.0 per unit (pu) can occur on unfaulted lines in series-compensated systems, following fault initiation on other lines in the system and subsequent bypassing of series capacitors. The cause of these overvoltages is an oscillatory discharge of the series capacitor banks initiated by sparkover of the protective gaps. As a consequence, a voltage standing wave will appear on the unfaulted lines, superimposed on the 60-Hz quantities, leading to high overvoltages in certain sections of the transmission lines. This phenomenon can result in cascading outage of more than one transmission line, and is particularly serious in case of long highly series compensated transmission lines. The theoretical investigation indicates that a particularly bad case will result when the length of the line is approximately equal to 3/4 wave length of the series capacitor discharge ringing frequency. Overvoltages of this type, rather than switching surges, may dictate transmission line insulation requirements in series-compensated systems. The effect of series capacitor discharge on system transients has not been reported previously in the literature and, therefore, it is the purpose of this paper to discuss key results of recent digital and Transient Network Analyzer (TNA) studies of this phenomena. Increased damping of the series capacitor discharge circuit is recommended to control this type of overvoltages on transmission lines.