B-dot Sensors Research Articles

A two-terminal fault location method for gas switch prefire in linear transformer driver.

Large-scale linear transformer drivers (LTDs) are composed of numerous high-power gas switches, and switch prefire is a frequent operational fault. To detect and locate the faulty switch accurately and efficiently, a two-terminal location method is proposed. A B-dot sensor is integrated on the gas switch's shell to collect the discharging signal. All the B-dot sensors are connected in parallel through cables of equal length. The fault position can be determined by the time delay of the signals at the two terminals. A diode is inserted between the B-dot sensor's coil and the cable core to ensure low-loss transmission of the signal. Two methods are applied in fault location, including time-of-arrival (TOA) and time reversal (TR). For the TOA method, an energy criterion and a phase criterion are applied and compared. The accuracy of the energy criterion is greatly influenced by the signal-to-noise ratio, while the phase criterion requires a reasonable estimate of the actual delay to account for the impact of phase periodicity. The TR method based on a precise simulation model is established, which demonstrates high precision in location. The TR method has been tested and validated on a single stage LTD module. Moreover, the location method for double switches prefire is discussed theoretically. The method proposed in this paper will be helpful to improve the efficiency of the commissioning, operation, and maintenance of the large-scale LTD devices.

Modular electromagnetic railgun accelerator for high velocity impact studies.

A modular electromagnetic railgun accelerator facility named "RAFTAR" (i.e., Railgun Accelerator Facility for Technology and Research) has been commissioned and its performance has been characterized for high velocity impact testing on materials in a single-shot mode. In the first tests, RAFTAR demonstrated an acceleration of more than 1000m/s for an 8g solid aluminum-7075 armature projectile. The current fed was 220kA, having a muzzle time of about 1.75ms. It is a single pulse breech-fed rectangular bore (14 × 13mm2) railgun, and its 1.15m long barrel assembly consists of two parallel copper bars with an inter-gap of 13mm that are encased within 50mm thick high strength reinforced fiberglass sheets (Garolite G10-FR4) and bolted from both the sides. RAFTAR is powered by two capacitor bank modules that have a maximum stored energy of 160 kJ each (containing eight 178 μF/15kV capacitors), two high power ignitron switches, and a pulse shaping inductor. To obtain consistent acceleration of the armature inside the barrel, reversal of driving current is prevented, and its pulse duration is stretched by tactical integration of the crowbar switch and bitter coil inductor in the circuit. Armature projectile velocity measurement in-bore and outside in free space was performed by the time-of-flight technique using indigenously made miniature B-dot sensors and a novel shorting-foil arrangement, respectively. The time resolved measurement of the in-bore armature evidenced a velocity-skin-effect in the high acceleration phase. There is good agreement between the experimentally measured and theoretically predicted efficiency, confirming the optimal choice of operating parameters. The conclusion summarizes important experimental findings and analyzes the underlying causes that limit the performance of railguns.

Study of the B-Dot Sensor for Aircraft Surface Current Measurement.

The B-dot sensor is a type of Rogowski coil widely used in the measurement of current. However, the accuracy of the B-dot for measuring aircraft high-frequency lightning current is greatly affected by factors such as numerical integration drift, high-frequency oscillation, and calibration. In this study, a new design and optimization for improving the B-dot measuring accuracy was carried out. To correct the drift of the numerical integral of the measurement signal in differential mode, the measuring current was reconstructed based on the nonlinear least squares method. The sensor was then optimized by isolating the sampling resistance and matching the impedance with a voltage follower. A low-cost coaxial loop calibration system was also designed to calibrate the high frequency and strong magnetic fields more accurately. Finally, the optimized B-dot sensor accuracy was greatly improved with a measuring range of 30 kA/m, an error of 3.1%, and a high-frequency response of 50 MHz. Our study greatly increases the accuracy of measuring aircraft high-frequency lightning current.

SIMULATION AND MEASUREMENT OF EXTERNAL ELECTROMAGNETIC ENVIRONMENT OF TOKAMAK DEVICE.

Magnetic confinement nuclear fusion is an important way to realize controllable nuclear fusion. Due to the large current and complicated coil arrangement, there is a complicated electromagnetic environment around the fusion device. In this paper, the B-dot sensor is used to measure the magnetic field, the D-dot sensor is used to measure the electric field, the MAXWELL electromagnetic simulation software is used to simulate the electromagnetic field strength; the simulation and measurement of the spherical Tokamak SUNIST device and the measurement of MARX generator are carried out, then we give corresponding electromagnetic protection suggestions.

Frequency Response Properties of the B-Dot Sensors Employed on a High Current Pulsed Power Facility

B-dot sensor, as an important device, is extensively employed to measure the current flowing in the magnetically insulated transmission line (MITL). To shield the output signal of the B-dot sensor from electron flow and high electrical field in the MITL anode-cathode (A-K) gap and also avoid the electrical breakdown in the A-K gap, the sensor is often covered with the metallic film and located in a conductor cavity penetrating the MITL electrode hole simultaneously. The frequency response properties of such kind of sensor are investigated. The experiment results indicate that the upper limited response frequencies of the sensors with and without the attached 5-μ m-thick Ni-Chrome foil are 50 MHz and 148 MHz, respectively. Moreover, as a result of penetration of magnetic flux into the cavity's wall and the wire loops wound to form the sensor, the low-frequency components of the B-dot sensor output signal are amplified, consequently the falling edge of the waveform given by the B-dot probe will be distorted. We have proposed a method to make a first-order correction for such effect by using a passive resistor-capacitor integrator instead of the numerical process suggested previously. The integrator combines the behaviors of integration and compensation of the B-dot sensor output. The actual measured current waveform can be directly obtained by multiplying the output signal of the integrator by the sensitivity of the B-dot monitor. The B-dot sensors and this post-processing method have been successfully used on a pulsed power accelerator.

Design of Transmission Line and Electromagnetic Field Sensors for DC Partial Discharge Analysis

In this paper, a testbed is designed and constructed for the investigation of DC PD pulses. The testbed is equipped with a 50 Ω transmission line (TL) that terminate to an oscilloscope for measuring the charge displacement current generated by PD pulses. Besides the oscilloscope measurements, two types of electromagnetic field sensors (D-dot and B-dot) were developed to capture the EM fields of the PD pulses propagating through the TL. The main goal of this paper is to investigate the DC PD pulses through the EM fields and the corresponding discharge current pulses that are considered as calibrating signals for the developed D-dot and B-dot sensors. The results of DC cavity discharge measured by the constructed testbed and the EM field sensors demonstrate close agreement with the reference PD pulses measured via oscilloscope.

Electromagnetic Pulse Sensor

ABSTRACTA simple dipole antenna maybe not intended to measure the transient electromagnetic pulse of sharp rise time in order of picoseconds. The pulse might be produced from numerous ways and their particular recognition is a need for proficient innovative work of the related systems. Magnetic-field sensor (B-dot) and Electric-field sensor (D-dot) are the more suited choice for precise detection of such pulse, owing to their pungent rise time. In this paper, the new conceptualized design of the B-dot sensor using a Mobius loop and detailed design of the Ground Plane D-dot sensors using an Equivalent Charge Distribution Method, followed by the principal, configuration, and calibration methods are introduced. Furthermore, an advanced prototype of the electric field sensor is developed based on simulation analysis and lightweight Aluminum (Al) Alloy. Moreover, the paper emphasizes the consequence of material choice on bandwidth and technically deployment of Al-Alloy in sensor manufacturing. The developed prototype is up to 3.0 times lesser in weight and up to 3.0 times, economical compared to a like Copper-based sensor and has a bandwidth greater than 12.1 GHz, which minimize multi-sensor requirement in the measurement of the electromagnetic pulse with wide-band EM systems.

Determination of Magnetically Insulated Transmission Line Cathode Current of a 1.0-MV Pulsed Power Generator

We have employed B-dot monitors to measure the magnetically insulated transmission line (MITL) cathode current on a 1.0-MV pulsed power machine, whose MITL anode and cathode electrodes are separated completely by the insulation stack. The MITL anode and cathode conductors are connected together by the B-dot sensors’ signal transmission cables, when transmitting the outputs of the B-dot probes out of the MITL anode–cathode (A-K) gap. It provides a leakage current path and disturbs the MITL power flow unavoidably. During our experiment, the impedance of the leakage current path is increased by winding the signal transmission cables along one helical isolated inductor frame. The leakage current flowing along the outer conductors of the cables is significantly reduced, and the outputs of B-dot sensors could be delivered out of the MITL A-K gap effectively. The experiment results suggest that the B-dot sensors are able to diagnose the MITL cathode current successfully, even though the performance of the B-dot probes on this generator is not as perfect as that of the monitors employed on an induction voltage adder. Finally, particle-in-cell simulation is performed to check the accurateness of the measured MITL cathode current. It demonstrates that the peak value of the cathode current given by the B-dot monitors agrees with the simulation result very well.

Design and Experimental Study of a Current Transformer with a Stacked PCB Based on B-Dot.

An electronic current transformer with a B-dot sensor is proposed in this study. The B-dot sensor can realize the current measurement of the transmission line in a non-contact way in accordance with the principle of magnetic field coupling. The multiple electrodes series-opposing structure is applied together with differential input structures and active integrating circuits, which can allow the sensor to operate in differential mode. Maxwell software is adopted to model and simulate the sensor. Optimization of the sensor structural parameters is conducted through finite-element simulation. A test platform is built to conduct the steady-state characteristic, on-off operation, and linearity tests for the designed current transformer under the power-frequency current. As shown by the test results, in contrast with traditional electromagnetic CT, the designed current transformer can achieve high accuracy and good phase-frequency; its linearity is also very good at different distances from the wire. The proposed current transformer provides a new method for electricity larceny prevention and on-line monitoring of the power grid in an electric system, thereby satisfying the development demands of the smart power grid.

B-Dot and D-Dot Sensors for (Sub)Nanosecond High-Voltage and High-Current Pulse Measurements

In this paper, we present a large-bandwidth, high-current, and high-voltage measuring system for pulse measurements in pulsed power systems. The developed sensors can be easily calibrated, require no extensive (3-D) modeling, are very compact, are inexpensive, and have a bandwidth of up to several gigahertz. Moreover, they can be used in any pulsed power system, where a pulse source is connected to its load by a coaxial cable (without disturbing the coaxial geometry). We developed this sensor system for the use with our nanosecond pulse source system. The type of sensors we used is D-dot and B-dot sensors, which are compactly mounted on the coaxial cable that connects our nanosecond pulse source to its load. This enables us to measure the characteristics of each sensor very precisely with a vector network analyzer. With these characteristics—combined with the characteristics of the measuring cable assembly—we can numerically reconstruct the voltage and current waveforms that passed the sensor positions. Our calibration approach, the mounting on the coaxial cable and the postprocessing of the results makes these sensors very flexible. While we use the sensors for energy measurements, camera triggering, and the general measurement of the pulses, other researchers can use these types of sensors as well in any system, where a (coaxial) cable connects a pulse source to its load.

High-Frequency CMR-B-Scalar Sensor for Pulsed Magnetic Field Measurement

To extend the frequency range of recently developed Colossal Magnetoresistance (CMR)-B-scalar sensors and to decrease the influence of induced picked-up (loop effect) voltages on the measured signal, a compensation method is used. A B-dot sensor of the same geometry as the CMR-B-scalar sensor's current leads is used for this purpose. This allowed the subtraction of the time derivative of the magnetic field signal from the total measurement signal obtained by the CMR-B-scalar sensor. This B-dot sensor is placed at a distance of only 0.25 mm from the CMR-B-scalar sensor. The signal from the B-dot sensor is measured directly using an oscilloscope and subtracted from the signal of the CMR-B-scalar sensor with an empirically determined weight factor. This allowed the investigation of the high-frequency behavior of the CMR-B-scalar sensor using an oscillating source of a magnetic field with a time-derivative equal to 1.7 T/μs. Using the compensation method described above, it is possible to measure the magnetic field pulses with amplitudes up to 10 T and rise times down to 3.5 μs with an overall accuracy of ~10%.

Application of W-band, Doppler Radar to Railgun Velocity Measurements

This paper describes a W-Band Doppler radar system in use at the electromagnetic launch facility at the Naval Surface Warfare Center in Dahgren, VA. Experimental results for a medium caliber railgun launch at 2000 m/s are presented. A discussion of the radar signal characteristics is given and the Doppler velocity profile during launch is computed. The in-bore time history of the launch package derived from the radar system is found to have good agreement with that derived from traditional magnetic field flux sensors (B-dot sensors). Given the increased temporal resolution of the radar system over B-dot measurements, time resolved friction coefficients can be estimated based on launch parameters. Due to the capabilities of the Dahlgren test range, the radar is able to track the projectile for a portion of flight just after the projectile exits the launcher. Drag coefficients for the projectile in hypervelocity free-flight are also presented.

Theoretical and Experimental Analysis of Breech Fed and 40-Distributed Energy Stage Plasma Arc Railguns

The design and experimental results of a 40-stage distributed energy store (DES) plasma arc railgun are presented. The railgun drives a free running hypervelocity plasma arc, one that is not pushing a payload, to velocities in excess of 10 km/s. These high velocities are of interest as they are required to successfully launch payloads into low earth orbit (LEO). The ability to launch payloads into LEO using a hypervelocity electromagnetic launcher has many financial benefits over the more conventional chemical combustion launchers. In collaboration with an Air Force Office of Scientific Research funded Multidisciplinary University Research Initiative project, the Center for Pulsed Power and Power Electronics at Texas Tech University in Lubbock, Texas has been responsible for developing and investigating a functional scale model of a multistage DES railgun to determine its effectiveness to suppress restrike phenomenon and increase plasma armature railgun performance. The distributed energy scheme is theorized to suppress restrike arc formation because the back emf voltage is localized to active stage regions where high gas density and low temperature inhibits breakdown. B-dot sensors positioned along the length of the launcher provide data to measure the plasma arc velocity and detect restrike, arc splitting, or additional secondary arc formation phenomena.

Measurement of Lightning Currents Using a Combination of Rogowski Coils and B-Dot Sensors

The paper presents laboratory tests and a characterization of commercial Rogowski coils and a specially designed B-dot sensor for the measurement of lightning currents on the Santis telecommunications tower in Switzerland. In order to overcome the limited high frequency response of the Rogowski coils, we propose to use magnetic loops located very close to the tower. We introduce the design of a B-dot sensor based on improvements proposed by C. E. Baum, the inductance of which is lowered by extending the vertical dimension of the loop, with the overall effect of extending the operating frequency range. To overcome the integrating behavior of the loop when connected to low-impedance loads (such as 50 Ohm), an arrangement of 100-Ohm-impedance cables connected across conical-transmission-line gaps is used. The designed sensor is characterized by an upper frequency cutoff of 20 MHz and a 50 Ohm matched termination. Laboratory tests carried out in the high voltage laboratory of the EPFL show the effectiveness of the simultaneous use of Rogowski coils and B-dot sensors for the measurement of lightning currents.

The Use of High-Speed Video as an In-Bore Diagnostic for Electromagnetic Launchers

One of the difficulties associated with electromagnetic launch is the lack of diagnostic tools capable of surviving the in-bore environment. In railguns, the principal diagnostics are current and voltage measurements, in addition to magnetic field (B-dot) sensors. X-rays and high-speed video are often taken of projectiles after exit, but imaging the in-bore performance of armatures has been limited. In those cases where high-speed cameras have been used to look down the bore of the launcher, the short depth of field ultimately limits their usefulness. High-speed video has been used to look at solid armatures from the side, though this has been limited to either stationary or hybrid solid/plasma armatures. In most cases, laboratory railguns are surrounded by high-strength laminated steel containments that preclude the kind of visual access required for imaging. The University of Texas at Austin and the Institute for Advanced Technology have performed experiments using a modified containment system that allows real-time imaging of the armature as it is accelerated down the launcher. Numerous tests were performed using aluminum armatures on copper and copper-alloy rails at currents up to 150 kA and observed velocities up to around 300 m/s. Grayscale imaging of the armature was performed using a Phantom imaging camera and a 10-ms high-intensity flash lamp, which provided frame rates in excess of 40 000 pps and integration times down to 2 μs. Successful imaging of the armature-rail interface was observed for a variety of test conditions including solid-state and transitioned armature-rail contacts. In addition to providing direct evidence of overall armature behavior, video was used to independently measure projectile velocity, which is usually inferred from relatively coarse B-dot measurements.

Experimental Studies of the Influence of a Resonance Cavity in an Axial Vircator

Experiments on an axial virtual-cathode oscillator (vircator) with a resonance cavity enclosing the virtual cathode are reported. The vircator is driven by a repetitive Marx generator operating in a single-shot mode. To be able to separate different radiation mechanisms, the design of the vircator allows adjustment of the cavity depth as well as the way microwave radiation is extracted. The microwave radiation is measured with a pair of free-field B-dot sensors. The maximum field strengths were registered when the bandwidth was very narrow.

The analysis and modeling of the ARDEC 2.5 km/s 20-mm plasma railgun shot

The 20-mm round-bore plasma railgun was successfully fired at the ARDEC electric gun facility. The 4-m gun with copper rails and alumina composite insulators was operated using a light-gas gun injector to start the projectile, already located in the gun, moving prior to the introduction of current. Current from the EMACK homopolar generator was commutated into the gun by an explosively actuated opening switch. The muzzle velocity was recorded by breakwires and flash X-rays at 2.5 km/s. B-dot sensors, rail current Rogowski coils, and breech and muzzle voltage measurements provided data on the in-bore dynamics of the armature. Postshot analysis using the ARMRAIL code successfully provided calculations reproducing all the main features of the data. Models account for the observed secondary arcs present throughout the shot, and the basis for the code and physics modeling is given. >

AIRBORNE PLATFORM FOR MEASUREMENT OF TRANSIENT OR BROADBAND CW ELECTROMAGNETIC FIELDS

ABSTRACT This paper deals with the development of an airborne platform for measuring transient or broadband CW electromagnetic (EM) fields. The considerations and guidelines [1] useful in designing such a platform are discussed. The EM problem consists of choosing the appropriate sensor type and location on an aircraft such that the scattered fields from the aircraft and the sensor mountings do not significantly couple to the sensor, while measuring the Incident field. These design considerations were applied In specifically designing a nose boom B-dot sensor for the NASA F-106 B aircraft [2]. Sample experimental results [3] with comparisons to calculated Incident fields are also presented.