Current Derivative Signal Research Articles

X-ray emission from copper X-pinches driven at a current rate of ∼1kA/ns

A characterization of Cu X-pinches experiments driven at current rates ∼ 1kA/ns is presented. The emission of two bursts of X-rays associated with hot-spot formation and electron beam-target mechanism are identified. The first burst, associated with the central hot-spot, appears consistently at currents in the range (154 +/- 20) kA, with time correlated noticiable dips in the current derivative signal, which indicate sudden changes in load impedance. This first burst emits photons pulses of ∼ 1–2 ns width and with source sizes ranging between 50 and 150 μm with emission in the 2–5 keV energy range combined with photons in the ∼ 8–9 keV range. The second burst, associated with electron beam-target mechanism, emits longer pulses of tens of nanoseconds, with source sizes larger than ∼ 0.75 mm and emission concentrated in energies>5 keV, reaching up to 9 keV or more. Spectroscopic data shows the presence of K-shell line emissions of Cu XXVII and Cu XXVIII, which combined with PrismSpect simulations indicate a electron temperature of ∼ 850 eV with an ion density of 1022 cm−3. Further details and analysis of the X-pinch plasma and its possible applications are presented and discussed.

Influence of electrode geometry on X-ray emission, plasma inductance, voltage, and current derivative signals obtained from a plasma focus device

Plasma focus devices have been investigated as a pulsed radiation source. Different studies have shown that the shape of the anode affects strongly the plasma dynamic and X-ray emission that takes place in plasma focus devices. In the present work, a hybrid-type plasma focus device (6 μF, 16 ± 1 kV), named PF-2kJ, is operated at various pressures of hydrogen gas with five cylindrical anodes of the same lengths but different in shapes, without cathode bars. The effect of cathode bars on various electric parameters is studied for the anode that has the maximum X-ray emission. The electrical signals of Rogowski coil (current derivative), voltage divider (voltage at the anode), and photomultiplier tubes (X-rays) were analyzed for all the anodes. Inductance and the voltage at the time of the pinch were derived from the measured voltage and current derivative signals. The X-ray dose measurements were performed using thermoluminescent dosimeters (TLD-100). The PF-2kJ performance is not the best while using the cathode bars. Our results show that by changing the anode shape just over the insulator sleeve the plasma focus devices can be switched between close energies stored in the capacitor bank.

Energetics X-Ray Burst Observation in the Collapse of an X-Pinch Conducted in a Small Capacitive Generator of Low Impedance

In this paper, the experimental results of the behavior of an X-pinch plasma conducted in a small capacitive generator of low impedance and inductance are reported. The generator produces currents of the order of 120 kA in times about 500 ns, when a charging voltage of 24 kV, and wires as load are used. W, Al, and Cu X-pinches of different diameters were tested. The experimental observations show a similar global dynamic to those observed in experiments of higher current and shorter quarter period, with an intense emission of energetics X-rays (>5 keV) generated by plasma dynamics in the cross point of wires. The fast plasma dynamic is evidenced from the strong change of frequency in the current-derivative signal, which is correlated with the X-ray signals observed from the radiation detectors. From the time-integrated pinhole images and time-integrated aperture images, filtered with different aluminum foils of different thicknesses, an estimated X-ray source size of $70~\pm ~20~\mu \text{m}$ with energies above 7 keV is obtained.

Evaluation of NALDN performance characteristics in the vicinity of the CN Tower based on tall-structure lightning

The North American Lightning Detection Network has been providing lightning data since 1998. Important applications, such as detection of lightning-caused forest fires, power line fault location and aviation safety procedures, have triggered a number of hardware and software upgrades for improvement of the network performance characteristics, including its detection efficiency and location accuracy. The performance characteristics of the network have been here evaluated by examining the reported North American Lightning Detection Network (NALDN) return-stroke data based on the lightning current measured at CN Tower during three major storms (2005, 2011 and 2014). Each of these three storms followed a substantial network upgrade in 2003–2004, 2010–2011 and 2013–2014, respectively. The objective of this investigation is to outline the evolution of the network and determine where there may be opportunities for additional improvements. (It is important to stress that the evaluation of NALDN in the vicinity of the tower is mainly based on tall-structure lightning, which is upward-initialed, due to the rarity of recording downward-initiated lightning at the extremely tall tower.) Since 1990, the CN Tower has been equipped to measure the lightning current derivative signals. Its 553-m height has allowed the recording of many hundreds of current derivative signals, mostly resulting from return strokes. Also, image recording systems have been used to capture images of strikes to the tower. The evaluation of the NALDN performance characteristics includes polarity, flash and stroke detection efficiency, location accuracy and peak current estimation. The 2013 deployment of LS7002 digital sensors with the enhanced embedded software has substantially improved the sensitivity of the sensors leading to greater flash and return-stroke detection efficiencies. Furthermore, the 2014 total lightning processor (TLP100), well designed with new algorithm and techniques, provides smaller time-of-arrival errors and better location accuracy. Based on this investigation, following the latest network upgrade, the polarities of the NALDN-detected return strokes were perfectly matched with the polarities of the corresponding return-stroke currents measured at the tower. In 2014, the NALDN detected every flash that contained at least one return stroke, as well as all recorded CN Tower return strokes, resulting in 100% flash- and stroke-detection efficiencies. The perfect stroke-detection efficiency in 2014 confirms a significant network improvement in comparison with the assessment in 2005 (55.3%) and 2011 (53.8%). In the 2014 evaluation, relative to the location of the tower, the NALDN is found to have a median absolute location error of 119m and an average absolute location error of 129m. The 2014 median absolute location error is found to be significantly lower than those for 2005 (289m) and 2011 (199m). Based on 2014 evaluation, the NALDN is found to overestimate the current peak measured at the tower by a factor of 3.89, which is due to field enhancement effect resulting from high speed propagation of the current within the tall tower. The presented analysis shows that the latest NALDN upgrade (2013–2014) following the 2003–2004 and 2010–2011 upgrades, has substantially improved the NALDN performance characteristics, especially in terms of stroke-detection efficiency and location accuracy.

The circuit parameters measurement of the SABALAN-I plasma focus facility and comparison with Lee Model

The circuit parameters are investigated in a Mather-type plasma focus device. The experiments are performed in the SABALAN-I plasma focus facility (2kJ, 20kV, 10µF). A 12-turn Rogowski coil is built and used to measure the time derivative of discharge current (dI/dt). The high pressure test has been performed in this work, as alternative technique to short circuit test to determine the machine circuit parameters and calibration factor of the Rogowski coil. The operating parameters are calculated by two methods and the results show that the relative error of determined parameters by method I, are very low in comparison to method II. Thus the method I produces more accurate results than method II. The high pressure test is operated with this assumption that no plasma motion and the circuit parameters may be estimated using R-L-C theory given that C0 is known. However, for a plasma focus, even at highest permissible pressure it is found that there is significant motion, so that estimated circuit parameters not accurate. So the Lee Model code is used in short circuit mode to generate the computed current trace for fitting to the current waveform was integrated from current derivative signal taken with Rogowski coil. Hence, the dynamics of plasma is accounted for into the estimation and the static bank parameters are determined accurately.

ATP Modeling of Tall-Structure Lightning Current: Estimation of Return-Stroke Velocity Variation and Upward-Connecting Leader Length

Electromagnetic Transients Program is used to model the lightning current distribution within the CN Tower and the attached channel. The tower is modeled by either three or five transmission line sections connected in series. On the assumption of the absence of an upward-connecting leader, the lightning channel is represented by a transmission line with a continuously expanding length. The presented model takes into account reflections within the tower and the lightning channel. Through rigorous analytical analysis, the model is used not only for estimating the return-stroke velocity variation profile, but also for determining the length of the upward-connecting leader. This detailed information is obtained using several lightning current derivative signals that were recorded at the tower. The model allows for the computation of the current at any point along the current path (the tower and the attached lightning channel), which is required for the calculation of the associated electromagnetic field. The simulation results are found to be in good agreement with measurements. They also verify that the return-stroke velocity initially increases rapidly with height, reaching a peak, and then decreases less rapidly with height. Moreover, the presented five-section model of the tower matches very well with the measured current derivative signal, especially when the estimated return-stroke velocity profile and the upward-connecting leader length are taken into consideration. The proposed model is also experimentally verified based on the comparison between the computed and measured magnetic fields. The simulated magnetic field waveform reproduces important details of the measured magnetic field signal, including the initial split peak that results from first channel-front reflection in the presence of an upward-connecting leader.

Wire array experiments in a low impedance and low current generator

In this work, a preliminary study about the behavior of a low impedance generator on different wire array configurations is reported. The experimental measurements were carried out on a small multi-purpose generator (1.2μF, 345J, 47.5nH, T/4 = 375 ns and Z = 0.2Ω in short circuit) which produces currents up to 122 kA with 500 ns quarter period, when a charging voltage of 24kV and a wire load are used. Two types of configurations were tested: parallel wires (two and four) and X-pinch configurations. The experiments were carried out on W, Al, and Cu wires with different diameters. The discharge was characterized by means of a set of diagnostics which included: Rogowski coil; filtered PCD detector; filtered PIN diode; gated VUV/soft X-ray pinhole camera, Shadow diagnostic and dark field Schlieren technique. From the set of experimental results, the following observations can be established: (i) The generator is highly sensitive to the changes of load impedance due to its low impedance design. (ii) Every shot shows a dip in the current derivative signal shortly after the discharge onset time (from 6 to 40 ns), which is inversely related to the load resistance. (iii) Both configurations show a similar dynamic to those observed in experiments of higher current and shorter quarter period. (iv) At the X-pinch experiments, two or more hard X-ray bursts are detected, around 200 ns from the current onset time. These X-ray bursts are correlated with the dips observed in the current derivative signal.

Non-radioactive Source for Field Applications Based in a Plasma Focus of 2J: Pinch evidence

A portable plasma focus for field applications operating at two joules have been designed and constructed. Pinch evidence in the current derivative signal have been obtained. Also the dependence of the time to pinch, tp, over the filling pressure, p0, (tp α □p0) was observed. Therefore, the device is working as a plasma focus.

Design and construction of a very small repetitive plasma focus device

In this paper the design and construction of a very Small Plasma Focus Device (20J) that can operate at repetition rate about 1 Hz are described. In the first experiments with it using Ar as working gas, high negative spikes in current derivative signals have seen, and the proper range of the operation using the Ar is identified.

On Probable Fusion Mechanisms in a Mather-Type Plasma Focus

An experimental study on plasma focus fusion mechanisms is presented in this paper. Simple diagnostic techniques such as current derivative, voltage, and time-integrated neutron detectors are used. This paper allows us to obtain information and fusion mechanisms present in a medium energy plasma focus [Auto Confined Plasma (PACO), 2 kJ, 31 kV, and T=2.9 μs]. The current sheet (CS) inductance is calculated through the anode voltage and current derivative signals, for some 80 shots performed within the pressure range 0.8-2.1 mbar of deuterium. It is concluded that the CS inductance is consistent with the behavior of a unique plasma sheet moving between two coaxial electrodes. From the information collected, it is concluded that the main fusion mechanism in this device is the beam-target one.

Manifestation of Constrained Dynamics in a Low-Pressure Spark

Some features of neutron emission from dense plasma focus suggest that the participating deuterons have energy in the range of 105 eV and have a directionality of toroidal motion. Theoretical models of these devices assume that the plasma evolves through a purely irrotational flow and thus fail to predict such solenoidal flow on the scale of the plasma dimensions. Predictions of a relaxation theory are consistent with experimental data [S K H Auluck, Physics of Plasmas,18, 032508 (2011)], but the assumptions upon which it is based are not compatible with known features of these devices. There is thus no satisfactory theoretical construct which provides the necessity for solenoidal flow in these devices. This paper proposes such theoretical construct, namely, the principle of constrained dynamics, and describes an experiment which provides support for this idea. The experiment consisted of low inductance, self-breaking spark discharge in helium at a pressure ~100 hPa between two pointed electrodes separated by 30-50 mm distance kept inside a vacuum chamber mounted on a low inductance high voltage capacitor. The current derivative signal showed reproducible sharp dips at all the extrema of the damped sinusoidal discharge. A planar diamagnetic loop centered with and perpendicular to the discharge axis consistently showed a signal representing rate of change of axial magnetic flux. A qualitative explanation of observed phenomena is obtained using a simple model using the proposed principle.

Preliminary Results of the 115 kJ Dense Plasma Focus Device IR-MPF-100

This work summarizes the design and construction of the first Iranian 115 kJ Mather type plasma focus (PF) machine (IR-MPF-100). This machine consists of a 6.25 cm radius and 22 cm height brass made anode with a 50 mm height insulator which separates the anode and cathode electrodes. Twelve copper made 22 cm height rods play the role of cathode with 10.2 cm radius. Twenty four 6 μF capacitors were used with the maximum charging voltage of 40 kV (maximum energy of 115 kJ) as the capacitor bank and maximum theoretical current around 1.224 MA. The total inductance of the system is 120 nH. By using NE-102 plastic Scintillator, Rogowski coil, current and voltage probes, hard X-ray, current derivative, current and voltage signals of IR-MPF-100 were measured. The primary result of neutron detection by neutron activation counter represents approximately 109 neutrons per shot at 65 kJ discharge energy while using deuterium filling gas. Also IR-MPF-100 PF has been tested successfully at 90 kJ by using the argon gas.

Temporal Correlations Between Hard X-Ray and Neutron Pulses in the PACO Plasma Focus Device

An experimental study on the fluctuations in the instants of time and amplitudes of the peaks in the voltage, current time derivative, X-rays, and neutron signals during a 150-shot series in a 2 kJ Plasma Focus device is presented. The results show a reasonable correlation between the time of the voltage peak and that of X-ray emission, in agreement with the picture of the formation of an electron beam accelerated by this voltage. However, the time differences between X-rays and neutron maximum emission are difficult to interpret in terms of a common origin.

Comparative study of adaptive techniques for denoising CN Tower lightning current derivative signals

The lightning current derivative data recorded at the CN Tower during the past 18 years contain different kinds of noise and needs to be denoised for accurately determining the lightning current waveform parameters. It is usually a challenging task to denoise transient signals having large bandwidth without altering their waveshapes or shrinking their amplitudes. This paper deals with denoising the CN Tower lightning current derivative signals using several adaptive techniques. A new adaptive denoising approach (Divide-and-Conquer) has been successfully used to denoise a vast variety of CN Tower lightning current derivative waveshapes. The supremacy of the new technique over the existing ones is outlined for a signal with a poor signal-to-noise ratio (SNR). While keeping the signal amplitude unchanged and preserving its waveshape, the new denoising technique improved its SNR from -22.93 dB to 71.41 dB.

Frequency Domain Approach to De-Noise the CN Tower Lightning Current Derivative Signal and Its Parameters Calculations

Frequency Domain Approach to De-Noise the CN Tower Lightning Current Derivative Signal and Its Parameters Calculations

Electrical characterization and experimental results in the SPEED4 plasma focus device

The pulsed power SPEED4 generator has a capacitor bank system of 1,25 μF in a Marx configuration to produce up to 100 kV, a maximum current up to ∼700 kA, and 6,25 kJ of stored energy. In the plasma focus mode, this device has been prepared for the study of the emission of X-rays and neutrons in the Laboratory of Plasma Physics and Plasma Technologies of the Chilean Nuclear Energy Commission. The present experimental results report about the electrical characterization as well as preliminary results using Hydrogen like filling gas. Pinch evidences in the high voltage and current derivative electrical signals are presented in this work and preliminary arguments are given in order to achieve an optimal operation.

Compact sub-kilojoule range fast miniature plasma focus as portable neutron source

As a first step toward the development of a compact and portable, quasi-continuous pulsed neutron source, we have demonstrated a ‘fast miniature plasma focus (PF) device’ as a compact nuclear fusion apparatus. The system operates with deuterium gas and produces an average neutron yield (Y90°) of (1 ± 0.27) × 104 neutrons/shot at ∼70 kA peak discharge current. In the range of 1.5–4 mbar, a distinct and sharp dip in the current derivative signal indicates a strong pinching action with subsequent emission of hard x-rays followed by a neutron pulse. The yield and the time history of the neutrons were measured by both active and passive detection techniques (such as a 3He proportional counter, NE102A plastic scintillator and CR-39 SSNTDs). The overall dimensions of the apparatus, which includes a capacitor bank, sparkgap switch and the focus chamber, are 0.2 m × 0.2 m × 0.5 m and the total mass of the system is ∼25 kg. The scope of this paper is to evaluate/demonstrate the potential of such a fast miniature PF device as a compact and portable fusion apparatus producing neutrons while operating at relatively low energy.

Pinching evidences in a miniature plasma focus with fast pseudospark switch

We report the observations of pinching in a miniature plasma focus (PF) (58—160 J) operated in repetitive mode using fast pseudospark switch (PSS). The size of the device, which includes the capacitor bank, PSS and the focus chamber, is of the order of 22 cm × 22 cm × 38 cm. Several diagnostic tools, the gated imager, streak camera, current and voltage probe, are employed simultaneously to confirm the occurrence of pinching in this fast miniature PF device. The device is optimized for operation in neon and hydrogen as the working gas. The best focus formation was obtained at pressures between 0.5 to 8.0 mbar for neon and between 7.0 to 15.0 mbar for hydrogen. When the system was operated at 100 J with hydrogen as the filling gas, the typical dip in the current derivative signal and the typical peak in the voltage signal associated with pinch compression, are observed to be most intense indicating efficient pinching in the miniature PF device.

Characterization of 100-kHz Noise in the Lightning Current Derivative Signals Measured at the CN Tower

Successful simultaneous measurements of the current derivative, the generated electromagnetic field, and the channel characteristics of CN Tower lightning strikes have been performed since 1991. A noise component in the lightning current derivative signals, oscillating around 100 kHz, has caused substantial limitations on the use of recorded lightning current data. The characterization and identification of noise has become necessary for its removal and possibly avoiding it altogether in future installations. This paper proves that this noise corrupting the current waveforms is the Loran-C signal. Loran-C is a radionavigation system using the frequency band of 90-110 kHz. Although it is known that lightning is the main source of noise in the reception of Loran-C, it is shown here that Loran-C signals are the source of a low-frequency noise corrupting the lightning current derivative measured at the CN Tower. This finding is a major contribution not only for the CN Tower lightning project but also for any other research related to the measurement of the lightning current at tall structures.

A simple criterion for the snowplowing efficiency of the working gas in a kJ plasma focus

Correlation between the appearance of the second peculiarity in the current-derivative signal and the drop in the neutron yield is observed experimentally in a plasma focus discharge. The time-resolved photographs of the current sheath lead to the conclusion that, at the instant of maximum compression, the main current is shunted by the second current sheath, which is formed from the rest of the gas after the passage of the first one. It is also suggested that the generation of the second current sheath is a consequence of two competing processes: filamentation of the first sheath, which thus becomes transparent to the working gas, and magnetization of the residual plasma by the field of the main discharge current.