Measurement and Processing of Fast Pulsed Discharge Current in Plasma Focus Machines

The fast pulsed electric discharge current drives all physical processes in the plasma focus device; in turn all physical processes in the focus affect the current waveform. Thus the discharge current waveform is the most important indicator of plasma focus performance. This underlies the importance of properly measuring, processing and interpreting the discharge current waveform. This paper reports the measurement of fast pulsed discharge current by the Rogowski coil, in two different modes: the current transformer, ??I?? mode, and current derivative, ??Idot?? mode. The processing and interpretation of recorded current waveform to obtain useful information about the physical processes in the plasma focus device are discussed. The current transformer with a large number of turns and a sub-1 Ohm terminator has good high frequency response, necessary for the sharp current dip region when dI/dt exceeds 2?×?10¹¹?A/s. However the signal is ??noisy?? in the current dip region. Several methods to extract the current dip from the noise are discussed and examples of how low pass filters affect the signals are shown. The dI/dt coil, the Rogowski coil in ??Idot?? mode, with a few turns terminated by 50-Ohm is also described. Integrating the 1?GSa/s digital waveform does remove the high frequency noise components, yet the extracted waveform shows sharp angular features indicative of the retention of short-time features. This makes the dI/dt coil superior to the current transformer. A 7-turn coil is tested against the Lee Model code and found to be suitable to measure the plasma focus discharge current.     

Stray Capacitance in a Plasma Focus Device: Implications on the Current Derivative Calibration and the Effective Discharge Current

The notion that PF discharge circuits should be represented by an equivalent circuit having two loops instead of the traditional single one is presented. This implies that two frequencies must be expected in the currents and voltages in these devices. Also, that the current flowing into the plasma is not the same as the current flowing from the capacitor bank. Finally, the difficulties for calibrating in situ a Rogowski coil are discussed.     

Current self-limitation in a transverse nanosecond discharge with a slotted cathode

A high-voltage transverse pulsed nanosecond discharge with a slotted hollow cathode was found to be a source of high-energy (few kV) ribbon electron beams.Conditions for the formation and extinction of electron beams were experimentally studied in discharges in helium at pressures of 1-100 Torr.It was found that interaction of fast electrons with a non-uniform electric field near the slotted cathode led to limitation of the magnitude of the discharge current.A physical model was developed to describe the discharge current self-limitation that was in satisfactory agreement with the experimental results.Some technical solutions that are expected to increase the upper current limits in transverse nanosecond discharge are discussed.     

Design and Development of Rogowski Coil Sensors for Eddy Currents Measurement on Toroidal Vessel

In tokamak, eddy currents are produced due to change in plasma positions during plasma instabilities that result into generation of electromagnetic forces on interaction with the induced currents. Measurement of this current is essential to design a mechanical structure that can withstand this force. Principle objective of this paper is the development of Rogowski coil sensor to measure eddy currents on a toroidal vessel. The paper presents an elaborative and practical construction technique of a Rogowski coil. The calibration method for the Rogowski coil is also presented. Rogowski coils as an eddy currents diagnostics are tested and experiments to measure induced currents on the toroidal vessel are performed using the coils. Experimental values of eddy currents are compared with the ANSYS simulation results.     

Design and Performance of a Novel Pancake Rogowski Coil for Measuring Pulse Currents

A novel pancake Rogowski coil without magnetic core is introduced in this paper. Owing to its special pancake winding structure, the coil is of low self-resista...     

Calibration of Power Systems and Measurements of Discharge Currents Generated for Different Coils in the EGYPTOR Tokamak

The EGYPTOR tokamak is a small device of rectangular cross section 25 × 20 cm. Its major (R) and minor (A) radius are 30 and 10 cm, respectively. The aspect ratio is 3. The present work is devoted to measure the absolute values of the discharge currents and to calibrate the different power systems used to supply different coils in the tokamak. Two different methods are applied to measure the discharge currents from different capacitor banks. The first is the direct method employing a calibrated resistance R=0.008 Ω; this resistance was prepared and calibrated in the IPP of the Czech Academy of Sciences, Czech Republic. The second is the indirect method using a calibrated Rogowski coil, fabricated and calibrated also in Prague (Czech Rogowski coil). Both methods are applied simultaneously. Details of each power system presently used are reported. Sensitivity calibration of two different Rogowski coils is also done, and comparison with the calculated sensitivity shows good agreement.     

强流快脉冲测量用罗果夫斯基线圈

文章叙述了一个用于测量毫微秒脉冲大电流的罗果夫斯基线圈的特点、结构和主要参数。该装置已被用在80GW强流相对论性脉冲电子束加速器上监测二极管产生的束电流。实验表明,在直到260kA范围内,该装置性能良好。上升时间约5ns。     

The Research of High-Performance Integrator for ITER Poloidal Field Converter Measurement System

Rogowski coil is used to measure the current of the thyristor in ITER poloidal field converter, the output of the Rogowski coil is the differential of current over time, so the result should be synchronized with input signal by using the integrator. In the process of measuring, the integral drift is caused by the input offset current, input offset voltage and the input bias current, which produces errors which will be larger after a long time, so something should be taken to decrease the errors. Then a real-time way of compensation was proposed in this paper based on the foregoing analysis. According to the method, the high-performance integrator was designed and tested to fulfill the demand of decreasing the errors by simulation analysis and experimental test.     

Experimental Verification of Modified Lee’s Model Using a Newly Designed and Constructed Filippov-Type Plasma Focus Device

A new 20 kJ Filippov-type plasma focus device has been designed and constructed in Isfahan University. The paper reports on the design and construction of the Iranian Filippov-type plasma focus device (UIPFF1) using modified Lee’s model. A Rogowski coil has been used to measure the experimental discharge current. Equivalent electric circuit of the device is RLC circuit; therefore the discharge current has a sinusoidal shape which its amplitude decreases exponentially during the time. The current signal contains a set of data from physical processes in the device as well as discharge current characteristics. In a typical discharge experiment these values were obtained: the discharge current was 181 kA, period of current signal 7.9 µs, the total inductance of the device 132 nH and electrical resistance of the circuit 77 mΩ. By averaging from data obtained with a set of five experiments the calibration factor was obtained 121 kA/V. Temporal changes in plasma focus discharge current, confirmed the occurrence of pinch at a specific pressure of argon, neon and nitrogen gases. UIPFF1 has been tested between 15 and 25 kV and wide range of pressure for various gases. Experiments at various pressures and voltages have also confirmed reproducibility and stability of the plasma focus device.     

Initial Plasma Formation in the GLAST-II Spherical Tokamak

This paper reports the initial plasma formation in glass spherical tokamak (GLAST-II) with electron cyclotron resonance pre-ionization assisted startup. Initially, a plasma current of 3 kA has been produced for duration of about 0.5 ms after establishing optimum conditions for microwave absorption at 2.45 GHz. Plasma current is then enhanced up to 5 kA by applying a small vertical magnetic field that provides additional plasma heating and shaping. Applied vertical field is optimized experimentally and optimal value is found to be 40 Gauss for this experiment. Plasma current and loop voltage are monitored by using Rogowski coil and toroidal loop of wire. A fast framing camera (5000 fps) is used for temporal investigation of plasma during the discharge scenario. A fast photodiode (BPX-65) and USB4000 spectrometer are used to record the signature of plasma current and the impurity content (O₂, H etc.). Cross-sectional average electron temperature is also estimated from plasma resistivity and found to be 6.1 eV for maximum plasma current of 5 kA.     

Magnetic Probe Measurements in INTI Plasma Focus to Determine Dependence of Axial Speed with Pressure in Neon

Current sheath dynamics generated in INTI plasma focus device operated with neon gas has been studied. A 3-turn Rogowski coil design has been used to measure derivative current. A new magnetic probe was designed and used to study of current sheath arrival time, current profile and velocity variation in the axial phase at different experimental conditions. The current sheath’s average velocity was found to vary with pressure^?0.51 with a R² value of 0.9 which agrees well with the theoretically expected variation of pressure^?0.5.     

Experimental study on nanosecond pulsed pin-to-plate discharge in supersonic air flow

Development of magnetohydrodynamic acceleration technology is expected to improve wind tunnel simulation capability and testing capability. The underlying premise is to produce uniform and stable plasma in supersonic air flow, and gas discharge is an effective way to achieve this. A nanosecond pulsed discharge experimental system under supersonic conditions was established, and a pin-to-plate nanosecond pulsed discharge experiment in Mach 2 air flow was performed to verify that the proposed method produced uniform and stable plasma under supersonic conditions. The results show that the discharge under supersonic conditions was stable overall, but uniformity was not as good as that under static conditions. Increasing the number of pins improved discharge uniformity, but reduced discharge intensity and hence plasma density. Under multi-pin conditions at 1000 Hz, the discharge was almost completely corona discharge, with the main current component being the displacement current, which was smaller than that under static conditions.     

A 7 T Pulsed Magnetic Field Generator for Magnetized Laser Plasma Experiments

A pulsed magnetic field generator was developed to study the effect of a magnetic field on the evolution of a laser-generated plasma.A 40 kV pulsed power system delivered a fast(~230 ns),55 kA current pulse into a single-turn coil surrounding the laser target,using a capacitor bank of 200 nF,a laser-triggered switch and a low-impedance strip transmission line.A one-dimensional uniform 7 T pulsed magnetic field was created using a Helmholtz coil pair with a 6 mm diameter.The pulsed magnetic field was controlled to take effect synchronously with a nanosecond heating laser beam,a femtosecond probing laser beam and an optical Intensified Charge Coupled Device(ICCD) detector.The preliminary experiments demonstrate bifurcation and focusing of plasma expansion in a transverse magnetic field.     

Research on Nanosecond Pulse Corona Discharge with Cross Magnetic Field Applied

An application of magnetic field to the nanosecond pulse corona discharge is investigated. A cylinder reactor with different corona electrodes is set up for experimental study. A magnetic field with its direction perpendicular to the corona discharge is applied. Different discharge images are taken under single nanosecond pulse with a high sensitive UV-visible light imagine recorder. Experimental results show that with a cross magnetic field the nanosecond corona discharge both generates paths and develops homogeneously in space more than that without the magnetic field. The results may lead to a possibility to apply a cross magnetic field on nanosecond pulse corona discharge for getting higher desulfurization efficiency.     

Atmospheric air dielectric barrier discharge excited by nanosecond pulse and AC used for improving the hydrophilicity of aramid fibers

In this paper, a long line-shape dielectric barrier discharge excited by a nanosecond pulse and AC is generated in atmospheric air for the purpose of discussing the uniformity, stability and ability of aramid fiber treatment. The discharge images, waveforms of current and voltage,optical emission spectra, and gas temperatures of both discharges are compared. It is found that nanosecond pulsed discharge has a more uniform discharge morphology, higher energy efficiency and lower gas temperature, which indicates that nanosecond pulsed discharge is more suitable for surface modification. To reduce the water contact angle from 96° to about 60°, the energy cost is only about 1/7 compared with AC discharge. Scanning electron microscopy,Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy are employed to understand the mechanisms of hydrophilicity improvement.     

A Calibrating Device for Rogowski Coil Development

A calibrating device for the Rogowski coil is developed, which can be used to calibrate the Rogowski coil having a partial response time within tens of nanoseconds. Its key component is a step current generator, which can generate the output with a rise time of less than 2 ns and a duration of larger than 300 ns. The step current generator is composed by a pulse forming line （PFL） and a pulse transmission line （PTL）. A TEM （transverse electromagnetic mode） coaxial measurement unit is used as PTL, and the coil to be calibrated and the referenced standard Rogowski coil can be fixed in the unit. The effect of the dimensions of the TEM unit is discussed theoretically as well as experimentally.     

Effect of Airflows on Repetitive Nanosecond Volume Discharges

Atmospheric pressure discharges excited by repetitive nanosecond pulses have attracted significant attention for various applications.In this paper,a plate-plate discharge with airflows is excited by a repetitive nanosecond pulse generator.Under different experiment conditions,the applied voltages,discharge currents,and discharge images are recorded.The plasma images presented here indicate that the volume discharge modes vary with airflow speeds,and a diffuse and homogeneous volume discharge occurs at the speed of more than 35 m/s.The role of airflows provides different effects on the 2-stage pulse discharges.The 1st pulse currents nearly maintain consistency for different airflow speeds.However,the 2nd pulse current has a change trend of first decreasing and then rapidly increasing,and the value difference for 2nd pulse currents is about 20 A under different airflows.In addition,the experimental results are discussed according to the electrical parameters and discharge images.     

Discharge Characteristics of SF6 in a Non-Uniform Electric Field Under Repetitive Nanosecond Pulses

The characteristics of high pressure sulphur hexafluoride (SF ₆ ) discharges in a highly non-uniform electric field under repetitive nanosecond pulses are investigated in this paper. The influencing factors on discharge process, such as gas pressure, pulse repetition frequency (PRF), and number of applied pulses, are analyzed. Experimental results show that the corona intensity weakens with the increase of gas pressure and strengthens with the increase of PRF or number of applied pulses. Spark discharge images suggest that a shorter and thicker discharge plasma channel will lead to a larger discharge current. The number of applied pulses to breakdown descends with the increase of PRF and ascends with the rise of gas pressure. The reduced electric field (E/p) decreases with the increase of PRF in all circumstances. The experimental results provide significant supplements to the dielectric characteristics of strongly electronegative gases under repetitive nanosecond pulses.     

Measurement of Plasma Energy Confinement Time in Presence of Resonant Helical Field in IR-T1 Tokamak

Plasma energy confinement time is one of the main parameters of tokamak plasma and Lawson criterion. In this paper we present an experimental method especially based on diamagnetic loop (toroidal flux loop) for measurement of this parameter in presence of resonance helical field (RHF) in IR-T1 tokamak. For this purpose a diamagnetic loop with its compensation coil constructed and installed on outer surface of the IR-T1. Also in this work we measured the plasma current and plasma voltage from the Rogowski coil and poloidal flux loop measurements. Measurement results of plasma energy confinement time with and without RHF (L = 2, L = 3, L = 2 & 3) show that the addition of a relatively small amount of RHF could be effective for improving the quality of tokamak plasma discharge by flatting the plasma current and increasing the energy confinement time.     

Investigation of self-generated magnetic field and dynamics of a pulsed plasma flow

Due to the growing interest in studying the compression and disruption of the plasma filament in magnetic fusion devices and Z-pinches, this work may be important for new developments in the field of controlled thermonuclear fusion. Recently, on a coaxial plasma accelerator, we managed to obtain the relatively long-lived (∼300 μs) plasma filaments with its self-magnetic field. This was achieved after modification of the experimental setup by using high-capacitive and low-inductive energy storage capacitor banks, as well as electrical cables with low reactive impedance. Furthermore, we were able to avoid the reverse reflection of the plasma flux from the end of the plasma accelerator by installing a special plasma-absorbing target. Thus, these constructive changes of the experimental setup allowed us to investigate the physical properties of the plasma filament by using the comprehensive diagnostics including Rogowski coil, magnetic probes, and Faraday cup. As a result, such important plasma parameters as density of ions and temperature of electrons in plasma flux, time dependent plasma filament's azimuthal magnetic field were measured in discharge gap and at a distance of 23.5 cm from the tip of the cathode. In addition, the current oscillograms and I–V characteristics of the plasma accelerator were obtained. In the experiments, we also observed the charge separation during the acceleration of plasma flow via oscillograms of electron and ion beam currents.