Current Sheath Dynamics Research Articles

Investigation of the Plasma Current Sheath Dynamics in the Run Down Phase of the Hemispherical Plasma Focus Devices Using Two-Dimensional Time Dependent Modeling

Investigation of the Plasma Current Sheath Dynamics in the Run Down Phase of the Hemispherical Plasma Focus Devices Using Two-Dimensional Time Dependent Modeling

Experimental investigation of a low-energy linear plasma propulsion device

This paper presents preliminary results for a new design of a pulsed linear plasma device that generates electromagnetic plasma propulsion. The current sheath dynamics and the plasma propulsion along an extension tube have been studied. Breakdown of helium gas occurs between coaxial electrodes when applying a high-voltage pulse. This paper presents measurements with a number of probes, as well as camera images of the plasma propagation. A plasma is ejected and fills a 40 cm length extension tube. Additionally, the measurements show that the length of the propelled plasma column and its intensity inside the expansion tube is increased by increasing the charging voltage, while the plasma propulsion current is decreased along the extension tube. It is noted that a lower critical time; 4.7 μs, is needed to ionize the gas and form the plasma current sheath.

BASINÇ VE VOLTAJIN KÜRESEL PLAZMA ODAK CİHAZI DİNAMİ ÜZERİNE ETKİSİ

A spherical plasma focus device with two concentric electrodes is simulated using equal amount of deuterium and tritium mixture as a working gas. The developed model is used to investigate the effect of gas pressure and charging voltage on dynamics of current sheath in spherical plasma focus. While charging voltage is varied from 15 kV to 30 kV to see the effect of voltage, gas pressure is increased from 1 Torr to 20 Torr for gas pressure study. In this work, it is found that there is a strong correlation between current sheath dynamics and investigated parameters which are gas pressure and charging voltage.

Plasma Current Sheath Dynamics and Energy Dissipation in a Low-Energy Plasma Focus Device

This paper reports the measurements of plasma current sheath (PCS) dynamics, the energy dissipation processes, and the plasma focus (PF) electrical characteristics, particularly during the axial phase discharge in a Mather-type PF device (EAEA-PF1) energized with a 30 µF capacitor bank charged with 8, 10 and 12 kV. All these works are carried out under discharge conditions where the optimal PF action is achieved. At each charging voltage (Vch), 8 kV, 10 kV and 12 kV, the optimal PF action is studied for different Argon gas pressures (P) ranging from 0.4 to 1.2 Torr. The results show that the best PF is formed at Vch = 8 kV and P = 0.6 Torr, Vch = 10 kV and P = 0.8 Torr, and Vch = 12 kV and P = 0.8 Torr. The implosion velocity (Vz) results of PCS show that the maximum value of Vz (4.48 cm/µs) occurs at the end of the axial phase (i.e., at the coaxial electrode muzzle), which is detected at Vch = 12 kV and P = 0.8 Torr. Moreover, a less inefficient snowplow action is observed under these discharge conditions. The energy dissipation process data indicate that at Vch = 12 kV and P = 0.8 Torr, the ratio between the total energy dissipation and the input energy has a maximum value of ≅ 90%, and the minimum residual energy left on the condenser bank (175.39 J) is also achieved under these discharge conditions.

Estimation of the Current Sheath Dynamics and Magnetic Field for Theta Pinch by Snow Plow Model Simulation

The snowplow model and shock wave equation are used to simulate theta pinch plasma experiment. The model calculates the sheath velocity, radial position, plasma current, plasma temperature, and the induced magnetic field of the current sheath. The model uses two phases: the radial inward phase and the reflected shock phase. The governing equations are derived by using the current factor and the mass factor. The model calculates the induced magnetic field of the plasma by the deriving equation and the magnetic field of the coil. Also, the model is operated by using the Hydrogen, Deuterium and Tritium gases at the same of the mass density. The results illustrate that the temperature and the induced magnetic field are higher for tritium gas, which has a higher molecular weight, while the current sheath velocity of tritium is lower. Some results of the model were compared with Lee model and other paper.

The Effects of Ferrites on Dense Plasma Focus Device: Using a Modification to the Lee Model

The Lee mode code has been promoted for better simulation of the dense plasma focus (DPF) device. At the first step, the calculation of magnetic field and tube inductance was improved to include the geometry of the rods forming the cathode. At the second step, the influences of the number of the cylindrical cathodes have been investigated. Ultimately, for enhancing the current sheath dynamics in the axial acceleration phase, the effects of ferrites located near the cathodes have been studied. Simulations were performed with integrating the modified generating equations. The influences of the ferrite’s susceptibility as well as saturation field on the current waveform, the tube inductance, and the current sheath speed in the DPF axial phase have been investigated. The numerical studies showed that increasing the number of cathodes would decrease the starting time of the pinch phase. The weak ferrites presented some enhancement in the total magnetic field and the axial speed of the current sheath.

Stable current sheath dynamics caused by preionization source in a small plasma focus device using He as an operating gas

ABSTRACTPinching of the current sheath in the plasma focus (PF) device is an important phase to transfer the stored energy to the high density short lived plasma for the production of a group of useful radiations. The current sheath formation during the initial breakdown of the gas is a critical process in the PF operation. In this regard, the understanding of the current sheath dynamics is of prime importance. In this paper, we present the effects of alpha particle induced preionizaion by U238 in 3.3 kJ He operated PF device on the dynamics of current sheath, focus amplitude and axial rundown velocity by employing Rogowski coil and High voltage probe as main diagnostic tools. The formation of parabolic axisymmetric sheath is confirmed by Rogowski coil and high voltage probe signals which also reveals the variation in focusing time as well as amplitude during collapse phase by using the preionization source. Time resolved X-ray diagnostics are also carried out using PIN diodes covered with Al and Mg filters of 12.5 and 25 µm which provide an important information about an early onset of X-ray generation. Time integrated X-ray imaging is also carried out to estimate X-ray production zone in the PF device indicating enlarged and symmetric images with preionization.

The effects of the cathode array on emitted hard x-ray from a small plasma focus device

In this study, the effects of the cathode array variations on emitted hard x-rays from a small Mather type plasma focus device (450 J) were investigated. The gradual elimination of the cathode rods inside the cathode array of the device lowered the quality and quantity of the emitted hard x-rays at different pressure values of argon gas. We theorized that the variations of the cathode array were able to change some discharge parameters that could vary the number of the energetic runaway electrons generated during the pinch phase which were responsible for the created features of the emitted hard x-rays. On the other hand, we hypothesized that the removal of the cathode rods could influence the current sheath dynamics during all the phases of a shot including its average axial velocity which was demonstrated by using two axial magnetic probes. We also theorized that cathode rod omission from the cathode array could also increase the initial inductance and the impedance of the system, and the impurities inside the plasma during the pinch phase which could lead to the growth of the instabilities. Moreover, by using the wavelet technique and studying the Mirnov signals, it was shown that the decrease of the cathode rod number increased the plasma magnetic field fluctuations or instabilities (MHD activities) that adversely affected the pinch quality, and reduced the emitted hard x-rays.

The influence of the cathode array and the pressure variations on the current sheath dynamics of a small plasma focus device in the presence of an axial magnetic probe

In this research, the influence of the cathode array and the pressure variations on the current sheath dynamics of a small plasma focus device (450 J) was investigated. For this purpose, the signals of an axial magnetic probe for two different gases (argon and nitrogen) were studied. The magnetic probe signals showed the slower movement of the current sheath layer when the number of cathode rods decreased. This was related to the increase in the circuit inductance, which caused the longer discharge time of the capacitor bank followed by the creation of runaway electrons. These electrons in turn produced the impurities that led to the appearance of the instabilities inside the plasma. On the other hand, in order to investigate the effect of the cathode array variation on the instabilities produced inside the plasma, the wavelet technique was used. With the aid of frequency analysis, this technique showed the increase in these instabilities, which was due to the non-uniform formation of the current sheath layer during the breakdown phase, and finally, the higher values of the pressure caused the slower movement of the current sheath due to the inverse relation of the current sheath velocity to the square root of the pressure.

The effects of pre-ionization on the impurity and x-ray level in a dense plasma focus device

In this study, the effects of pre-ionization on the reduction of the impurities and non-uniformities, the increased stability of the pinch plasma, the enhancement of the total hard x-ray yield, the plasmoid x-ray yield, and the current sheath dynamics of the argon gas at different pressures in a Mather type plasma focus device were investigated. For this purpose, different shunt resistors together with two x-ray detectors were used, and the data gathered from the x-ray signals showed that the optimum shunt resistor could cause the maximum total hard and plasmoid hard x-ray emissions. Moreover, in order to calculate the average speed of the current sheath, two axial magnetic probes were used. It was revealed that the pre-ionization could increase the whole range of the emitted x-rays and produce a more uniform current sheath layer, which moved faster, and this technique could lead to the reduction of the impurities, creating a more stabilized pinched plasma, which was capable of emitting more x-rays than the usual case without using pre-ionization.

Study of the interrelation between the electrotechnical parameters of the plasma focus discharge circuit and the plasma compression dynamics on the PF-3 and PF-1000 facilities

The main stages of the plasma current sheath (PCS) dynamics on two plasma focus (PF) facilities with different geometries of the electrode system, PF-3 (Filippov type) and PF-1000 (Mather type), were studied by analyzing the results of the current and voltage measurements. Some dynamic characteristics, such as the PCS velocity in the acceleration phase in the Mather-type facility (PF-1000), the moment at which the PCS reaches the anode end, and the plasma velocity in the radial stage of plasma compression in the PF-3 Filippov-type facility, were determined from the time dependence of the inductance of the discharge circuit with a dynamic plasma load. The energy characteristics of the discharge circuit of the compressing PCS were studied for different working gases (deuterium, argon, and neon) at initial pressures of 1.5–3 Torr in discharges with energies of 0.3–0.6 MJ. In experiments with deuterium, correlation between the neutron yield and the electromagnetic energy deposited directly in the compressed PCS was investigated.

Influence of Kr doping on neon soft X-rays emission in fast miniature plasma focus device

Abstract An investigation on the possibility of enhancement of soft X-ray (SXR) (900–1600 eV) emission from a fast miniature plasma focus (FMPF) device of 235 J (at 14 kV) storage energy through doping of operating gas was performed. Neon (Ne), the operating gaseous medium, was doped with krypton (Kr) in different volumetric ratios at various operating pressures ranging from 2 to 14 mbar. The 1% Kr doping increased the average optimum SXR emission efficiency from 0.47% to 0.6% without enhancing the hard X-ray (HXR) (>1600 eV) emission. The Kr doping influenced the major pinching characteristics such as focusing efficiency and time to pinch with consequential effect on X-ray emissions. Synchronous operation of the 4 pseudo-spark gap (PSG) switches was mandatory for efficient discharge current delivery to the electrodes. A drastic improvement in the pinching efficiency was obtained with replacement of old and worn out PSG switches with the new ones. Optical imaging of current sheath dynamics was performed using gated ICCD camera to verify the normal operation of the device after the PSGs replacement. A numerical simulation analysis on the 2 cm long stainless steel tapered anode, used in this study, was done to predict the maximum SXR emission efficiency and the peak operating gas pressure. An analysis on the amount of SXR fluence generated at the source position and the proportion of it reaching the target position is also reported.

Order of Magnitude Enhancement in Axial Run-Down Velocity of a Current Sheath and the Focus Duration of a PF Device with Preionization

The effects of the neon gas pressure and preionization on the current sheath dynamics in a low energy (3.3 kJ) Mather type plasma focus (PF) are investigated. The formation and dynamics of the current sheath are monitored by using a Rogowski coil and a high voltage probe. The experiment is performed to explore the evolution of the plasma sheath and to estimate the range of its velocity during the run-down phases. The Rogowski coil and high voltage probe signals reveal an axisymmetric parabolic current sheath, which propagates down the co-axial tube, and a longer confinement time during the collapse phase with a preionization source.

Effect of preionization on the dynamics of current sheath in a small plasma focus device

In the dense plasma focus (PF) device, the dynamics of the current sheath is an important parameter for good focusing or pinching. In this paper, we present the effect of preionization on current sheath dynamics, its pinching evidence and details of its filamentary structure by measuring the axial run-down velocity, confinement time and focus amplitude in a miniature PF device using a Rogowskicoil and high-voltage (HV) probe. For this purpose, the discharge is produced in deuterium gas with deposited energies lying in the range of 3.3 kJ. The Rogowski coil and HV probe signals reveal the formation of an axisymmetric parabolic current sheath propagating down the coaxial tube, and longer confinement time of it during the collapse phase and increase in focus amplitude with preionization source. The axial run-down velocity of the current sheath is increased from 2.1 to 3.2 cm/μ s. The focus duration is from 45 to 72 ns and the focus amplitude is from 1.8, which increases to 2.9 with preionization at the optimum pressure. The effect of the working pressure on the current sheath dynamics and its spatial evolution are also studied.

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.

Filamentary Structure of Current Sheath in Miniature Plasma Focus

Current sheath dynamics is an important parameter for good focusing or pinching in a dense plasma focus device. In this paper, we present pinching evidence and details of the filamentary structure of the current sheath in a miniature plasma focus device using a time-resolved laser shadowgraphic technique and time-integrated optical imaging.

Study of plasma sheath dynamics by using two magnetic probes in a low energy plasma focus device

The effects of the argon gas pressure, charging voltage and anode shape on the current sheath dynamics in a low energy (4.9 kJ) Mather type plasma focus (PF) were investigated. The formation and dynamics of the current sheath were monitored by using two magnetic probes, which were inserted radially and axially to explore the evolution of the plasma sheath and to estimate the range of its velocity during the break-down and run-down phases. The radial magnetic probe measurements showed a rather constant current sheath velocity near the insulator, which was more sensitive to the variations of the gas pressure than the charging voltage, and the current sheath did not lose its uniformity by expanding away from the insulator during the break-down phase. The results found from the axial magnetic probe signals revealed a higher current sheath velocity inside the step region of the step anode than the cylindrical one. The simulated axial current sheath trajectories (Lee's model) that were obtained after the fitting process of the current signals showed good agreement both for the cylindrical anode throughout the run-down phase and the step anode before the step region. Inside the step region, the separation between the simulated and the experimental trajectories of the step anode was increased at greater axial distances.

Current Sheath Dynamics and its Evolution Studies in Sahand Filippov Type Plasma Focus

One of the most important factors for optimizing the plasma focus device operation is the dynamics of the plasma. In this paper, we investigated the profile and dynamics of the current sheath by measuring the velocity and distribution of current sheath in Sahand as a Filippov type plasma focus device. For this purpose, the discharge is produced in pure neon gas with capacitor bank stored energies in the range of 14–50 kJ. The current sheath is monitored using two sets of magnetic probes, one with four and other with three equi-distant probe coils. These probes, installed in both radial and axial directions near the edge of the interior electrode (anode), are used for monitoring the distributions and dynamics of the current sheath. The maximum current sheath velocities at radial and axial phase are 4 ± 0.13 and 3.51 ± 0.22 (cm/μs) respectively for 0.25 Torr. The decreasing of CS velocities in going move away from anode surface is one of the our results in this paper. In this paper we conclude that the current sheath velocity at radial phase in Sahand is greater than axial phase. The effect of the neon working gas pressure and working voltage on the current sheath dynamics and its spatial evolution is investigated and presented.

Study of a cumulative gas phenomenon in plasma focus discharge

A cumulative phenomenon of nitrogen gas at a pressure of 3.3 torr in a 1.2 kJ coaxial plasma focus discharge device was investigated experimentally. Variations of nitrogen gas density in the axial phase along the inter-electrode region were estimated experimentally from the plasma current sheath (PCS) dynamics in terms of its velocity, acceleration as well as axial magnetic force data. An inclination angle of PCS with the axial distance was estimated. Investigations of the ratios of radial and axial magnetic force as well as the magnetic pressures coupled to PCS were studied. The variation of the trapping fraction η with the axial distance was computed theoretically based on a snow-plow model and was compared with experimental results.

The effect of anode shape on neon soft x-ray emissions and current sheath configuration in plasma focus device

The effect of three different anode shapes, flat, tapered and hemispherical, on the x-ray emission characteristics of a neon filled UNU–ICTP plasma focus device is investigated. The current sheath dynamics, in the radial collapse phase, has been simultaneously interrogated using the laser shadowgraphy method to understand the variation in x-ray emission characteristics for anodes of different shapes used in the experiments. The maximum neon soft x-ray (SXR) yield for the flat anode is about 7.5 ± 0.4 J at 4 mbar, whereas for hemispherical and tapered anodes the neon SXR is almost halved with the optimum pressure shifting to a lower value of 3 mbar. The laser shadowgraphic images confirm that the reduction in the overall neon SXR yield is due to the reduced focused plasma column length for these anodes. The relative HXR yield was the highest for the hemispherical anode followed by the tapered and the flat anodes in that order. The shadowgraphic images and the voltage probe signals confirmed that for the hemispherical anode the multiple-pinch phenomenon was most commonly observed, which could be responsible for multiple HXR bursts for this anode with maximum HXR yields.