Dynamics Of Current Sheath Research Articles

MHD Simulation of Physical Processes in Spherical Plasma-Focus Chambers with Allowance for Neutron Generation

The results of the development of a two-dimensional MHD code for carrying out computational studies of the dynamics of plasma current sheath in spherical chambers with a plasma focus are presented. Equations of magnetohydrodynamics with allowance for magnetic field diffusion, thermal conductivity and plasma radiation are used in this work. An implicit scheme is used in the calculation of the magnetic field, which makes it possible to describe the motion of plasma in a low-density region behind the plasma sheath. The formulas that take into account the possible appearance of anomalous resistance in the plasma are used to calculate the plasma conductivity. The neutron yield is calculated with allowance for thermonuclear and beam–target neutron generation mechanisms. The effect of the minimum residual gas density behind the plasma sheath on the cumulation of the plasma sheath is studied. The effects of magnetic field diffusion, thermal conductivity and anomalous plasma resistance on the plasma sheath dynamics are considered. The calculations are performed for two spherical plasma-focus chambers operating with currents up to 1 and 2 MA and neutron yields to 1012 and 1.5 × 1013 DT neutrons, respectively. The comparison of the calculated dependences with experimental data on the current, voltage and neutron yield made it possible to refine the parameters used in the calculations and achieve a satisfactory agreement between the simulation and experiment.

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.

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.

Dynamics of current sheath in a hollow electrode Z-pinch discharge using slug model

The hollow electrode Z-pinch (HEZP) experiment is a new construction for the electromagnetic propulsion application in which the plasma is formed by the discharge between a plate and ring electrodes through which the plasma is propelled. The experimental results for 8 kV charging voltage shows that the peak discharge current is about 109 kA, which is in good agreement with the value obtained from the simulation in the slug model that simulates the sheath dynamics in the HEZP. The fitting of the discharge current from the slug model indicates that the total system inductance is 238 nH which is relatively a high static inductance accompanied with a deeper pinch depth indicating that the fitted anomalous resistance would be about 95 mΩ. The current and mass factors vary with the changing the gas pressure and the charging voltage. The current factor is between 0.4 and 0.5 on average which is relatively low value. The mass factor decreases by increasing the gas pressure indicating that the sheath is heavy to be driven by the magnetic pressure, which is also indicated from the decreases of the drive factor, hence the radial sheath velocity decreases. The plasma inductance and temperature increase with the increase of the drive factor while the minimum pinch radius decreases.

Axial Distribution Profile of Magnetic Field and Plasma Sheath Velocity in Argon Plasma Focus Device

The main aim of this work is to study dynamics of plasma current sheath, PCS during the axial acceleration phase in term of its axial velocity, Vz and the induced azimuthal magnetic field induction, Bθ in 2.2 KJ plasma focus, PF device of Mather type. It powered by a capacitor bank of 30 µf capacity with charging voltage of 12 KV. In the present paper all the experimental work are performed for Argon gas pressure in the range of 0.2 – 1.8 Torr. The length of the inner electrode, IE has been changed to 9.5, 10.5 and 11.5 cm respectively. To detect, Bθ and Vz three identical magnetic probes are located at radial distance r1 = 2.75 cm, r2 = 3.5 cm and r3 = 4.25 cm from the IE axis and at axial distance z = 8.15 cm from coaxial electrodes breech.The dependence of Vz and Bθ on gas pressures are measured and the results of them clear that, Vz is decayed with increasing of gas pressures and it has a maximum value for IE length of 10.5 cm, P = 0.2 Torr and at r1, while Bθ has a maximum value for IE length = 11.5 cm, 1.2 Torr at r1 . Also distribution of Vz with p in linear fitting shows that efficient a snow-plough action is obtained approximately for IE length = 10.5 cm. Results of Vz versus radial distance demonstrated that the PCS has a parabolic shape especially at lower Argon gas pressures. Results of modification factor, F as a function of gas pressures show that at IE = 10.5 cm, r1 and for most Argon gas pressures F has a higher values than any others IE lengths and radial distances. Also the decay of dF/dp along radial distances has a smaller values at IE length = 11.5 cm than any other lengths (9.5 and 10.5 cm).

Plasma Kinetics in Deuterium-Filled Plasma Focus with Step Anode Configuration

A numerical analysis of a plasma focus device with step anode as the central electrode is carried out with Lee Model Code for a stepped central electrode configuration. The snowplow model is employed to describe the dynamics of current sheath between the electrodes using momentum conservation of the swept gases. The discharge current and the tube voltage are determined by circuit equation. Different plasma parameters including plasma temperature and speed of current sheath are simulated for the step anode configuration. The step anode model is compared with the standard electrode configuration to observe the dynamical behaviour of the plasma during the axial and radial phase. The results indicate that the current dip and the spike voltage are slightly higher and an increase in the speed of current sheath at both axial and radial phases are observed. The step anode has shown peak current of 0.4% less than the cylindrical anode. However, the current sheath speed is enhanced at both axial and radial phases up to 20.1% for step anode configuration. The step anode configuration is also contributed to rise the plasma pinch temperature. The new configured anode in Lee's model can carry a new tube parameter for optimizing the plasma focus device for SXR yield and neutron yield based on numerical experiment.

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.