Plasma Focus Chamber Research Articles

A pulsed neutron source with a sealed dense plasma focus chamber

Compared with other pulsed neutron sources, dense plasma focus (DPF) devices have the advantages of simple in construction, extremely compact and cost-effective. In this paper, a DPF device was developed for used as pulsed neutron source. The Mather-type sealed chamber was adopted for long duration and multishot operations without vacuum system, which is convenient for transport, use and storage. The maximum D-D neutron yield of the device is approximately 1.1 × 109 n/pulse when the storage capacitors are charged with 22 kV and the deuterium gas filling pressure is 750 Pa. The neutron pulse width (FWHM) was measured to be 45 ns ± 10 ns.

Особенности применения установок Плазменный фокус в радиационном и космическом материаловедении

The features of the use of Plasma Focus (PF) devices in the field of radiation and space materials science related to the specific generation of several types of radiation in the working chambers of the PF are considered. It is noted that the main role in the damage and modification of the structural state of the surface layers of materials irradiated in the cathode zone of the PF working chamber is played by powerful pulsed beams of working gas ions and flows of high-temperature plasma in the nano- and microsecond range of pulse duration. It is shown that in addition to the common combined beam-plasma effect on a target sample, PF installations allow the possibility of separate irradiation of the target material with ion and plasma flows. Three methods for separating pulsed flows of hot plasma and fast ions in space and time in a PF installation, described in the literature, are presented. It is noted that PF installations are a promising tool for solving fundamental and applied problems in radiation and space materials science, including for simulation experiments focused on problems of thermonuclear energy and aerospace engineering. It is also indicated that hard X-ray pulses with a duration in the nanosecond range, arising from pulsed discharges in the PF chamber, can be used for diagnosing fast processes, in particular for non-destructive flaw detection of machines and mechanisms during their operation.

Neutron and Hard X-Ray Pulses of the Plasma Focus Chamber at Discharge Currents of 100–200 kA

Neutron and Hard X-Ray Pulses of the Plasma Focus Chamber at Discharge Currents of 100–200 kA

Operation of Plasma Focus Chamber as a Part of a Subkilojoule Pulsed Neutron Generator

Operation of Plasma Focus Chamber as a Part of a Subkilojoule Pulsed Neutron Generator

Pressure change dynamics of plasma focus chambers as a part of the pulse neutron generators

The dynamics of working gas pressure changes in the plasma focus (PF) chambers were studied by using the chamber with a built-in manometer as part of the ING-102 neutron generator (2.5 MeV neutron yield 106÷107 neutrons/pulse). Investigations were carried out during high-voltage training and neutron emission operation mode. The studies were executed while filling the chamber with various gas mixtures. For operation with a deuterium-tritium mixture, a sealed-off chamber with a built-in gas generator was used. After the primary filling of the PF chambers with a gas mixture, there is a systematic decrease in pressure by Δp = (5 ÷ 50) ·10−3 Torr (∼0,7 ÷ 7 Pa) after each shot of the chamber.The change in the shape of breakdown curves Ubr(p) in the interelectrode gap of the chamber after each series of shots was studied. That made it possible to assess the optimal number of shots during high-voltage training of the PF chamber.

Особенности повреждаемости ниобия импульсным лазерным излучением в сравнении с пучково-плазменным воздействием

The features of damageability of niobium by pulsed fluxes of laser radiation (LR) in the free-running modes (power density qFR = 105 – 106 W/cm2 with pulse duration τFR = 700 μs) and Q-switch mode (q = 108 – 109 W/cm2, τMD = 80 ns) in comparison with the pulsed effects of helium ion (HI) and helium plasma (HP) fluxes in the Plasma Focus (PF) setup at a flux power density qi ~ 108 W/cm2 and qp ~ 107 W/cm2, respectively and pulse durations τi ≈ 30 – 50 ns and τp ≈ 100 ns. LR fluxes were exposed to Nb in air; the working gas in the PF chamber was helium. It is shown that, in contrast to the effect of IG and HP fluxes on niobium in the PF installation, which contribute to the erosion of the material, the irradiation of niobium by pulsed LR in air fluxes under the conditions implemented does not cause noticeable surface erosion. When Nb is exposed to pulsed LR in the FR mode, the melt interacts with the air and forms on the irradiated surface of a thin film of elements of the liquid and gas phases. A similar nature of damage to Nb under the conditions of laser and beam-plasma treatment was found: a wavy relief of the irradiated surface with the presence of drop-like fragments on it, extended wave crests and microcracks. Irradiation of Nb by pulsed LR fluxes in the FR mode leads to the formation in the surface layer (SL) of sections with block and cellular structures, which are also formed after experiments in the PF chamber. It was found that after laser treatment in the FR and Q-switch modes, bubbles (blisters) are not formed in the SL of niobium, which are always present on the irradiated surface when exposed to pulsed fluxes of HI and HP in the PF chamber due to the implantation of helium ions in Nb. It is noted that in laser experiments there is no possibility of implanting working gas ions into the material, which is characteristic of beam-plasma impacts in PF devices, which affects the parameters of damageability and modification of the structure of the irradiated SL.

A Gas Generator Based on Porous Titanium in the Composition of Sealed-Off Plasma Focus Chambers

A Gas Generator Based on Porous Titanium in the Composition of Sealed-Off Plasma Focus Chambers

Shelf life of plasma focus chambers with deuterium-tritium filling

The paper presents the results of an experimental study of the shelf life of sealed plasma focus chambers with deuterium-tritium and deuterium fillings. Shelf life is defined as maintaining the level of neutron yield during operation of PF chambers as part of pulsed neutron generators after long storage intervals. The release of impurities from the inner surfaces of the PF chamber and the accumulation of he-lium He3 due to the decay of tritium in the volume of the sealed-off PF chambers leads to a significant decrease in the neutron radiation yield several years after the chamber is filled with the working mixture. The paper shows that the shelf life of the chambers is significantly increased when using a gas generator, in which hy-drogen isotopes are contained in a bound state, and are released into the inner volume of the chamber only for the duration of operation as part of neutron gen-erators. It has been shown experimentally that spherical chambers of the PF9 type provide a level of neutron radiation yield Y, close to the initial value in the manu-facture of chambers, after more than 10 years of storage.

Structure of Neutron Emission Pulses in Plasma Focus Chambers with Deuterium–Tritium Filling

Structure of Neutron Emission Pulses in Plasma Focus Chambers with Deuterium–Tritium Filling

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.

Повреждаемость и деформационные эффекты в поверхностных слоях меди и сплава системы медь – галлий при импульсном облучении в установке Плазменный фокус

The results of experiments concerning the irradiation of copper and Cu-10% Ga alloys by pulsed fluxes of deuterium plasma (DP) and deuterium ions (DI) performed using a Plasma Focus (PF) unit are presented. The technique of investigation is described. Damage and deformation effects in the surface layers of the mentioned materials after the irradiation of each material in two modes are studied. In one case, the irradiation is performed by a pulsed deuterium plasma at a power density of qpl = 107 W/cm2 and at pulse duration of τpl = 100 ns. In another case, the pulsed fluxes of deuterium ions at qi = 108–1011 W/cm2, τi = 50 ns act simultaneously with a dense deuterium plasma at qpl = 108–109 W/cm2, τpl = 100 ns. Under a less severe irradiation mode (only plasma flux), the levels of damage in the case of both materials are close to each other. In the melted surface layer (SL), there are an undulated surface, craters, and micropores. Under the influence of thermal stresses, a plastic deformation is observed in the SL of the alloy, whereas in the case of pure copper this process is not observed under this irradiation mode. Damage to both materials in a more severe irradiation mode with the pulsed fluxes of deuterium ions and deuterium plasma is enhanced and accompanied by the SL erosion and sometimes by a deposition of microparticles of elements contained in the functional materials of the PF chamber onto the irradiated surface. The most significant damage is observed in the SL of Cu–10% Ga alloy, which, along with the action of powerful plasma beam fluxes, experiences a shock-wave impact. In this irradiation mode, plastic deformation is observed in the SL of each of the materials under consideration. In the case of pure copper (at q = 108–109 W/cm2), the plastic deformation is observed in separate local microvolumes of SL, whereas in the case of copper-gallium alloy at q = 1010–1011 W/cm2 this process occurs within the entire irradiated SL. In this case, plastic deformation occurs both under the action of shock-wave mechanical loads and under the action of thermal stresses.

Experimental Dependence of the Neutron Yield on the Discharge Current for Plasma Focus Chambers Filled with Deuterium and Deuterium–Tritium

Results of measurement of the mean neutron yield from plasma focus (PF) chambers filled with deuterium and deuterium–tritium are compared for various Mather-type and spherical PF devices at discharge currents in the range of 200−1000 kA. On the basis of the experimental results, an expression for the ratio of the neutron yields $${{\xi }_{{{\text{exp}}}}} = \left\langle {{{Y}_{{{\text{DT}}}}}} \right\rangle {\text{/}}\left\langle {{{Y}_{{\text{D}}}}} \right\rangle $$ in the D + T and D + D reactions as a function of the discharge current is derived, according to which ξexp decreases from 150 to 110 (by ≈1.4 times) as the PF discharge current increases from 200 to 1000 kA. Assuming the beam–target mechanism of neutron generation in the PF, the ratio ξexp for D + T and D + D reactions is compared with the ratio σDT/σDD of the cross sections for the corresponding nuclear reactions at different mean energies of accelerated ions. Taking into account the mean energies of accelerated ions (~50–70 keV) determined from the measured space–energy anisotropy of neutron emission, it is suggested that the mean effective energy of accelerated D+ and T+ ions in the plasma beam formed in the pinch decay stage increases with increasing PF discharge current, which leads to a decrease in the ratio $${{\xi }_{{{\text{exp}}}}} = \left\langle {{{Y}_{{{\text{DT}}}}}} \right\rangle {\text{/}}\left\langle {{{Y}_{{\text{D}}}}} \right\rangle $$ with increasing current.

A Plasma Focus Device with a 2-MA Discharge Current as a Hard X-Ray Source

A device based on a pulsed current generator with capacitive energy storage loaded on a plasma focus (PF) chamber is described. The device provides a discharge current amplitude of up to 2 MA in the PF chamber at a stored energy in the capacitor bank of up to 150 kJ. The PF chamber is designed to study hard X-ray (HXR) emission. It has windows for output of HXR emission in the cathode direction, as well as a special insert for output of HXR emission into the anode cavity. A study of operation of the chamber as a part of the setup with the use of various X-ray targets on the anode has been carried out. At a discharge current of 1.5MA, an HXR pulse with an average duration of 16 ns and energy spectrum from 10 to 200 keV, which provides an absorbed dose in the irradiated samples on the order of 1 Sv, is generated in the PF chamber.

Lifetime and shelf life of sealed tritium-filled plasma focus chambers with gas generator

The paper describes the operation features of plasma focus chambers using deuterium–tritium mixture. Handling tritium requires the use of sealed, vacuum-tight plasma focus chambers. In these chambers, there is an accumulation of the impurity gases released from the inside surfaces of the electrodes and the insulator while moving plasma current sheath inside chambers interacting with β-electrons generated due to the decay of tritium. Decay of tritium is also accompanied by the accumulation of helium. Impurities lead to a decreased yield of neutron emission from plasma focus chambers, especially for long term operation. The paper presents an option of absorption type gas generator in the chamber based on porous titanium, which allows to significantly increase the lifetime and shelf life of tritium chambers. It also shows the results of experiments on the comparison of the operation of sealed plasma focus chambers with and without the gas generator.

Specific features of X-ray generation by plasma focus chambers with deuterium and deuterium–tritium fillings

The process of hard X-ray (HXR) generation in plasma focus (PF) chambers was studied experimentally. The radiation was recorded using scintillation detectors with a high time resolution and thermoluminescent detectors in combination with the method of absorbing filters. Time-resolved analysis of the processes of neutron and X-ray generation in PFs is performed. The spectra of HXR emission from PF chambers with deuterium and deuterium–tritium fillings are determined. In experiments with PF chambers filled with a deuterium–tritium mixture, in addition to the HXR pulse with photon energies of up to 200–300 keV, a γ-ray pulse with photon energies of up to 2.5–3.0 MeV is recorded, and a mechanism of its generation is proposed.

Thermal loads of plasma focus chambers

The passage of current through the electrodes of a plasma focus chamber is analyzed taking into account the skin effect; the thermal load on the elliptical shaped electrodes is studied in the interaction with the current-plasma sheath, calculated within the two-dimensional ideal magnetohydrodynamic model. The heating of the anode of the plasma focus chamber by the discharge current is assessed under the limit modes of operation. Signs of the thermal action of plasma particles on the electrode surface are demonstrated.

Theoretical and experimental study of inertial gases admixtures influence on the hard x-ray emission of plasma focus

This paper studies the influence of inertial gases admixtures (Ar, Kr, Xe) to deuterium in plasma focus (PF) chambers. Experiments were realized in PF chambers with discharge currents of 350, 650 and 1000 kA. The measurements of the hard x-ray (HXR) emission were carried out by the scintillation detector SSDI38 with time resolution of 2.5 ns. Experiments show the existence of optimum amount of inertial gases, which corresponds with the atomic number of added gas. At the optimum amount of inertial gas and deuterium in PF chamber, the HXR yield rises up to 10 times in comparison with HXR yield only for deuterium filling. This work shows the dependence of HXR emission on PF device stored energy. The mechanism of inertial gases admixtures influence that leads to rise of HXR yield has been discussed. The mechanism concerns with different behavior of deuterium ions and ions of inertial gases during the pinch decay phase when the discharge current compression force has reduced. Inertial gas ions locate near the axis of the pinch and deuterium ions go to the near plasma area. Local positive charge in plasma forms on this axis because of multiply charged ions of inertial gases. Then electrons gather to the axis area and electron density increases. This electrons form high current electron beam under the influence of the induced electromotive force during the pinch decay phase. HXR emission is generated after the electron beam interaction with the anode target in PF chamber.

Pulsed neutron generators based on the sealed chambers of plasma focus design with D and DT fillings

Development of neutron generators using plasma focus (PF) chambers is being conducted in the All-Russia Scientific Research Institute of Automatics (VNIIA) during more than 25 years. PF is a source of soft and hard x-rays and neutrons 2.5 MeV (D) or 14 MeV (DT). Pulses of x-rays and neutrons have a duration of about several tens of nanoseconds, which defines the scope of such generators—the study of ultrafast processes. VNIIA has developed a series of pulse neutron generators covering the range of outputs 107–1012 n/pulse with resources on the order of 103–104 switches, depending on purposes. Generators have weights in the range of 30–700 kg, which allows referring them to the class of transportable generators. Generators include sealed PF chambers, whose manufacture was mastered by VNIIA vacuum tube production plant. A number of optimized PF chambers, designed for use in generators with a certain yield of neutrons has been developed. The use of gas generator based on gas absorber of hydrogen isotopes, enabled to increase the self-life and resource of PF chambers. Currently, the PF chambers withstand up to 1000 switches and have the safety of not less than 5 years. Using a generator with a gas heater, significantly increased security of PF chambers, because deuterium-tritium mixture is released only during work, other times it is in a bound state in the working element of the gas generator.

Results of Ultracompact Plasma Focus Operating in Repetitive Burst-Mode

The results of a miniature plasma focus are presented in this paper which is operated with energy less than or equal to 150 J. The miniature plasma focus is driven by a small capacitor bank and the peak current delivered in the focus is 75–80 kA. The deuterium gas is filled with a pressure range of 5–7 mbar inside the plasma focus chamber. The quartz glass is used for generating initial surface breakdown at a 4–5-kV discharge, which is a typical value for low-voltage plasma focus discharges. The repetitive operation (record 50 Hz) of the device is achieved by a combination of a simple and high power (5 kW) supply with the synchronized triggering of the capacitor bank at the time of isolation between supply and the capacitor bank. The diameter of cathode is 25 mm and anode diameter is 8–12 mm and both of them are made of stainless steel. The length of anode and gas pressure is adjusted in such a way that the pinching occurs at the time of occurrence of the peak of current. It enhances the neutron emission from the device. The time-of-flight diagnostic is used to distinguish neutron and X-ray emission from the plasma focus. The neutron measurement using a helium-3 detector and also a fast-scintillator-backed photomultiplier tube demonstrates production of neutron pulses in the time separated by nearly 20 ms, which corresponds to 50 Hz in the burst mode. The device can serve the purpose of being a portable and compact repetitive neutron source for various applications as the flux of the radiation is comparable with that of bigger plasma focus operating at the similar current.

A KPU-200 movable capacitor installation

A movable electrophysical capacitor installation with a 250-kJ maximum bank energy, which generates intense neutron pulses, is described. A current pulse generator with a capacitive energy storage forms the basis of the installation. When the initial voltage at the capacitor bank is up to 35 kV, the installation ensures a flow of current pulses with amplitudes of up to 2 MA in a gas-discharge plasma-focus chamber, which is filled with an equal-component deuterium-tritium (DT) mixture. Under these conditions, the chamber is capable of repeatedly generating single fast-neutron pulses with an energy of 14.1 MeV, a duration of ∼70 ns, and an integral yield over 1013 neutrons/pulse.