Uses Of A Plasma Focus Device Engineering Essay

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A dense plasma focus is a plasma machine that produces by electromagnetic acceleration and compression, short lived plasma that is so hot and dense that it becomes a copious multi-radiation source. Plasma Focus (PF) devices appeared as potential nuclear fusion devices based in the pinch phenomenon occurring during the path of high electric currents through the working gas. The operation of PF has been extensively studied and several PF configurations have been developed over the years aiming to increase the neutron emission.

The dense plasma focus device was developed in 1960s independently in USA and the former Soviet Union. Filippov working on a modified design of Z-pinch with a metal wall vacuum chamber noticed that a noncylindrical, high density, high temperature plasma filament was formed close to the anode. The efficiency of the device was further improved when other electrode of the device was removed and ground terminal of capacitor bank was connected to chamber itself. Mather working on a coaxial accelerator, noticed that under certain conditions, a high temperature high density plasma develops at open end, in front of anode. These configurations were termed as plasma focus devices [1].

The dense plasma focus device is a simple pulsed plasma device in which the electrical energy of a capacitor bank, upon discharge, is initially stored as the magnetic energy behind the moving current sheath as the sheath is accelerated along the coaxial electrode assembly. A portion of this magnetic energy is then rapidly converted into plasma energy during the collapse of the current sheath towards the axis beyond the end of the central electrode resulting in the formation of short lived, but hot and dense plasma. Historically, the dense plasma focus device was developed as a fusion device with most of its studies being done in hydrogen and its isotopes in relation to the neutron emission with the aim of determining neutron production mechanisms. During its early stages of development, the focus has been used as a neutron source for pulsed activation analysis, as a spectroscopic source for production of highly ionized species and as a pump source for lasers. The focus device, however, not only is a source of neutrons but also emits highly energetic ions, relativistic electrons and abundant amount of X-rays. Being a source of such a wide range of phenomena, the plasma focus device has found applications in other areas too. More recently, the energetic ions of the plasma focus device have been used for inducing a change of phase in thin films for causing the diode like behavior of polyaniline films and for deposition of thin films [2].

Besides its own perspectives renovated during the last few years due to impressive experiments provided in Sandia lab (U.S.A) with multi arrays pinching assemblies it can serve as a powerful source of various types of ionizing radiations. It is a type of plasma accelerator producing nanosecond pulses of directed powerful hot streams, highly energetic ions and electron beams, soft X-Rays and fusion neutrons.

They were originally developed in the early 1960s independently in the former Soviet Union (Filippov type) and theUSA (Mather type) with D/H > 1 and D/H < 1, respectively, where D and H are the diameter and the height of the anode, respectively. In Mather and Filippov geometries, plasma focus consists of two coaxial electrodes (anode and cathode) separated by an insulator sleeve made up of Pyrex. The Mather type geometry has an anode aspect ratio (diameter/length) < 1 whereas Filippov is characterized by an aspect ratio > 1. In Mather and Filippov type geometries, the density and temperature of the particles are found to be (2-3-1019 /cm3), (1-3) keV and 1019 /cm3, 1 keV respectively [3,4]. The devices have significantly different geometries as shown in the figure

Figure 1.1: Configurations of Mather type and Fillipov type plasma focus device.

1.1 Plasma focus dynamics

A conventional dense plasma focus device consists of two coaxial electrodes (cathode and anode) separated by an insulator sleeve. The plasma focus device is usually operated with the inner electrode as anode and the outer electrode as cathode. Decker et al. [5] reported that in case of negative central electrode x-ray emission and neutron yield is reduced. We use positive central electrode because in this case the electric field helps to concentrate electrons towards insulator sleeve resulting in low inductance breakdown. The device involves current sheath formation whose dynamics is divided into three different phases.

. 1. Breakdown phase or inverse pinch phase

2. Axial acceleration phase

3. Radial phase.

The first one is the initial gas breakdown and current sheath build-up phase. In the second phase (acceleration phase), the sheath is formed and pushed by the j-B force (where j is the current and B is the magnetic induction) towards the open end of the electrode. The third part of the discharge is the rapid collapse of the current sheath toward the axis in the form of a dense plasma column (pinch). The pinch formation and consequently the efficient particle acceleration, is strongly related to the initial stage of the discharge. The current sheath dynamics of plasma focus device is indicated in fig 1.2.

1.1.1 Breakdown phase

The operation of a plasma focus (PF) device begins with the application of a voltage pulse between the coaxial electrodes which produces the dielectric breakdown of the filling gas and the formation of a plasma zone in which the discharge current circulates (the current sheath, CS). This initial phase, referred to in the literature as the 'breakdown phase', conventionally ends when the CS starts to move, pushed by the electromagnetic forces (the CS 'lift off). The ensuing formation of highly energetic, dense magnetized plasma in the device axis (the focus) is known to depend strongly on this phase.

When a high voltage pulse is applied between the coaxial electrodes of the device filled with a working gas at an appropriate pressure an electric discharge is initiated. At the beginning of this discharge, an electric breakdown occurs due to polarization along the insulator sleeve surface after a time delay of few nanoseconds [6] This time delay strongly depends on initial gas pressure [7], but is almost independent of applied voltage (within a reasonable range) during this time delay more electrons are created by the field emission from the metallic edges of cathode plate and ionization of residual gas. These electrons further ionize the gas during their acceleration towards central anode. On reaching anode surface they are recombined with positive charges. Now the surface of the sleeve starts gradually charging while the highest electric field remains at the front of discharge. The development of high current discharge depends on solid insular sleeve [8,9] and gas conditions [10,11].

Figure 1.2: Current sheath dynamics in plasma focus

When the sliding discharge reaches at the end of sleeve both electrodes are connected, breakdown takes place just about the sleeve surface between anode and back plate of cathode. Self generated J-B force can explain this phenomenon. As electrons are more mobile than ions so we consider only electronic currents. The magnetic field due to these currents is in azimuthally direction; hence J-B force acting along the insulator sleeve drives plasma along the insulator surface. As siding discharge reaches at closed bottom end of sleeve surface, it connects the two electrodes. The J-B force directed radially outward lifts the current discharge upward near the cathode surface in the reverse pinch manner. This current sheath reaching at the inner surface of outer electrode consists of two layers, ionization front and magnetic piston. On reaching inner surface of outer electrode initial phase of current sheath dynamics is completed and enters into second phase.

Rawat reported that proper shaping of central electrode of a low energy plasma focus can establish high mode of operation, enhances ion beam and x-ray emission manifold [12]. Optimum length of insulator sleeve gives an azimuthally symmetric and uniform current sheath and any departure from optimum value causes spoke formation on sleeve surface [13].

Zakaullah studied the performance of a low energy plasma focus with the insulator sleeve contamination. It is found that Cu evaporated from the electrode material and deposited on sleeve surface improves the breakdown conditions. A small level of sleeve contamination is essential for good focusing action and high neutron yield [14].

1.1.2 Axial acceleration phase

The breakdown phase ends with the arrival of current sheath at the inner surface of outer electrode, the second phase called axial acceleration or axial rundown phase of plasma focus dynamics starts with the acceleration of current sheath towards s the open end of inner end of outer electrode by its own JB force This phase ends with the arrival of current sheath at anode tip.

J-B force consists of two components, axial component and radial component. The axial component moves the current sheath in axial direction (open end of electrode), while radial component is responsible for pushing current sheath towards outer electrode. Because of 1/r2 dependence of force , axial component is stronger than the radial component near anode which leads to high velocity of current sheath near anode surface which makes current sheath parabolic.

At the end of this phase, inner end of current sheath sweeps around the top of anode and the outer end moves along the tube continuously to accumulate the greater portion of the plasma in the axial direction. Only small fraction of plasma contributes to the final focus at the end of the axial acceleration phase.

There are two conditions which are responsible for subsequent formation of hot and dense plasma.

The arriving time of current sheath at anode tip should be in consistent with the arrival of first maximum of discharge current. This adjustment depends on different parameters applied voltage, gas pressure and electrode geometry. It represents energy transfer from capacitor bank to pinch plasma.

For good focusing, the structure and profile (r,z) parameters of current sheath must have certain characteristics, which ensures axial symmetric and uniform current sheath.

Two important phenomena take place in axial acceleration phase, namely the current shedding and mass loss. Current shedding is the loss of discharge current during the current sheet movement. On the other hand, mass loss is the escape of mass, carried by current sheet, through the transparent outer electrode. The current shedding occurs due to the slow moving diffused current layer behind the main current sheet and the current retained by insulator surface whereas mass loss results from the pressure gradient in the canted current sheet [15]. The mass loss can be realized from the decrease in current sheet thickness as it moves towards the open end of the electrodes.

Hawat et al[16] used a magnetic probe to study the current sheath in plasma focus device. In axial phase of plasma, current sheath, axial velocity and magnetic field profiles were theoretically predicted by snow plow model and experimentally measured using magnetic probes, for a filling gas at a specific pressure and voltage.

1.1.3 Radial collapse phase

As the current sheath reaches at the tip of anode the J-B force acts radially inward on the current sheath and converges it on the top of central electrode and all the magnetic energy is converted into plasma kinetic energy. This phase is indicated in fig[1.2]. Radial collapse occurs in a very short interval of time that is within 50-200nsec depending on device characteristics.

The velocity of this collapse ranging from 7-60cm/μsec depending on different conditions such as insulator sleeve length, current sheath profile, electrode geometry, gas pressure and device electrical characteristics[17] . During radial compression inductance of the system increases resulting in magnetic field diffusion into plasma column. The plasma sheath ion temperature does not exceed 300ev and electron temperature is much lower before reaching the axis [18]. This temperature difference creates an energy exchange between electrons and ions through collisions resulting in plasma column expansion in axial direction and hence inductance rises. This change in inductance induces an electric field, which is enhanced due to the growth of some instabilities.

E=I dL / dt

Where I is discharging current and dL/dt is rate of change of inductance. the radial collapse phase is further divided into four phases.

Rawat et al [19] studied plasma sheath structure and its dynamics in the presence of external axial magnetic field (Bz) in dense plasma focus with the help of N2-laser shadowgraphic technique. The shadowgraphs of axial phase show that in the presence of external Bz the current sheath gets diffused near the end attached to the central electrode when it reaches to the top of the central electrode. The shadowgraphs of radial collapse phase show clearly distinct hump formation above central electrode under the influence of Bz. Moreover, during the radial collapse phase a part of the current sheath lying close to central electrode gets highly diffused in comparison to the rest of the current sheath. Diffused bubble formation on the ionization wave front and dominantly visible remnant of plasma sheath above central electrode have also been observed during the quiescent phase (the expansion phase after the peak compression).

1.1.3.1 Compression phase

The compression phase starts with the sweeping of the current sheath toward the central electrode (anode) due to inward force. So the current sheath collapses radially with a non-cylindrical, funnel-shaped profile. For the formation of final focus, plasma column is compressed adiabatically. The pinch plasma column having diameter 1mm and length 7 mm is formed that the whole process lasts (time between the breakdown and collapse phase) for 850 ns. At the end of this phase magnetic field penetrates the plasma column rapidly.

1.1.3.2Quiescent phase

This phase starts with the expansion of plasma column in both axial and radial direction. In radial direction, expansion rate is hindered by magnetic pressure confinement while due to fountain like geometry of current sheath plasma column can easily expand in axial direction. A sharp increase in plasma inductance during compression phase induces an electric field which accelerates ions and electrons in opposite direction. At this stage micro-instabilities starts to grow and plasma confined is magnetically unstable.

1.1.3.3Unstable phase

Due to the electric field produced, electron and ions are accelerated in opposite direction. As accelerated electrons hit the anode, a large amount of impurities are injected in plasma column, disruption is produced, which continues until plasma column is broken completely, consequently plasma density decreases. In this phase electron velocity increases (4-5kev), so that large amount of radiations are emitted.

1.1.3.4 Decay phase

Due to complete disruption of plasma column a very large, extremely hot and thin plasma cloud is produced, during this phase. A large number of soft x rays, Bremstrahlung radiations, energetic ions and electrons are emitted during this phase. The neutron phase started in unstable phase reaches its peak value. For a Mather type device with an anode radius of 1cm, the implosion time is ~100ns while lifetime of plasma column is ~20ns [20]. This is the last stage of plasma focus dynamics.

1.2 Radiation emission from dense plasma focus device

Different types of radiations are emitted from hot plasmas. These radiations include electromagnetic radiations, neutrons, electrons and ion beams as well as high energy neutral particles, that is atoms and molecules. Plasma emits electromagnetic radiation in a broad frequency range. This radiation includes the line radiation as well as the continuum radiation. The relative strength of continuum and line emission depends on how plasma is formed. Continuum emission dominates in plasma of high Z-material whereas line emission is stronger in case of low Z-material plasma [21].

It is possible to determine the properties and characteristics of plasma from the examination of these electromagnetic radiations emitted from the plasma. The intensity of the radiation at a given wavelength, or its spectral distribution over a given wavelength band can be determined experimentally, and these measurements can be related to the relevant plasma parameters, that is electron temperature, etc. If the plasma temperature is greater than about 100 eV, the most of the radiation from plasma is bremsstrahlung [22].

During the last 20 years, attempts have been made to enhance the X-ray yield by adjusting different parameters such as discharge energy, operating voltage, filling pressure and different machine parameters in addition to normal operating conditions. It is a well-known fact that the production of stable pinch plasma by minimizing the axial and azimuthal non-uniformities leads to enhanced soft and hard X-ray emission [desktop dimple]. Several efforts were made to study the x-ray emission from DPF.

Burkhalter et al [23] investigated the x-ray emission from neon plasma by varying bank energy and anode/cathode diameter. They observed that soft x-ray yield was increased by the reduction of anode diameter and increase of bank energy.

Belyaeva et al [24] reported x-ray pinhole photograph of plasma focus. In single structure energy of x-ray emission reaches values of the order of 300ev. For large quantities of structures x-ray emission was in the range of ~200kev.

Zakaullah et al [25] investigated x-ray and ion beam emission from a low energy Mather type plasma focus employing three different anode shapes. They observed that for good focusing the radiation yield and the filling gas pressure were strongly dependent on anode shape. Furthermore, the time integrated pinhole images indicated the origin of x-rays from anode end surface.

1.3 Applications of plasma focus device

For nuclear weapons, as an external neutron source.

Simulation of nuclear explosions (testing of electronics equipments) and a shoot and intense neutron source.

Suitable to be used as a pulsed point source for some special applications such as microscopy, x-ray lithography, x-ray micromachine, x-ray backlighting and x-ray radiography.

It can be used for detection of illicit materials just in a single ns shot of the device that shorten the whole procedure, in particular in a case of hidden fission materials. It can be applied in low-dose medical X-Ray diagnostics as well as in micro-radiography. It looks promising to use a DPF for irradiation malignant tumors in BCNT by thermal neutrons as well as in a therapy by fast neutrons, in particular in combination with hard X-Ray photons generated by it in the same shots; it opens perspectives in a low-dose therapy. It has good opportunities in a production of short-lived isotopes for the aims of PET. DPF has attractive perspectives for application in nuclear physics, in particular in combination with sub-critical assemblies.

1.4 Stainless steel

In 1913, English metallurgist Harry Brearly, working on a project to improve rifle barrels, accidentally discovered that adding chromium to low carbon steel gives it stain resistance. In addition to iron, carbon, and chromium, modern stainless steel may also contain other elements, such as nickel, niobium, molybdenum, and titanium. Nickel, molybdenum, niobium, and chromium enhance the corrosion resistance of stainless steel. In metallurgy, stainless steel is defined as a steel alloy with a minimum of 11% chromium content by mass. Stainless steel does not stain, corrode, or rust as easily as ordinary steel (it stains less, but it is not stain-proof). Stainless steel is frequently used as construction material in aggressive environment. Their generally excellent resistance to corrosion is due to the spontaneous formation of a protective film with a thickness of a few nanometers on the surface.

Among the materials that withstand corrosion, stainless steel shows an excellent resistance in a large number of atmospheres, due to a phenomenon known as passivity. Physical and mechanical properties (toughness, strength and ductility), ease of fabrication (particularly ease of forming) excellent fatigue resistance and energy absorption capability are some of the properties of Stainless Steel which enable the specific requirements of structural components to be met. Common uses of stainless steel are cutlery and watch straps

1.4.1 Types of stainless steel

Stainless steel alloys are austenitic, ferritic, martensitic, precipitation hardened, and duplex metals that are available in a wide variety of grades, shapes, and sizes. These types of steels are identified by their microstructure or predominant crystal phase.

1.4.1.1 Austenitic:

Austenitic steel have austenite as their primary phase (face centered cubic crystal). These are alloys containing chromium and nickel (sometimes manganese and nitrogen), structured around the Type 302 composition of iron, 18% chromium, and 8% nickel. Austenitic steels are not hardenable by heat treatment. The most familiar stainless steel is probably Type 304, sometimes called T304 or simply 304. Type 304 surgical stainless steel is austenitic steel containing 18-20% chromium and 8-10% nickel. Good to excellent corrosion resistance combined with very good weldability and formability characterize the austenitic stainless steels. The austenitic structure has good creep resistance and good oxidations resistance that makes them useful in elevated temperatures. Austenitic steel can also be used in cryogenic applications and is in annealed condition the only non-magnetic steel.

1.4.1.2 Ferritic:

Ferritic steels have ferrite (body centered cubic crystal) as their main phase. These steels contain iron and chromium, based on the Type 430 composition of 17% chromium. Ferritic steel is less ductile than austenitic steel and is not hardenable by heat treatment. Ferritic stainless steels have good corrosion resistance, especially towards stress corrosion cracking. Lower carbon and nitrogen contents improve both weldability and toughness which otherwise can be limited. Ferritic stainless steels have good corrosion resistance, especially towards stress corrosion cracking. Lower carbon and nitrogen contents improve both weldability and toughness which otherwise can be limited.

1.4.1.3 Martensitic

The characteristic orthorhombic martensite microstructure was first observed by German microscopist Adolf Martens around 1890. Martensitic steels are low carbon steels built around the Type 410 composition of iron, 12% chromium, and 0.12% carbon. They may be tempered and hardened. Martensite gives steel great hardness, but it also reduces its toughness and makes it brittle, so few steels are fully hardened. These stainless steels are characterized by high strength and high wear resistance. The corrosion resistance is limited and the weldability degrades with increasing strength, i.e. increasing carbon content.

1.5 STAINLESS STEEL 303

Type 303 austenitic stainless steel, with an appropriate sulfur content that enhances its machinability, is frequently used as an easy machining alloy. The good resistance to atmospheric corrosion, and too many other chemicals, combined with a high machinability, makes it suitable for mechanical industries applications. Typical applications include: nuts and bolts, Bushing, Shafts, Aircraft Fittings, Electrical Switchgear Components and Gears. Its chemical composition is

Cr=17.5%, Mn=2.0%, Ni=8.5%, Mo=0.4%, P=0.2%, C=0.15%

1.6 Transition metals

Transition metals are the elements characterized by a partially filled d-subshel. All the transition metals have the same arrangement of outer electrons; only the 3d orbitals lower down are different. They are very hard, with high melting point, boiling point and high electrical conductivity. They have high ionization energies and exhibit a wide range of oxidation state. Copper, cobalt, iron zinc and tungsten are the examples of transition metals.

1.7 Tungsten

Tungsten also known as wolfram is a chemical element that has the symbol W and atomic number 74. The history of the development of tungsten and its alloys for practical use is more than two centuries old. C.W.Sheele discovered tungsten in 1981. J.J.de elhuyer published the first account regarding the isolation of tungsten from ore in 1783. The importance of tungsten hardened steel was recognized immediately, the manufacture of rails being the practical application of that product.

1.8 Industrial and other applications of tungsten

The physical attributes of tungsten alloy's density, hardness and heat-resistance have led to a multitude of peacetime uses. Modern uses are in aircraft, automotive and power components, for shielding and arc welding. The use of tungsten or its alloy in our daily lives is becoming increasingly common. For example it is used in manufacturing of some hand tools such as chisels and saw blades, and sporting equipments. Because of its ability to produce hardness at high temperature and its high melting point elemental tungsten is used in light bulb, cathode ray tube , vacuum tube filaments as well as heating elements and nozzles on rocket engine.

1.9 Tungsten carbide

Carbides have a unique set of properties necessary for many applications such as cutting tools and dies. These properties include great hardness and wear resistance, good thermal shock and thermal conductivity and good oxidation resistance. It is well established that tungsten carbide maintains its hardness at high temperature, and is characterized by their hardness and strength, are used in applications where materials with high wear resistance and toughness are required [1]. In hard metals industry, it is considered that nanostructured cemented carbides as an important branch of nanocrystalline materials could offer new opportunities for achieving superior hardness and toughness combination.

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