High Current And High Voltage Pulsed Power Sources

Abstract

This paper deals with circuits and systems for producing electrical impulses having desired characteristics and especially to systems for producing synchronizing impulses and the like for use in a television system. In the Bedford system the various voltage impulses, such as synchronizing and blanking impulses, are generated by means of photoelectric cells and rotating discs having openings therein through which light beams are projected onto the said cells. As mentioned above developing with those characters and the generator is free from moving parts for generating electrical impulses. A further object of the paper is to provide an improved circuit including an electric discharge tube oscillator and associated circuits for producing square-topped electrical impulses having the desired width and having the desired time relation to each other. A further object of the paper is to provide an improved “peak clipping” circuit. A further object of the paper is to provide an improved method of and means for narrowing electrical impulses. A further object the paper is to provide an improved method of and means for delaying or shifting the phase of an electrical impulse. A further object the paper is to provide an improved method of and means for slotting an electrical impulse or a group of electrical impulses. A still further object of this paper is to provide an improved method of and means for choosing a desired number or group of electrical impulses from electrical impulses which are continuously generated.

Table of Contents

TOC o “1-3” h z u HYPERLINK l “_Toc372708948″Introduction PAGEREF _Toc372708948 h 2

HYPERLINK l “_Toc372708949″The voltage current characteristics PAGEREF _Toc372708949 h 10

HYPERLINK l “_Toc372708950″The spark PAGEREF _Toc372708950 h 12

HYPERLINK l “_Toc372708951″Glow discharge PAGEREF _Toc372708951 h 15

HYPERLINK l “_Toc372708952″The arc discharge PAGEREF _Toc372708952 h 18

HYPERLINK l “_Toc372708953″Electric arcs and cathode spots PAGEREF _Toc372708953 h 20

HYPERLINK l “_Toc372708954″Discharges in dielectric liquids PAGEREF _Toc372708954 h 26

HYPERLINK l “_Toc372708955″Electric discharge machining device PAGEREF _Toc372708955 h 31

HYPERLINK l “_Toc372708956″Other applications PAGEREF _Toc372708956 h 47

HYPERLINK l “_Toc372708957″Introduction to high pressure water jets PAGEREF _Toc372708957 h 50

HYPERLINK l “_Toc372708958″Physical background processes of high pressure water jets PAGEREF _Toc372708958 h 52

HYPERLINK l “_Toc372708959″Applied nozzle design PAGEREF _Toc372708959 h 63

HYPERLINK l “_Toc372708960″Modeling of fluid structure interaction problem PAGEREF _Toc372708960 h 87

HYPERLINK l “_Toc372708961″Physical and numerical model PAGEREF _Toc372708961 h 111

HYPERLINK l “_Toc372708962″Electrical circuits PAGEREF _Toc372708962 h 127

HYPERLINK l “_Toc372708963″The schematic PAGEREF _Toc372708963 h 141

HYPERLINK l “_Toc372708964″Setup description sample PAGEREF _Toc372708964 h 146

HYPERLINK l “_Toc372708965″Conclusion PAGEREF _Toc372708965 h 165

HYPERLINK l “_Toc372708966″References PAGEREF _Toc372708966 h 166

Introduction

Creation of a conducting path between two points of different electrical potential in the medium in which the points are immersed leads to electrical discharge. The discharge is permanent if the supply of electrical charge is continuous, but otherwise it is temporary, and serves to equalize the potentials. Normally, the medium is a gas, often the atmosphere, and the potential difference ranges from a few hundred volts to millions of volts. For the discharge to occur there must be a medium between the two points. Only matter can carry electric charge, hence the transfer of matter between the two points is necessary. The matter between the two points is generally electrons which each carry charge of 4.803 x 10-10 esu. Electrons can be moved with little effort due to the very light, 9.109 x 10-28 g. Ions can also be used to carry charge and are 1836 times heavier than electrons, and in some cases are important carriers.  In cases where both ions and electrons are available as the medium, the electrons carry the majority of the current (Nakata, T., & Takahashi, N, 1986). Ions can either be negatively or positively charged, usually positively, and can carry small multiples of the electronic charge.

Electrical discharges have been studied since the middle of the 19th century, when sources of current electricity and vacuum pump became available. Lightning is the primary of electrical discharges in nature, others includes crackling sounds when clothes fresh from the dryer are separated. In laboratory electrical discharges mostly takes place in partially-evacuated tubes. Technology offers a wealth of examples, such fluorescent lamps, arc welding , the corona discharge on high-tension lines, including their automatic starters, argon and neon glow lamps, neon advertising signs, sodium and mercury lamps, mercury-arc lamps for illumination and UV, vacuum tubes ,carbon arc lights, including gas-filled rectifiers, Nixie numerical indicators and similar devices.

For electric discharge to take place, the potential difference must exit between the two electrodes. The higher potential is at the anode, while the negative or lower potential is at the cathode. The anode and the cathode are mostly conductors. In simple term the anode is “way in” and cathode is “way out”. Making the assumption that the medium is gas is composed of neutral molecules. The pressure p of the gas is related to its number density n by p = nkT, where k is Boltzmann’s Constant, 1.38 x 10-16 erg/K and T is the absolute temperature. The pressure p is in dyne/ cm2 if n is in cm-3 and T is in K. 1.0123 x 106 dyne/cm3 is equivalent to 760mmHg which is the atmospheric pressure and this can be used for conversion as in technical work, gas I measured in mmHg. At 273 K and 760mmHg, the number density in a gas is 3.22 x 1018 cm-3, which is the same for all gases according to the Avogadro’s Law. The gas is generally electrically neutral as it contains neither electrons nor ions, and so is a nonconductor. Hence air is an insulator.

For electric discharge to take, place there must be a source of electrons at the cathode, the nature of this source of electrons controls the form of the electric discharge. Small air conductivity is as result of natural radioactivity and cosmic rays and which continually produce a small number of ions and electrons in all gases at the surface of the earth. As the ions move to the cathode and the electrons to the anode, a small current flow. For a good discharge a more copious sources of electrons are necessary. An example of the source is the photoelectric effect, when light of sufficiently short wavelength falls on a semiconductor or metal and liberates a photoelectron. When molecules absorb photons, the molecule releases an electron and become a positive ion. The photon energy must be more than the energy required to free the electron that is greater than the work function. The emission of electrons by a heated body known as thermionic emission, can supply heavy currents. The body to be heated work function must be low for effective emission. Also the body to be heated should have a high melting point. The tungsten which for long has been used as electron emitter has a work function of 4 eV, and high melting point. The secondary electron as result of electron striking a metal surface, has little use for discharge, since the electron impact the anode and the secondary electrons would simply fall back into the anode, not add to the discharge current. Also, positive ions can create secondary electrons. Though not an effective process, ions produces electrons at the right place and can support an electrical discharge. Electrons already in the discharge, such as the random electrons produced by radioactivity and cosmic rays, can increases their number by ionizing gas molecules through collision. Each ionizing collision produces a positive ion and a new electron that moves the other way, an ion pair. For electron to do this it should have acquired sufficient kinetic energy through being accelerated in an electric field. This can be done in two ways. If the electron makes no collisions, even a small electric field will allow it to accumulate energy in a long-enough run. In this case, KE = mv2/2 = eEx, where x is the distance travelled and E is the field. Electron volt, abbreviated as eV, is often used to quote the electron energies. The probability of collision of the electron in a distance dx is given by dx/Le, where Le is known as the electron mean free path, and the speed of the electron is given by v = KE if Le is much smaller than the distance x, where K is the electron mobility, in cm/s per V/cm, for example. Then, the only way for the electron to accelerate is to find a larger field E. The pressure mean free path L is inversely proportional to the mean free path L, so pressure has a great effect on how an electron gains energy. The molecules of the gas also have a mean free path, but since molecules are larger, the molecules mean free path L is shorter than Le. As an approximate, we can take Le = 5.64L. In Ne, the mean free path at 273K and 760 mmHg is 1.93 x 10-5 cm, while in air it is 9.6 x 10-6 cm. Making the assumption that air has the usual mixture of nitrogen and oxygen, and the values are an average.

The ionization energy, in the reaction Ne → Ne+ + e-, is 21.559 eV, and 41.07 eV is required to knock two electrons off requires. To raise a Ne atom to its first excited state requires 16.58 eV, which is called the resonance energy. This gives an idea of the energy required to produce an ion pair. Since most collisions do not result in ionization, and there are many ways to fritter energy away uselessly, the average energy per ion pair produced is greater than the ionization potential, rather closer to twice this value. To give an electron sufficient energy to ionize Ne at one atmosphere, the field strength would have to be E = 21.559/ (5.64)(1.93 x 10-5), or about 200,000 V/cm, an extremely high field that would have some untoward effects. At 1 mmHg, the field would have to be 260 V/cm, a more tractable value.

Resonance energy is the energy required to excite an atom or molecule to its first excited state above the ground state, and is less than the ionization energy. Due to the closed shell of electrons in the ground state of the inert gases, makes inert gases to have very large resonance energies. For neon, it is 16.62 eV and for helium, it is 19.81V. The transition of these levels to the ground state by radiation is difficult, and they may retain their excitation energy for an extended period, perhaps until they collide with a wall, or experience another collision with an atom or electron this state is known as metastable. This makes cumulative ionization possible, where an atom can be ionized by multiple collisions in which the electrons have insufficient energy to ionize in a single collision. The energy of a metastable can be transferred to a different molecule or atom through a collision of the second kind. Alkali metals, with a single s electron outside a closed shell, have very low resonance potentials. For sodium, Vi = 5.138V and Vr = 2.102V. For caesium, the Vi = 3.893V and Vr = 1.39V. Mercury, frequently used in discharges, has Vi = 10.43V and Vr = 4.67V, and the lowest excited states, 3P’s are metastable to the 1S ground state. The spectroscopic notation is included for those who will appreciate it. The fundamentals of spectra and atomic structure are crucial in understanding discharges. The transition of these levels to the ground state by radiation is difficult, and they may retain their excitation energy for an extended period, perhaps until they collide with a wall, or experience another collision with an atom or electron this state is known as metastable. This makes cumulative ionization possible, where an atom can be ionized by multiple collisions in which the electrons have insufficient energy to ionize in a single collision. The energy of a metastable can be transferred to a different molecule or atom through a collision of the second kind

One of the principal characteristics of discharges is the emission of light. When an excited atom falls to a lower energy level, light of a definite frequency is emitted. If there is an electric dipole transition moment, then the transition is called allowed, and takes place in about 10-8 s if nothing interferes. The collision frequency is about 1011 per second at atmospheric pressure, so generally the excitation energy is lost in a collision before it can be radiated. At 1 mmHg, nevertheless, the collision frequency is comparable to the radiation lifetime, and radiation is a possibility. Radiation is always a competition between de-excitation processes. If the dipole transition moment is forced to be zero by symmetry considerations, then radiation may occur by other means, such as quadrupole radiation or magnetic dipole or, but the radioactive lifetime for these is much longer, so they are not seen even at 1 mmHg pressure. These are forbidden transitions. They are not really forbidden, just improbable. At higher pressures, excited atoms are continually affected by collisions, which broaden the lines emitted. The atom states are smeared out, and the radiation begins to assume the characteristics of black-body thermal radiation at still higher pressures. Electron avalanche, is created when an electron frees another by an ionizing collision, then these two became both free additional electrons, and so on. Electron avalanche may send a burst of electrons toward the anode, leaving in their wake a cloud of slow positive ions that will make their way to the cathode. The net result is to multiply the original electron current. Gas phototube uses this effect to increase the photocurrent for a given amount of light. This merely increases the current that otherwise would be available but does not start a sustained discharge. This type of discharge produces little light, so it is called a Townsend or dark discharge.

That cloud of positive ions will later or sooner collide with the cathode. It is rather unlikely for a positive ion to snatch an electron from the few that are available while it is moving through the gas. It is hard to conserve both energy and momentum as recombination is a very difficult process, since only one particle is the outcome, rather than the three particles that come out of ionization. Therefore, most of the positive ions created in an electron avalanche reach a surface finally, and they are driven to the cathode by the electric field. When they arrive, they recombine at the surface, and in some cases eject an electron. In case, the electron avalanche produces more than 45 electrons, then there will be sufficient positive ions to replace the electron that originally left the cathode or came in from elsewhere. Now the discharge produces its own electrons, without relying on natural radioactivity or cosmic rays, and becomes self-maintained. This is a crucial event in the life of a discharge, and usually means that the discharge becomes evident by noise or light. The potential between anode and cathode at which discharge becomes evident by noise or light is called the sparking potential Vs. Now the whole path between cathode and anode becomes conducting because of the ions and electrons distributed along it.

Current increases rapidly and without bounds, unless something limits the, such as the disappearance of the potential difference. The ion bombardment heats up the cathode surface, which becomes incandescent, and begins to emit electrons thermionic ally, without efficiency of the electron avalanche or reference to the number of ions coming in. Any spot that becomes hotter than its neighbor tends to become even hotter as the extra thermionic electrons attract the positive ions to the spot. This, the final state of the discharge, is called an arc. The name came from the way the path of the discharge, when arranged to be horizontal, rose in a flaming arch, or arc. It needs very little potential difference to support the arc, mainly just enough to keep the path of the discharge supplied with ions to replace those lost in various ways. A lightning stroke is an example of such a discharge, but with cathode and anode that are quite different from those in a carbon arc light. In the carbon arc light, the discharge is initiated by drawing the carbons apart, which produces an arc at once, since the discharge does not have the difficult task of establishing a conducting path over a great distance, as in lightning. An arc is also produced whenever an electric circuit is interrupted, and must be extinguished before it does any damage.

The nature of a discharge depends, on how the discharge is confined and the method for supplying electrons at the cathode. The carbon arc and the lightning stroke are both unconfined arcs. The lightning stroke draws its electrons from its cathode, the cloud, and transmits them to the earth, its anode. The carbon arc obtains its electrons from the cathode spot on the negative carbon, which it heats to incandescence. Both are self-confined, the surface of the conducting channel arranging itself so that the net outward current is zero. A discharge between metal electrodes in a glass tube that gets its electrons from positive-ion bombardment of the cathode, and is confined by the glass walls, is called a glow discharge. Glow discharges are useful and convenient to study, so their properties are very familiar, if not those of the majority of discharges. It is rather unlikely for a positive ion to snatch an electron from the few that are available while it is moving through the gas. It is hard to conserve both energy and momentum as recombination is a very difficult process, since only one particle is the outcome, rather than the three particles that come out of ionization. A discharge may exist in the vicinity of a sharp point, or other place with a small radius of curvature where the electric field is increased significantly from its average value. A negative potential on the point makes it a cathode, while the anode is an indefinite volume in the surrounding gas. A positive potential makes it an anode, and attracts electrons from an indefinite surrounding volume, which becomes the cathode. These two discharges look quite different with constant potentials, but with alternating current the opposites succeed one another and make an average impression. If the discharge occurs at about atmospheric pressure, it is called corona.

Any discharge, multiple processes compete at the gas and in the electrodes, so theories and explanations can become subjects of dispute. A theory normally takes into account only the principal process operating under the conditions of the problem, and this is often quite acceptable. Sometimes different mechanisms and assumptions can result to the same outcome, which further complicates things. The reader should keep in mind that complete explanations are probably impossible in many cases, and we must be satisfied with qualitative or semi-quantitative results. Also, the varieties of phenomena in discharges are very rich and depend on many factors, such as surface preparation and purity that are difficult to quantify. There is great scope for reasoning and thought in this field, which makes it fascinating, along with the beauty of the phenomena.

The voltage current characteristics Considering a general laboratory discharge, taking place in a glass tube with metallic electrodes. The nature of the electrodes has less effect on the characteristics of the discharge. The most used materials are platinum, carbon, iron, tungsten or nickel. The voltage source E is connected in series with a current limiting resistance R, so that the voltage between cathode and anode is V = E – IR. This relation is expressed by the load lines in the diagram, for values of R equal to R3 > R2 > R1. The irregular curve is the V-I characteristic of this device, distorted to show the various regions conveniently. Point A where R = R1 is a stable point of operation. This can be seen as follows: suppose the current I to be slightly reduced for some reason. Then V becomes greater, according to the load line, while the voltage between cathode and anode becomes smaller. The difference in voltage acts to increase the current, restoring it to the value before the disturbance. If the current is slightly increased, we find a voltage deficit, which reduces the current, again bringing the operating point back to the original place. This will always happen if the V-I curve is more steeply inclined than the load line. At point A, the current is no more than a microampere; the discharge is not self-sustained, and is dark. We are in the dark region also known as Townsend region.

In case where, resistance reduces steadily from R1 to R2 . Point A moves up the curve until the sparking potential is reached. Now the voltage is sharply reduced, and the operating point is changes from A to B, which is stable. The discharge is now self-sustaining as a glow discharge, and cathode heating is not enough to cause transition to an arc. If R is further decreased, towards R3, the voltage across the discharge increases until point B’ is reached. Although B’ is stable with respect to small changes, cathode heating may be enough to lower the discharge voltage and increase the electron supply. This change is cooperative, and the discharge quickly moves to point C, where V is lower and I is greater. This is the arc, and operating point C is stable. Nevertheless, if R is further reduced, the current will increase without bound until something melts. The regions where the discharge type changes are shown as cross-hatched, to show that the actual values may not be clearly defined . The difference in voltage acts to increase the current, restoring it to the value before the disturbance. If the current is slightly increased, we find a voltage deficit, which reduces the current, again bringing the operating point back to the original place. This will always happen if the V-I curve is more steeply inclined than the load line. This characteristic tells a lot about the circuit behavior of discharges, but it does not say much about the dynamic relations, only about the stable operating points.

The spark Analyzing the initial breakdown of the discharge, that produces the spark. We assume that every electron emitted from the cathode creates an avalanche, and that the positive ions from this avalanche liberate new electrons to join the discharge and also return to the cathode. In the case where no electrons start at the cathode, and at a distance x they have multiplied to n . The electrons added to the avalanche in a distance dx will be dn = αndx, proportional both to the distance dx and to the number of electrons. The factor α is the average number of electrons created per cm of path or the probability of creating a new electron per unit length and is known as the first Townsend coefficient. If α is constant, we can integrate the equation to find that ln n = αx + C, and compute that C = ln no, so that ln (n/no) = αx or n = noeαx, the equation for exponential growth. The number of electrons that arrive at the anode will be n = noeαd, where d is the distance from the anode to the cathode. More often than not, α will be a function of the electric field E, but here we make the assumption that E is constant, so our equation holds exactly only for plane-parallel electrodes and in the absence of space-charge effects. However, it will give us order-of-magnitude results. The number of positive ions produced in the avalanche will be equal to n – no. Assuming that all return to the cathode, where they release γ(n – no) new electrons. The factor γ expresses the efficiency of the ions in liberating electrons. This means that the net number of electrons leaving the cathode will be no + γ(n – no), and the number eventually reaching the anode will be n = [no + γ(n – no)]eαd. If ones find n, one find that n = noeαd/[1 – γ(eαd – 1]. If eαd is much greater than 1, we have simply n = noeαd/(1 – γeαd). In case, eαd increases to 1/γ, the denominator disappear, and the number of electrons reaching the anode increases without limit. This is the breakdown or sparking. The dependence of α on the electric field E is given by the empirical formula α/p = Ae-Bp/E, where A and B are constants, E the electric field and p is the pressure. The pressure comes in because the important thing is the energy gained in a mean free path, EL, and pressure is inversely proportional to L. In case the pressure changes, Bp/E will remain constant. α itself depends on collisions, and will be proportional to the pressure for the same reason. Therefore, α/p will be constant, as will Ap, as the pressure changes. Hence, the constants A and B need be determined for only one pressure. The dimensions of A and B are (cm-mmHg)-1 and V/cm-mmHg, respectively. For air, A = 14.6 and B = 365, and for helium A = 2.8 and B = 34. These figures hold only over certain ranges of electric field, of course. The factor γ for air on a nickel cathode is 0.036, for neon 0.023, which are the general figures. The sparking voltage as a function of pd for air is indicated in the graph at the right, showing a minimum at 327 V at pd = 5.67 mmHg-mm.

From the ionization constants for air, we find 266 V using the above equation, which is not bad agreement. At pd = 2000 mmHg-mm the sparking voltage is 10kV, and at 4500 mmHg-mm it is 20kV. The figures are for plane-parallel electrodes, so they give the sparking voltages for the corresponding values of field strength. We have assumed the mechanism of breakdown to be electron avalanches and positive-ion production of electrons at the cathode. The excited positive ions could also emit radiation that would eject photoelectrons from the cathode with the same effect. Therefore, the fact that we have cooperative amplification of the electron current does not unambiguously determine the mechanism. This occurs frequently in the study of electrical discharges, and often mechanisms are obscure while their effects are well-known. The minimum of the sparking potential has a strange consequence. For values of pd to the left of the minimum, if the discharge has a choice of two paths of different lengths, it will choose the longer path because it breaks down at a lower voltage, as indicated in the figure. In this case, bringing the electrodes closer together can actually increase the breakdown voltage.

At low pressures, breakdown takes place with a silent spark of fine filamentary form. At high pressures, the spark is noisy and bright. Breakdown can occur as the pressure reduced, or the cathode-anode distance is increased or the voltage is raised. Space charges can cause the voltage distribution to change, and increased fields have the same effect as an increase in the overall voltage. Lightning shows continuous breakdown over a long path by this mechanism. A simple increase in primary ionization that raises no will not cause breakdown by itself. Sparks have chemical effects, creating nitrogen oxides and ozone in air because of the excitation and ionization, and initiating chemical reactions. An interesting example of breakdown characteristics is the Geiger-Müller counter tube. It consists of a cylindrical metal cathode with a fine wire anode on the axis, as shown in the diagram. The thin window for entry of β electrons is not shown, if the counter is designed for this purpose. The counter also detects γ rays, that eject photoelectrons from the cathode, undergo create electron-positron pairs or Compton scattering. Even though the GM counter is not a sophisticated instrument, it has the great advantage of giving a large pulse that can even be heard directly through earphones without amplification. It is filled with Argon and a little vapor of ethyl alcohol.

The creation of a free electron at the cathode or in the volume of the counter starts an avalanche discharge that typically involves the whole length of the tube. The electrons are collected by the anode wire causing the leading edge of the pulse, and then the ions move more slowly to the cathode, making the pulse tail. The ions are swept out in about 100 μs, during which the tube is insensitive. The maximum counting rate is about 5000 counts/s, and the loss of counts because of the dead time is called the coincidence loss. If the counting rate with a certain source is measured as a function of the voltage applied to the tube, a characteristic like the one in the figure is found. Counting begins at the starting potential, when the electric field is first strong enough to support an avalanche. The counting rate increases until the Geiger threshold, and remains nearly constant across the Geiger plateau. The plateau existence allows the instrument to be calibrated. All the discharges in this region are of equal strength. If the ions liberated a sufficient number of electrons at the cathode, the discharge would become self-sustaining, rendering the tube inoperative. To prevent this, a quenching gas, often ethyl alcohol, but also a halogen such bromine or chlorine or, that sucks up electrons is added. The GM counter is between a Townsend discharge on one hand in which the current depends on the, breakdown and the ionization, where the discharge is self-maintained. At a sufficiently high applied voltage, nevertheless, a glow discharge cannot be avoided.

Glow discharge In case the gas pressure is reduced between 1 mmHg and 1 cmHg, we get a glow discharge that looks like the one in the diagram, a generally low-pressure glow discharge. If we started with the pressure at atmospheric, we would find it impossible to initiate a discharge with, say E = 300 V. As one pumpes the tube down, at some point a discharge would start, filling the tube with pink light, if the gas was air, but the light not intense and the current would be low. With continued evacuation, the current would increase as the voltage across the tube decreased, and we would see a dark region coming out of the cathode. Continuing, the dark region would increase in width, and the cathode would seem to be covered with a soft bluish light. The light of the pink column may begin to fluctuate in moving waves. At the pressure mentioned above, the voltage across the tube would be minimum and the current maximum, and this is the discharge state shown in the diagram. At lower pressures, the glow around the cathode would also expand, the dark region would expand proportionally to the reciprocal of the pressure, and perhaps a dark region between it and the cathode would become evident. The pink column of light would grow steadily shorter, and eventually be swallowed up by the dark zone. Now the glass of the tube might start to fluoresce green where fast electrons struck, and as the current fells, the voltage across the discharge would rise as the current fell. The electron mean free path is now comparable to the dimensions of the tube. Finally, the glow at the cathode would flicker and go out, and the discharge would cease, as the electrons could find no molecules to ionize as they traversed the tube from end to end.

There are two principal parts of the discharge. At the left, the region between the Faraday dark space (D.S.) and the cathode and is the engine that drives the discharge, creating the required electrons. If we lengthen the tube, this region does not change, but remains the same. At the right, the region between the anode and the Faraday dark space serves to connect the electron engine with the anode with a conducting path. It is almost electrically neutral, a plasma confin