The Townsend discharge or Townsend avalanche is a gas ionisation process where free electrons are accelerated by an electric field, collide with gas molecules, free additional electrons. Those electrons are in free additional electrons; the result is an avalanche multiplication. The discharge requires a source of a significant electric field; the Townsend discharge is named after John Sealy Townsend, who discovered the fundamental ionisation mechanism by his work circa 1897 at the Cavendish Laboratory, Cambridge. The avalanche occurs in a gaseous medium; the electric field and the mean free path of the electron must allow free electrons to acquire an energy level that can cause impact ionisation. If the electric field is too small the electrons do not acquire enough energy. If the mean free path is too short, the electron gives up its acquired energy in a series of non-ionising collisions. If the mean free path is too long the electron reaches the anode before colliding with another molecule; the avalanche mechanism is shown in the accompanying diagram.
The electric field is applied across a gaseous medium. An original ionisation event produces an ion pair. If the electric field is strong enough, the free electron can gain sufficient velocity to liberate another electron when it next collides with a molecule; the two free electrons travel towards the anode and gain sufficient energy from the electric field to cause further impact ionisations, so on. This process is a chain reaction that generates free electrons; the total number of electrons reaching the anode is equal to the number of collisions, plus the single initiating free electron. The number of collisions grows exponentially; the limit to the multiplication in an electron avalanche is known as the Raether limit. The Townsend avalanche can have a large range of current densities. In common gas-filled tubes, such as those used as gaseous ionisation detectors, magnitudes of currents flowing during this process can range from about 10−18 amperes to about 10−5 amperes. Townsend's early experimental apparatus consisted of planar parallel plates forming two sides of a chamber filled with a gas.
A direct current high-voltage source was connected between the plates. He forced the cathode to emit electrons using the photoelectric effect by irradiating it with X-rays, he found that the current I flowing through the chamber depended on the electric field between the plates. However, this current showed an exponential increase as the plate gaps became small, leading to the conclusion that the gas ions were multiplying as they moved between the plates due to the high electric field. Townsend observed currents varying exponentially over ten or more orders of magnitude with a constant applied voltage when the distance between the plates was varied, he discovered that gas pressure influenced conduction: he was able to generate ions in gases at low pressure with a much lower voltage than that required to generate a spark. This observation overturned conventional thinking about the amount of current that an irradiated gas could conduct; the experimental data obtained from his experiments are described by the following formula I I 0 = e α n d, where I is the current flowing in the device, I0 is the photoelectric current generated at the cathode surface, e is Euler's number αn is the first Townsend ionisation coefficient, expressing the number of ion pairs generated per unit length by a negative ion moving from cathode to anode, d is the distance between the plates of the device.
The constant voltage between the plates is equal to the breakdown voltage needed to create a self-sustaining avalanche: it decreases when the current reaches the glow discharge regime. Subsequent experiments revealed that the current I rises faster than predicted by the above formula as the distance d increases: two different effects were considered in order to better model the discharge: positive ions and cathode emission. Townsend put forward the hypothesis that positive ions produce ion pairs, introducing a coefficient α p expressing the number of ion pairs generated per unit length by a positive ion moving from anode to cathode; the following formula was found I I 0 = e d α n − α p e d ⟹ I I 0 ≅ e α n d 1 − ( α p / α n
Photography is the art and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It is employed in many fields of science and business, as well as its more direct uses for art and video production, recreational purposes and mass communication. A lens is used to focus the light reflected or emitted from objects into a real image on the light-sensitive surface inside a camera during a timed exposure. With an electronic image sensor, this produces an electrical charge at each pixel, electronically processed and stored in a digital image file for subsequent display or processing; the result with photographic emulsion is an invisible latent image, chemically "developed" into a visible image, either negative or positive depending on the purpose of the photographic material and the method of processing. A negative image on film is traditionally used to photographically create a positive image on a paper base, known as a print, either by using an enlarger or by contact printing.
The word "photography" was created from the Greek roots φωτός, genitive of φῶς, "light" and γραφή "representation by means of lines" or "drawing", together meaning "drawing with light". Several people may have coined the same new term from these roots independently. Hercules Florence, a French painter and inventor living in Campinas, used the French form of the word, photographie, in private notes which a Brazilian historian believes were written in 1834; this claim is reported but has never been independently confirmed as beyond reasonable doubt. The German newspaper Vossische Zeitung of 25 February 1839 contained an article entitled Photographie, discussing several priority claims – Henry Fox Talbot's – regarding Daguerre's claim of invention; the article is the earliest known occurrence of the word in public print. It was signed "J. M.", believed to have been Berlin astronomer Johann von Maedler. The inventors Nicéphore Niépce, Henry Fox Talbot and Louis Daguerre seem not to have known or used the word "photography", but referred to their processes as "Heliography", "Photogenic Drawing"/"Talbotype"/"Calotype" and "Daguerreotype".
Photography is the result of combining several technical discoveries, relating to seeing an image and capturing the image. The discovery of the camera obscura that provides an image of a scene dates back to ancient China. Greek mathematicians Aristotle and Euclid independently described a pinhole camera in the 5th and 4th centuries BCE. In the 6th century CE, Byzantine mathematician Anthemius of Tralles used a type of camera obscura in his experiments; the Arab physicist Ibn al-Haytham invented a camera obscura and pinhole camera. Leonardo da Vinci mentions natural camera obscura that are formed by dark caves on the edge of a sunlit valley. A hole in the cave wall will act as a pinhole camera and project a laterally reversed, upside down image on a piece of paper. Renaissance painters used the camera obscura which, in fact, gives the optical rendering in color that dominates Western Art, it is a box with a hole in it which allows light to go through and create an image onto the piece of paper.
The birth of photography was concerned with inventing means to capture and keep the image produced by the camera obscura. Albertus Magnus discovered silver nitrate, Georg Fabricius discovered silver chloride, the techniques described in Ibn al-Haytham's Book of Optics are capable of producing primitive photographs using medieval materials. Daniele Barbaro described a diaphragm in 1566. Wilhelm Homberg described how light darkened some chemicals in 1694; the fiction book Giphantie, published in 1760, by French author Tiphaigne de la Roche, described what can be interpreted as photography. Around the year 1800, British inventor Thomas Wedgwood made the first known attempt to capture the image in a camera obscura by means of a light-sensitive substance, he used paper or white leather treated with silver nitrate. Although he succeeded in capturing the shadows of objects placed on the surface in direct sunlight, made shadow copies of paintings on glass, it was reported in 1802 that "the images formed by means of a camera obscura have been found too faint to produce, in any moderate time, an effect upon the nitrate of silver."
The shadow images darkened all over. The first permanent photoetching was an image produced in 1822 by the French inventor Nicéphore Niépce, but it was destroyed in a attempt to make prints from it. Niépce was successful again in 1825. In 1826 or 1827, he made the View from the Window at Le Gras, the earliest surviving photograph from nature; because Niépce's camera photographs required an long exposure, he sought to improve his bitumen process or replace it with one, more practical. In partnership with Louis Daguerre, he worked out post-exposure processing methods that produced visually superior results and replaced the bitumen with a more light-sensitive resin, but hours of exposure in the camera were still required. With an eye to eventual commercial exploitation, the partners opted for total secrecy. Niépce died in 1833 and Daguerre redirected the experiments toward the light-sensitive silver halides, which Niépce had abandoned many years earlier because of his inability to make the images he captured with them light-fast and permanent.
Space charge is a concept in which excess electric charge is treated as a continuum of charge distributed over a region of space rather than distinct point-like charges. This model applies when charge carriers have been emitted from some region of a solid—the cloud of emitted carriers can form a space charge region if they are sufficiently spread out, or the charged atoms or molecules left behind in the solid can form a space charge region. Space charge only occurs in dielectric media because in a conductive medium the charge tends to be neutralized or screened; the sign of the space charge can be either positive. This situation is most familiar in the area near a metal object when it is heated to incandescence in a vacuum; this effect was first observed by Thomas Edison in light bulb filaments, where it is sometimes called the Edison effect, but space charge is a significant phenomenon in many vacuum and solid-state electronic devices. When a metal object is placed in a vacuum and is heated to incandescence, the energy is sufficient to cause electrons to "boil" away from the surface atoms and surround the metal object in a cloud of free electrons.
This is called thermionic emission. The resulting cloud is negatively charged, can be attracted to any nearby positively charged object, thus producing an electric current which passes through the vacuum. Space charge can result from a range of phenomena, but the most important are: Combination of the current density and spatially inhomogeneous resistance Ionization of species within the dielectric to form heterocharge Charge injection from electrodes and from a stress enhancement Polarization in structures such as water trees. "Water tree" is a name given to a tree-like figure appearing in a water-impregnated polymer insulating cable. It has been suggested that in alternating current most carriers injected at electrodes during a half of cycle are ejected during the next half cycle, so the net balance of charge on a cycle is zero. However, a small fraction of the carriers can be trapped at levels deep enough to retain them when the field is inverted; the amount of charge in AC should increase slower than in direct current and become observable after longer periods of time.
Hetero charge means that the polarity of the space charge is opposite to that of neighboring electrode, homo charge is the reverse situation. Under high voltage application, a hetero charge near the electrode is expected to reduce the breakdown voltage, whereas a homo charge will increase it. After polarity reversal under ac conditions, the homo charge is converted to hetero space charge. If the "vacuum" has a pressure of 10−6 mmHg or less, the main vehicle of conduction is electrons; the emission current density from the cathode, as a function of its thermodynamic temperature T, in the absence of space-charge, is given by Richardson's law: J = A 0 T 2 exp where A 0 = 4 π e m e k 2 h 3 ≈ 1.2 × 10 6 A m−2 K−2 e = elementary positive charge, me = electron mass, k = Boltzmann's constant = 1.38 x 10−23J/K, h = Planck's constant = 6.62 x 10−34 J s, φ = work function of the cathode, ř = mean electron reflection coefficient. The reflection coefficient can be as low as 0.105 but is near 0.5. For Tungsten, A0 = 0.6 to 1.0 × 106 A m−2 K−2, φ = 4.52 eV.
At 2500 °C, the emission is 28207 A/m2. The emission current as given above is many times greater than that collected by the electrodes, except in some pulsed valves such as the cavity magnetron. Most of the electrons emitted by the cathode are driven back to it by the repulsion of the cloud of electrons in its neighborhood; this is called the space charge effect. In the limit of large current densities, J is given by the Child-Langmuir equation below, rather than by the thermionic emission equation above. Space charge is an inherent property of all vacuum tubes; this has at times made life harder or easier for electrical engineers who used tubes in their designs. For example, space charge limited the practical application of triode amplifiers which led to further innovations such as the vacuum tube tetrode. On the other hand, space charge was useful in some tube applications because it generates a negative EMF within the tube's envelope, which could be used to create a negative bias on the tube's grid.
Grid bias could be achieved by using an applied grid voltage in addition to the control voltage. This could improve the engineer's fidelity of amplification, it allowed to construct space charge tubes for car radios that required only 6 or 12 volts anode voltage. Space charges can occur within dielectrics. For example, when gas near a high voltage electrode begins to undergo dielectric breakdown, electrical charges are injected into the region near the electrode, forming space charge regions in the surrounding gas. Space charges can occur within solid or liquid dielectrics that are stressed by high electric fields. Trapped space charges within solid dielectrics are a contributing factor lea
A gas-filled tube known as a discharge tube, is an arrangement of electrodes in a gas within an insulating, temperature-resistant envelope. Gas-filled tubes exploit phenomena related to electric discharge in gases, operate by ionizing the gas with an applied voltage sufficient to cause electrical conduction by the underlying phenomena of the Townsend discharge. A gas-discharge lamp is an electric light using a gas-filled tube. Specialized gas-filled tubes such as krytrons and ignitrons are used as switching devices in electric devices; the voltage required to initiate and sustain discharge is dependent on the pressure and composition of the fill gas and geometry of the tube. Although the envelope is glass, power tubes use ceramics, military tubes use glass-lined metal. Both hot cathode and cold cathode type devices are encountered. Hydrogen is used in tubes used for fast switching, e.g. some thyratrons and krytrons, where steep edges are required. The build-up and recovery times of hydrogen are much shorter than in other gases.
Hydrogen thyratrons are hot-cathode. Hydrogen can be stored in the tube in the form of a metal hydride, heated with an auxiliary filament. Deuterium is used in ultraviolet lamps for ultraviolet spectroscopy, in neutron generator tubes, in special tubes, it has higher breakdown voltage than hydrogen. In fast switching tubes it is used instead of hydrogen. For a comparison, the hydrogen-filled CX1140 thyratron has anode voltage rating of 25 kV, while the deuterium-filled and otherwise identical CX1159 has 33 kV. At the same voltage the pressure of deuterium can be higher than of hydrogen, allowing higher rise rates of rise of current before it causes excessive anode dissipation. Higher peak powers are achievable, its recovery time is however about 40% slower than for hydrogen. Noble gases are used in tubes for many purposes, from lighting to switching. Pure noble gases are employed in switching tubes. Noble-gas-filled thyratrons have better electrical parameters than mercury-based ones; the electrodes undergo damage by high-velocity ions.
The neutral atoms of the gas slow the ions down by collisions, reduce the energy transferred to the electrodes by the ion impact. Gases with high molecular weight, e.g. xenon, protect the electrodes better than lighter ones, e.g. neon. Helium is used in helium–neon lasers and in some thyratrons rated for high currents and high voltages. Helium provides about as short deionization time as hydrogen, but can withstand lower voltage, so it is used much less often. Neon has low ignition voltage and is used in low-voltage tubes. Discharge in neon emits bright red light; this is exploited in the decatron tubes, which act as both displays. Its red light is exploited in neon signage. Used in fluorescent tubes with high power and short length, e.g. industrial lighting tubes. Has higher voltage drop in comparison with argon and krypton, its low atomic mass provides only a little protection to the electrodes against accelerated ions. In fluorescent tubes it is used in combination with mercury. Argon was the first gas used in fluorescent tubes and is still used due to its low cost, high efficiency, low striking voltage.
In fluorescent tubes it is used in combination with mercury. It was used in early rectifier tubes. Krypton can be used in fluorescent lamps instead of argon; the voltage drop per lamp length is however lower than with argon, which can be compensated by smaller tube diameter. Krypton-filled lamps require higher starting voltage. In fluorescent tubes it is used in combination with mercury. Xenon in pure state has high breakdown voltage. Xenon is used as a component of gas mixtures when production of ultraviolet radiation is required, e.g. in plasma displays to excite a phosphor. The wavelength produced penetrates the phosphors better. To lower the ionization voltage, neon-xenon or helium-xenon are used. At concentrations of 1% and less of xenon, the Penning effect becomes significant in such mixtures, as most of xenon ionization occurs by collision with excited atoms of the other noble gas. Radon, despite being a noble gas, is dangerously radioactive and its most stable isotope has a half-life of less than four days.
It is not used in electronic devices. Penning mixtures are used where lower ionization voltage is required, e.g. in the neon lamps, Geiger–Müller tubes and other gas-filled particle detectors. A classical combination is about 98–99.5% of neon with 0.5–2% of argon, used in, e.g. neon bulbs and in monochrome plasma displays. Mercury vapors are used for applications with high current, e.g. lights, mercury-arc valves, ignitrons. Mercury is used because of its hi
Electrostatic discharge is the sudden flow of electricity between two electrically charged objects caused by contact, an electrical short, or dielectric breakdown. A buildup of static electricity can be caused by electrostatic induction; the ESD occurs when differently-charged objects are brought close together or when the dielectric between them breaks down creating a visible spark. ESD can create spectacular electric sparks, but less dramatic forms which may be neither seen nor heard, yet still be large enough to cause damage to sensitive electronic devices. Electric sparks require a field strength above 40 kV/cm in air, as notably occurs in lightning strikes. Other forms of ESD include corona discharge from sharp electrodes and brush discharge from blunt electrodes. ESD can cause harmful effects of importance in industry, including explosions in gas, fuel vapor and coal dust, as well as failure of solid state electronics components such as integrated circuits; these can suffer permanent damage.
Electronics manufacturers therefore establish electrostatic protective areas free of static, using measures to prevent charging, such as avoiding charging materials and measures to remove static such as grounding human workers, providing antistatic devices, controlling humidity. ESD simulators may be used to test electronic devices, for example with a human body model or a charged device model. One of the causes of ESD events is static electricity. Static electricity is generated through tribocharging, the separation of electric charges that occurs when two materials are brought into contact and separated. Examples of tribocharging include walking on a rug, rubbing a plastic comb against dry hair, rubbing a balloon against a sweater, ascending from a fabric car seat, or removing some types of plastic packaging. In all these cases, the breaking of contact between two materials results in tribocharging, thus creating a difference of electrical potential that can lead to an ESD event. Another cause of ESD damage is through electrostatic induction.
This occurs when an electrically charged object is placed near a conductive object isolated from the ground. The presence of the charged object creates an electrostatic field that causes electrical charges on the surface of the other object to redistribute. Though the net electrostatic charge of the object has not changed, it now has regions of excess positive and negative charges. An ESD event may occur. For example, charged regions on the surfaces of styrofoam cups or bags can induce potential on nearby ESD sensitive components via electrostatic induction and an ESD event may occur if the component is touched with a metallic tool. ESD can be caused by energetic charged particles impinging on an object; this causes deep charging. This is a known hazard for most spacecraft; the most spectacular form of ESD is the spark, which occurs when a heavy electric field creates an ionized conductive channel in air. This can cause minor discomfort to people, severe damage to electronic equipment, fires and explosions if the air contains combustible gases or particles.
However, many ESD events occur without a audible spark. A person carrying a small electric charge may not feel a discharge, sufficient to damage sensitive electronic components; some devices may be damaged by discharges as small as 30 V. These invisible forms of ESD can cause outright device failures, or less obvious forms of degradation that may affect the long term reliability and performance of electronic devices; the degradation in some devices may not become evident until well into their service life. A spark is triggered when the electric field strength exceeds 4–30 kV/cm — the dielectric field strength of air; this may cause a rapid increase in the number of free electrons and ions in the air, temporarily causing the air to abruptly become an electrical conductor in a process called dielectric breakdown. The best known example of a natural spark is lightning. In this case the electric potential between a cloud and ground, or between two clouds, is hundreds of millions of volts; the resulting current.
On a much smaller scale, sparks can form in air during electrostatic discharges from charged objects that are charged to as little as 380 V. Earth's atmosphere consists of 78 % nitrogen. During an electrostatic discharge, such as a lightning flash, the affected atmospheric molecules become electrically overstressed; the diatomic oxygen molecules are split, recombine to form ozone, unstable, or reacts with metals and organic matter. If the electrical stress is high enough, nitrogen oxides can form. Both products are toxic to animals, nitrogen oxides are essential for nitrogen fixation. Ozone is used in water purification. Sparks are an ignition source in combustible environments that may lead to catastrophic explosions in concentrated fuel environments. Most explosions can be traced back to a tiny electrostatic discharge, whether it was an unexpected combustible fuel leak invading a known open air sparking device, or an unexpected spark in a known fuel rich environment; the end result is the same if oxygen is present and the three criteria of the fire triangle have been combined.
Many electronic components microchips, can be damaged by ESD. Sensitive components need to be protected during and after manufacture, during shipping and device assembly
Electrical breakdown or dielectric breakdown is when current flows through an electrical insulator when the voltage applied across it exceeds the breakdown voltage. This results in the insulator becoming electrically conductive. Electrical breakdown may be a momentary event, or may lead to a continuous arc if protective devices fail to interrupt the current in a power circuit. Under sufficient electrical stress, electrical breakdown can occur within solids, gases or vacuum. However, the specific breakdown mechanisms are different for each kind of dielectric medium. Electrical breakdown is associated with the failure of solid or liquid insulating materials used inside high voltage transformers or capacitors in the electricity distribution grid resulting in a short circuit or a blown fuse. Electrical breakdown can occur across the insulators that suspend overhead power lines, within underground power cables, or lines arcing to nearby branches of trees. Dielectric breakdown is important in the design of integrated circuits and other solid state electronic devices.
Insulating layers in such devices are designed to withstand normal operating voltages, but higher voltage such as from static electricity may destroy these layers, rendering a device useless. The dielectric strength of capacitors limits how much energy can be stored and the safe working voltage for the device. Breakdown mechanisms differ in solids and gasses. Breakdown is influenced by electrode material, sharp curvature of conductor material, the size of the gap between the electrodes, the density of the material in the gap. In solid materials a long-time partial discharge precedes breakdown, degrading the insulators and metals nearest the voltage gap; the partial discharge chars through a channel of carbonized material that conducts current across the gap. Possible mechanisms for breakdown in liquids include bubbles, small impurities, electrical super-heating; the process of breakdown in liquids is complicated by hydrodynamic effects, since additional pressure is exerted on the fluid by the non-linear electrical field strength in the gap between the electrodes.
In liquefied gases used as coolants for superconductivity – such as Helium at 4.2 K or Nitrogen at 77 K – bubbles can induce breakdown. In oil-cooled and oil-insulated transformers the field strength for breakdown is about 20 kV/mm. Despite the purified oils used, small particle contaminants are blamed. Electrical breakdown occurs within a gas. Regions of intense voltage gradients can cause nearby gas to ionize and begin conducting; this is done deliberately in low pressure discharges such as in fluorescent lights. The voltage that leads to electrical breakdown of a gas is approximated by Paschen's Law. Partial discharge in air causes the "fresh air" smell of ozone during thunderstorms or around high-voltage equipment. Although air is an excellent insulator, when stressed by a sufficiently high voltage, air can begin to break down, becoming conductive. Across small gaps, breakdown voltage in air is a function of gap length times pressure. If the voltage is sufficiently high, complete electrical breakdown of the air will culminate in an electrical spark or an electric arc that bridges the entire gap.
The color of the spark depends upon the gases. While the small sparks generated by static electricity may be audible, larger sparks are accompanied by a loud snap or bang. Lightning is an example of an immense spark. If a fuse or circuit breaker fails to interrupt the current through a spark in a power circuit, current may continue, forming a hot electric arc; the color of an arc depends upon the conducting gasses, some of which may have been solids before being vaporized and mixed into the hot plasma in the arc. The free ions in and around the arc recombine to create new chemical compounds, such as ozone, carbon monoxide, nitrous oxide. Ozone is most noticed due to its distinct odour. Although sparks and arcs are undesirable, they can be useful in applications such as spark plugs for gasoline engines, electrical welding of metals, or for metal melting in an electric arc furnace. Prior to gas discharge the gas glows with distinct colors that depend on the energy levels of the atoms. Not all mechanisms are understood.
The vacuum itself is expected to undergo electrical breakdown near the Schwinger limit. Before gas breakdown, there is a non-linear relation between voltage and current as shown in the figure. In region 1, there are free ions that can induce a current; these will be saturated after a certain voltage and give a constant current, region 2. Region 3 and 4 are caused by ion avalanche. Friedrich Paschen established the relation between the breakdown condition to breakdown voltage, he derived a formula that defines the breakdown voltage for uniform field gaps as a function of gap length and gap pressure. V b = B p d ln ( A p d ln ( 1 +
Dielectric barrier discharge
Dielectric-barrier discharge is the electrical discharge between two electrodes separated by an insulating dielectric barrier. Called silent discharge and known as ozone production discharge or partial discharge, it was first reported by Ernst Werner von Siemens in 1857. On right, the schematic diagram shows a typical construction of a DBD wherein one of the two electrodes is covered with a dielectric barrier material; the lines between the dielectric and the electrode are representative of the discharge filaments, which are visible to the naked eye. Below this, the photograph shows an atmospheric DBD discharge occurring in between two steel electrode plates, each covered with a dielectric sheet; the filaments are columns of conducting plasma, the foot of each filament is representative of the surface accumulated charge. The process uses high voltage alternating current, ranging from lower RF to microwave frequencies. However, other methods were developed to extend the frequency range all the way down to the DC.
One method was to use a high resistivity layer to cover one of the electrodes. This is known as the resistive barrier discharge. Another technique using a semiconductor layer of gallium arsenide to replace the dielectric layer, enables these devices to be driven by a DC voltage between 580 V and 740 V. DBD devices can be made in many configurations planar, using parallel plates separated by a dielectric or cylindrical, using coaxial plates with a dielectric tube between them. In a common coaxial configuration, the dielectric is shaped in the same form as common fluorescent tubing, it is filled at atmospheric pressure with either a rare gas or rare gas-halide mix, with the glass walls acting as the dielectric barrier. Due to the atmospheric pressure level, such processes require high energy levels to sustain. Common dielectric materials include glass, quartz and polymers; the gap distance between electrodes varies from less than 0.1 mm in plasma displays, several millimetres in ozone generators and up to several centimetres in CO2 lasers.
Depending on the geometry, DBD can be generated on a surface. For VDBD the plasma is generated between two electrodes, for example between two parallel plates with a dielectric in between. At SDBD the microdischarges are generated on the surface of a dielectric, which results in a more homogeneous plasma than can be achieved using the VDBD configuration At SDBD the microdischarges are limited to the surface, therefore their density is higher compared to the VDBD; the plasma is generated on top of the surface of an SDBD plate. A particular compact and economic DBD plasma generator can be built based on the principles of the piezoelectric direct discharge. In this technique, the high voltage is generated with a piezo-transformer, the secondary circuit of which acts as the high voltage electrode. Since the transformer material is a dielectric, the produced electric discharge resembles properties of the dielectric barrier discharge. A multitude of random arcs form in operation gap exceeding 1.5 mm between the two electrodes during discharges in gases at the atmospheric pressure.
As the charges collect on the surface of the dielectric, they discharge in microseconds, leading to their reformation elsewhere on the surface. Similar to other electrical discharge methods, the contained plasma is sustained if the continuous energy source provides the required degree of ionization, overcoming the recombination process leading to the extinction of the discharge plasma; such recombinations are directly proportional to the collisions between the molecules and in turn to the pressure of the gas, as explained by Paschen's Law. The discharge process causes the emission of an energetic photon, the frequency and energy of which corresponds to the type of gas used to fill the discharge gap; the electrical diagram of the DBD device at the absence of discharge can be presented in the form shown in Fig. 1 where C 1 is capacitance of dielectric adjacent to one of two electrodes and C 2 is capacitance of the air gap between the dielectric within the adjacent electrode footprint and the ground electrode.
C p and R p are resistance modeling electric response of plasma. If a switch S connects the capacitors C 1 and C 2 shown in Fig. 1, the voltage generator is connected to a circuit comprising two capacitors C 1 and C 2 connected in a series circuit. A capacitance of this circuit can be expressed as C s = C 1 C 2 C 1 + C 2, the electric current I through this circuit can be expressed in the form I = C s d U d t, where U is a generator voltage. Oscillograms U and