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General Aspects of Vacuum Interrupter Application
Published in Paul G. Slade, The Vacuum Interrupter, 2020
Watanabe et al. [59] using AMF contact structures show that for a metal vapor density greater than 1020 m−3, the contact gap will fail to interrupt the current. They state that this occurs when the contact surfaces reach a temperature of about 1750K. Table 4.2 shows that the vapor pressure of Cu plus Cr at this temperature is about 33Pa. Niwa et al. [60] indicate that a somewhat higher temperature of 2150K is required. Here Table 4.2 shows that the vapor pressure of Cu plus Cr is now about 2 × 103Pa. At this temperature, Table 4.1 shows that there is a gas density of 7 × 1022 m−3 and a small electron mean free path (about 10−2mm). Thus, it is possible for a Townsend avalanche to be established and the vacuum arc to reignite. It can therefore be seen that for both the TMF and AMF contact structures there will be a limited current that can be interrupted for a given contact diameter.
Interaction of Neutrals, Charged Particles and Radiation with Gases in Vacuum
Published in Pramod K. Naik, Vacuum, 2018
The gas breakdown is not necessarily limited to vacuum environment. It can also occur at atmospheric and higher pressures. The Townsend discharge theory 8 forms the basis of the gas breakdown. Pairs of charged particles, comprising positive ions of the residual gas and the electrons, are present in the atmospheric air due to the stray ionizing agencies. The theory states that a small number of free stray electrons and positive ions, accelerated by an electric field, give rise to electrical conduction through a gas by an avalanche process. A Townsend avalanche is a cascade reaction involving electrons in a region within an electric field. The positive ions generated by electron–impact ionization drift towards the cathode, while the free electrons drift towards the anode. Consider one such electron that will undergo an ionizing collision with another atom/molecule of the medium. The two free electrons then travel together some distance before another collision occurs. The number of electrons travelling towards the anode is multiplied by a factor of two for each collision, so that after n collisions there are 2 n free electrons. Progressively, this results in an electron avalanche. Electrons are also emitted by other processes such as photoelectric emission and secondary electron emission due to positive ion bombardment of the cathode. , called the first Townsend ionization coefficient, corresponds to the number of ionizing collisions per unit length in the direction of the field. It is given by
Atmospheric Plasmas for Carbon Nanotubes (CNTs)
Published in R. Mohan Sankaran, Plasma Processing of Nanomaterials, 2017
Jae Beom Park, Se Jin Kyung, Geun Young Yeom
A corona discharge is defined as a luminous glow localized in space around a sharp tip in a highly nonuniform electric field. Corona discharges are electrical discharges formed by ionization of a fluid surrounding a conductor, which occurs when the potential gradient at the sharp tip exceeds a certain value but is not sufficient to cause complete electrical breakdown or arcing [58–60]. The Siemens research team [61] was the first to propose the use of a corona discharge to generate ozone for disinfecting water. This was the first report that applied plasmas for inactivation of microorganisms. Later, Menashi [62] used a pulsed RF-driven corona discharge to form a plasma at atmospheric pressures. This plasma could be described as a Townsend or negative glow discharge depending upon the field and potential distribution. By applying a high voltage to the sharp electrode, small localized discharges of very short duration can be observed in the gas gap of about 1 to 10 mm. A schematic diagram of a corona discharge system is shown in Figure 7.4. The system consists of a line of pins fastened to a power electrode. If a nonelectronegative gas such as He or Ar is used as the supply gas instead of air, the discharge is enhanced and can be operated at a relatively low voltage. The pin array is biased by a DC, AC, or pulsed power supply. The plasma usually extends about 0.5 mm from the metal tips. In the drift region outside this volume, charged species diffuse toward the planar electrode and are collected. Corona discharges in air are commonly used for ozone production [63] or for the activation of polymer surfaces before printing, pasting, or coating [64,65].
Simulation study of ternary gas mixture transport properties and their gain in GEM detectors used for muon tracking
Published in Molecular Physics, 2022
Badria Al Rashdia, Amr Radi, Abbasher Gismelseed, Ahmed Al Rawas
The detection performance of a gaseous detector is characterised by its time, spatial, and energy resolutions besides their gain. The transport properties of the study, which are drift velocity, longitudinal and transverse diffusion coefficients, Townsend coefficient, and attachment coefficient, affect the detector performance in a way or another. For example, having a high drift velocity of electrons in the gas leads to a good time resolution that is one of the detector requirements [27]. The higher diffusion coefficients on the other hand are lowering the gain and worsening the spatial resolution. If the gas atoms or molecules have a high ability to attach the electrons, this means that they have a high attachment coefficient and if so, the result will be loss of electrons and low energy resolution [28]. The Townsend coefficient is defined as the number of electron-ion pairs produced per unit length. It is related to the gain by: Where G is the gain, n is the number of electrons produced, n0 is the number of primary electrons, α is the Townsend coefficient and Δx is the whole length travelled by the electrons. So, the Townsend coefficient should be high to have a high gain.