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Cathode Phenomena In A Vacuum Discharge: The Breakdown, The Spark, And The Arc

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Cathode Phenomena in a Vacuum Discharge: the Breakdown, the Spark, and the Arc G. A. Mesyats Nauka, Moscow, 2000 In this monograph the author considers for the first time on inified physical grounds the mechanisms for all the three stages of a vacuum discharge: the breakdown, the spark, and the arc, each being discussed in a separate part of the book. Behind these stages are two physical processes occurring at the cathode. First, these are microscopic explosions at the cathode surface and the resulting explosive electron emission which proceeds in portion referred to as ectons. Second, this is the interaction of the plasma generated upon microexplosions at the cathode surface. The interaction of this plasma with liquid-metal microjets originating from the cathode or with the dielectric films and inclusions present on the cathode surface gives rise to energy concentration in microvolumes at the cathode surface and to new microexplosions. Thus, the discharge is self-sustaining. The microscopic explosions occur due to Joule heating resulting from the high current density in microvolumes at the cathode surface. This approach makes it possible to clearly distinguish the three stages of a vacuum discharge: the breakdown, the spark, and the arc. During the first stage the energy necessary for a microexplosion to occur is accumulated. The second stage begins with an explosion and ends with the closure of the cathode(anode gap with plasma. The third stage, the arc, takes place under the conditions that a plasma column exists between the cathode and the anode. The cathode(plasma junction is referred to as a cathode spot. It is this place where microexplosions are reproduced. Based on the above ideas, the author has been able to account for the cyclic operation of cathode spots and their cellular structure, the cathode fall voltage, the threshold current, the limiting current, the spontaneous extinction of an arc, the erosion rate, etc. The monograph consists of three parts which include 22 chapters. The first part entitled The Vacuum Breakdown consists of seven chapters. In these chapters the properties of the electrode surface are described, the criteria for pulsed and dc vacuum breakdowns are established, the problem of the initiation of breakdown with plasma and by other means is considered, and it has been demonstrated that eventually everything is reduced to microexplosions occurring at the cathode. This part is concluded by the simplest model of an ecton which represents a portion of electrons (1011—1012 in number) generated by an electrical explosion of the cathode material in microvolumes, since ectons are the basic phenomenon in the physics of cathode processes in a vacuum breakdown. The second part of the monograph, The Vacuum Spark, consists of seven chapters as well. Here, the instrumentation for studying the spark on the nanosecond scale is described, the studies on the cathode erosion and the properties of the cathode plasma are presented, the mechanisms for the passage of the vacuum spark current and for the self-sustaining of the ecton processes are discussed, etc. In conclusion, a description is given of the anode processes at the spark stage of a vacuum discharge. The third part, The Vacuum Arc, contains eight chapters. These chapters describe the general properties of vacuum arcs and the methods allowing an investigation the arc instabilities with nanosecond time resolution; special attention is given to the spontaneous extinction of an arc and the cyclic processes occurring in a cathode spot and the cathode spot cells, and the reasons for the chaotic motion of cathode spots and for the droplet and plasma cathode erosion are analyzed. Also, measurements of the current density and the properties of cathode plasma jets are discussed. This part of the monograph is concluded by a description of the ecton mechanism for a vacuum arc, which is based on microexplosions occurring at
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