Initiation of ecton processes by interaction of a plasma with a microprotrusion on a metal surface
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AL, NONLINEAR, AND SOFT MATTER PHYSICS
Initiation of Ecton Processes by Interaction of a Plasma with a Microprotrusion on a Metal Surface S. A. Barengoltsa, G. A. Mesyatsb, and M. M. Tsventoukhb* a
Prokhorov Institute of General Physics, Russian Academy of Sciences, Moscow, 119991 Russia b Lebedev Physical Institute, Russian Academy of Sciences, Moscow, 119991 Russia *e-mail: [email protected] Received April 14, 2008
Abstract—Evolution of rapid (~10 ns) Ohmic overheating of a microprotrusion on a surface in contact with a plasma by emission current is studied taking into account the energy carried by plasma ions and electrons, as well as Ohmic heating, emissive source of energy release (Nottingham effect), and heat removal due to heat conduction. Plasma parameters were considered in the range of n = 1014–1020 cm–3 and Te = 0.1 eV–10 keV. The threshold value of energy transferred to the surface from the plasma is found to be 200 MW/cm2; above this value, heating becomes explosive (namely, an increase in the temperature growth rate (∂2T/∂t2 > 0) and in passing current (∂J/∂t > 0) is observed in the final stage at T ~ 104 K and j ~ 108 A/cm2). In spite of the fact that Ohmic heating does not play any significant role for plasmas with a density lower than 1018 cm–3 because the current is limited by the space charge of electrons, rapid overheating of top of microprotrusion is observed much sooner (over a time period of ~1 ns) when the threshold is exceeded. In this case, intense ionization of vapor of the wall material leads to an increase in the plasma density at the surface, and the heating becomes of the Ohmic explosion type. Such conditions for the formation of a microexplosion on the surface and of an ecton accompanying it can be created during the interaction of a plasma with the cathode, anode, or an insulated wall and may lead to the formation of cathode and anode spots, as well as unipolar arcs. PACS numbers: 52.40.Hf, 52.77.Fv, 52.80.Qj, 52.65.-y DOI: 10.1134/S1063776108120133
1. INTRODUCTION Surface microexplosions play an important role in the interaction of a plasma with the wall. It is known from analysis of electric discharge in vacuum that operation of cathode spots is ensured by a sequence of microexplosions on the surface. Each microexplosion is accompanied by the ejection of a plasma bunch from the cathode material and a portion of electrons of explosive electron emission (ecton) [1]. Initiation of such microexplosions and ectons may take place at the surface of the wall under the action of various factors such as breakdown of dielectric films, impact of microparticles, or overheating of microinhomogeneities. The latter are always present and may be formed additionally under the action of the plasma. For example, sputtering may lead to the growth of whiskers and cones, while blistering and flaking lead to substantial (large-scale) erosion of the surface. It was proposed [2] that spots of unipolar arcs are also a sequence of microexplosions because the spots of unipolar as well as bipolar arcs exhibit si
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