Electric-field enhancement and ion-flow focusing at the multiwire cathode of a high-current plasma-filled diode
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MA–SURFACE INTERACTION
Electric-Field Enhancement and Ion-Flow Focusing at the Multiwire Cathode of a High-Current Plasma-Filled Diode E. V. Nefyodtsev and G. E. Ozur Institute of High-Current Electronics, Siberian Branch, Russian Academy of Sciences, Akademicheskiœ pr. 4, Tomsk, 634055 Russia Received November 11, 2007; in final form, December 24, 2007
Abstract—The time evolution of an unsteady ion sheath in the presence of cylindrical protrusions at the cathode is studied numerically in a two-dimensional model. The calculations demonstrate the enhancement of the electric field and the focusing of the ion flow at the protrusion vertices. It is shown that, in the stage of the sheath formation, in addition to the electrostatic focusing of the ion flux, there is also focusing caused by the curvature of the plasma boundary around a protrusion. The results of calculations agree satisfactorily with the experimental data on the delay time of explosive emission at the multiwire cathode of a high-current plasma-filled diode. PACS numbers: 52.25.Fi, 52.65.-y DOI: 10.1134/S1063780X08080035
1. INTRODUCTION Studies of the dynamics of the cathode ion sheath are important for understanding the mechanism for generating low-energy (10–30 keV) high-current electron beams (HCEBs) in plasma-filled diodes [1, 2] and the processes occurring in plasma immersion ion implantation devices [3, 4] and other electrophysical devices, including vacuum circuit breakers, switches, and pseudospark gaps. The dynamics of the ion sheath determines the electric field strength and the ion current density at the cathode during the generation of an HCEB and, therefore, the excitation efficiency of explosive electron emission (EEE). In studying these processes, it is important to know the relative contributions from the main mechanisms governing the formation of emission centers, namely, the explosion of micropoints by a high-density field-emission current and the breakdown of nonmetal inclusions and films charged by the plasma ion current. The delay time of micropoint explosion depends strongly on the electric field strength near the cathode surface. To enhance this field, emitting elements with small-radius-curvature vertices (such as ring protrusions, fine wires, blades, teeth, or graphite filament bunches) are placed at certain intervals on the cathode substrate [5]. For vacuum diodes, there are calculated data [6] on the field strength for different emitter geometries. For plasma-filled diodes, the corresponding calculation technique is still lacking. Depending on the ratio between the voltage rise time τ and the ion transit time θ across the sheath, three types of ion sheath are distinguished. If τ θ, then the sheath is called the ion-matrix sheath (the ion density in
such a sheath is nearly constant) [7]. When τ θ, the sheath is quasi-steady and obeys the Child–Langmuir law. For simple geometries, the field strength and current density in ion-matrix and quasi-steady sheaths can be calculated analytically [3, 4, 8]. However, these quantities a
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