Boundary conditions on the plasma emitter surface in the presence of a particle counter flow: I. Ion emitter
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MA AND ION SOURCES
Boundary Conditions on the Plasma Emitter Surface in the Presence of a Particle Counter Flow: I. Ion Emitter V. T. Astrelin* and I. A. Kotelnikov Budker Institute of Nuclear Physics, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia Novosibirsk State University, Novosibirsk, 630090 Russia *e-mail: [email protected] Received May 4, 2016
Abstract―Emission of positively charged ions from a plasma emitter irradiated by a counterpropagating electron beam is studied theoretically. A bipolar diode with a plasma emitter in which the ion temperature is lower than the electron temperature and the counter electron flow is extracted from the ion collector is calculated in the one-dimensional model. An analog of Bohm’s criterion for ion emission in the presence of a counterpropagating electron beam is derived. The limiting density of the counterpropagating beam in a bipolar diode operating in the space-charge-limited-emission regime is calculated. The full set of boundary conditions on the plasma emitter surface that are required for operation of the high-current optics module in numerical codes used to simulate charged particle sources is formulated. DOI: 10.1134/S1063780X17020027
1. INTRODUCTION Two boundary conditions on the emitter surface are required to numerically simulate sources of charged particle beams. For the case of a solid-state emitter, the first condition specifies the electric potential of the emitting surface. The second condition specifies the emission current density or the electric field strength in case of a diode operating in the spacecharge-limited regime. This formulation of the problem can also be applied to sources with a plasma emitter. In this case, as a rule, emission of particle of only one species (ions or electrons) from the plasma surface and their subsequent acceleration in the unipolar sheath is considered in the theory of plasma emitters [1–3]. In actual sources, the beam is usually transmitted to the destination location through the so-called secondary plasma. This plasma is generated when the residual (or specially injected) gas in the transport channel undergoes impact ionization by the beam. It can also come from the target irradiated by the beam or arrive along the magnetic field from an open plasma trap into which an electron beam is injected. The experiments carried out at the GOL-3 facility at the Budker Institute of Nuclear Physics, Siberian Branch, Russian Academy of Sciences, can serve as an example [4–6]. The counter particle flows arriving from the secondary plasma and penetrating into the beam source can substantially affect the source operation by dis-
torting the distributions of electric fields and emission flows in it. This is of crucial importance for systems with plasma emitters, because the electric field in them determines the position of the emitting plasma boundary, i.e., the optical characteristics of the system, which depend on the geometry of all surfaces and the boundary conditions on them. In some cases, intense
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