Process of compensation of the space charge of a negative ion beam in a gas
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Process of Compensation of the Space Charge of a Negative Ion Beam in a Gas V. N. Gorshkov, A. M. Zavalov, and I. A. Soloshenko Institute of Physics, National Science Academy of Ukraine, Kyiv, Ukraine Received February 8, 2007; in final form, April 19, 2007
Abstract—The process of compensation of the space charge of a negative ion beam propagating through a neutral gas is investigated numerically. A comparison of the results obtained with experimental data unambiguously proves that, at high gas pressures, when the beam space charge is overcompensated, the electric field within the beam is determined by Coulomb collisions of the beam ions with plasma electrons. At low pressures, when the space charge is undercompensated, the field within the beam is determined by the dynamic processes related to oscillations of the beam current. PACS numbers: 52.40.Mj DOI: 10.1134/S1063780X07120082
1. INTRODUCTION Efficient transportation of a high-current ion beam requires that the space charge of the beam ions be substantially compensated for by electric charges of opposite sign. When the beam propagates through a neutral gas, these charges arise due to gas ionization by the beam ions. In the initial stage of this process, the “anticompensating” particles (i.e., those having the charge of the same sign as the beam ions) are expelled from the ionization region (r < r0, where r0 is the beam radius) by the radial electric field. In a steady (generally, quasisteady) state, transverse fluxes of particles having charges of both signs are established that counterbalance (on the average) the generation of plasma particles (electrons and ions) in the axial region of the beam. Such a balance is the most general feature of the beam– plasma system. The space-charge distribution that is established in the beam channel depends on the generation rate of plasma particles (and, accordingly, on their averaged radial fluxes) and determines the main sought-for parameter—the radial potential drop ∆ϕ within the beam.1 This parameter is directly related to the mechanisms governing the radial transport of the plasma components because it is these mechanisms that determine the level to which the beam channel is filled with plasma in a steady state. The specific features of transportation of positive and negative ion beams are related to the large difference in the masses of the electrons and ions produced by gas ionization. In a positive ion beam, both the beam charge and the charge of the positive ions formed by gas ionization are neutralized by light electrons. Therefore, in the absence of a magnetic field, the beam space average electric field in the beam is Er ∝ ∆ϕ = ϕ(r = 0) – ϕ(r = r0).
1 The
charge is always undercompensated. In the case of a negative ion beam, when heavy positive ions act as neutralizing charge particles, two regimes characterized by different mechanisms for generating radial chargedparticle fluxes (and, consequently, by very different compensation parameter ∆ϕ) can be established. Depending on the gas pressure P, the
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