A study of two-stage ion-beam optics

Abstract

Ion-beam optics of a two-stage electrostatic accelerator (i.e., the extraction-accel-decel column) is studied by three approaches: (i) a linear optics analytic solution based on the paraxial equations which account for lens effects and beam space charge, (ii) a nonlinear optics solution based on the Poisson-Vlasov equations and including the two-dimensional source-plasma sheath, and (iii) a 60-kV single-beamlet experimental study. Principal results are that in certain operating modes, the minimum beam divergence in a two-stage source is significantly less than in the conventional single-stage source and that the optics of the two-stage source is less sensitive to variation in extraction perveance about the divergence minimum. This is because the optimum extraction perveance in such modes of operation is relatively high, causing the crucial residual aberration fields near the first electrode to be smaller.

An ion optics study for KSTAR neutral beam injector development

“…4 In the source plasma, the ions are supposed to be well thermalized and thus the perpendicular component poses as an intrinsic divergence against the acceleration voltage. We assumed the ion temperature in the source plasma to be 0.4 eV ͑Ref.…”

Development of a radio-frequency ion beam source for fast-ion studies on the large plasma device

“…Divergence of a space charge limited ionbeam has been estimated by Coupland et al, 19 Ray et al, 20 and Kim et al 18 (more references therein). It was analytically derived and experimentally demonstrated that a minimum beam divergence is achieved when j j max /2.14.…”

The Applicability of Dual Stage Ion Optics to Ion Engines for High Power Missions

“…3 the lens corresponding to the second grid is either convergent or divergent depending on the relative strength of the electric field in the first and second gap. When the field is weaker in the first gap than in the second (E 1 < E 2 ), the lens is converging, hence a flatter shape of the plasma sheath (and consequently a higher perveance ratio) is required to obtain a collimated jet, whereas if the contrary happens (E 1 > E 2 ), the plasma sheath needs to penetrate more into the plasma to produce more converging ion trajectories to compensate the presence of two diverging lens; this causes a reduction of perveance and hence a reduction of the total extracted current [34].…”

“…This change in optimal perveance can be studied as a function of the ratio of the potential drop applied to the acceleration stage to the one applied in the extraction stage. To do so, the analytical model developed by Kim, Whealton, and Schilling [34] will be used. According to this model, the beam divergence can be expressed as…”