Ion‐beam optics of a two‐stage electrostatic accelerator (ie, 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

A study of two‐stage ion‐beam optics

An experimental investigation of the physical processes governing ion extraction from a plasma is presented. The screen hole plasma sheath of a multiaperture ion accelerator system is defined by equipotential plots for a variety of accelerator system geometries and operating conditions. A sheath thickness of at least 15 Debye lengths is shown to be typical. The electron density variation within the sheath satisfies a Maxwell‐Boltzmann density distribution at an effective electron temperature dependent on the discharge plasma primary‐to‐Maxwellian electron density ratio. Plasma ion flow up to and through the sheath is predominately one dimensional and the ions enter the sheath with a modified Bohm velocity. Low values of the screen grid thickness to screen hole diameter ratio give good ion focusing and high extracted ion currents because of the effect of screen webbing on ion focusing.

Ion extraction from a plasma

Ion extraction and optics arithmetic

A finite difference 3-D Poisson-Vlasov algorithm for ions extracted from a plasma

The effect of an insulator coating on the plasma electrode of a duoPIGatron ion source on ion‐beam optics was examined theoretically and experimentally. The effect of a preacceleration potential, applied between the insulated electrode and the target cathode of the source plasma, on ion optics was also examined. A principal result is that this electrode arrangement with precel gives a much lower beam divergence than the same arrangement without precel. Detailed comparisons between the data and calculations are presented.

Effect of preacceleration voltage upon ion‐beam divergence

Several different axisymmetric aperture shapes have been studied, experimentally and theoretically, for use in the plasma electrode employed in extracting ions from a plasma. Compared to a cylindrical bore aperture, a class of shapes opening away from the source plasma resulted in smaller beam divergence (due to reduced aberration fields), while a class opening toward the source plasma resulted in higher beam power density at optimum divergence. The minimum half‐width‐half‐maximum divergence obtained for the former class was 0.57° at a beam energy of 27 kV, as compared to 1° obtained for a conventional cylindrical bore aperture.

W. L. Stirling

Papers

Effect of emission aperture shape upon ion optics

Effect of preacceleration voltage upon ion‐beam divergence