Elektronen- und Ionenoptik

Aberration of a negative ion beam caused by space charge effect

Aberrations are inevitable when the charged particle beams are extracted, accelerated, transmitted, and focused with electrostatic and magnetic fields. In this study, we investigate the aberration of a negative ion accelerator for a neutral beam injector theoretically, especially the spherical aberration caused by the negative ion beam expansion due to the space charge effect. The negative ion current density profiles with the spherical aberration are compared with those without the spherical aberration. It is found that the negative ion current density profiles in a log scale are tailed due to the spherical aberration.

Electron microscope calibration for the Lorentz mode

The calibration of a modern electron microscope for Lorentz microscopy observations has been performed using diffractogram, Fresnel diffraction fringe analysis and low-angle electron diffraction methods. An example related to the observations of electrostatic fields associated to a thinned reverse-biased p-n junction is also reported.

Aberration correction for high-voltage electron microscopy

The simultaneous correction of third order aperture aberration and axial chromatic aberration is studied theoretically to determine practical upper limits of the accelerating voltage. Aberrations of a round objective lens are compensated by a quadrupole-octopole corrector similar in design to the Archard, Deltrap and Hardy systems. Severe tolerances on lens stability, large electrostatic fields in the quadrupoles, and chromatic aberration of magnification are the primary factors that adversely affect the performance of these systems at high voltage. The variational calculus is used to compute correctors with low electrostatic fields in the quad­rupoles. When lens stability requirements are also considered, the general configuration of high performance systems is deduced. In addition to internal corrector structure, five parameters including voltage and magnification are used to define a system. Optical and field properties are calculated for a wide variety of these parameters. It is concluded that chromatic correction is feasible for accelerating voltages up to 500 kV. Lens stability requirements become very severe for beam energies greater than the electron rest energy, 511 keV. Tabular results are given for systems designed for resolutions of 1, 0.5 and 0.1 nm. In these examples lens stability requirements are one part in 5 x 105, 106and 5 x 106re­spectively. Configuration changes and their effects on stability and field of view are discussed.