Space Charge Aberrations Research Articles

Space-Charge Aberrations in Single-Shot Time-Resolved Transmission Electron Microscopy

In this paper, we discuss effects of space charge fields on imaging performance in a single shot time-resolved electron microscope (TEM). Using a Green's function perturbation method, we derive analytical estimates for the effects of space charge nonlinearity on the image formation process and the associated aberration coefficients. The dependence of these coefficients on the initial beam phase space distribution is elucidated. The results are validated by particle tracking simulations and provide fundamental scaling laws for the trade-off between temporal and spatial resolution in single-shot time-resolved TEM.

Discrete space charge aberrations in spherical photoelectric emission structures

Concentric spherical photoelectron diode structures can be used to generate high current densities when illuminated with intense laser beams. For applications in electron microscopy and for beam source applications, trajectory aberrations of the emitted particles are important. This paper describes Monte Carlo estimations of trajectory fluctuations produced by discrete-charge interactions in a laser-driven spherical photoemission structure. The trajectory of one photoelectron traveling in the presence of 125 randomly positioned neighbors is computed, and its apparent source position is determined as it arrives at the anode. Hundreds of such passes are averaged to estimate the aberration. Apparent source blur sizes range between 1 nm and 3.5 µm, for emitter radii between 160 µm and 2.82 mm, as the current density increases from 1 to 1000 A/cm2. Such small aberrations imply that discrete space charge effects do not significantly degrade the performance of these electron sources under most conditions.

Combined calculation of lens aberrations, space charge aberrations, and statistical Coulomb effects in charged particle optical columns

Optimizing a charged particle optical system utilizing a high brightness source requires the inclusion of the effect of Coulomb interactions in the evaluations. A new computer program package, named ANALIC, has been developed to perform the combined calculation of lens aberrations, space charge aberrations, and statistical Coulomb effects in a complete instrument. By making use of an analytical slice method, valid for weak/incomplete collisions, to calculate the Coulomb interactions, the program combines reasonable accuracy with high speed. Using ANALIC, an optical system with an arbitrary number of lenses and apertures, an arbitrary mode of imaging, and an arbitrary distribution of the beam energy can be analyzed directly. The functions, features, organization, and calculation approach of the program are reported. As an example of the use of the program, a four lens electron probe instrument is analyzed for the demonstration of the combined calculation and optimization process of a particle optical system.

Space-charge aberrations in the photoelectron microscope

The contributions of space charge to chromatic and trajectory aberrations of accelerating electrons in a laser-illuminated photoelectron microscope are calculated. Average and fluctuating components of the electrostatic forces on the particles are considered spearately. The average forces produce a magnification change but no chromatic spread. Fluctuating effects include relaxation of potential to kinetic energy in a beam obeying Poisson emission statistics, and transverse-to-longitudinal velocity relaxation by multiple scattering. The potential energy relaxation is treated in a new way by modifying the Poisson distribution with position along the beam axis. The field fluctuations produce both chromatic and trajectory spreading effects, the latter being much larger at high currents. The resolution limit is about 500 Å for 30-kV electrons in a beam of 1-A/cm2 current density.