Characterization of Charged Particle Beam Sources
TL;DR: In this article, it is shown how a quantity R v ∗ with dimensions of A/m2 sr V (or W/m 2 sr V2) can be used for a quantitative characterization of all electron and ion sources, thermionic emitters, field emitters and TF emitters.
About: This article is published in Nuclear Instruments and Methods.The article was published on 1975-12-15. It has received 4 citations till now.
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TL;DR: In this paper, the authors defined the "practical brightness" of a probe, which is defined as the amount of current contained in the probe when the virtual source is imaged onto the target.
Abstract: Probe size, shape, and current are important parameters for the performance of all probe forming systems such as the scanning (transmission) electron microscope, the focused ion beam microscope, and the Gaussian electron beam lithography system. Currently, however, the relation between probe current and probe size is ill defined. The key lies in a lacking definition of “size.” This problem is solved with the introduction of the “practical brightness.” In literature, many different definitions of “brightness” can be found, but for systems in which the whole of the virtual source is imaged onto the target, it is the practical brightness of a source that determines how much current is in the probe. This means that only with the practical brightness the performance of a probe forming system can be calculated quantitatively. The beauty of the practical brightness is that this source property is unaffected by the quality of the column: without interactions between electrons in the beam, the practical brightness is conserved down to the target. This makes it the only relevant brightness for probe forming systems to be used to compare different sources. The practical brightness can be measured, but can also be calculated when the source intensity profile is known. The Gaussian source intensity profile of thermionic, Schottky, and cold field emitters yields a practical brightness of 1.44ej/????, where j is the current density on the emitting surface and ??? is the average tangential electron energy.
60 citations
TL;DR: In this paper, the authors proposed a beam extraction method based on magnetic transport of slow positrons down to field strengths of ∼100 G and passage through an aperture grid of ∼10 cm diameter in a field termination shield followed by some brightness enhancement stages.
Abstract: In a hybrid slow positron beam, extraction of the positrons from the magnetic field (∼1 T for the PSI beam) to field‐free space is a necessary operation. A theoretical and experimental demonstration is given which shows that the following proposed beam extraction method works as predicted: magnetic transport of the slow positrons down to field strengths of ∼100 G and passage through an aperture grid of ∼10 cm diameter in a field termination shield followed by some brightness enhancement stages. The simulation and measurement of the magnetic‐field distributions along the beam axis show a steep drop down of the field from ∼100 G to a few gauss within 1 cm of shield thickness and a quasiuniform spreading of the transverse field strength across the grid opening. Measurement of transmission and divergency (transverse energy) of the beam exiting the extraction aperture confirmed theoretical estimations and ray tracing calculations for the aperture design used to be of the order of 75% and 20 eV, respectively. These data as a function of field strength and beam energy are used for optimization of the final extraction aperture design (≳85% transmission) to be used in the PSI high intensity beam facility.
21 citations
TL;DR: A review of the most recent technological developments in the field of ultrafast structural dynamics with focus on the use of ultrashort X-ray and electron pulses is provided in this paper.
Abstract: A review that summarizes the most recent technological developments in the field of ultrafast structural dynamics with focus on the use of ultrashort X-ray and electron pulses follows. Atomistic views of chemical processes and phase transformations have long been the exclusive domain of computer simulators. The advent of femtosecond (fs) hard X-ray and fs-electron diffraction techniques made it possible to bring such a level of scrutiny to the experimental area. The following review article provides a summary of the main ultrafast techniques that enabled the generation of atomically resolved movies utilizing ultrashort X-ray and electron pulses. Recent advances are discussed with emphasis on synchrotron-based methods, tabletop fs-X-ray plasma sources, ultrabright fs-electron diffractometers, and timing techniques developed to further improve the temporal resolution and fully exploit the use of intense and ultrashort X-ray free electron laser (XFEL) pulses.
16 citations
01 Sep 1995-Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment
TL;DR: In this article, the problem of extracting the amplitude and phase of the electron wavefunction from an intensity record is considered. But the problem is not a form of Newton's equation but Schrodinger's equation.
Abstract: Traditionally, the study of the optics of lenses, guns, energy analysers, spectrographs and accelerators is based on the solution of Laplace's or Poisson's equation followed by the calculation of trajectories and hence of paraxial properties and aberration coefficients, emittances and the like. Instrumental optics especially for high-resolution microscopy requires other approaches since the point of departure is not a form of Newton's equation but Schrodinger's equation. Thus highly perfected programs are available for calculating the propagation of waves through specimens, for example, and transfer theory and electron holography are expressed in the language of wavefunctions. In between these two extremes is a less well-explored territory. The study of brightness, partial coherence and the radiometric quantities is one example. The proper way of treating grossly under-sampled images is another; in the STEM, for example, a very interesting approach to the so-called “phase problem” (the problem of extracting the amplitude and phase of the electron wavefunction from an intensity record) involves recording the full far-field diffraction pattern from each object-pixel and manipulating the resulting template (image-valued image) in the computer. Such records will have very few electrons/pixel; what does this mean in practice? These and the related problem of discrete formulation of the image-forming process are evoked.
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TL;DR: In this article, a field emission electron source operating at a pressure of 10−9 Torr was used to produce a focused spot having a radius smaller than 50 A and providing 1000 times more intensity than a hot filament system having a similar final spot size.
Abstract: A new electron gun has been built which features mechanical and optical simplicity. Theoretically, it can produce a focused spot having a radius smaller than 50 A and provide 1000 times more intensity than a hot filament system having a similar final spot size. The increase in intensity is made possible by using a field emission electron source operating at a pressure of 10−9 Torr, which is provided (without baking) using commercially available pumps. The small spot is produced by using two properly shaped electrodes which accelerate and focus the electrons from the tip. It would take a hot filament gun and at least two additional lenses to replace this field emission gun when a spot radius less than 100 A is required. Even then the brightness of the conventional source would be too low to make use of the small spot size obtained. The optical properties for the new gun were predicted on a computer and experimentally confirmed in a new scanning electron microscope. The aperture aberration coefficient was measured to be no more than a factor of two greater than the theoretical value of 1.5 cm. A spot radius of 250 A has been measured, and this value is to be compared with the theoretical value of 150 A. Although it was convenient to measure the spot directly only at a relatively large image distance (11.3 cm), calculations imply that the gun can provide a spot radius less than 25 A when very small image distances are used. The gun can be used in pulsed operation because all optical properties are constant for a given voltage ratio so that application of the electrode voltages by means of a voltage divider provides automatic focusing for arbitrary changes in the applied voltage. The methods used to make and operate reliable high field emission tips are reviewed, and a technique is described for changing the required tip voltage to obtain a given emission current.
348 citations
TL;DR: In this article, the current density and the distribution in energy of electrons emitted from metals are calculated for various combinations of temperature, applied surface electric field, and work function using numerical integration.
Abstract: Both the current density and the distribution in energy of electrons emitted from metals are calculated for various combinations of temperature, applied surface electric field, and work function. A wider range of those variables than previously achieved is made possible by use of numerical integration. The integrand is the usual function based on the free-electron theory of metals and the wave-mechanical barrier transmission coefficient of Sommerfeld and Bethe which assumes a classical image force and a plane surface. Results, which are presented in graphical form, are consistent with the Fowler-Nordheim field emission equation for low temperatures, and with the Richardson thermionic emission formula at low fields. Predicted emission at temperatures up to 3000\ifmmode^\circ\else\textdegree\fi{}K is compared with cold emission at fields between ${10}^{7}$ and ${10}^{8}$ v/cm. A qualitative comparison is made between the present results and previous experiments on the transition between field emission and the vacuum arc.
130 citations
TL;DR: In this paper, the thermodynamic Green's function method was used to treat field emission from superconductors, and nonideal metals in which electrons collide with phonons, impurities, and lattice imperfections.
Abstract: Publisher Summary The chapter explains how field electron microscopy has established itself as a powerful tool for elucidating a variety of phenomena occurring at metal and semiconductor surfaces. The quasielectron momentum components perpendicular to the tunneling direction are conserved during electron tunneling through a finite potential barrier. The chapter talks about the many-body approach that involves the thermodynamic Green's function method to treat field emission from superconductors, and nonideal metals in which electrons collide with phonons, impurities, and lattice imperfections. The method was also applied to the effect of a finite analyzer resolution. The atomic potential was represented both by a square well with an attractive core parameterized by its depth and width, and by a repulsive delta function potential that was equivalent to orthogonalization of the tunneling electron wavefunction to the occupied, tightly bound adsorbate electron orbitals. Plausible forms for the pseudopotential for both metallic and neutral adsorbates were suggested. The chapter also states that a Stark splitting of an electron–phonon transition can be envisioned in the case of degenerate vibrational, rotational, or bending modes.
115 citations
TL;DR: In this paper, the properties of vacuum when it behaves as an electrical insulator, the factors which determine breakdown, and the hypotheses advanced to account for the breakdown are discussed. But, the authors do not consider the effect of the type of vacuum used.
40 citations