Spot Size Of Incident Beam Research Articles

Aperture effects and mismatch oscillations in an intense electron beam

When an electron beam is apertured, the transmitted beam current is the integral of the incident beam current density over the aperture area. Space charge forces generally cause an increase in incident beam current to result in an increase in incident beam spot size. Under certain circumstances, the spot size will increase faster than the current, resulting in a decrease in current extracted from the aperture. When using a gridded electron gun, this can give rise to negative transconductance. Here, this effect is explored in the case of an intense beam propagating in a uniform focusing channel. It is shown that proper placement of the aperture can decouple the current extracted from the aperture from fluctuations in the source current, thereby isolating systems downstream of the aperture from upstream current fluctuations. It is also shown that apertures can serve to alter longitudinal space charge wave propagation by changing the relative contribution of velocity and current modulation present in the beam.

Portable autocollimators using the laser diode and the position sensitive detector

We were able to simplify the structure of the autocollimators and enhance the performance by using one- and two-axis PSDs (position-sensitive detectors) which have spatial resolution of 0.1 and 2.5 μm, respectively, with little dependence on the spot size of incident beam and the visible laser diodes as the light source. Two kinds of portable autocollimators which can measure angles with resolutions 0.01″ and 0.1″, respectively, were made. They were calibrated by the reference angle generator which employed the laser interferometer. The characteristics of temporal stability were evaluated and the profile of a test surface was measured.