Electron current reflected from a machined copper reflector electrode obtained with and a diameter Gaussian laser spot. An optoisolator amplifier with resolution was used to measure the photocathode current.
Apparatus consists of a photocathode source illuminated with a laser and simple electron optics to measure the electrons reflected colinearly with the incident beam.
Values of vs indicating a linear relationship as assumed in the text.
Derivative of the reflected current curve is consistent with the narrow energy spread from the photocathode for and a Gaussian laser spot diameter (FWHM). Note that the symmetry of the derivative curve is due to the reduced axis voltage span as compared to Fig. 2.
Current density profile at the target obtained with a pinhole and a Faraday cup adjacent to the decelerator electrode.
Variation of photocathode current with reflector electrode voltage caused by reflected electrons passing through the extractor aperture. The relevant trajectories are shown in Fig. 7(b).
(a) Equipotential lines obtained with the voltage configuration shown in Fig. 2 and . (b) Simulation of electron trajectories assuming a uniformly emission spot diameter, a energy spread, and initial energy. A 30° half emission angle was assumed to visualize the electron trajectories in the scale of the apparatus. The actual emission half angle of the photocathode is less than 2°. Note that some reflected electrons that pass through the extractor opening are returned to the photocathode and collected by the extractor electrode.
Simulations to show the effect of the electron source parameters on the apparatus performance. (a) Simulation of 10 000 electron reflections from a source with an energy spread of , initial energy of , and emission half angle of 30°. The source emission is assumed uniform over a diameter area. A step wide is obtained which is much wider than the experimentally observed. (b) Simulation of 10 000 electron reflections with negligible electron energy spread and initial energy. The emission angle is 0° and the source size is diameter. In this case a step wide is obtained which is sharper than the experimentally obtained shown in Fig. 2 also shown in this figure for direct comparison. (c) The following values were assumed: 10 000 electrons with energy spread, initial energy, 2° emission half angle, and diameter uniformly emitting spot.
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