Abstract
The performance of free electron laser x-ray light sources, and systems for ultrafast electron diffraction and ultrafast electron microscopy, is limited by the brightness of the electron sources used. The intrinsic emittance, or equivalently, the mean transverse energy (MTE) of electrons emitted from the photocathode determines the maximum possible brightness in such systems. With ongoing improvements in photocathode design and synthesis, we are now at a point where the physical and chemical surface roughness of the cathode can become a limiting factor. Here we show how measurements of the spatially dependent variations in height and surface potential can be used to compute the electron beam mean transverse energy (MTE), one of the key determining factors in evaluation of brightness.
Highlights
Photoinjectors can provide high-brightness electron beams that are suitable for use in a wide variety of applications, from free electron lasers [1] to ultrafast electron diffraction and microscopy [2] setups
We show that for a real alkaliantimonide surface the combined effect of physical and chemical roughness would initially cause the mean transverse energy (MTE) to reduce with increasing electric field, go to a minimum and increase again
We developed a MATLAB® [22] based program which allows us to calculate a lower bound to the MTE due to such surface variations at various accelerating electric fields
Summary
Photoinjectors can provide high-brightness electron beams that are suitable for use in a wide variety of applications, from free electron lasers [1] to ultrafast electron diffraction and microscopy [2] setups. Codeposition methods have reduced the physical roughness and the extent of local changes in potential in alkali-antimonides [11], it is common to find height variations of a few nm and chemical potential amplitudes of 0.1 V over length scales of 50–200 nm These variations can be measured quite accurately using atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM). Gorlov used a more precise, combined analytic numerical method of the 3-D field calculation close to a realistic 2-D surface [18] This method has been used to obtain the MTE increase from the physical roughness on realistic photocathode surfaces measured using an AFM [19,20]. We numerically calculate the trajectories of electrons in these fields and calculate the expected MTE variation with the accelerating gradient due to the combined effect of physical and chemical roughness. All computation work has been conducted using MATLAB® [22]
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