Evaluation of electron energy spread in CsBr based photocathodes
Electron source development test stand (SDT) installed at the Stanford Synchrotron Radiation laboratory (SSRL) was modified to perform energy spread measurements. Compact energy spread analyzers were fabricated and mounted in an manipulator to measure the energy spread of the electron beam emanating from the photoemitter sources.
(A) Results of photocathode and extractor electrode current vs photocathode voltage for a sample utilizing a simple parallel plate analyzer. (B) The derivative of the extractor current trace relative to the photocathode voltage shown in (A) is depicted. The data were obtained after of activation with a laser at a current density of .
Retarding field analyzer installed in the SDT system. The structure of the analyzer electrode was calculated as shown in (B) with a ratio of thickness to aperture radius of 6 to get a resolution . Care was taken to cover all the insulators to minimize charging effects. (B) Calculated electron trajectories in the analyzer electrode. The incident beam is completely blocked with a potential change of .
(A) Effect of beam tilting in the analyzer’s resolution. Without tilting the beam, the energy resolution is . Tilting the beam more than 0.01°, a significant portion of the beam does not reach the Faraday cup collector even when the applied voltage is less than the beam energy because the tilted beam hits the sidewalls of the analyzer. This creates secondary and back scattered electrons affecting the analyzer’s performance. The calculations show that the cathode and analyzer unit should be aligned within for an analyzer to photocathode distance of about . In principle, the SDT piezostage has motion capabilities of fractions of a micron in a field of view. However, the measurement is further complicated by the beam misalignment effect shown in (B) Two electron trajectories are shown for a misalignment of from the optics axis of the electron collector aperture. The collector acts as a divergent lens for a misaligned beam and creates spurious electrons at the analyzer’s entrance aperture on the front electrode.
(A) Data obtained in two samples with the analyzer shown in Fig. 3(a). One new sample had a photoyield of and another aged one with several hundred hours of operation and exposed to air several times had a photoyield of . For both samples (the new one operating at ), in addition to the large peak at the acceleration energy of with a width , a broad electron distribution at lower energies is present. This is attributed to backscattered electrons due to beam misalignment. The aged sample operated at a current density of only . (B) shows reduction in secondary and backscattered electrons biasing the shield electrode before the Faraday cup in the analyzer.
(A) Beam profile obtained by scanning the analyzer relative to the incident electron beam in two perpendicular directions. A diameter aperture was utilized to measure the beam. (B) The FWHM beam dimensions from (A) were utilized to estimate the energy spread of the assumed Gaussian shaped beam corrected by the aperture. An energy spread of was estimated from the data.
(A) Energy analyzer consisting of a Faraday cup covered with a Au mesh. The Faraday cup is shielded by a Copper cylindrical electrode. The electrons travel in a field free region until they reach the shield electrode aperture. A uniform retarding field is applied between the Faraday cup mesh and the shield electrode. (B) Data obtained in a sample utilizing the Faraday cup/mesh analyzer at . Low energy secondary electrons due to beam misalignment present in the tails prevented fitting the data with a simple Gaussian function.
(A) Model used for the calculations of the resolution of the Faraday cup analyzer using the MEBS SOELENS program. (B) Results of the calculations for two shield electrode apertures. The larger aperture appears to have a slightly better resolution. This may due to some lensing effect or a better averaging of the electron beam reaching the mesh.
Summary of energy spread results.
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