Measurement of the longitudinal energy distribution of electrons in low energy beams using electrostatic elements

The Transverse Energy Spread Spectrometer (TESS) was designed primarily to study the mean transverse energy spread of electrons emitted from photocathode electron sources at both room and liquid nitrogen temperatures as a function of quantum efficiency through analysis of the photoemission footprint. By reconfiguring the potentials applied to different detector elements, TESS can also be used to measure the mean longitudinal energy spread of photoemitted electrons. Initial plans were to use electrostatic wire meshes as a retarding element which prevents the detection of electrons with insufficient energy to overcome a variable potential barrier. However, this method has proved impractical and a new method has been proposed in which the photocathode bias potential is swept (effectively from a state of no electron emission to full emission) and the emitted photocurrent is then detected by using a photoemitted charge collector. In this article, we present the TESS set-up and analyze this new method to measure the longitudinal energy distribution curve. Experimental results are presented and compared to simulated results by utilising a custom designed tracking code.