Conservation of Charge in the Secondary Electron Emission Test Stand at UNLV

Abstract

Summary form only given. Secondary electron emission (SEE) can potentially lead to beam instabilities, material degradation and r.f. breakdown. The past SEE studies have deduced secondary electron yield by invoking a conservation of charge without the need to experimentally account for all measurables. In the spatial distribution studies of SEE, one may not invoke conservation of charge without some knowledge leading to wall loss effects. An electron gun in the SEE test stand at UNLV directs the primary electron beam through a beam drift tube piercing the center of a particle position detector with controlling grid towards the sample under test. Secondary electrons emitted by the sample either drift freely or are drawn by a grid potential to the particle position detector. The location and number of electrons impinging on the detector records the number of electrons and the locations of electron impact. Low primary beam currents are necessary in order to perform these measurements within the resolution of the detector. Because the detector surface is flat and finite in dimension, some loss to the chamber walls will result. Consequential SEE loss to the walls is not measurable in a noise environment even if the experimental apparatus (the SEE test stand) is isolated from the earth ground. To monitor the primary beam current, a C shaped ferromagnetic core coil fitted to the beam drift tube was designed to divert the primary beam to the beam drift tube for measurement with minimal SEE loss from the tube. Assuming successive shots are similar, the total number of incident charge can be deduced statistically. The coil current is then changed to partially null the Earth's magnetic field when experiments are performed. Beam drift tube, grid, and sample currents are monitored and integrated to yield the number of charges collected by each component over time. Along with time resolved, spatially distributed, SEE count from the detector, wall loss may be deduced by invoking conservation of charge. Spatial distributions of secondary electron yield and secondary' electron emission wall loss counts will be reported. Such distributions offer insights in determining the initial conditions on the emitted secondary electrons.