Primary Stabilization of a 1 MeV Electron Accelerator

Chromatic aberration is a major obstacle in the attempt to achieve near atomic resolution in electron microscopes. In analogy to dispersion of colors in a glass lens, nonuniform velocities of the illuminating electrons will reduce the resolution of an electron microscope lens. Instability of the accelerating potential is an important cause of these nonuniformities. To achieve resolution on the order of 1 to 5 Å, accelerating voltage must be stable to approximately 3 parts per million. To achieve these resolutions, a highly stabilized 1 MeV linear electron accelerator was developed. One of its two 15 in. (38 cm) accelerating tubes will provide a velocity stabilized electron beam source for a high resolution electron microscope. Electron acceleration occurs through a constant potential divider. A Van de Graaff type charge carrying pellet string provides the accelerating voltage. The primary or coarse control system for the linear electron accelerator compares the high voltage terminal voltage as detected with an accurate voltage divider to a reference cell voltage. Error signals are amplified to control the voltage on a capacitive liner. Control is accomplished by a voltage control of the cylindrical capacitive liner surrounding the high voltage terminal. The low frequency fluctuations in accelerator voltage maintained by this primary loop are less than 0.1% to permit utilization of a fine control loop with a spectrometer beam analysis control to achieve fine voltage stability of 3 parts per million.