EXPERIMENTAL STUDY OF LONGITUDINAL ENERGY SPREAD IN SPACE-CHARGE- DOMINATED BEAMS

Abstract

A compact high-resolution variable-focusing retarding field energy analyzer has been developed to measure the energy spread of space-charge-dominated electron beams. A cylindrical focusing electrode is used to overcome the beam expansion inside the device due to space-charge forces, beam emittance, etc. The focusing voltage is independently adjustable to provide proper focusing strength. Single particle simulation and theoretical error analysis using beam envelopes show that this energy analyzer can get very high resolution for low-energy beams, which was found to be in good agreement with experimental results. The measured beam energy spectrum is both temporally and spatially resolved. In addition, a computer-controlled automatic system is developed to provide real-time data acquisition and processing. The measurements of the beam energy spread are compared with the theoretical predictions.

It is believed that coupling between the transverse and longitudinal directions via Coulomb collisions will cause an increase of the beam longitudinal energy spread. At the University of Maryland, experiments have been carried out to study the energy evolution in such intense beams with a high-resolution retarding field energy analyzer. The temporal beam energy profile along the beam pulse has been characterized at the distance of 25 cm from the anode of a gridded thermionic electron gun. The mean energy of the pulsed beams including the head and tail is reported here. The measured rms energy spread is in good agreement with the predictions of the intrabeam scattering theory.

In order to study the scaling of the beam energy spread, a two-meter long linear solenoid focusing system has been set up and beam energy spread is measured after the beam passes through the long transport line. Again, the beam energy spread is measured under different beam conditions (energy, current, and density). The experimental results here are in remarkable agreement with the lower limit of the beam energy spread set by the intra beam scattering theory at the lower beam current density. Under some controlled conditions, energy spread larger than the predictions of the intra beam scattering theory are also observed.