Measurement of spiraling in a magnetically confined electron beam for use in collision studies

A method is described to directly measure mean spiral diameter in a magnetically confined electron beam. In collision experiments, spiraling affects polarization measurements and effective path length assessment; and in experiments using colliding beams, the collision energy or energy spread can be significantly altered. The technique described here allows experimental correction for these effects. Magnitudes of transverse velocities from various causes leading to spiraling are estimated on the basis of models. A gun designed for use in crossed beam electron-ion excitation experiments is described, and results of tests on spiraling and space charge effects for this gun are given. The tests generally indicate the modeling to be correct. Judicious choice of operating conditions led to path length corrections as small as 4±2% at 3 eV, decreasing to 0.25±0.2% at 200 eV.