Adiabatic Beam Collapse in the Cryogenic Electron - Ion Source.

The Cryogenic Electron-Beam Ion Source (CRYEBIS), which was constructed in 1978 at the Institut de Physique Nucleaire at Orsay, France, uses a low-energy (1--10keV), low-current (0.1--2.0A) magnetically focused electron beam to produce highly charged ions for atomic and nuclear physics experiments. Results of preliminary tests on CRYEBIS indicate that the electron-beam current density increased by almost two orders of magnitude during operation of the source. Since an increase in the current density corresponds to a decrease in the beam radius when the current is constant, this phenomenon is known as beam collapse. Using both a single-particle model and a fluid model, we show that neutralization of the electron-beam space charge by ions in the beam can lead to beam collapse. We also describe how ions are focused in the beam by stepwise ionization and discuss the role ion focusing plays in beam collapse. Finally, we present results of a one-dimensional magnetized electrostatic particle code which was written to simulate beam collapse.