Nonlinear focusing by residual-gas ionization in long-pulse electron linacs - art. no. 046501

For short electron bunches in accelerators, the radial ion density, due to residual-gas ionization faithfully reproduces the radial electron bunch dis tribution for time scales similar to the electron bunch length. If the elec tron bunch length is sufficiently long, however, the ions focus and, even f or radially uniform electron beams, tend to form a very nonuniform equilibr ium distribution. This ion distribution, in turn, leads to nonlinear focusi ng forces on the electron bunch itself. In this paper, we find the equilibr ium distribution when the electron distribution is uniform, and calculate t he emittance growth for axial slices in the electron bunch later in time. A regime is found in which the emittance growth is quadratic with both resid ual gas pressure and electron bunch length.