Self-pinching of a relativistic electron bunch in a drift tube

Electron bunches with charge densities ρ of the order of 102 to 103 [nC/cm3], energies between 20. and 100. [MeV], peak current > 100 [A], bunch lengths between 0.3 and 1.8 [cm], and bunch charge of 2.0 to 20. [nC] are relevant to the design of Free Electron Lasers and future linear colliders. In this paper we present the results of numerical simulations performed with a particle in a cell (pic) code of an electron bunch in a drift tube. The electron bunch has cylindrical symmetry with the z-axis oriented in the direction of motion. The charge density distribution is constant in the radial and Gaussian in the longitudinal direction, respectively. The electron bunch experiences both a radial pinch in the middle of the pulse, corresponding to the peak electron density, and a significant growth of the correlated emittance. This behavior is explained, and an approximate scaling law is identified. Comparisons of the results from the pic and PARMELA codes are presented.