Use of radial distributions of the beam current density for evaluation of beam emittance and brightness

Radial distribution measurements of the density j(r) in two (at a known foc us plane position) or three beam cross-sections are enough for estimation o f the angular distribution of the beam current density j(dr/dz) (z coincide s with the beam axis) when the electron beam current density can be charact erised by a 2-D Gaussian distribution in the phase-space plane. The followi ng assumptions are made: a linear approximation of the electron trajectorie s in the treated field-free drift region can be made and the effects due to collisions and space charges can be neglected. In the paper, we use the Mo nte Carlo procedure to process the experimental data j(x, y) to obtain a st andard normal distribution j(r) for every plane z, where experimental inves tigations of the j(x,y) are made. A system of two (or three) equations is u sed to find the standard deviation sigma (dr/dx) = sigma (r), of the normal distribution j(dr/dz). rising known values of sigma (r0) and sigma (r '0) (in the focal plane) and a chosen portion p of the beam current, the import ant parameters: the emittance epsilon (p), the normalised emittance epsilon (np) and the brightness per 1V accelerating voltage (B/U)(p) for this beam portion are estimated. (C) 2001 Elsevier Science Ltd. All rights reserved.