Calculations of the electric field in the excitation pulse of a 8‐cm‐diam copper‐vapor laser are presented. The electric field evolution is governed by a diffusion equation. Both radial and axial diffusions are considered. The boundary conditions are determined by the geometry of the tube, which is coaxially shielded by a metal sleeve, considering the tube inductance and parasitic capacitance. The diffusion coefficients are determined by the electrical conductivity of the plasma that is calculated based on measured plasma parameters. These calculations are essential both for modeling the excitation process and for estimation of the overall tube impedance in such lasers. Inadequate penetration of the electric field into the plasma because of high pre‐pulse conductivity turns out to be a main cause of power and pulse repetition rate limitations of this laser. The optimization of the tube geometry in order to minimize the field penetration problem is discussed. ©1995 American Institute of Physics.