A time-dependent numerical model of the astron, with which injection and trapping can be studied in detail, has been developed. The effects due to the resistors and neutralization have been included. The model is axially symmetric. The E-layer electrons are simulated by many thousands of finite-size superparticles, which move in the r−z domain and have velocity components ν r , ν θ , and ν z . The model is relativistic and the electromagnetic fields are obtained by solving four wave equations — three for the vector potential and one for the scalar potential. The E-layer current and the current induced in the resistor wires are included in the above field equations. The computed self-fields are added to the external field to give the field configuration as a function of time. Results of multiple pulse injection are presented.