AbstractA one-dimensional radial hybrid code was written to study the start-up of the field-reversed mirror via neutral beam injection. This code, named FROST (Field-ReversedOne-dimensionalSTart-up), models the plasma as azimuthally symmetric with no axial dependence. A multigroup method in energy and canonical angular momentum describes the large orbit ions from the beam. This method is designed to be more efficient than those employing particle tracking, since the characteristic time scale of the simulation is the ion slowing down time, rather than the much shorter cyclotron period. A time-differentiated Grad-Shafranov equation couples the ion current to massless fluid equations describing electrons and low-energy ions. Flux coordinates are used in this fluid model, in preference to a Eulerian framework, so that coupling of plasma at the two different radii of a closed flux surface can be treated with ease. Since a fluid treatment for electrons is invalid near afield null, a separate model for the electron current was included for this region, which is a unique feature. Results of simulation of injection into a 2XIIB-like plasma are discussed. Electron currents are found to retard, but not prevent, reversal of the magnetic field at the plasma center.