A relatively long‐pulse width (0.1–1 &mgr;s) intense ion beam accelerator has been built for materials processing applications. An appliedBr, magnetically insulated extraction ion diode with dielectric flashover ion source is installed directly onto the output of a 1.2 MV, 300‐kJ Marx generator. The diode is designed with the aid of multidimensional particle‐in‐cell simulations. Initial operation of the accelerator at 0.4 MV indicates satisfactory performance without the need for additional pulse shaping. The effect of a plasma opening switch on diode behavior is considered.