We solve the Poisson–Vlasov equations for volume‐produced ions, fast neutrals, and backstreaming electrons in a Mirror Fusion Test Facility 80‐keV preprototype neutral beam source. Neutral pressure distributions are estimated from known gas feed rates and calculated accelerator column conductance, and are calibrated by comparing measured and calculated electron backstreaming power (computed by solving another Vlasov equation). The depletion of atomic yield from these volume processes is computed for sources in which the atomic yield is 100% in the extraction region. The energy distribution of neutrals and ions is also established.