Strong beam–plasma interactions occur when beam densitynbapproaches plasma density,nb/np>0.01. Energy flow from resonant waves to short wavelengths is modeled with existing theory, using instability rates from linear and nonlinear (Zakharov) dispersion relations. Langmuir waves lose energy by wave convection, through observed electromagnetic emission, and to heating by induced return currents acting on anomalous resistivity. Inputs to the model equations are current, voltage, and other parameters of a 600 keV, 5 kA electron beam. The beam propagates in 10 mT helium preionized to 2×1012cm−3, emitting radiation of peak power 100 kW at an efficiency of 3×10−5. Turbulent electric fields reach 27 kV/cm [Phys. Fluids B1, 2488 (1989)]. Comparison between experiment and model calculations shows satisfactory agreement.