Three-dimensional ion acoustic turbulence is excited by injecting a cold ion beam into a nonmagnetized plasma with $\frac{T_e}{T_i}>5\mathrm{}$. A rapid, approximately exponential heating of the ion beam is observed along the beam path. The heating rates level off when the ion temperature approaches $0.2T_e$. The turbulent ion acoustic wave energy density does not exceed $\mathrm{}(1-3)\mathrm{}\times {10}^{-3\mathrm{}}{T}_e$. Wave saturation, which occurs before significant dissipation of ion beam energy, is consistent with orbit-diffusion strong-turbulence theory.

• Received 28 January 1972

DOI:https://doi.org/10.1103/PhysRevLett.29.34