Electron beam duration at different plasma frequencies, assuming second-harmonic emission. The free-streaming curve is calculated assuming v₁ = 0.20 c and v₂ = 0.15 c. The gas-dynamic fit is calculated assuming free-streaming to 100 MHz using u_1,max = 0.24 c and u_2,max = 0.19 c, then plateaus with average velocities u₁ = 0.20 c and u₂ = 0.15 c. The simulation with n_beam = 10^4.5 cm⁻³ (black) has u₁ = 0.18 c and u₂ = 0.14 c between 70–30 MHz. The simulation with n_beam = 10^5.0 cm⁻³ (red) has u₁ = 0.22 c and u₂ = 0.14 c between 70–30 MHz.