The performance of a pulsed D2O laser pumped longitudinally by a CO2 laser and emitting at 385 μm is investigated numerically. These studies point out the limitations in pumping efficiency due to the ‘‘bottleneck’’ effect in the vibrational deexcitation of D2O. The influence of a buffer gas on the saturation behavior is discussed. The results of this numerical simulation are in good agreement with experimental measurements. Design parameters are predicted for a D2O laser system suitable for Thomson scattering diagnostics of a Tokamak plasma.