An ablation-dominated capillary discharge using low atomic number elements for plasma formation to flow into an ablation-free extension barrel is a concept that provides a high energy–density plasma flow sufficient to propel fuel pellets into the tokamak fusion plasma chamber. In this concept, the extension barrel is made from a non-ablating material by coating the interior wall of the barrel with nanocrystalline diamond to eliminate mixing the propelling plasma with any impurities evolving from the barrel ablation. The electrothermal plasma code ETFLOW models the plasma formation and flow in the capillary discharge and the flow into the extension barrel to accelerate frozen deuterium pellets. The code includes governing equations for both the capillary and the extension barrel, with the addition of the pellet’s terms. It also includes ideal and non-ideal plasma conductivity models. The joule heating term in the energy conservation equation is only valid in the capillary section. The pellet momentum and kinetic energy are included in the governing equations of the barrel, with the addition of the effect of viscous drag terms. The electrothermal capillary source generates the plasma via the ablation of a sleeve inside the main capillary housing. The acceleration of the pellet starts in the extension barrel when the pressure of the plasma flow from the capillary reaches the release limit. The code results show pellet exit velocities in excess of 2 km/s for source/barrel systems with low-Z liner materials in the source for 5, 20, 45, and 80 mg pellets. The study shows that an increase in the length of both the source and the extension barrel increases the pellet exit velocity with the limitation of slowdown effects for plasma expansion and cooling off inside the barrel.