A local energy conversion efficiency for a confined-discharge plasma generator is defined as the fraction of local power input which is transferred to the gas. The local efficiency is a maximum near the gas inlet and diminishes with axial distance.

The thermal development length is the axial extent of that region in the plasma generator duct where the local efficiency is nonzero. This length varies with mass flow rate, current, and duct diameter. Nondimensionalization of the energy equation results in a dimensionless S parameter which determines the thermal development length. The S parameter is the product of mass flow rate, electrical conductivity, enthalpy, and duct diameter divided by the arc current squared. The thermal development length is the product of two times the S parameter and the duct diameter.

Below-atmospheric static pressures occur when a calorimeter is installed at the generator plasma exit. These pressures result from deceleration of the plasma in the calorimeter.

Empirical expressions for enthalpy and electrical conductivity as functions of current and mass flow rate are substituted into the S parameter, allowing the parameter to be evaluated directly from the mass flow rate and current.

The S parameter is effective in evaluating redevelopment thermal lengths downstream of local radial fluid injection into the plasma. Also, the S parameter is found to be effective in evaluating thermal development lengths of independent data from a previous investigator.