Cascaded arc plasma sources with channel diameters between 4 and 8mm were experimentally investigated at discharge currents up to 900A and hydrogen (H2) flow rates up to 10 slm. Pressure measurements at the arc exit showed that the heavy particle temperature in the discharge channel was about 0.8 eV. The electron temperature was calculated from the electron mass balance, taking into account electron losses due to ambipolar diffusion and convection out of the source channel. This calculation showed that the electron temperature was 1.5-4 eV, increasing with decreasing density in the channel (i.e. with decreasing H2 flow rate and increasing diameter). The results of Thomson scattering measurements at 1 and 5 cm distance from the source exit showed the same trends. Using measurements of the average axial electric field, the effective size of the current-carrying 'active' plasma was calculated, expressed in terms of the filling fraction ¿2 = (reff/R) 2. The data showed that the filling fraction increased linearly with the input power and was independent of the diameter and flow rate. The ionization degree in the active center was estimated to be 20-30% from an evaluation of the electron energy balance, Thomson scattering measurements and Hß emission measurements. The highest gas efficiency was obtained when the channel was completely filled at a maximum current of 900A (65 kW input power, 8mm channel, 4 slm flow rate) and was 19%. The highest energy efficiency was 7%. © 2010 IOP Publishing Ltd.