We investigated the pressure dependence of the gas flow and the field ion intensity of a coaxial ion source operating at room temperature over a wide pressure range, testing various gases and ionization voltages. Flow conductance measurements taking into account the different gases’ viscosity and molecular mass consistently exhibit a generic pattern. Three different flow regimes appear with increasing upstream pressure. Since the coaxial ion source supplies the gas locally, very near the apex of the tip where ionization occurs, large ionization currents can be obtained without degrading the propagation conditions of the beam. Compared with field ionization in a partial pressure chamber, using the coaxial ion source increases the ion current a hundredfold for the same residual low pressure. We also show that the gas flow regime does not impact the ionization yield. Although a fuller characterization remains to be performed, brightness reaches 3×1011 A/m2/sr at 12 kV extracting voltage.