Stagnation probe measurements in flowing plasmas

Measurements of the ion current to a circular flat stagnation probe have been performed in a laboratory flame of known ionization density. The measurements were performed over an ionization density range from 10¹⁵ to 10¹⁸ m⁻³, probe bias voltages from 5 to 400 V and probe/flame velocities from 3 to 19 m s⁻¹. The measurements show good agreement with the calculated sheath/convection currents: I=(VR)^0·8 (n_eeuinfinity)^0·6π(6₀μ)^0·4 (mks) for the case where the sheath is thick compared with the probe, and I=(72R⁸n_e³e³uinfinity³ V²μ₀πa⁻³)^0·25 (mks) for the case where the sheath is thin compared with the probe. Here R is the radius of the flat conduction face of the probe, n_e the ionization density, e the electronic charge, μ the ion mobility, uinfinity the plasma flow velocity relative to the probe, far from the probe, V the probe bias voltage, ₀ the permittivity of free space, and α the radius of the insulator surrounding the conducting face of the probe.