Transition from sheath-convection to saturation-current behaviour of a Langmuir probe in a flowing plasma

The transition between diffusive (saturated-current) and sheath-convection [current ¿(voltage)¿] behaviour of the current to a negatively biased Langmuir probe in a plasma jet has been studied experimentally by varying the electron density over a wide range. The measurements verify for the first time the expected change in the dependence of current on voltage. However, this change occurs at higher electron densities than expected, and, when saturated-current behaviour is finally obtained, these currents are significantly lower than their calculated values and, surprisingly, remain close to the sheath-convection currents calculated for moderate voltages.