Effect of space-charge limited emission on measurements of plasma potential using emissive probes

Effects of space-charge limitation have been considered for an analytical estimation of plasma potential with use of emissive probes, taking account of a correct expression of emission current under space-charge limited condition. The analytical results are obtained for the floating potential method of emissive probe and for the differential emissive probe method, both without and with an oscillating ac potential. In the case of the floating potential method, the operating condition is discussed for different plasma parameters at several probe temperatures. These results indicate that the floating potential of the emissive probe gives us the plasma potential to an accuracy of the order of the plasma electron temperature Te/e for a strong probe emission. In the case of the differential emissive probe method, the relations among space-charge effects, the ratio of thermoelectron emission current to plasma electron collection current, the plasma electron temperature, the probe temperature, and the amplitude of the oscillating ac potential are discussed in terms of determination of plasma potential. In addition, the analytical results also indicate that the midpoint of the two peaks in the derivative d〈Iem〉/dV curve is a good method for measuring the time-averaged plasma potential.