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 e stimation of plasma potential with use of emissive probes, taking account o f a correct expression of emission current under space-charge limited condi tion. The analytical results are obtained for the floating potential method of emissive probe and for the differential emissive probe method, both wit hout and with an oscillating ac potential. In the case of the floating pote ntial method, the operating condition is discussed for different plasma par ameters at several probe temperatures. These results indicate that the floa ting potential of the emissive probe gives us the plasma potential to an ac curacy of the order of the plasma electron temperature T-e/e for a strong p robe emission. In the case of the differential emissive probe method, the r elations among space-charge effects, the ratio of thermoelectron emission c urrent to plasma electron collection current, the plasma electron temperatu re, the probe temperature, and the amplitude of the oscillating ac potentia l 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 < I-em>/dV curve is a good method for measuring the time- averaged plasma potential. (C) 2000 American Institute of Physics. [S1070-6 64X(00)03008-1].