Automatic Plotting of Langmuir-Probe Susceptance- and Conductance-Voltage Curves

Langmuir-probe susceptance- and conductance-voltage measurements have been made using a system which automatically plots the in-phase and quadrature components (simultaneously) of a two-terminal complex admittance, as a function of applied dc potential. The operation of the system, which utilizes phase-sensitive detection to separate the admittance components, is discussed. Measurements for a planar guardsleeve probe in a mercury-vapor plasma (collisionless) and for a spherical probe in an argon plasma (collisional) are presented. Positive and negative peaks observed in the low-frequency susceptance-voltage characteristic are discussed in terms of ion-response, probe-contamination, and plasma-perturbation effects.