Comparative measurement of plasma potential with tube probe and Langmuir probe

Plasma potential measurements using the conventional Langmuir probe may cause an error due to the space charge effect. To solve the problem, a tube probe is proposed in this study which can minimize the space charge effect by collecting electrons with an orifice instead of the solid surface of the Langmuir probe. The I-V characteristic of the tube probe exhibits a clear turning point, accurately indicating the plasma potential. Comparing with the results of the conventional Langmuir probe, it suggests that the plasma potential measured by the Langmuir probe may be underestimated by about 0.1-0.2 Te/e, which may cause underestimation of the electron density by about 10%-20%. Combination use of the tube probe and the Langmuir probe is suggested for accurate measurement of the electron density.