Figure 1. An illustration of different types of collecting Langmuir probes with radius rp ; a) Spherical, b) Cylindrical and c) Planar geometries

From

Theoretical Study of Spherical Langmuir Probe in Maxwellian Plasma

Shankar Bhattarai, Lekha Nath Mishra

International Journal of Physics. 2017, 5(3), 73-81 doi:10.12691/ijp-5-3-2