Guardring Probes in Hot-Cathode Hg-Vapor Discharges

The probe curves J(V)=Je(V)+Ji(V) for planar guardring probes (GP) are found to approximate ``ideal plane probe'' (IPP) behavior only in Je≈const.≈Je0 [=zero-field electron (e-) current] for e-attraction; for ion (i-) attraction Ji≈const.≳5Ji0, in columnless Hg-vapor discharges with hot cathode. The sharp knee of the IPP is prevented mainly by perturbations of the plasma by the large G whose radius rG≫ Debye length, and rG≫rP. The perturbations are proportional to the surface area of G+P. The planar GP reacts strongly to a weak fast group of electrons of perpendicular incidence, while small spherical probes (SP) react only weakly. The probe curves for planar and spherical GP's and for small SP's and their second derivatives (SD) are automatically plotted and discussed. Peculiar distortions of the SD's of the GP's (double hump shape) are found to be caused by the perturbations of the plasma by the GP's.