  The electric field in the vicinity of the top of an emitter with a profile
consisting of a triangular protrusion on an infinite line is analytically
obtained when this system is under an external uniform electric field. The same
problem is also studied when the profile features a two-stage system,
consisting of a triangular protrusion centered on the top of a rectangular one
on a line. These problems are approached by using a Schwarz-Christoffel
conformal mapping, and the validity of Schottky's conjecture (SC) is discussed.
The authors provide an analytical proof of SC when the dimensions of the
upper-stage structure are much smaller than those of the lower-stage structure,
for large enough aspect ratios and considering that the field enhancement
factor (FEF) of the rectangular structure is evaluated on the center of the top
of the structure, while the FEF of the triangular stage is evaluated near the
upper corner of the protrusion. The numerical solution of our exact equations
shows that SC may remain valid even when both stages feature dimensions of the
same order of magnitude, reinforcing the validity of SC for multistage field
emitters.
