  Complex structures on a material surface can significantly reduce the total
secondary electron emission yield from that surface. A foam or fuzz is a solid
surface above which is placed a layer of isotropically aligned whiskers.
Primary electrons that penetrate into this layer produce secondary electrons
that become trapped and not escape into the bulk plasma. In this manner the
secondary electron yield (SEY) may be reduced. We developed an analytic model
and conducted numerical simulations of secondary electron emission from a foam
to determine the extent of SEY reduction. We find that the relevant condition
for SEY minimization is $\bar{u} \equiv AD/2 >>1 $, where $D$ is the volume
fill fraction and $A$ is the aspect ratio of the whisker layer, the ratio of
the thickness of the layer to the radius of the fibers. We find that foam can
not reduce the SEY from a surface to less than 0.3 of its flat value.
