I can only answer in terms of Bragg's Law. There are more correct models out there, but this one is easiest to work with :)
n * Lambda = 2* d * sin(theta)
where
n is the "order" of the diffraction. Diffraction does get weaker at higher n, as you predict. First order (n=1) is usually focused on because the angles are smaller, the points are brighter, and higher order spots contain mostly redundant information about the crystal structure.
Lambda is the wavelength. For 200 keV electrons, the wavelength (lambda = h*c/E; lambda = 1240/200000 nm) is crazy short.
d is the repeated spacing in the sample creating diffraction. Usually around 1-10 Angstrom.
The picture I think of for this equation is a theta-theta X-ray diffraction setup.
In this picture you can see that the incoming "beam" is bent by 2-theta.
It's hard to picture it applied to electron diffraction. As you saw if you started playing with the Bragg Law, in this case the diffraction angle is tiny. The schematic you posted is not drawn to scale :)
Anyway- in electron diffraction, electrons are diffracted by planes essentially parallel to the incident beam. I picture electrons that find themselves flying through a hallway in the structure of the sample veering through the walls (like the Kool Aid man) at an angle of 2-theta.
Awwwwww I just noticed this thread is nearly 4 years dead :(
