Also, I’m looking at the chart of x-ray absorption cross-sections, and notice that Carbon might not be very bad as arterial structure, graphene tubes helping conduct waste heat very quickly away from the tip, with much less radial heat flow to the helium input.
Here’s the comparison…
Beryllium
keV µ (cm-1)
3.0 37.9
10 1.08
30 .324
100 .247
Carbon
3.0 203.9
10 4.951
30 .567
100 .339
Copper
3.0 7034
10 1960
30 96.6
100 4.06
And, if you arrange the tubes parallel to the high-energy electrons, you’ll also drastically reduce erosion. That’s what leads me to the conical shape, ~20 degree angle, to agree with the electron exit beam