Rest asssured that about 99.9999999% of the grid heating in the early stages is purely due to ion bombardment. Once the grid gets into a range where electron emission is possible, the machine will run away and the grid melt solely due to emission current reducing the impedance of the machine. Also, metal ions and desorbing gases will enter the chamber gas raising the pressure and further reducing the impedance causing more run away. This latter scenario is not a normal mode, but is where more amateur built fusors wind up if pushed a bit hard.
As the grid is negative, exiting electrons from within the grid should not collide with it.
Neutral collisions should also account for nearly zero heating compared to ion bombardment which are eager to hit the negative grid with huge nergies.
As you say neutrons should acount for no heating, perhaps .0000000000000000000324% if that much.
Absorbed photons should also be near zero on the heating scale as well.
Richard Hull
Created on Tuesday, December 12, 2000 10:42 AM EDT by Richard Hull