I received the new SCR. It can handle 3000 amps peak current. It is much larger than the SCR I recently destroyed:
Here it is installed. Everything works:
I also added leads to the current sense resistor to avoid touching the back of the power supply:
The pressure necessary for plasma is still too high. I tried another tweak to the electron gun:
No joy. It requires about 24 millitorr for a stable plasma. UG.
Prometheus Fusion Perfection 
Try thinner wire, thinner wire has greater local electric fields and should emit better. (lookup field emission) better yet, use a filament, it will then emit into pure vacuum.
Steve.