That x-ray transparent fluid (helium gas, I don’t know at which pressure it gets fluid, but I suspect the pressure has to be extremely high at 1000K) is required for cooling the electrodes (canals within the electrodes), and also the onion (also with canals).
Maybe also lithium would be possible: https://focusfusion.org/index.php/forums/viewthread/457/#4213
The patent has a drawing of the onion with canals in it for cooling (figure 13). Look for a reference in the text to figure 13.
