desired properties include:
* high melting point
* high electrical conductivity
* high thermal conductivity
* low neutron absorption cross-section
* low xray absorption cross-section
* good stability in vacuum
lithium melting point is quite low. anything exposed to the plasma is going to get to over 1000° C.
heavy elements would absorb a lot of xray.
wouldn’t hydrides bleed off their hydrogen when you put them in vacuum?