Since ICRH antennas and their coaxial feeding lines are necessarily va
cuum insulated, their RF electric strength depends mainly on the resid
ual gas pressure, which can be severely impeded during high power RF a
pplication. This interaction between RF, vacuum and electric strength
and the consequences on design and operation are discussed in general
and in the case of ICRH at ASDEX Upgrade, whose feeding design is desc
ribed in detail.