Ok I know the shield is balanced. Just thought I say that before I go spewing my opinions and have crap loads of people spewing facts back at me. but here are my thoughts.
1. having a potential of 18 turns of immunity to physical damage is a lot. It's so much that if I didn't just read the forum rules I might describe it using much more colorful language.
2. 6/5 quanta cost for a shield that only last three turns is a lot as well.
These are two user friendly problems and opponent friendly problems that I think I have an idea for. Instead of 3 turns per shield, make it 2. Instead of 6/5 quanta, make it 4/3. And lastly to buff the shield up so that users of the shield don't feel too cheated, make the shield immortal that way it feels right at home with the rest of the Aether element.
P.S. Here's a fun little idea to try. Take a graviton salvager and immortalize. Take a Pulverizer and immortalize it. play a dimensional shield and on the last turn destroy the shield with your pulvy and then realize what I just realized by reading the description on the salvager that this strategy doesn't work and you just wasted a lot of time making a deck centered around this idea. Come on "Elements," this should totally be allowed, I don't actually think its overpowered at all.