The idea itself is good, but from the name, "field", shouldn't it be in the same class as the other "field" as a shield? Otherwise with less PC (...people use SoFo anyways...) plus a shield (diamond, permafrost etc) this might get a tad bit imba.
Btw, this+lobo/flayer=lock.
Well I see dissipation field as a force field around the user. Unupped, it is dissipation shield. Null field is a "field" just like in the case of flooding, a card that covers and affects the battlefield.
Lock? If that's the case, it only acts like a cloak that also prevents you from using spells. If two rushes go up against each other, and one carries this, the one that carries this will be slower.