Oh, for the record:
The portal Tesla opened is to the ghost world, aka the ethereal.
When a portal is opened, it rips a small tear a pocket of space time. Basically, it creates its own small gravitational field to create a miniature black hole (think particle colliers, but being fictional and that it uses magic, it doesn't nearly need as much energy to crate the speed).
The machine acts as a door frame around the black hole, stretching it outward until there's a visible plane of space warping. The frame acts as both the stretcher and the stopper, containing it in the rectangular green swirl. This allows for the user to step through the gateway without being crushed by the immense gravity.
However, Tesla is actually retrofitting old runes he found in Croatia that were attached to an ancient doorway much to the same properties of this artificial one. But he only knows what he can gather from his studies. He didn't invent these portals, he's only recreating one. He doesn't know the exact nuances of this kind of magic yet. The portal he created is unstable and corroding the space-time around itself. If given more time to tinker with it, he should be able to create a stable one soon.
Turning it off in this realm causes a drastic chain of power failures that will trigger a feedback explosion. Turning it off on the other side where the connected portal is constructed correctly and won't feedback (at the very least, even if it does, it's in another dimension and won't harm anyone but already dead ghosts; meaning they won't be harmed).
Plus, it's still possible that tethering a rope between the two gateways will be able to pull Sam back into the corporeal world.
Also, update in a few minutes.