You can use AC on steel, but DC+ is preferred. Stick and TIG machines use constant current and yo can use an AC stick welder, right?? The only difference with stick is the flux on the filler rods, but with TIG, there are no fluxes. If you're worried about penetration, weld up a joint, I'd do a lap joint, let it cool, and break it. Don't make the weld too big and only weld one side but after breaking it, look at the to plate and If there is a jagged line, which shows where you tied in at the root, then you pass the test. If there is a distinct straight line, where the weld didn't fuse properly to the root, you fail. Just practice until you pass. Do it again with a different joint, different position. If you have any Q's about the testing, just ask. If I don't know, I'll ask my instructor. He's a certified welding inspector in several area's.
But here's my best advise: GET A FOOT PEDAL!!! Or at least a rig with a finger rheostat so you can regulate the amperage.