Electronic – SCR in this circuit exploded – why

With separate grounds, there is no guarantee that a pulse generated with respect to the first ground will have any effect on a circuit referenced to the other. There are at least two ways to handle this. The most obvious is to use an optocoupler to transfer the pulse to the SCR. The other, older, technique is to use a pulse transformer.

^{simulate this circuit – Schematic created using CircuitLab}

You will need to research optocouplers, since some are too slow for your application. Also, if your grounds really get to swinging wildly, a given optocoupler has a certain rated isolation voltage which you need to respect. Given that you have not destroyed your scope, your grounds are probably not that far apart, but it's something to think about.

As a side note, are you sure you need a 33 ohm resistor to the gate of your MOSFET? It pulls 150 mA when the 2N2222 is on, and that's a good deal of current. It's entirely possible that you do it to pull up the gate quickly during turn-off, but it still seems excessive.

When connected the circuit draws upwards of 10A

I'm not surprised. Consider the transformer primary as an inductor and forget about the secondary or any loading. The primary is still an inductor and you have to get the primary inductance high enough so that the current taken (with no secondary winding/load) is reasonably small.

Primary inductance (in fact it's called magnetization inductance) is proportional to turns squared so doubling the turns makes the inductance 4 times greater and, would reduce the intake of current by 4.

Eighteen turns is never going to be enough. A typical ferrite core (for example) might have an A\$_L\$ value of 10 micro henries per squared turn and this means 18 turns is going to be about 3 milli henries. At 60 Hz this has an impedance of just over an ohm and without a fuse you'd have a fire.

Usually AC transformers have round about 1000 turns on the primary and this of course means a primary inductance of 10 henries and a very much reduced magnetization current. Try looking at this site for some information on transformers.

Also note that transformers do not pass DC because the primary inductance becomes a short i.e. the lower the operating frequency you use the more turns you need to avoid excessive primary (only) current.

If you are still intent on rolling your own, here's the math: -

Also take a look at this medium power transformer: -

Note the number of primary turns. A quick estimation I did tells me there are about 12 layers of about 90 turns each. That's 1080 turns!