The bottom of the secondary has to go somewhere. Where it goes is not the wall socket, and understanding why is the difference between a coil that works and a house full of interference.
Why the secondary needs a ground at all
Every streamer that leaves the topload is current, and that current has to come back. It travels through the air, into whatever it strikes or simply into the surroundings by displacement, and then back to the bottom of the winding. The resonator is not a closed loop of wire; it is a loop that goes out through space and returns through the ground connection.
So the ground is not a safety afterthought. It is one side of the circuit.
Not mains earth
A counterpoise instead
A large sheet of metal under the coil, which the secondary's base connects to. Roofing aluminium beats foil or mesh, because it does not crumple and does not have a thousand joints. Round the corners, or they will corona.
The lead to it should be short. A long wire has inductance, and inductance at these frequencies is impedance, which defeats the whole point of having a counterpoise at all. Do not run it alongside anything else either, because the secondary will fire into whatever it is running alongside.
The base of the secondary connects to both the counterpoise and the strike ring.
And the frame is not a ground
The cure is to break the loop: nylon bushes, washers and Kapton, so that each upper rail is grounded at one end only. How much voltage appears across those gaps, and why two millimetres will not hold it, is in three grounds and a shorted turn.
The three grounds inside the electronics
Separately from the coil's ground, the driver has three of its own and they are not interchangeable: the high side device's emitter, which floats with the switching node; the low side's, which is fixed; and logic ground. Logic joins power at one star point, so that power currents have no path through the logic. That is its own piece, and it is where more QCW drivers die than anywhere else.
One thing that will surprise you
Metal closer than half the secondary's diameter kills the Q. A chassis, a case, a rack, a radiator: they load the resonator and steal energy into induction heating, and the coil simply performs worse for no reason you can see on a scope.
The earth symbol under the secondary on every department diagram is this, and what it is not connected to is the more important half.