I have been reading about Tesla coils and their principle of operation - if I have understood correctly - after the primary is sufficiently charged, its circuit is closed (by means of a spark gap or a solid state device) which much like an LC circuit generates an oscillatory circuit.
Due to the magnetic coupling between primary and secondary coils however, there's exchange of energy towards the secondary (and backwards, after the first half of the entire cycle) - the secondary acts like an LC circuit too. Since the capacitance of the secondary is low, the energy that is transferred in its entirety from the primary after some point, generates a massive voltage which can ionize the surrounding air and cause discharges.
My questions are the following:
Why does the primary have to be intermittently charged in order to cause resonance on the secondary? Isn't the primary always oscillating at its natural frequency? Why can't it be driven at that frequency by the supply transformer constantly?
Is the frequency of voltage in the secondary the same as the frequency of its excitation in the primary? What determines how fast energy from the primary transfers to the secondary and vice versa?
The Wikipedia page on Tesla coils mentions:
[...] Current flows rapidly back and forth through the secondary coil between its ends [...] The secondary current creates a magnetic field that induces voltage back in the primary coil, and over a number of additional cycles the energy is transferred back to the primary.