How to find the ideal resonance frequency for a transformer to be used for a High Voltage circuit

Please read the description first.
This video shows a test circuit with which you can test unknown transformers (thus also reversed 230 V/110 V AC transformers, made for 50 Hertz or 60 Hertz & 6 Volt or 12 Volt or 24 Volt or other transformers) on their properties to act as a High Voltage transformers on their resonance frequency (when they have that, their physical construction is key).
Next video (how to make a 300 Hz-15 KC squarewave oscillator with a high voltage output) is here (first stage of that project) • Avoid contact resistan...
To show what is happening in this video and what I am talking about here (the principles) is this:
Say when you put in AC current/voltage (50 Hertz - 100 KHZ, I used a square wave oscillator here in this video) into the low resistive winding (say the 6/12/24/30 Volt AC winding of a (here 50 Hertz) transformer and then look/measure at the high-resistive winding (the secondary, between say 500 Ohm and 10000 Ohm) how much voltage you can get out.
Because I used a 1N5408 Silicon rectifier diode (it can handle max. 1000 Volt) and a 3.3 uF (microfarad) capacitor made for 300 Volts DC, you can see that the DC voltage paralleled to that 3.3 uF capacitor slowly builds up to approx. 150 DC Volt at a frequency of 13 KC (Kilohertz).
This in this demo with a (tiny) transformer developed for 50 Hertz and 230 V AC.
Thus, in this case, 13 KC was the best frequency to drive this mini 30 V-230 V with its tiny core to its maximum high voltage out. Key reason: low loss in the metal (iron laminated) core.
The core (how it is made in practice + also the material itself) of the transformer is (thus) key, to the maximum frequency that can be reached and thus the core material has a severe impact on the power transmission from the primary winding into the secondary winding.
For higher frequencies and better power transmissions, often ferrite materials (ferrite cores) are used.
You can see them everywhere nowadays, especially in HF oscillators that work to transfer the DC out of solar panels (in whatever way) to push current (especially generated via sine waves) into the (our) mains supply net/mains, be it in Europe, America, Australia, India, South America.
Thus everywhere in the world, DC solar energy is pushed into AC systems via DC-AC converters.
Ferrite materials were in a specific way developed by the Philips factories in Eindhoven, in the Netherlands, during the 1940’s-1960’s. They did a lot of research, finally leading to produce the ferrite cores/blocks that were used in nuclear research in the United States in 1954-1955, a particle accelerator. Source: Philips technical magazine, 1954 or 1955.
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Thanks 👍