The core of the flyback converter is a switching transistor and a resistor. The voltage supply, trafo, cascade and control (train of pulses) are all separate parts. In order to be able to quickly change transistors, flyback core is assembled on a protoboard. The whole high-voltage generator is then assembled from parts (see the photo).
The schematic is available in Fig. 12 in the following document
https://o-esd.etf.bg.ac.rs/forgejo/dragan.olcan/O-ESD/raw/branch/main/Documents/O-ESD-02.pdf
Currently considered transistors are
TIP122
https://eu.mouser.com/ProductDetail/STMicroelectronics/TIP122?qs=ljbEvF4DwOPuWPwhe20bIQ%3D%3D
MOSFET N-CH 200V 18A TO220AB
https://www.digikey.com/en/products/detail/infineon-technologies/IRF640NPBF/811884
MJE15030G NPN 150 V 8 A 30MHz 50 W
https://www.digikey.com/en/products/detail/onsemi/MJE15030G/919491
N MOSFET, 800 V, 11 A, 0.35 ohm, TO-247
https://uk.farnell.com/stmicroelectronics/stw11nm80/mosfet-n-ch-800v-11a-to-247/dp/1752196
MOSFET N-CH 200V 9.9A TO220-3
https://www.digikey.com/en/products/detail/vishay-siliconix/IRLI640GPBF/812191
