The PWM controls the supplys on and off time which in turn forms its voltage and current levels. It is indirectly related to this problem but electrically quite distant from the coolant circuit.
I don’t see how there are eddy currents in a tube of water and how that would translate to something that damages a power supply.
Eddy currents are localized to specific kinds of material in a moving magnetic field resulting in localized mechanical forces.
The eddy current created within the copper by a moving magnet creates a mechanical braking force that slows its down. The currents are localized in the material.
I also can’t image much of a magnetic field due to the coolants low inductance and low current.
For there to be back emf there would have to be a changing magnetic field, no? Which means there needs to be adequate inductance. I don’t see how there would be much inductance in this circuit and how much of a magnetic field would be created. Keeping in mind that even conductive coolant is a pretty good electrical insulator negating much current.
That said this is a unusual condition of very high voltages which at times seems to defy normal electrical behavior.
I will have to think about this idea more.
My working theory:
On the other hand you have 20,000 volts across surface of glass (laser tube) on the other side of which is an area of coolant whose other surface is connected to air (another capacitor)
The tubes surface is large and the coolant itself has substantial surface that is connected to air. I would expect the flow of current to be higher if that water was more of a conductor than a dielectric. i.e. a more leaky capacitor.
The area represented by the surface of the water jacket is substantial, the voltage is substantial so its not hard to imagine a small change in the dielectric could increase the current beyond the supply’s design capacity which is in the range of 32ma.
Also consider that the voltage applied to the tube of water is a large spike of 20,000 volts with low current until the gas ionizes at which time there is massive current and a huge drop in voltage. So there is a period of time, albeit short, that this capacitor has large voltage across it.
All theory so far, as there is no real empirical proof this is the circuit mechanism.
As an interesting side my research showed that professional laser cutter systems all use distilled water or coolants that have careful control of their conductivity.
All theory’s are still in play… I will have to noodle your ideas more.