Is there such a thing as a 'Constant Power Source'?
It's pretty simple to do with circuitry. The below circuit provides constant power at  the output provided OA1 does not saturate, which limits it to something like 15A.
In reality the VCCS would also have limited voltage output compliance, which would show up in an inductor circuit.



simulate this circuit &ndash; Schematic created using CircuitLab
Here is what it looks like charging a 1F capacitor with 1W:

P.S. I used an op-amp with a multiplier because CircuitLab does not have a divider function.