Alternative to a potentiometer to regulate 50 W
Don't try to scale down the 50 W signal coming out of the driver.  Instead, control the driver circuit to produce a lower amplitude signal.

This is the same concept as a volume control for audio.  There isn't some big fat pot between the power amp output and each speaker.  Instead, the volume control adjusts the amplitude of the low power signal going into the amplifier.

Show the power driver, and how it is controlled.  From that we can probably recommend ways to cause it to make less power.  Without those details, there is little to say.