Engraving is getting dimmer after a while


My CO2 laser engravings get dimmer after 15 min, no matter the position in bed. I’m desperate. If I engrave five pieces, 5 min each, at the same position, it gets dimmer from the 4th. If I turn off the machine and turn on after some minutes, it works again for 15 min.
Water temperature is constant at 18-20ºC. Checked and cleaned lens and mirrors, they are in very good shape. Object are aligned within ± 0,25 mm aprox or better with a 2.5’’ focal length lens.

Can anyone imagine what could be happening?
Thank you!

I’d lay long odds on the power supply fading away.

The only way to know is with a milliammeter displaying the tube current, which should be pretty close to the same for each piece. If it declines steadily, then the power supply is unable to maintain the proper tube current.

AFAICT, power supplies are consumables, so you may as well order one, install it, and see if the situation improves.

Another possibility is the laser tube fading away, but that seems unlikely. Given that you’d want to replace the power supply before assuming the tube has failed, getting one now makes as much sense as anything.

The tube temperature also affects the power output. If you’re running an active water chiller with a thermostatic control, then seeing a constant water temperature makes sense: the tube is being properly cooled. In contrast, I’m using a low-buck icemaker, so the temperature tends to rise during extended sessions; if it remained constant, that would be an indication both the water pump and the water flow sensor failed, overheating the tube. This is almost certainly not your problem, but worth ruling out Just In Case.