K40 Laser Beam Focus Issues

[mcdanlj (Michael K Johnson)]

25⁰ is probably too hot, even if you got a high-quality tube. Once the tube shifts mode, it doesn’t shift back; the damage has been done.

Cooling the K40 laser tube

@keen

How much cooling is enough?

Enough cooling to keep your cooling water bucket under 20C/68F and enough pumping to keep about one liter of water per minute flowing through your laser tube.

The laser tube makers recommend water at 18-20C (64-68F) and about 1 liter per minute. Actually, the laser tube designers for the K40 laser tube didn’t say a word about it, because no one knows who exactly they are. The 18-20C temp is a common recommendation from other laser tube makers, and the 1 L/m is based on commercially available 80-100W tubes. But they’re really good guesses.

@keen

Why is there a minimum temperature for the cooling water? Why can’t I run cooling water at 15C? 10C?

Doing this flirts with causing thermal expansion/contraction cracks in the glass.

The innermost glass tube contains the lasing material at a power density twice (or more) the density of the actual external beam and also contains the 10,000V electrical arc that excites the gas to lase. It’s really hot inside that tube. The next tube out contains the reserve gas, where the tired (Lase-y? ) gas from the inner tube goes to rest. The outermost tube contains the cooling water. It’s job is to keep the middle reserve gas tube cool enough for the CO and O2 wastes from the lasing gases to recombine into CO2 to live to lase again. With very hot gasses and glass on the inside, and cold water on the outside, all meeting at the ends which make the tube supports, the temperature differences get critical.

There’s a lot of good information in the K40 Laser: Getting Started category, worth reading even if you’ve already gotten started.