Buffo, Hu[man} did this. Hu was having a bad day with various social frustrations. Hu, tired of the hourly alignment touchups of the motion stage, somehow aimed M2 back into the beam path towards the laser head. Normally M2 aligned the beam along path "A" into the cutting head to hit fold mirror M3, which deflected the beam into the lens, which could also move in three axis. Hu had to contend with 15 optical adjustment screws, not counting the CNC motion, laser power and assist air velocity.
Remember when I said, almost resonant? Hu's action resulted in the spot "stacking" itself into a row of beams on Laser two's OC mirror and at M2. The beam waist from the laser head focused right before M2, so it had a really small spot size.
Hu set the power to 100% on both tubes and started a long cutting run.
Hu was at his part time job, Hu is not a grad student.
You know when your aligning a cavity with an alignment HENE, and your close to having the mirrors aligned, but one axis is still a little off and you see a close together pattern of Dots in a Row? That is what Hu ended up doing, with the ~ 3 mm diameter beams almost but not quite totally overlapping.
Eventually the waveplate W1 could not take the flux and turned into a weak lens. Laser tube One's OC is also damaged. So at first it melted the rounded "burn" spot, and then the beam from one tube diverged from the optics damage and heated the whole mirror and mirror mount.
So 180 Watts minus the per pass adsorption bounced "N" times and built up to a huge amount of power, by reflecting from M2 to the OCs and back. There is a Quantum Mechanical Quirk where in some cases the OC is not all that reflective when it is intracavity and exactly aligned to form an Etalon. So he had a long, long, cavity. But OC two was just perfectly aligned enough to not have damage.
This was CW Co2, no Q-Switching. Really freaky situation, and I doubt I could ever reproduce it in the lab. I've had a few people with decades of experience look at that picture. They are all scratching their heads wondering just how that happened.
The three mirror linear cavity is well known to having energy enhancing effects for things like intracavity frequency doubling, when well designed, and is in fact patented long ago. It is just really difficult to "accidently" make one.
Back of the envelope calculation is 1121'F / 650'C if its aluminum, and 785'F / 385'C if its Zinc Alloy for die casting. I'm not sure it 100% melted, it may have gotten just hot enough for the strain from the springs to warp it. But the "burn spot" is a melt.
Jury is still out if Hu had a really freaky day and forgot to tighten the M2 mounting post, or if Hu wanted to see what would happen if the beam was folded back on itself. The laser head and M1, M2, M3, move on a gantry with frightening acceleration levels for such an assembly. I'd like to give Hu the benefit of the doubt, but I'm not going to bet either way.
The system is back up and running on tube two only in an emergency manufacturing situation. That's why I got the call. Tube two was undamaged and running nearly TEM00.
Tube one still lases at a surprising amount of power, but the beam is divergent crap and is not aligned with Tube two. The laser head is buried deep in the machine, and the judgment call is to run on the remaining tube. Only way to get a new tube is a long wait for one to be flown in from overseas.
Sorry for using the "woke" New Age sexless Pronoun "Hu", but Hu is reasonably young, and we all make mistakes.
See Attached.
Steve