Laser power suddenly reduced

Hey everyone

I bought myself a 50W CO2 laser cutter from china about two years ago (pic 1). I used it about one to two hours every week.
I had several problems during this time which I managed to solve:
- Broken pilot laser cable in the drag chain caused shortage at a certain x position. This resulted in a "restart" of main power supply and hence interruption of laser cut.
- Water cooling ring at laser beam output side fell off. This resulted in power drop of laser beam.

Some weeks ago I suddenly noticed a permanent drop of the laser power. The laser still managed to cut paper but nothing more. I checked the water cooling ring again but this was still in its position. Water tempearture was normal around 18°C. I could not see any damage on the laser tube. When I held a wood board into the laser beam directly at the output side of the tube (before the first mirror) I got the pattern you see in pic 2. When I did this to adjust the mirrors when the laser was new and intact the result was (I think to remember) a whole round circle of burnt wood. The electric arc inside the laser tube was visible between anode to cathode which looked good to me. Next I checked the power supply and noticed that a small part of the ceramic insulation of the high voltage transformator had broken off. To me the case seemed clear and I ordered a new power supply. I got it today and tried it out.

Unfortunately whith the exact same result. The power is still reduced the pattern on the wood still the same. The laser still manages to cut paper but nothing more. Next I checked the electrical connections of the laser tube. The cathode is connected by a alligator clip. The anode was hidden by a rubber tube and rubber mass all over and inside it. I removed the rubber. The high voltage cable was just spun around the anode several times! Again I thought to have found the cause of my problem and fixed the connection. Unfortunately nothing changed. I then dismounted the laser tube.

This was when I discovered a spot (or hole?) in the mirror at the anode side of the laser. I'm not sure whether this is normal or not. I frankly am not even sure whether I really see something on the mirror or whether it is just a reflexion of some sort. I made tree pics (3..5) which was not easy. I hope you can see what I mean.

My questions:
- Could this be a damage on the back mirror of the laser tube? Could this be what you call 'normal wear' of such a laser tube?
- Is there any way of checking the mirror is damaged and this actually is the problem?
- Would there be any way to replace this mirror or is it adviseable to order a new tube?

Thank you very much, your help is much appreciated.
Marcel