After doing a lot of reading on space warfare on Atomic Rockets, ToughSF and other websites, I was almost sold on laser dominance. Sure, a continuous wave laser could be countered by cooling the hull and hinders itself with the plasma it creates from penetrating deeply, but pulsed lasers circumvent these issues neatly.
However, then I came across Children Of A Dead Earth, which is a physics simulation of space combat. Kerbol Space Programm, except that Jebediah Kerbin doesn't try to get himself killed anymore, instead he kills others with high yield rail-guns. In CoadE lasers are only viable at really short distances because apparently the beam quality factor, M2 is horrible for high powered lasers.
M2 is the beam quality factor, which can be considered a multiplier of the beam waist. So, an M2 of 5 means the beam waist is 5 times that of a diffraction-limited beam. In terms of area, this means the beam is 25 (52) times the area of a diffraction-limited beam, or 25 times as weak. As you can see, having an M2 even in the high single digits will yield beams a far cry from “perfect” diffraction-limited beams. In practice, it is not the pumping efficiency, nor the power supply, nor diffraction, which ultimately limits lasers. It is the beam quality factor. In the end, M2 ends up being the number one limit on laser damage in combat. In small lasers, M2 close to 1 is easily achieved without issue, but in high power lasers, M2 can easily reach into the millions if not accounted for. This is because generally, M2 scales linearly with laser power. Each optical component of a laser affects the M2. In particular, using a deformable mirror to focus a laser at arbitrarily long ranges (such as from 1 km to 100 km) is measured at reducing M2 to between 1.5 to 3. Problematic, but not exactly debilitating. But the main issue is Thermal Lensing (Note that this is different from Thermal Blooming, which only occurs outside the laser in the presence of an atmosphere). The heating of a laser gain medium generates a thermal lens that defocuses the beam, ultimately widening the beam waist, preventing the beam from focusing properly. Also note that thermal lensing actually occurs in every single optical component of the laser, though it is strongest in the lasing medium. Thermal lensing increases M2 roughly linearly with input power. This means if you have 1 kW laser with an M2 of 1.5 (which is reasonable), this means dumping 1 MW into that same laser will yield an M2 of about 1500.
Other sources only discuss the diffrection limit, meaning a perfect laser and then they call it a day. M2 actually seems to mean that a more powerful laser will perform worse than a less powerful one.
M2 can apparently be fought by cooling the laser (which means even more radiators) or making it physically bigger (more mass). None of these solutions are great.
Does this really ruin the long-range death-ray, continuous wave or pulsed beam?