The Thaser was born from the deficiencies of laser weaponry, and the inherent flaws they had. Lasers had no knock down potential, had no discriminatory capacity, and in close quarters it was prone to overpenetration. On top of this, conventional laser weaponry was expensive, required a large amount of maintenance, and was easily damaged in the field. Combat lasers that were reliable and effective required large anti-shock housings to protect the optics, and were also power hungry.
Not a great combination for a general issue weapon.
The Thaser was introduced. It was a THermal Amplification weapon, and functioned very similar to a laser, but had a much larger aperture. The beam a thaser generated was much more diffuse and less focused than a laser, and as such was much less adept at cutting things, or overpenetrating targets. It was much more useful for inflicting 3rd degree burns, setting things on fire, melting things, and detonating ordnance.
Aside from a more diffuse beam, a thaser uses a gas charge. This gas, pumped into the targeting chamber, acts like a diet form of plasma. When fired, the energy beam charges the gas and it is discharged with the beam, giving the thaser a minor punch. The real bonus is that this superheated gas transfers energy well and allows for relatively short beam bursts to kill, while using less power.
Thasers deal a moderate amount of kinetic damage and a large amount of heat damage. Freestanding objects can be very rapidly heated by a thaser, effectively melting holes in them, or in the case of things containing large amounts of water, flashed to steam. Being shot with a thaser is both incredibly painful and generally lethal as the bodily fluids are flashed to steam, the flesh is effectively cooked, and survivors generally have to undergo amputation or organ replacement.
The thaser, despite being fairly common, is considered a cruel weapon.
The Thaser was quickly adopted. It was considered an excellent weapon by the higher ups, and it was quickly developed into a large number of models.
Thaser Pistols - popular weapons, more commonly known as Blasters. A blaster has limited range, and a limited power pack, but are still popular weapons in space. The blaster is unlikely to blow holes in hulls, which tend to be thermoablative, and are good at killed foes even in armor.
Thaser Rifles - blaster rifles are likewise popular in space. It has better range and more power than the pistol version.
Thaser Carbine - halfway between a pistol and rifle, the carbine is a compromise between both. The typical carbine has a higher rate of fire, making them a tad more intimifating.
Thasers have been widely adopted, especially by offworld units. The weapons are readily available on the Black Market, and the biggest restriction on them isnt the weapons themselves, but finding the compressed gas cylinders the weapons use. A large number of terrestrial governments make use of thasers, but generally for special forces operations.
The only weapons more common than thasers are magnetic firearms and conventional firearms. They are the most common energy weapon available.
Being a heat beam, thermoablative armors offer excellent protection against thasers, and most SLAB armor suits will stop a thaser. Also, being a heat beam rather than a true laser any sort of military armor, which is almost by default going to have a thermoablative layer, will stop a thaser. The weapon is also remarkably ineffective against very large objects, such as tanks and mecha.