>I've considered describing a plasma weapon that would create a containment field all the way to the target
This is where the "scoffing by people with a knowledge of physics" part comes into play. All too often the term field is used to denote something that simply acts like a invisible container or wall, but nothing like the macroscopically observable everday (force) fields. (And, yes, I am aware that those "walls and containers" satisfy the basic definition of "field")
So if you say "containment field":
- What property of matter does it affect? (charge, mass, ..)
- If you can project it from your weapon to the target, why can't you use it to interact with the target directly?
- How is the field influenced by matter? If it can't pentrate denser matter, raindrops and foliage will pose a problem. (On a sidenote, David Drake occasionally mentioned this as a drawback of his powerguns)
>Or one that fires a small projectile that's basically a containment field generator
If the projectile can carry enough energy to generate your field why not use this energy as an substitute explosive? If one can manufacture a generator that small, your tech level might be advanced enough for bullets containing metallic hydrogen or a nuclear isomer (both of which can be used as powerful explosives).
>[Power and cooling] Such things are fairly easy to explain by making up exotic materials.
They are easy to handwave using technobabble, like everything else.
But high performance materials only go so far (insulation). In the end you have to dump all your used energy.
So with high powered direct energy weapons your soldiers will at least be a shining beacon in the IR spectrum.
>[...] make it dangerous to use anywhere near innocent civilians.
In really don't see a way around that.
Projectiles will go through their target (that's an issue even today and the only resort are bullets that can be stopped by armoured vest), lasers will blind bystanders, plasma has IR, UV, and gamma radiation as sideeffects (depending on power this might lead to vision loss and/or burns) and stray rounds are prone to start fires.
I'd be interested what kind of armour you have in mind. As long as it's not some fully enclosing powersuit, smart ammunition might an alternative.