I need some help designing a type of plasma-based weapon that cannot burn through armor but can damage soft-targets(i.e. people). I want it to be based somewhat in science, but all I can think of is a "cold plasma", which (in reality) is non lethal and medical in nature. Please help (creative block is a b****).
I think the problem with this question here is that we're being to nitpicky about our definition of plasma. Yes, plasma is unstable and dangerous as hell (the comments on this question appropriately describe how bad of an idea it is to get hit by it), but in pop culture we don't describe it as such - it's typically something cool and glowy. So I'm going to work with that definition and mix science into it.
- Lasers: Lasers are cool in that scientifically speaking they're just high-energy light. If it's of high-enough energy, it could give you a sunburn, or ionize the blood in your body, or destroy a solar system (gamma ray burst). You can have soldiers fire such weapons at people. No need for extremely high energy, but high enough to cause burns and such on bare skin, set fire to flammable materials like hay. Your defense? Anything that has enough of a heat capacity to absorb the light being beamed at you, or just use some nice mirrors. Reflect the light right back at your energy (given that you angle yourself right). You can have your armor be completely reflective or just have a reflective shield. This is bound to lead to some cool outfit ideas for your armies.
- Electrocution Plasma: Plasma is basically an ionized gas. And ionized gas is a terrific conductor. So instead of having true, hot plasma, you can invent your own version of a cool plasma weapon that technically fires a long focused burst of gas. But that's not all, because 'relatively' ionized gas itself can't do much except in its purest/best form. Let's say it takes 1 second to fire this weapon (because you're firing a stream of gas, not a single bullet). We can say that 99% of the second after you pull the trigger is just your weapon passing gas. For the 1% remaining, we have your weapon fire a significant enough charge of current into this stream of gas. If done right, this could look like lightning being fired from your weapon, made ever-spookier by your choice of colored gas. For your reference, 100 mA of current can kill a person, and a person's skin has a resistance of approximately 5kOhms. Assuming no current-loss to the air around you (impossible to assume in a combat situation but still) your weapon needs to generate a potential difference of 500 V to kill a person. Buy a whole lot of batteries. The way to prevent this from killing you is to literally wear clothes - the thicker and more resistive the better.
- Missiles: Have RPGs and grenade launchers firing projectiles filled with small amounts of white-hot plasma. This will be destructive and has no defense against it, but may just blow up upon exiting the chamber of the weapon. There's no defense against it, but maybe the enemy might accidentally kill themselves off this way lol.
Hope this helps! Great question!
Usually a plasma weapon is envisioned as a way to burn stuff, because plasma is usually very hot. It is sort of a sucky way to burn stuff because plasma is also nondense and so although the component ions are hot, there are a lot less of them and so less total energy. Some people think about using plasma to transfer electrical charge which seems to me an unnecessarily fussy way to do that.
Consider though the chemical reactivity of plasma ions. Plasma is made of ions, produced because of thermal or electrical forces adequate to tear molecules apart. Ions do not like to be ions. Ions want to be part of molecules and will join with themselves or anything else in their desperation to not be lonely ions.
This vision of a plasma weapon uses the reactivity of the component ions to cause damage. Chlorine plasmas, for example, are real and are used industrially as etching agents. A weapon which fired a chlorine plasma vortex would be a phenomenal antipersonnel weapon. The putative advantage of plasma over just chlorine is you need less of it to do damage, and so when the cloud disperses there will be less free chlorine to blow back.
Other gases could be used but chlorine gas is a bonafide chemical weapon in use today. It burns exposed tissues and causes characteristic damage which can be recognized after the fact. Armor (don't forget respiratory armor!) is protective. Chlorine will keep its terrifying historical gravitas when it appears in a science fiction weapon.
Additional thought on armor: chlorine plasma etches, and it would etch the armor. These etchings would be visible and might be colorful chlorine compounds depending on the composition of the armor. The soldiers are issued a spray that they are supposed to apply to the armor to serve as a sacrificial barrier to chlorine. Many of them prefer to use surfboard wax.
There are laser diodes which, with sufficient energy source, can ionize the air enough such that it will send a pulse of plasma through (or at least deep within) steak. Videos on YouTube generally show how to make such "weapons." Such things are banned in war thanks to the Geneva Convention, but anything harder than the meat will absorb the blast.
In the real world, such things are impractical weapons (non-lethal, non-incapacitating injuries) but it's not hard to imagine them being scaled up.
Existing (experimental) plasma weapons like PEP and PIKL can damage people but will only burn very very gradually through armour. They won't do much damage even through a shirt and take several shots to burn through a paper target. But the flash-bang effect may be enough to stun or do greater damage to unarmoured individuals.
I have written quite a lot about these over many years -- https://www.popularmechanics.com/military/research/a25091957/plasma-weapon-history/