Now, that I have a proper plasma rifle, I only just have to kill/disable/make my enemy disable his/her/its magnetic shielding for enough time to properly roast it to a smoldering pile of ash!
How does the magnetic shield work?
The plasma rifle shoots a ball of charged plasma. When the charged particles enter the field, which is polar (magnetism works that way), the Lorentz force deflects them onto a spiral trajectory which makes them impact either to the North or South poles of the field. In this scenario the plasma ball impacts all the same, but not where you aimed. Presumably, the "poles" of the field would be strengthened and possibly protected with some kind of ablative layer that would be too expensive/impractical to use on the whole body.
A shield of this kind can be defeated by saturating it and aiming for the poles, so that as little energy as possible is lost anywhere else.
What if the "polar" areas are actually invulnerable, and the reason the whole body is not covered is that the duranium shield is too heavy/rigid/whatever to make a suit out of? In that case you're out of luck. A plasma rifle against a magnetic field plus unsatiable plasma sump is outmatched.
A workaround could be to use heavier plasma. The Lorentz deflection depends on the mass/charge ratio, and heavier elements are less deflected. Of course you need more energy, but you can save on that by using less plasma. Half as much plasma, twice as heavy, should be around sixteen times more effective in terms of transferred energy. After some mass/charge threshold is reached, it gets deflected so little that it actually impacts.
If the sump is not invulnerable, we can saturate it. The shield manufacturers knew this was a possibility. What countermeasures might have they put in?
One clever trick is to use a rotating magnetic field that couples with the plasma bolt and actually accelerates it. It's much more complicated to build. The shield "waits" in its static state until the plasma bolt is near enough to influence the field itself (that's how the field "knows" it has caught itself a bolt). When this happens, a secondary field is spun up and coupled with the first, so that the poles shift away from the plasma bolt. Now the bolt effectively "orbits" the shielded person. This shield is the unholy get of a cyclotron and an electric motor, and in theory (and in a vacuum) could form a Tokamak bottle around the shielded person where the plasma bolt would orbit until it radiated all its energy. Also, by collapsing the field after the bolt has orbited around once, some part of the bolt could be clumsily redirected towards the shooter, or anyway far from the shielded person.
This kind of Tokamak shield would probably not work unless we knew very well the incoming bolt's characteristics. To defeat it, we just need two plasma bolts made up of plasmas with different mass to charge ratio, either using a reconfigurable rifle, or more simply a two-barreled rifle, or two separate rifles. The field parameters that lock one bolt will not lock the other, and two magnetic shields would merge into an averaged shield that would not lock either bolt. The bolts would still be deflected somewhat, but not enough.
Another very simple tactic is having three people shooting synchronously at right angles against the victim (one from above, necessarily), so that there is no field configuration where all three bolts lie in the same plane and can be deflected together - a sort of "plasma lock" position.
Other kinds of deflections (e.g. timed deflection) can be defeated by either firing two or three closely spaced bolts, so that they sync with the magnetic field "wave" needed to bounce them; or shooting more energetic plasma. One possible design would have a first pilot bolt drive a ionization tunnel through the atmosphere, so that a heavier ion stream may follow. A ionizing laser (ultraviolet or more) could be used for the same purpose.
How does the plasma bolt work?
While spectacular, the plasma bolt can't be a weapon for that reason alone - you can't awe the enemy to death.
One "plasma bolt" is a small mass of accelerated ions, and it gives problems from the start because we need it to remain in one piece up to impact. The only "internal" force we have available to do this is speed: when electric charges travel in space, they form currents, and while currents of the same sign repel each other due to electrostatic repulsion, they also attract each other due to electrodynamic attraction. The former depends on charge and density, the second depends on charge, speed and density. So for any given charge unit and density, the the ratio between the stabilizing and destabilizing forces depends on the bolt speed.
Unfortunately, speed will decrease during travel, so the stabilizing force will decrease as well. The bolt will expand and lose both momentum and strength.
Now the effects of a plasma bolt on a target are mainly electric, secondarily kinetic, and finally thermal.
- electric: of course we have a lot of charge hitting the other guy. We can't store all that charge ourselves, what we are doing is separate positive and negative charges, and exhale the one while launching the other. The two charges are equal, since we, the attacker, are on average electrically neutral. The mere fact that a plasma bolt exists means that there is a way of harmlessly dissipating the charge. The bolt's damage stems from the fact that our discharge is controlled, while the discharge on impact is uncontrolled. Even so, it can't be a world-shattering difference (unless we're hitting someone much less technologically advanced, or way less protected). Most unprotected electronics will be fried by the EMP or by the stray currents, and the shock might daze or even knock out a human.
- kinetic: the plasma bolt is a very small mass moving at enormous speed. The effect, however, will be significantly less than that of the same mass delivered at the same speed in the form of a shaped tungsten penetrator (which also transforms into a kind of "plasma" on impact). The advantage here is that you can design armor to deflect a solid kinetic impact, or a shaped charge jet, or a plasma kinetic impact, but few armors can be proofed against all of them - see also patent US4463678A - so it makes sense to have a weapon capable of, say, disrupting reactive armor).
- thermal: contrary to popular belief, a plasma bolt is unlikely to be splitting the darkness like demonic lightning bolts, turning the river valley's towering coniferlike trees into roaring torches (David Weber, In Fury Born). The reason is the same why you can open your electric oven when it is 250 °C hot -around 480 °F - and wave your hand inside without receiving the least damage, while touching the metal inside wall of same oven will burn you badly: even if the air is at the same temperature of the wall, and its specific heat is twice that of iron, its low density means that you're actually "touching" very little air; one kilogram of air is one cube about one meter in side. If the plasma bolt was one gram in mass (and that's a lot!), at a temperature of one hundred thousand K, and the heat got somehow all transferred instead of mostly going away to thermal bloom and radiation, it would supply around two million Joule of energy. Tap water has a specific heat of four thousand Joule per Kelvin kilogram, so one kilogram (about one liter) of water would soak up the plasma bolt and get its temperature increased by 2,000,000/4,000 = 500 K, or 500 °C. But by doing that, it would hit the 100 °C boiling point, so the first 400 KJ or so would go into heating the water, the remaining 1.6MJ into making it change phase. And water has a latent heat of 2.2 MJ per kilogram, so 1.6MJ is not enough. Our integrally transferred plasma bolt will flash-boil about 1.6/2.2 = 0.7 liters of water, leaving the remaining 0.3 liters hovering at 100 °C.
Of course, if heat loss, reflection and reradiation aren't significant (one purpose of the magnetic shield could be that of diffusing the bolt to make it lose energy) and you can rain bolts on a target, even a backpack with ten kilograms of ice at -20 °C is only good for about twenty hits. The twenty-first will increase the temperature of the depleted armour.