True, that there energy requirement is exceptionally high for the purposes of armour.

However allow me to hypothesize for a moment. Let us imagine a high powered laser used for point defence on a nuclear warship. Due to the nature of this laser being the last line of defence against incoming missiles its power out put is rather high. Due to this laser's power output it is physically rather large and the military has decided not to place any additional weight upon it due to the distorting effects this would have on the highly sensitive optics. This creates an interesting problem that the laser is now  unarmoured and even small hail storm, could damage the weapon. A light plasma field is placed round portions of the laser allowing it to fire out and offering modest protection against small shrapnel, hail, salt water, etc? This example may prove unfeasible/unneeded but I?m merely pontificating.

Presumably the incoming projectiles charge distribution will be 0 with a rigid structure of electrons, in which case no EM force will be able to interact with it, for instance: A lump of granite.

Since this is a sci-fi forum if my memory serves me right, the Culture, in I M Banks novels, warships main armaments are EM fields which they can manipulate locally to the point of being able to separate individual electrons from atoms in combat.

The maximum speed that a plasma can expand at is limited by the average velocity of the particles in it, which is solely a function of temperature.

 

The Joule-Thompson effects dictates that any expansion driven by pressure will cause the plasma to cool as it expands.

Quite so. Thankfully the plasma in this case if confined to a specific volume by an exceptionally strong EM force, so the talk of dissipation is largely a moot point.

However allow me to hypothesize for a moment. Let us imagine a high powered laser used for point defense on a nuclear warship. Due to the nature of this laser being the last line of defense against incoming missiles its power out put is rather high. Due to this laser's power output it is physically rather large and the military has decided not to place any additional weight upon it due to the distorting effects this would have on the highly sensitive optics. This creates an interesting problem that the laser is now  unarmored and even small hail storm, could damage the weapon. A light plasma field is placed round portions of the laser allowing it to fire out and offering modest protection against small shrapnel, hail, salt water, etc? This example may prove unfeasible/unneeded but I?m merely pontificating.

Interesting, I had not even considered the effect of rain on plasma armor.  It should be spectacular, possibly on a par with grade B movie self destructs.

 

There is no reason that the exterior of a HEL cannot be adequately weather protected.  Armoring a device that has to shift it's aim point quickly and precisely is another matter.

 

Where the plasma window will likely have a significant impact is on the emissions window.  Not as armor, but as a non-material pressure seal.  This is a major problem/limitation currently.

It merely strikes me as an elegant solution to the compounding issue of increasing the weight of large optical devices causing distortions. As you say it is at the emissions point the plasma window?s unique properties will shine.