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As an example of a plasma beam: in the Star Wars franchise, lightsabers are used as the multipurpose weapons of Jedi. The Jedi lightsabers have beams which seem to have be around 1 to 2 inches in diameter. They are used to deflect lasers, deflect other lightsabers, and to cut through a large variety of materials. In the franchise, the Jedi are also capable of adjusting the length of the saber, as well as the shape of the tip, but the width adjustment is not mentioned.

Some cases I can think of for variable adjustment of are adjusting the beam width to become closer to a shield in width to protect against laser barrages, adjusting it to be closer to a rapier for precise cutting operations, and perhaps even adjusting the width to allow the weapon to target gaps in the enemy’s armour.

Assume that operation time of the bean is not indefinite. A beam on a personal weapon can only be maintained for eight hours per energy cell. Increasing the surface area of the beam will increase the required energy output due to dissipation exponentially. Eg, if the surface area is doubled, the operation time will be 1/4th.

If in a world, a lot of warriors do employ the beams in weapons for combat, would a variable width plasma beam have an advantage over an invariable one? Also, what width is suitable for incapacitating other lifeforms?

If anyone is interested, here is how such a weapon would employ the beam, using plasma contained with magnetic fields, as stated by Halfthawed below.

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    $\begingroup$ Lightsabers are more or less on the far side of 'hard vs soft' when it comes to science fiction, so the better question is just 'how effective will it be', and just handwave how the build works. For instance, Kylo Ren's saber-hilt monstrosity thing goes against the previously established rules for building sabers, and nobody cared. $\endgroup$
    – Halfthawed
    Jan 6 '20 at 0:55
  • $\begingroup$ @Halfthawed good point. Will revise, it is rather difficult to understand the specific mechanics of a laser sabre $\endgroup$
    – Enthu5ed
    Jan 6 '20 at 0:56
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    $\begingroup$ Lightsabers are actually plasma swords bound in magnetic fields these days, which they swapped to when they realized how impractical a laser sword would be. Plasma swords aren't much better, but it's an improvement at least. $\endgroup$
    – Halfthawed
    Jan 6 '20 at 0:58
  • $\begingroup$ @Halfthawed I did a search based on that, and there were some nice articles on how to replicate it, thanks for the info :) $\endgroup$
    – Enthu5ed
    Jan 6 '20 at 1:16
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    $\begingroup$ @BKlassen as in the Discworld, the photons of starwars travel at the speed of plot. $\endgroup$ Jan 6 '20 at 17:05
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I'm going to base this answer on how lightsabers appear to work in Star Wars - not on the actual physics of plasma constrained within magnetic fields. These two different lightsaber concepts produce drastically different results - and the "real" version linked in the question is not an effective weapon.

Width doesn't matter for melee

The Star Wars wiki says that aside from a few special substances, lightsabers can cut through anything. If lightsabers were relatively weak, and you had to target gaps in armor, greater width would allow you to do more damage. But we'll only need to do slashing, so we don't need to poke wide holes at all. We could hypothetically enter combat with a saber of infinitely small radius and it would be no different from using a 1-inch saber because we're cutting, not stabbing.

Width does matter when cutting through metal

We know that lightsabers take longer to cut metal than flesh, which is important when using them to cut through thick armor, tanks, or buildings. A thick blade would leave a larger hole, separating dense material more effectively. If you cut through a brick wall with an infinitely thin blade, the "hole" you cut would be difficult to displace. If the shape you cut had a one-inch margin of melted / evaporated material, entry would be easier. A thicker blade might also get hotter - or at least distribute more heat, because a wider cylinder would have a larger surface area. Therefore, thickness might mean faster melting.

Width does matter for ranged combat

A thin blade would be useless at deflecting lasers; once again, surface area is the key factor here. A wider blade has more surface, so it works as a better shield to intercept a thin ray of light. Using a lightsaber to deflect lasers doesn't make a lot of sense in the first place, assuming that "lasers" move at the speed of light and thus can't be reacted to. You would need a serious Jedi-reaction speed to notice a laser - or even a bullet - in time to deflect it like a baseball.

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    $\begingroup$ As it was shown in VII episode when Kyle time-stops blaster shot - blasters are not lasers. And they are moving at much lower than bullet speed (about 20-50 m/s). MG 42 would be a devastating uber-anti-infantry deathbringer in a galaxy far, far away! $\endgroup$
    – ksbes
    Jan 9 '20 at 8:01
  • $\begingroup$ Thanks for pointing out some of my incorrect assumptions about blaster fire; I'm still getting into the sci-fi universe. Good answer overall, pointing out the various scenarios of useful beam-width. Thanks! $\endgroup$
    – Enthu5ed
    Jan 9 '20 at 14:22

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