As far as fiction is concerned, plasma weapons look better than modern-day metal weapons. However, I'm curious about their functionality in reality as depicted in fiction.

What mechanism will allow a plasma blade to melt a bullet before it can pass through the blade itself?

The mechanism I'm looking for should include answers to these.

1. What elemental/compound gas ionized into plasma will do the trick?

2. At what temperature should the plasma be heated?

3. How can the sword use magnets to maintain the plasma blade?

4. How strong a magnet should be to maintain the blade shape of the plasma?

Clarifications:

• Plasma, not lasers. Although if the design includes lasers, feel free to use them. Just keep in mind the blade is plasma.
• plasma is the 4th state matter behind solid, liquid, and gas. Laser is Light Amplified by Stimulated Emissions of Radiation. They atre two different things. Can you please clarify things? – Michael Kutz Sep 30 '18 at 18:07
• "We all know lasers look better than modern-day metal weapons." Do we? – Renan Sep 30 '18 at 18:22
• Plasma , being charged particles, can be contained in a "shape" by a magnetic field. There are a number of articles on the physics and probable effectiveness of light sabers, including a number on SE in the last 9 months. – pojo-guy Sep 30 '18 at 19:09
• There is a Because Science episode on this. The answer is that even though you can melt the bullet, it is going so fast that if you are even quick enough to move the balde into the path of the bullet, you will stillend up with liquid lead flying into your face. – John Locke Sep 30 '18 at 19:46
• The biggest problem with these weapons is that if you have a sword hot enough to vapourise a bullet before it makes contact with you and you hit a big skin bag of dirty water (read humanoid creature) the water in them goes from liquid to gas, and right on to dissociated ionised plasma, instantly for a reasonable percentage of their body and you died a horrible agonising death combining full thickness burns with extreme, acute radiation poisoning. – Ash Oct 1 '18 at 11:58

The Math

So here is some math. We are going to assume the bullet is a 9mm projectile made from pure lead. (Yes, bullets aren't made from pure lead much anymore, but for ease of energy calculations I will posit that the bullets are pure lead) Since bullets travel very fast we are going to assume that your plasma sword is transferring enough energy to the projectiles to vaporize them instantly.

The latent vaporization point of lead is 177KJ/mol, 1 gram of lead is equal to .0048 mol of lead. A typical 9mm bullet weighs in at 8 grams. this gives us .038 mols that we must convert from a solid to a gas. This will require 6.79kj of energy. This means that the bullet will not so much vaporize as explode in your face with the equivalent of about 2 grams of TNT. Not exactly catastrophic, but you might lose your eyes. Maybe ought to be wearing some protective equipment and definitely would want to avoid breathing any of the vapor.

But lets say you are vaporizing a bigger bullet than a fairly small 9mm projectile. Lets say its a .45 ACP which weighs in at 15 grams. That will be requiring 12.7 kj of energy, which means the bullet will explode with the equivalent of 5 grams of TNT, about enough to take your fingers off. As the bullets get larger you have a significantly more dangerous explosion happening basically right in front of your face or right next to your hands. Not exactly conducive to user safety. We are already pushing into the realm of loss of life limb and eyesight and we haven't even gotten into the much much heavier rifle projectiles yet. Also, these are pure lead bullets, which have a significantly lower latent vaporization point than metals like copper, tin, or steel which are all commonly used in the construction of modern bullets. A copper jacketed steel core and lead/zinc alloy 7.62x39mm bullet that is standard military issued load-out for the AK-47 is going to detonate with the yield of approximately 35 to 40 grams of TNT. Smaller Antipersonnel landmines designed specifically to mangle a persons legs contain about that much explosive in them. So when you start trying to block heavier rifle calibers constructed from much sterner stuff than our purely though experimental pure lead bullets you are going to lose an arm.

I don't really know anything about plasma physics, but I can say that even without such knowledge I would not rate this as a viable defensive method. To see what your average plasma sword user's hands and face would look like after defecting a few low caliber bullets just look up the fun accidents people have had with M-80's.

• So I was going to ask how can plasma shields work, then I found this... Nevermind. – Mr.J Sep 30 '18 at 23:34
• Well.... If you have a spare nuclear reactor or two laying around to power it I guess it could.... – TCAT117 Sep 30 '18 at 23:57
• Even if I have reactors small and light enough to be shields... The damage of incoming projectile is quite interesting. – Mr.J Oct 1 '18 at 1:56
• Well, as a personal shield it kinda makes bullets more lethal, which is not great. But as an area denial device it would probably work pretty well. Plus it would be wicked cool to see birds and stuff run into it. Or enemy henchmen. Or your henchmen when they annoyed you. – TCAT117 Oct 1 '18 at 2:02
• then maybe a GATE is more plausible for plasma? But AFAIK, plasma is used for cutting and with current technology, its used besides water when cutting things. – Mr.J Oct 1 '18 at 2:07

The real issue is heat transference. You can wave your hand through an open flame, walk across coals or even quickly dip you fingers into a pot of liquid nitrogen or boiling lead without sustaining any damage, despite the extremes of heat or cold being described.

Red hot glowing coals are 800C, the proper temperature for cooking meat-guess what your foot is made out of? Liquid nitrogen is -196C, and often used to freeze flowers solid so they shatter like glass in physics demos.

The reason you don't suffer agonizing burns or frostbite is the time you are in contact with the coals/lead/nitrogen is far too short for the heat energy to be transferred from the material to your body (or the other way around in the case of liquid nitrogen).

Similarly, bullets are moving at a high rate of speed, with the typical 5.56mm rifle round moving at @ 900 m/s, while 9mm bullets have a typical range of velocities from 360 m/s to 400 m/s. Obviously, handloaded and wildcat rounds can be made with velocities outside these parameters, but this gives you an idea.

Assuming you intend to melt the bullet by passing it through a plasma "blade" about the same thickness as a knife or sword blade, the bullet would pass through a plasma of anywhere between a few mm to maybe 6mm for a Katana to perhaps 8mm for an Arming Sword. The bullet will pass though the plasma in a very small fraction of a second, and there will be little to no heat transference to the projectile (and precious few microseconds for the surprised look on your face to form as the bullet passes through and strikes you).

The only work around would not be to count on heat transfer at all, but think of the monster magnetic field which is keeping the plasma together in spite of it's natural tendency to fly apart due to the electrical charges of the freed electrons and ions repelling each other, and the high velocity of the particles (shorthanded as "temperature" in the plasma state). The magnetic field "should" draw the bullet if it has a steel penetrator, and possibly deflect its path. You might also get an eddy current in the copper jacket as the bullet flies past or through the magnetic field, which may also work to deflect the bullet.

Don't forget the bullet has considerable momentum, and could continue on its path deflected by only a few degrees due to the very short interaction time between the bullet entering and leaving the magnetic field (once again, we are talking between a few millimetres and a few tens of millimetres at most). You will have a satisfied look on your face for a fraction of a second as the bullet enters the magnetic field, then that shocked expression as the bullet impacts you shoulder, rather than the centre of your chest cavity.

Overall, I would suggest that plasma based weapons are not going to be very practical in the real world. If you really want to keep from being shot, a 50 KW laser aimed at the enemy shooter might work out very well for you....

• Mythbusters had a go at this - discovery.com/tv-shows/mythbusters/mythbusters-database/… - and the results were underwhelming even with a row of ridiculously powerful magnets. Agree that using the plasma weapon to neutralise the shooter is a much better idea than trying to deflect the bullet. – KerrAvon2055 Oct 1 '18 at 13:51

I got your laser. I got your plasma too. That's because I got you a

LASER-DRIVEN PLASMA BASED ELECTRON ACCELERATOR!

https://journals.aps.org/rmp/abstract/10.1103/RevModPhys.81.1229

Laser-driven plasma-based accelerators, which are capable of supporting fields in excess of 100 GV∕m, are reviewed. This includes the laser wakefield accelerator, the plasma beat wave accelerator, the self-modulated laser wakefield accelerator, plasma waves driven by multiple laser pulses, and highly nonlinear regimes.

That's how it works. Moving on - the bullet? You do not vaporize the bullet. You cause it to miss, which is what you really want from an incoming bullet. You deflect the bullet by swinging your field at it and through it. Your weapon consists of a very strong magnetic field confined within the plasma blade. A conductive object (a bullet) moving through a magnetic field will induce an electrical current within itself which produces a magnetic field that opposes the original external magnetic field.

By swinging your plasma magnetic field through the bullet, you push the bullet off course with magnetism. It takes much less energy to divert a bullet than to destroy it. It is basically laser driven plasma aikido via magnets, which is the dream we all dream of.

I know you want to cut through battleship hulls with this plasma blade. Maybe that could work via ohmic heating and currents induced within the hull, just as the current is induced within the bullet. It would probably take more than one cut. I think probably it would not work at all with the dead tauntaun so don't throw out your Gerber Minimagnum.

• Generally speaking, the magnetic field has to be external to the plasma to contain it. Also, Wakefield accelerators are generally used to drive electron beams to relativistic speeds in a small amount of space and time, meaning particle beam accelerators could be built on a tabletop, in theory. Using this as the basis of an electron beam weapon to fire at enemy positions and prevent them from shooting at you in the first place seems to be much more sensible. – Thucydides Oct 1 '18 at 14:23