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Ability to produce an energetic plasma

  1. Jan 19, 2017 #1
    I've run into an issue that the internet seems ill-equipped to answer, so I turn to the experts.

    While I'm aware that most fusion experiments to date have used deuterium and deuterium-tritium blends as their fuels, I find myself wondering: can any dielectric material produce energetic plasma under the right conditions? Ignoring the engineering headaches in pumping enough energy into the fuel, attaining necessary compression, and other limiting factors for science, is plasma a fair state of matter for any element and compound, or is there a critical attribute?
    Moreover, is there any benefit in shifting to a different source material for generating your plasma? If you could produce plasma from, say, sulfur hexafluoride, would it effectively be any different from the plasma produced by a D-T fusion test fire? Or would the only difference be more waste neutron radiation?
    I really want to get this right (I like my writing to be scientifically accurate as far as the state of the art will allow) and I'm hoping someone can shed some light on this. Many thanks in advance!
     
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  3. Jan 19, 2017 #2

    Drakkith

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    Any material can be turned into plasma simply by heating it to a high enough temperature.

    There's a difference between "producing plasma" and "producing plasma to be used in nuclear fusion". The latter has much tighter constraints since certain elements/isotopes release more energy than others and are easier to fuse. Also note that all molecules will be broken apart into their constituent elements well before you reach the temperature required for nuclear fusion.
     
  4. Jan 19, 2017 #3
    Good point; I should've clarified to start with. My fault.

    I'm not actually looking into fusion power. I'm trying to write accurate sci-fi (which need not be an oxymoron), and since plasma weapons are such a trope, I'm trying to get my fingers into the nuclear science that could form its foundation. Specifically, what could be used as a fuel to generate that plasma and, in turn, if there are any advantages in one fuel or another in producing a hotter/denser/"better" plasma than another fuel can.
     
  5. Jan 19, 2017 #4

    Drakkith

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    Unfortunately I have little knowledge of the physics that would govern plausible plasma weapons. Hopefully someone else here can assist you. Also, I've gone ahead and moved your thread from General Physics to the Sci-Fi Writing and Worldbuilding forum.
     
  6. Jan 19, 2017 #5
    Thanks, although I was really hoping to get information from someone with a nuclear/plasma physics background rather than fiction. It's why I avoided even mentioning it to begin with; I was hoping I'd be able to obtain pure info without tainting it with the "oh, it's sci-fi" contaminant. (I really detest the "anything goes" school of sci-fi writing.)

    In that vein, you said that my example of sulfur hexafluoride had "tighter constraints" than a D-D or D-T plasma. Which constraints are you referring to? Pressure/temperature points or something else?
     
  7. Jan 19, 2017 #6

    Drakkith

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    No worries. I don't think any regular members that I know of have backgrounds in fiction. :wink:

    The problem is that general properties are often hard to match up with specific needs. So what you learn about plasma in a general sense may or may not be useful for your story. It's far more likely you'll just end up writing things which are obviously false to anyone familiar with plasma physics.

    I was referring to plasma produced with the intent of being used for nuclear fusion, not the sulfur hexafluoride. But I don't have any idea what kind of constraints are needed for plasma weapons, so take what I said with a grain of salt. And yes, I meant pressure, temperature, composition, and other factors.
     
  8. Jan 19, 2017 #7
    Precisely what I'm hoping to avoid. :wink: Even though it means more work for me, I'd much rather produce something that an expert can read and think "okay, maybe" and not something that gives them immediate stomach cramps. In the process, I get to learn new things, so it's a win-win.

    (Incidentally, I love the energy quote!)
     
  9. Jan 20, 2017 #8

    mfb

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    While everything turns into a plasma if you heat it enough, with some elements it is easier than with others. To make a plasma, you have to free some electrons from the atoms, which means you have to ionize them. A low ionization energy helps. here is a list, most of the elements with a low ionization energy are in the first and second group of the periodic table.
    If you want to use plasma as a weapon, however, you probably want it to be very hot. So hot that the first ionization energy doesn't matter, and many atoms will lose more than one electron. A plasma will lose energy via radiation. Heavier elements tend to emit more radiation that can escape from the plasma, so lighter elements could be better for a weapon.
     
  10. Jan 20, 2017 #9

    ZapperZ

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    The problem that I see here is that the OP is confused between generating a plasma, versus generating a plasma for nuclear fusion.

    A plasma, simply by definition, is ANY ionized gas. This means that I only need to either add, or remove, at least ONE electron from each atom of the gas and voila! I have a plasma!

    The plasma to cause fusion is a more energy consuming effort because for fusion to take place more efficiently, you need to collide BARE nucleus, meaning you have to strip away ALL of the electrons. It is why we tend to use H and He atoms or isotopes. We won't have to remove that many electrons.

    So you can create a plasma with ANY vapor. Your fluorescent lamps have a plasma in it when it is ignited. But this is simply the removal of one electron for the gas atom inside the lamps (for ordinary fluorescent, it is Hg). You don't remove ALL of the electrons from the atoms.

    Creating a plasma is not that exotic.

    Zz.
     
  11. Jan 20, 2017 #10

    mfb

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    By that definition basically every macroscopic amount of gas would be a plasma. Usually it is called "gas" until the number of ions and free electrons is large enough to play a role.
    You don't have to do that. Electrons can even help a bit in shielding the charge of the nucleus for a while. This effect is much stronger with muons, which leads to the idea of muon-catalyzed fusion: The muon removes the need to have high-energetic collisions.

    The ionization is just a side-product: at the temperatures necessary for fusion, all fuel atoms tend to be fully ionized, but the ionization status doesn't matter much for fusion.

    Hydrogen (deuterium+tritium) is used for fusion because the nucleus has the lowest possible charge, and hydrogen fusion releases a lot of energy.
    He-3 as fusion fuel suffers from the higher electric charge of the nucleus already, which reduces reaction rates a lot. He-4 is not suitable as fuel.
     
  12. Jan 21, 2017 #11
    There are plenty of reasons for generating plasma in substantial quantities other than fusion. There are propulsion methods under study that could harness directed plasma as reaction mass, using it to create thrust, not a fusion reaction. At the moment, the state of the art with usable plasmas may circle fusion power, but there are other potential applications. But laying that aside, the key elements of the original post were the possibility of converting any material into plasma (which was answered as true) and whether generating a plasma from one feedstock or another would yield any effective difference in the plasma created (which appears to be a no, but if there are functional differences, believe me, I'd like to know).

    mfb, I'm curious: since a plasma approaching fusion-friendly conditions tends to be more or less fully ionized, would it be theoretically possible to contain the ionized components in Penning traps? One trap for electrons, the other for positively-charged nuclei? Or would separating the plasma in that state be effectively impossible?

    You definitely got my attention with this! Please explain if you would, because it makes the interest in He3 for fusion fuel seem a little...misguided? Is the per-unit energy released in an He3 reaction substantially greater than that released in a D-T reaction? Or is it strictly the appeal of not having prompt neutrons irradiating your reactor gear? Considering that our best "local" source of He3 would be lunar regolith, that's a lot of effort for a fuel that doesn't seem to want to be fuel, if I'm reading you correctly.

    mfb, I'm afraid I'm really going to annoy you, but I do need to ask an additional question. I was reading through the ionization energy table you linked to, and I have to say I'm, well, confused. A higher ionization energy means that it's more difficult to strip away the valence electron, right? If that is the case, then Hydrogen's ionization energy of 13.598 eV is much, much higher than, say, Cesium at 3.8939 eV. Are we targeting hydrogen in research because it's so plentiful, or is it a case of the listed value being for the valence electron, and stripping away each successive electron (of which cesium has plenty more) becomes substantially more and more difficult such that the net energy needed to achieve a fully ionized state ends up being far greater than hydrogen? (I hope that made more sense than I'm afraid it did.)
     
    Last edited: Jan 22, 2017
  13. Jan 22, 2017 #12

    Drakkith

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    I believe it's mostly the possibility of using He-3 for aneutronic fusion.
     
  14. Jan 22, 2017 #13
    Wow, no love for the neutrons. Come on Drakkith! High-speed neutrons are fun! Okay, not really. :wink:
     
  15. Jan 22, 2017 #14

    mfb

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    The properties of the plasma do depend on the atoms it is made out of.

    Macroscopic amounts of only positive ions or only electrons lead to strong electrostatic repulsion - no trap can contain that.

    He-3 is interesting for aneutronic fusion only. It is harder to get, it needs higher pressures and temperatures and leads to lower reaction rates. A helium plasma has higher energy losses. It is not even clear if the Lawson criterion, necessary for useful long-term fusion, can be achieved at all with it.


    Fully ionizing heavier elements like cesium needs much more energy than fully ionizing lighter elements (orders of magnitude more). Removing the first electron tends to get easier for heavier elements. Where would that comparison be important? Ignoring that cesium is extremely reactive (chemically): it is easier to make a cesium plasma. What are you going to do with it?
     
  16. Jan 22, 2017 #15
    Well, insofar as I can reason it out, when scifi authors talk about plasma weapons, they're talking about using energetic plasma to inflict damage, and the most reasonable means for that to occur is thermally rather than kinetically (as plasma dissipates without continual confinement). So unless I'm way off base (which is possible), the goal would be the hottest possible plasma.

    But, building off an idea that Drakkith planted in my reasoning, it would appear that, regardless of plasma temperature, hydrogen is the only reasonable fuel; all other elements have neutrons, and the waste neutrons being liberated from the plasma could be as much a risk to the operator as the plasma could be to the target. Would that be a fair statement?
     
  17. Jan 22, 2017 #16

    mfb

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    Which waste neutrons? There is a huge range (5-9 orders of magnitude) between "it is becoming a plasma" and "nuclear reactions become common". No nuclear reactions, no neutrons.
     
  18. Jan 22, 2017 #17
    Uhoh, then I may've run myself down the wrong road.

    If I'm creating a plasma, I'm ionizing the atoms of my fuel. While I understand that would separate my fuel into positive and negative components, does the nucleus not necessarily lose cohesion? The neutrons and protons remain as they were when the atom was "whole?" I was under the impression that as the fuel ionized into plasma, the nuclear force would weaken and the neutrons would be lost.
     
  19. Jan 22, 2017 #18

    Drakkith

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    Nope. I think a few tens of thousands of kelvin is enough to completely ionize most elements, but you need to get to millions of kelvin to get nuclear fusion. For comparison, the peak reaction rate for D-T fusion (by far the easiest nuclei to fuse) occurs at 800 million kelvin.

    Nope. I don't know what exact conditions are required to separate neutrons from protons, but I know it is WELL outside the range of conditions we're talking about.
     
  20. Jan 23, 2017 #19

    Ryan_m_b

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    Other than being cool (which is a perfectly fine answer in settings where stylistic consistency is more important than internal or scientific consistency; like Warhammer 40K) is there any advantage to a plasma weapon that outweighs the downsides and alternatives? Once it's shot out of the container the plasma is going to bloom, not sure how much but it could be so bad that it's a very short range weapon. On top of that if you have to pump so much energy in why not use that energy in a more direct manner, like propelling a projectile?
     
  21. Jan 23, 2017 #20
    Honestly, Ryan, I'm right there with you in your line of thinking. I've always thought the concept of plasma weapons to be a little, well, off. Unless you're going to really cook the books and claim there's some free-flying magnetic field that keeps the plasma bunched up until impact (and good luck justifying that with any kind of known science), it's just going to sort of...poof...right out the muzzle. Hard lines for anyone standing right in front of it, but pretty much useless at any kind of range. The only quasi-scientific approach to making a ranged plasma weapon work that I can see would be if you claimed that the plasma "bolt" were nucleated around some massive praticle, like if you could create gravity at will, and that kept the plasma confined through gravity...but even that is really, really shaky. (Plus, that method is going to tear up a lot of stuff all the way downrange just from gravity effects, which makes the plasma pretty much redundant.)

    Personally, I'm more of a classical DEW concept guy. Lasers, particle beams, masers, even x-rays or electrolasers if you want to get really fancy. But plasma...never made much sense to me. Unfortunately, in this case, it's not my choice; I didn't choose the tech. I was just tasked with writing the flavor text that supports it. If I'd been asked in the first place, I'd've said no to plasma in favor of a nice, understandable PBC. I always prefer substance over style. But some people, God love 'em, want their classic scifi tropes.

    Along those lines, though...
    Drakkith, I'll try to make this the last question and stop bothering people with this. Assuming (which is a huge assumption) the plasma could be kept in a cohesive state until it hits its target, which would you consider to be more valuable? A plasma that's hotter or one that has heavier nuclei? My gut is saying heat, but the question of collective kinetic impact is still tickling at the back of my brain. Of course, since the whole thing is hypothetical, straight opinion is fine by me; don't go to any inconvenience. (But if you have a handy link to a table of predicted plasma temperatures for different elements, I'd be really grateful! That ionization energy table was very useful.)

    Ryan, I'd be happy if you'd weigh in, too! :smile:
     
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