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As the title suggests, this is a conceptual physics kinda question.

What is the feasibility of using an electrically charged (as in not neutral) plasma as the working fluid in a turbine where the blades are permanent magnets rather than hydrodynamic fins? In my head the charge of the plasma repels the magnets causing the turbine to spin, in turn spinning an alternator. This may not be the most efficient method, but it does sound “cool” and curiosity got the better of me.

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    $\begingroup$ Welcome to Worldbuilding, Malcolm! I feel like this question is better fitted for physics.stackexchange.com, because this is a real-world science. As for the answer, I think that in theory this turbine will work, however, there may be a practical problem for the plasma to maintain its charge while going through a turbine. $\endgroup$ – Alexander Jul 5 '18 at 16:25
  • $\begingroup$ Thanks Alexander, you may be right! But I’ve been doing a lot of fusion research lately and wanted a more science fiction response than I’m likely to get at the physiscs stack. $\endgroup$ – Malcolm Jul 5 '18 at 16:29
  • $\begingroup$ The plasma would have a typical magnetic field which would push the blade above it upwards. However it would also push the blade below it downwards. $\endgroup$ – P.Lord Jul 5 '18 at 16:32
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    $\begingroup$ Probably not a good idea. First, the plasma would erode the blades, probably fairly quickly: en.wikipedia.org/wiki/Plasma_cutting Second, there are much better ways to extract energy from plasma: en.wikipedia.org/wiki/Magnetohydrodynamic_generator $\endgroup$ – jamesqf Jul 5 '18 at 17:49
  • $\begingroup$ Please take our tour and visit the help center when you have a moment to learn more about the site. Have fun! $\endgroup$ – FoxElemental Jul 5 '18 at 22:46
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What is the feasibility of using an electrically charged (as in not neutral) plasma as the working fluid in a turbine where the blades are permanent magnets rather than hydrodynamic fins

You are in a vacuum? So the plasma flow is, essentially, a current in vacuum.

The bad news: the turbine won't work

A charged particle does not exert useable force on a static magnetic field, nor does the static magnetic field exert useable force on the particle. It's a consequence of the equation for the Lorentz force. The plasma current will be curved around the magnetic field without losing energy, as it happens with the solar wind when it impacts the Earth's magnetosphere.

You might build something like a Crookes' radiometer running on ions instead of a thermal exchange, but the plasma would rapidly ruin the blades.

The good news: you don't need a turbine

A plasma flow in vacuum is a current. By intercepting or modulating the flow you can make it a variable current. A variable current will induce a variable magnetic field, and a variable magnetic field can be used to produce energy at the expense of the kinetic energy of the charged plasma.

Your machine is essentially a hollow cylinder through which the variable plasma flow runs. The inner walls of the cylinder could be covered with ice, or some low-melting non-magnetic metal (GalInStan, Rose's metal, Wood's metal, mercury) in order to easily counteract erosion by stray particles. Or you could re-sheath them when needed.

Inside the cylinder you have large superconducting coils running all around the cylinder, circuiting around the incoming current and making a large solenoid. Every oscillation in the plasma current will induce a voltage in the solenoid; you've essentially built a large and inefficient transformer in space, but if the plasma flow comes from some large natural phenomenon, inefficiency isn't really an issue, and you get lots of energy for free.

Plasma jet engine

With a bit of handwaving, you might imagine a first stage that will both compress the plasma flow and induce in it a series of rapid oscillations through three-dimensional vortex shedding. The oscillating plasma flow can then be exploited using the cylindrical transformer described above.

The overall scheme would be somewhat similar to that of a scramjet propulsor.

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The power input might as well come from solar wind... so it would be a "solarwind-turbine", featuring a magnetic magnus-effect rotor, running on the right-hand-law. In this case, the blades won´t have to spin physically.

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    $\begingroup$ I have high hopes for you, @MAX HOFBAUER BALMORI. I am optimistically upvoting this in the hope that you will expand your answer to a page of sage exposition, with diagrams. Solar wind turbine? Magnus-effect rotor? Right-hand-law? You can draw the diagrams yourself if you like. $\endgroup$ – Willk Jul 5 '18 at 22:20
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Unfortunately I'm on my phone so cannot draw a diagram

Fuel

Where is the energy coming from? And why do you get energy out?

The act of charging plasma takes energy and so how does this give you more energy than you put in?

If nothing is losing energy then you have made a perpetual motion machine. Which cannot work.

Magnets

Right so without diagrams imagine this.

  • S - this is the south pole of the blade above
  • P - the charged plasma
  • N - the north pole of the blade below

Either P is positively or negatively charged.

Either way the magnet below will attract or repel the plasma whilst the magnet on top does the opposite.

This means the blades are pushing against eachother therefore the turbine is moving equal parts up and equal parts down.

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  • $\begingroup$ Could you explain this further for me? I think I get what you mean, but its a little vague. I also don’t see how it relates to perpetual motion? The limit there is the ever presence of friction or some other energy loss. $\endgroup$ – Malcolm Jul 5 '18 at 16:34
  • $\begingroup$ Right but first I will explain why this is perpetual motion. $\endgroup$ – P.Lord Jul 5 '18 at 16:35
  • $\begingroup$ The limit with perpetual motion machines is the belief that you can extract energy from aomething without changing the energy of said thing. E.g. increasing total energy. Or the belief we can reverse entropy. $\endgroup$ – P.Lord Jul 5 '18 at 16:44
  • $\begingroup$ I don't believe Malcolm is trying to build a perpetual motion machine here. Conventional turbines, where we have a stream of gas, are working irrespective of where this gas is coming from. The system may be very inefficient as a whole, but that's Ok. $\endgroup$ – Alexander Jul 5 '18 at 16:54
  • $\begingroup$ Ok but the lack of explanation of how the plasma will be charged and the fact you cannot charge plasma without already having electricity make it seem as if it is perpetual motion. Why turn electricity into less electricity? $\endgroup$ – P.Lord Jul 5 '18 at 16:55
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It's not clear what you mean by a "charged plasma." Plasmas generally are neutral, and being electrically conductive fluids, would be difficult to maintain a charge on them. (The charge causes the plasma to repel itself, and destabilise.)

It is also unnecessary for your question. Magnets aren't affected by static charge; they are affected by currents. And a plasma moving in a magnetic field contains currents.

The short answer is that there already exist generators that exploit these effects to generate power from a plasma. The general discipline is called magnetohydrodynamics, or MHD for short, and the generators are MHD generators. A great deal of work was done in the field in the 1980s and '90s, but none of the designs were sufficiently efficient to be commercialised.

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