My friend was describing a Pulsed Plasma Thruster (PPT) to me, and we couldn't figure out how it actually worked. The setup involves two conducting plates with a potential difference and a solid, insulating fuel block sandwiched in between.

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From the Wikipedia page:

The first stage in PPT operation involves an arc of electricity passing through the fuel, causing ablation and sublimation of the fuel. The heat generated by this arc causes the resultant gas to turn into plasma, thereby creating a charged gas cloud. Due to the force of the ablation, the plasma is propelled at low speed between two charged plates (an anode and cathode). Since the plasma is charged, the fuel effectively completes the circuit between the two plates, allowing a current to flow through the plasma. This flow of electrons generates a strong electromagnetic field which then exerts a Lorentz force on the plasma, accelerating the plasma out of the PPT exhaust at high velocity.

As I understand it, the current is directed vertically between the two plates, which creates a magnetic field in the horizontal direction - call it $x$ (left to right in the diagram). The electrons would seemingly then experience a Lorentz force in the $y$-direction (into the plane). What am I missing here? It seems in order for the plasma to be accelerated along $x$, we would need $\mathbf{B}$ directed along $y$.

  • $\begingroup$ Well, rotate the chamber by 90 degrees in the xy plane and it should drive the plasma in the direction you "expect" :-) $\endgroup$ – Carl Witthoft Sep 3 '15 at 12:03
  • $\begingroup$ Glancing through, it appears to work the same way as a rail gun, replacing the metal projectile with a plasma. I'll look again on my lunch break. $\endgroup$ – Asher Sep 3 '15 at 13:10
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    $\begingroup$ Ah okay. Dumb of me. The arcing happens in the $yz$ plane, and current flows along this sheet, creating a $\mathbf{B}$ field in the $y$-direction, so the Lorentz force is indeed in $x$. Thanks! $\endgroup$ – Kyle Arean-Raines Sep 3 '15 at 13:28
  • $\begingroup$ When plasma is pulled by the magnetic field created by the current in the plasma, where does the plasma move? Nowhere. $\endgroup$ – stuffu Sep 3 '15 at 14:47
  • $\begingroup$ @user7027 if plasma is pulled by a magnetic field, it moves in the direction the field pulls it. Why wouldn't it? $\endgroup$ – Asher Sep 3 '15 at 16:02

The current in the plates creates a magnetic field, the charges in the plasma move in that magnetic field. That causes a Lorenz-force on the particles.

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  • $\begingroup$ There is no current in the plates. The plasma provides a conductive path between the plates, which causes electrons to flow from one plate to the other. The flow of electrons generates a magnetic field, which in turn exerts a force on the plasma. I wrote the answer in a comment. $\endgroup$ – Kyle Arean-Raines Sep 3 '15 at 17:33
  • $\begingroup$ Let's consider a tram overhead wire and and tram rail. We put a metal rod between the wire and the rail. To which direction does the rod start to move? For example does it start to move along the rail or sideways? Can anybody here answer that very simple question? $\endgroup$ – stuffu Sep 3 '15 at 22:54

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