I recently learned about (and am now fascinated by) spheromaks. I am no expert on plasma physics, but as I understand they are analogous to propagating toroidal vortices (smoke rings), but with plasma instead of air! Because we are talking about plasma physics, they are not very stable. They propagate for a small time, but they loose energy through thermal radiation until they become unstable.

My question is the following: Would it be possible to have a spheromak propagating through empty space over large distances? Imagine that it is big and hot enough for fusion reactions to take place while it propagate. Could this balance the energy loss by radiation and stabilise it over 'human' time scales?

I know that we are not able to produce such plasma configurations. My question is not about our current technological abilities. However, I would very much like to see this as a plot element in some science-fiction story. Could it be possible to either make (in the distant future) a propagating spheromak or observe one passing by earth?

  • $\begingroup$ So, you would not be able to do this by introducing such a configuration into a pure vacuum. The particle would quickly spread out diffusively. In a real plasma, like in space, we observe things kind of like these spheromak thingys, but not the same at all. What we observe are called ring-beam velocity distributions. Think of a doughnut-shaped clump of charged particles drifting along the background magnetic field (i.e., the center of the ring moves parallel to $\mathbf{B}$). $\endgroup$ – honeste_vivere Nov 19 '15 at 13:04
  • $\begingroup$ @honeste_vivere So am I using the wrong terminology? Should I edit my question to replace 'spheromak' by 'ring-beam velocity distribution'? $\endgroup$ – Steven Mathey Nov 19 '15 at 13:36
  • $\begingroup$ No, it appears that spheromaks may be a real phenomena. If so, they are distinctly different, I think. I was only commenting that we do observe distributions of particles in a slightly similar geometry. $\endgroup$ – honeste_vivere Nov 19 '15 at 13:54

Yes, spheromaks can and do propagate in a vacuum. Large distances... that is another issue. One popular way to form spheromaks is with magnetized coaxial plasma guns. The formation process gives the spheromak a large velocity out of the gun, on the order of 10's of km/s.

While spheromaks do have self generated magnetic fields that confine the plasma (not indefinitely of course due to non-ideal effects), they will expand until something confines the fields. A highly conducting 'flux conserver' is often used, i.e. a copper can. In other words, the spheromak expands to fill the volume you shoot it into.

In a thought experiment of shooting a spheromak into a very large or infinite vacuum, it would presumably continue to expand. As it did so, its field strength, particle density, and energy density, amongst other quantities would decrease. I would think that even if it was hot and dense enough for fusion initially, it would expand and at some point critical parameters would drop below the fusion threshold.

Factor in the non-ideal MHD effects that cause spheromaks to decay naturally, and the human time scales thing might be tough.


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