Author Topic: "Bottling" high temperature plasma from the Sun  (Read 8232 times)

Offline 93143

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Re: "Bottling" high temperature plasma from the Sun
« Reply #20 on: 05/22/2010 11:33 pm »
"the stability of spheromak plasmas also allows their creation by 'gun' devices. In a plasma gun, a pulse of electricity ionizes some gas, which is expelled from the barrel and coalesces into a stable spheromak."

The principle here is no different.  If you want fusion to happen you have to supply most of the energy required to get it going at the desired level.  It won't spontaneously "grow" from a small amount of fusion or a small amount of high-quality heat; you have to keep heating it until it reaches the ignition condition (which BTW no tokamak has ever done).  This is why fusion weapons have fission triggers - a fission device can be set up so that a relatively small input triggers a runaway reaction.  A fusion device can't.

A spheromak, while stable (and thus not requiring the fancy active control needed to make a tokamak work), is generally a fairly low-energy configuration.  Fusion schemes that use them tend to utilize secondary methods, like General Fusion's steam rams, to compress and heat the spheromaks.

Just for the sake of completeness, I hope you realize that a spheromak gun has to supply all of the energy present in the spheromak...

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I believed a similar process could work with 20MK plasma from the corona but I'm wrong.

You can't just intake plasma and have it form a spheromak; you need to generate it properly.  Pulsed spheromak generation from dense feed gas is not at all similar to coronal exposure, and I'm not sure how you would bridge this gap.

The corona is virtually empty space.  The density is extremely low, eight or nine orders of magnitude lower than the density in a modern tokamak, or even lower if you want some distance from the photosphere.  Collecting plasma from the corona is about the most difficult and inefficient method of fueling a reactor that I can imagine - even if you did somehow manage to make it form a spheromak, the rate of intake would be so low as to render it useless.

Also, I don't believe the bulk of the corona is quite that hot.  Wikipedia says 1-3 MK, not 20.  That's not even 300 eV; it's nowhere near hot enough to fuse at a significant rate, never mind ignite.  Plasmas in that range are very easy to generate - Ohmic heating alone can get a tokamak plasma past 20 MK, which BTW is still insufficient for ignition with a modern magnetic confinement scheme - so there really doesn't seem to be any point to this exercise at all.
« Last Edit: 05/23/2010 12:12 am by 93143 »

Offline cosmicpax

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Re: "Bottling" high temperature plasma from the Sun
« Reply #21 on: 05/25/2010 06:45 pm »
Thanks a lot for the clear explanations!
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Also, I don't believe the bulk of the corona is quite that hot.  Wikipedia says 1-3 MK, not 20. 
The article http://en.wikipedia.org/wiki/Sun in the paragraph Atmosphere says:
"The low corona, which is very near the surface of the Sun, has a particle density around 1015–1016 m−3.[53][note 2] The average temperature of the corona and solar wind is about 1 million–2 million kelvins, however, in the hottest regions it is 8 million–20 million kelvins.[54] "
I can't access the paper on reference 54 anyway so I can't be more precise.

Offline 93143

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Re: "Bottling" high temperature plasma from the Sun
« Reply #22 on: 05/25/2010 09:07 pm »
"The low corona, which is very near the surface of the Sun, has a particle density around 1015–1016 m−3.

Looking at the referred paper (ref. 53), it appears the density drops from 10^16 close to the edge of the chromosphere to below 10^15 in a matter of 500 km or so.  I'm not sure you want to get that close to the sun, especially at almost 600 km/s...  also, the proton temperature there is in the range of 0.1 MK...

If you treat the coronal plasma as an ideal gas, your scheme does produce a ~20 MK hydrogen plasma (with a potentially unacceptably high helium fraction) at >1e20 density via simple isentropic stagnation.  Of course, you can't treat a plasma this hot and tenuous as an ideal gas on the scale of a spacecraft, so you won't get that result in real life.

Not that even 20 MK really helps; as I noted before, we can already do better than that with Ohmic heating in a tokamak.

One solar radius away from the surface, the density is below 10^11, or 8-9 orders down from a tokamak, and it doesn't get past 10^12 until you get quite close to the chromosphere.  To collect a significant amount of plasma relative to the capacity of the reactor, you'd have to capture a significant fraction of the coronal plasma encountered by your spacecraft over the whole course of its swingby (simple stagnation isn't going to cut it; you need to magnetically capture and sequester it against an increasing pressure gradient, ultimately reaching multiple atmospheres). The proton temperature at this altitude is about 6 MK (or around 20 MK after isentropic stagnation at 500 km/s), and the helium (ash) fraction is predicted to approach 30%.

Another thing.  If my calculations are correct, an ITER-sized 500 MW reactor consumes its fuel at such a rate that at 500 km/s, one solar radius away from the sun, you have to sweep several thousand square metres of coronal plasma just to replace the hydrogen being consumed.  Given the strength of the solar magnetic field at that altitude, this may or may not be technically possible with a magnetic scoop using modern superconducting magnets, and ITER isn't a particularly powerful reactor - it doesn't even operate in the ignited regime...  This conclusion holds (assuming I didn't screw up the numbers) for any reactor in ITER's power range, regardless of whether it burns continuously or uses pulsed spheromak compression.

...so yeah.  I underestimated the density somewhat (and casually used a too-low value of the near-Sun orbital velocity in my stagnation calculation), but it doesn't really change my conclusions.  I'm afraid your scheme is a preposterously difficult (maybe impossible) and involved way to do what we've been able to do here on Earth for decades - in fact, we can already do significantly better, and we'll need to do better still to achieve ignition.

In short, you might be able to do this stunt with a lot of brilliant engineering, but it wouldn't give you a burning plasma.

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The average temperature of the corona and solar wind is about 1 million–2 million kelvins, however, in the hottest regions it is 8 million–20 million kelvins.[54] "
I can't access the paper on reference 54 anyway so I can't be more precise.

According to that paper, the 8-20 MK temperatures happen in closed-loop magnetic structures.  Not the sort of thing you want to count on for a scheme like this.  And even if you could count on 20 MK everywhere, that still only gives you 35 MK at best, which isn't much of an improvement; for D-T you want 100 MK.  Any other reaction needs even higher temperatures.
« Last Edit: 05/25/2010 09:44 pm by 93143 »

Offline mlorrey

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Re: "Bottling" high temperature plasma from the Sun
« Reply #23 on: 05/26/2010 12:41 am »
Fusion doesn't take place in the outer layers of the Sun.  Only in the core, and you can't duplicate the conditions and confinement there.
I knew that fusion doesn't take place in the corona but I also know that you can find plasma from 1MK to 20MK there, and it's actually a scientific question how it reaches similar temperatures at such a great distance from the center, especially since there is a lower layer as cold as 4100K.

Ok WHY THE CORONA IS SUPERHOT is rather simple: because its so empty.

Bear with me. At the surface the sun is about 8000 degrees or so. It is also very dense material constrained by very intense magnetic field. Compared to this, the corona is a very very thin atmosphere of gasses that is constantly being microwaved by the sun's electromagnetic field, but at the same time because the corona is so non-dense compared to the surface material, but the same amount of heat has to radiate through it, you have less matter handling the same amount of calories. That automatically means that the corona will be much much hotter because it has to handle the same number of calories as the solar surface with much much less matter.

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