Author Topic: "Bottling" high temperature plasma from the Sun  (Read 8235 times)

Offline cosmicpax

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"Bottling" high temperature plasma from the Sun
« on: 05/22/2010 04:39 pm »
Please don't laugh. ok you can laugh, but would it be possible to trigger a fusion reactor inside a spaceship with material sampled and "magnetically bottled" by a probe during a close solar flyby? Material could be transferred by the probe to the spaceship and there a fusion reactor could be ignited. What do you think?
I know that the probe would need some extraordinary shielding but it doesn't need to take a dip into the surface of the Sun, it could collect a sample from the hot but "lower density" plasma in the corona, which can reach 20 million K in some regions. I think a very high speed flyby would not destroy the probe if it is protected in the direction of the Sun by a radiation shield that would be disposed of after the flyby for the return trip.
« Last Edit: 05/22/2010 04:47 pm by cosmicpax »

Offline kfsorensen

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Re: "Bottling" high temperature plasma from the Sun
« Reply #1 on: 05/22/2010 04:58 pm »
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.

Offline cosmicpax

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Re: "Bottling" high temperature plasma from the Sun
« Reply #2 on: 05/22/2010 05:08 pm »
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.

Offline kfsorensen

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Re: "Bottling" high temperature plasma from the Sun
« Reply #3 on: 05/22/2010 05:14 pm »
Fusion isn't just temperature.  Density and confinement are the other two parameters of the Lawson criterion.  The coronal plasma has neither in sufficient amounts for fusion.

Offline Jim

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Re: "Bottling" high temperature plasma from the Sun
« Reply #4 on: 05/22/2010 05:14 pm »

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.

Plasma can't be stored.

Offline Jorge

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Re: "Bottling" high temperature plasma from the Sun
« Reply #5 on: 05/22/2010 05:41 pm »
The act of confining the plasma (by whatever method) will take enough energy out that it won't be plasma any more.

Learn about the second law of thermodynamics to find out why this won't work. Entropy always beats you. You can't win, you can't break even, you can't leave the game.
JRF

Offline 93143

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Re: "Bottling" high temperature plasma from the Sun
« Reply #6 on: 05/22/2010 05:46 pm »
There's nothing special about a solar plasma; higher temperatures (and far higher densities than what you'd find in coronal plasma) have already been achieved in the lab.

The final component of the Lawson criterion, confinement time, should tell you something:

Plasma can't be stored.

Well, it can - but not very well, or for very long, which incidentally is the single biggest reason why net-power fusion reactors are so hard to build.

Offline Robotbeat

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Re: "Bottling" high temperature plasma from the Sun
« Reply #7 on: 05/22/2010 05:55 pm »
Sounds infinitely easier to "just" harvest gas from a gas giant. And, as people have noted here, fusion only happens in the center, and even there, it is rather slow. The energy-production density of the core is no greater than the human metabolism... rather low performance! You'd need to heat it up to far higher temperatures anyways to get enough fusions-per-second.
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Offline cosmicpax

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Re: "Bottling" high temperature plasma from the Sun
« Reply #8 on: 05/22/2010 06:05 pm »
What if the reactor is contained in the probe? So the reaction is started during solar flyby then continues on it's own as it should? The small tokamak inside the probe could initially contain only high density but cold tritium and deuterium that would be ignited by the injection of 20MK plasma. Then the reaction would continue. The magnetic field in the Tokamak would be activated only after the reaction has started.

That said, I think a Spheromak would be better suited for a space probe than a Tokamak.
« Last Edit: 05/22/2010 06:20 pm by cosmicpax »

Offline 93143

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Re: "Bottling" high temperature plasma from the Sun
« Reply #9 on: 05/22/2010 06:20 pm »
What if the reactor is contained in the probe? So the reaction is started during solar flyby then continues on it's own as it should? The small tokamak inside the probe could initially contain only high density but cold tritium and deuterium that would be ignited by the injection of 20MK plasma. Then the reaction would continue. The magnetic field in the Tokamak would be activated only after the reaction has started.

No, it doesn't work that way.

The density in the corona is too low.  The energy available from coronal plasma particles is too low.  And how, pray tell, are you going to ignite a plasma without the magnetic field turned on?  I can only conclude that you have no idea what the magnetic field is for...

You're describing a method that (a) could never work, and I mean never, and (b) even if it did work, would be vastly more difficult and expensive and complicated than just starting the reactor normally.

Fusion is not a chain reaction.  The main difficulty is not in starting it up; it's in keeping it going.
« Last Edit: 05/22/2010 07:16 pm by 93143 »

Offline Robotbeat

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Re: "Bottling" high temperature plasma from the Sun
« Reply #10 on: 05/22/2010 06:21 pm »
The act of confining the plasma (by whatever method) will take enough energy out that it won't be plasma any more.

Learn about the second law of thermodynamics to find out why this won't work. Entropy always beats you. You can't win, you can't break even, you can't leave the game.

What if the reactor is contained in the probe? So the reaction is started during solar flyby then continues on it's own as it should? The small tokamak inside the probe could initially contain only high density but cold tritium and deuterium that would be ignited by the injection of 20MK plasma. Then the reaction would continue. The magnetic field in the Tokamak would be activated only after the reaction has started.
How does this help anything versus just collecting the gas using a fusion reactor on your main spacecraft?
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Offline cosmicpax

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Re: "Bottling" high temperature plasma from the Sun
« Reply #11 on: 05/22/2010 06:27 pm »
What if the reactor is contained in the probe? So the reaction is started during solar flyby then continues on it's own as it should? The small tokamak inside the probe could initially contain only high density but cold tritium and deuterium that would be ignited by the injection of 20MK plasma. Then the reaction would continue. The magnetic field in the Tokamak would be activated only after the reaction has started.

No, it doesn't work that way.  You really need to learn a bit of fusion physics before getting into this sort of discussion.

The density in the corona is too low.  The energy available from coronal plasma particles is too low.  And how, pray tell, are you going to ignite a plasma without the magnetic field turned on?  I can only conclude that you have no idea what the magnetic field is for...

I understand the problem of the density, that's the real problem I think. I was thinking that an high speed passage could capture enough hot particle to created an higher density in a sort of preliminary container, before the hot, high density plasma is injected in the reactor. I realize that activating the magnetic field of the Tokamak, or better, the Spheromak must be done before the chain reaction takes place, but superconducting magnets require so much energy that makes sense to wait until the probe is very close to the Sun to use as much energy density as possible on the special PV panels of the probe.

Offline 93143

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Re: "Bottling" high temperature plasma from the Sun
« Reply #12 on: 05/22/2010 06:30 pm »
chain reaction

Okay, suspicion confirmed.  Go back and read my post again (I edited it).

Quote
superconducting magnets require so much energy

Once started, superconducting magnets require no energy input beyond their cooling system requirements (which are fairly modest if the insulation is well designed, especially before the reaction starts up).

Also, about the high-speed passage - the density of the solar corona is extremely low and the temperature is extremely high.  Stagnation of a 3e6 K, 2e-16 kg/m^3 monatomic hydrogen gas at 35 km/s would increase the pressure and density by less than 2%.

Not that the corona is really dense enough to treat as an ideal gas on the scale of a space probe...

EDIT:  35 km/s is too slow.  Try 400-500 km/s.  That does increase the temperature quite a bit, and the density by a significant factor - but not significant enough...
« Last Edit: 05/25/2010 10:11 pm by 93143 »

Offline cosmicpax

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Re: "Bottling" high temperature plasma from the Sun
« Reply #13 on: 05/22/2010 06:37 pm »
How does this help anything versus just collecting the gas using a fusion reactor on your main spacecraft?
[/quote]

If the main spacecraft is manned I wouldn't send it so close to the Sun. Even if the main spacecraft is robotic, only a very special probe could resist such a close flyby to the Sun. That said, the specialized igniting probe could be used in a cyclic way, repeating the solar flybys for starting the reactors of many fusion propelled spacecrafts.

Offline Robotbeat

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Re: "Bottling" high temperature plasma from the Sun
« Reply #14 on: 05/22/2010 06:53 pm »
How does this help anything versus just collecting the gas using a fusion reactor on your main spacecraft?

If the main spacecraft is manned I wouldn't send it so close to the Sun. Even if the main spacecraft is robotic, only a very special probe could resist such a close flyby to the Sun. That said, the specialized igniting probe could be used in a cyclic way, repeating the solar flybys for starting the reactors of many fusion propelled spacecrafts.
[/quote] I'm just saying, why not just have the probe collect the gas and deposit it at the main spacecraft?
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Offline Sparky

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Re: "Bottling" high temperature plasma from the Sun
« Reply #15 on: 05/22/2010 06:57 pm »
This sounds oddly similar to plot device in Stargate Universe...

Offline Patchouli

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Re: "Bottling" high temperature plasma from the Sun
« Reply #16 on: 05/22/2010 07:05 pm »
The Sun's gravity well is so deep getting to Saturn and back would be easier.
You can actually try this in the orbiter sim with something with tons of delta V like the nuclear Orion.
« Last Edit: 05/22/2010 07:08 pm by Patchouli »

Offline cosmicpax

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Re: "Bottling" high temperature plasma from the Sun
« Reply #17 on: 05/22/2010 07:13 pm »
chain reaction

Okay, suspicion confirmed.  Go back and read my post again (I edited it).
[/quote]

Thank you, I get it now that fusion require a constant external source of energy (gravitational in the case of stars) and I apologize about my ignorance (which is even more humiliating as I'm a physics student who has studied stellar and Big Bang fusion processes for astrophysics and cosmology exams). I clearly had learned the details ignoring the essential :(
« Last Edit: 05/22/2010 07:18 pm by cosmicpax »

Offline 93143

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Re: "Bottling" high temperature plasma from the Sun
« Reply #18 on: 05/22/2010 07:25 pm »
Well, technically the gravitational effect in a star is what supplies confinement.  The fusion reaction itself produces enough energy to keep the star hot.

So it's kind of like a chain reaction, once you've reached ignition conditions (enough fusion going on to keep the plasma hot).  But it's not something you can grow from a small 'spark' like with fission - if you're not at ignition yet and you let the reaction go without any more external heating, it will die back down.  The corona is not nearly dense enough (or hot enough, for that matter) to provide the brute force necessary to get a tokamak up to ignition conditions, and if it was it would instantly destroy any spacecraft that ventured near...

The main difficulty is confinement.  Reaching ignition is easy if the confinement problem is solved, which is why your scheme wouldn't really address the issue even if it could work.
« Last Edit: 05/22/2010 07:42 pm by 93143 »

Offline cosmicpax

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Re: "Bottling" high temperature plasma from the Sun
« Reply #19 on: 05/22/2010 07:52 pm »
it's not something you can grow from a small 'spark' like with fission.
Thank you, can you explain than this quote from:
http://en.wikipedia.org/wiki/Spheromak
"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."
I believed a similar process could work with 20MK plasma from the corona but I'm wrong.

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...

Quote
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!
Quote
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.

Quote
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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