Magrid Vs electrically biased grid

Discuss the technical details of an "open source" community-driven design of a polywell reactor.

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sd_matt
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Post by sd_matt »

Orient the chamber so that no one standoff is horizontal. A slight reduction of the forces from the cantilever.

KitemanSA
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Post by KitemanSA »

MSimon wrote:The "around" idea conflicts with the oscillation idea.
Not sure what you mean by the "oscillation" idea. The electrons exit a cusp gyrating around a field line and extend out away from the MaGrid a distance that is dictated by the distance to the ground state barrier (the Faraday cage or the chamber wall, whichever) OR by the distance the field line traces.

If the field line being followed curves around and back into another cusp before the ground state is achieved, the electron will follow it around; unless it runs into a support first.

If the field line crosses the ground state distance, the electron will follow it out, stop, and come back in. That field line may or may not curve into a support before it crosses the ground stte. If it does, the electon could run into a support on that line too.

An electron does NOT exit and then turn around immediately; unless it is very cold. A hot electron (the energy state desired of the electrons to maintain the desired well depth) must reach almost to the ground state.

The only way to truly protect the supports from electrons is to tuck as much of it as possible outside the Wiffleball but inside the outer surface of the MaGrid. JMHO.

D Tibbets
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Post by D Tibbets »

My take on oscillation is that the escaping electron follows a field line (gyrates along it) untill it reverses due to some type of mirror reflection (bounce) or untill it follows the field line into a structure or back into the machine through another cusp. But, this discribes the situation in which the magrid casing is neutral or grounded. Any charge on the magrid will change the dynamics. eg: with a potential well of ~ 10 kV and a magrid charge of ~ +12 kV, when the escaping electron passes the magrid with the full acceleration provided by the potential well it sees the 12 kV positive potential on the grid (Gauss's law stuff) and it is quickly decelerated by (or more precisely accelerated towards) the magrid. The brakes are put on, it reverses quickly and it renters the internal magrid volume through the same cusp. What I'm unsure of is how far it will travel before this charge reverses it. Is it 1 mm, 1 cm, 10 cm... ? Also, I think that once it reverses it will gain a velocity dependent on the potential, and the distance it traveled before reversing does not change this (Gauss's law again) so, the recirculating electron that returns through the same cusp is reset at the original energy (12 kV (actually KeV but I'm sticking with volts to keep it simple)) even if they were mildly upscattered. In this regard, the inefficiency of creating the potential well (80-85% efficient) is actually an advantage as it permits the recapture (recirculation) of mildly upscattered electrons without the penalties of mild, but progressive upside thermalization.
Mildly upscattered electrons 10.1 to 11.9 kV will travel a little farther before reversing. Upscattered electrons above 12 kV would slow but not stop. These electrons would continue to follow the field line till it hit something or reentered through another cusp. The problem if it recirculates in this fashion is that it would reenter the magrid at it's residual velocity plus the mageid accelerating velocity. A limited amount of this escalation may be tolerable, but there are limits to how much of this sequential upscattering could be allowed before undesirable thermalization effects reared it's ugly head. In that regard, some limit on the efficiency of recirculation may be desired. Of course, external structures like electron guns, ion guns, supports, etc. may eat/ deflect these electrons, once they climb far enough beyond the magrid mid line, depending on their placement and shielding. Another example of the synergistic compromises that make up the Polywell.

[EDIT]
It occurs to me that if mildly upscattered electrons are reset to the drive potential with recirculation, then downscattered escaping electrons would also be reset as they traveled a short distance past the magrid midline and were reversed. The part of Gauss's law that says that a mobile charge is accelerated the same amount whether it starts a short distance or a large distance from the charged plate would dictate this.
This fits the picture that the Wiffleball is important for the obtainable density, not for energy efficiency. In fact, if Wiffleball containment is to great, the electrons may thermalize too much before escaping and exceed the capacity of the magrid potential to reset their energy. In essence the recirculation does two critical things. It conserves energy (decrease electron losses) and it serves as a voltage (energy) regulator.


Dan Tibbets
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MSimon
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Post by MSimon »

In previous discussions (FWIW) the conclusion was reached that it was unlikely for electrons to follow the field lines from one cusp to another. Most likely is exiting and entering the same cusp. Thus oscillation.

In fact I first had this discussion with Tom Ligon and the NASA spaceflight folks back in early 2007. The consensus (FWIW) is that recirculation was a misleading name for the process. Oscillation was better. For better or worse we seem to be stuck with recirculation - for now.

Edit: fixed typo
Last edited by MSimon on Fri Jul 23, 2010 4:39 pm, edited 1 time in total.
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KitemanSA
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Post by KitemanSA »

D Tibbets wrote: ... The brakes are put on, it reverses quickly and it renters the internal magrid volume through the same cusp. What I'm unsure of is how far it will travel before this charge reverses it. Is it 1 mm, 1 cm, 10 cm... ?
The last I looked, the physical distance would be dependant on the gradient of the potential and is approximated by the distance to the ground state barrier. That distance is what defines the gradient. If the electron has 10keV of energy it will get ~5/6th of the way to ground state from the 12kV MaGrid. This will be about 5/6th of the distance to the barrier, typically.
There was some talk of having the electron source ata slightly positive potential so that fewer upscttered electrons would be lost, but I am not sure if anything ever happened with that.
But the take away is, it will go a goodly portion of the distance to whatever defines the ground state. If it can't get that far due to tighter looped field lines, it will circulate into another cusp. If the lines go further than it can climb, it will reverse itself and reenter along that field line. Simple physics, no?
Ok, well perhaps not QUITE that simple because I was describing a linear condition, not a spherical one but the situations are still similar.

ladajo
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Post by ladajo »

WizWom wrote:
ladajo wrote:Fair point. Shock rating does tend to throw in a monkey wrench or two, and a dollar or ten.
Actually, a jiggling would not be as bad as all that... the core would flex. The precision of the placement affect the whiffleball factor, and that dropping in a production machine a few percent would be seen as a drop in fusion rate during the stress event.

As far as it goes, the design shown in the picture has the 4 supports going in at what looks like a 45 degree angle, which with 4 supports would be quite rigid.

And remember that a high vacuum chamber is going to have to be pretty rigid itself. All in all, I'd expect a Polywell to continue to fuse unless the vacuum vessel is breached.
I've been out for a 10,000lb shot close aboard for shock testing. Can you say, "Hammer of God"? Jiggle is not a term that comes to mind.
One thing all navy ship shock testing has in common...stuff breaks. It is all in how fast you can recover.

KitemanSA
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Post by KitemanSA »

ladajo wrote: I've been out for a 10,000lb shot close aboard for shock testing. Can you say, "Hammer of God"? Jiggle is not a term that comes to mind.
One thing all navy ship shock testing has in common...stuff breaks. It is all in how fast you can recover.
And just think, that wasn't even design level!

And they (the Program Offices) want us to stop doing Full Scale Shock Trials because "MIL-S-901D qualification testing will always assure that nothing will break". Ha!!

mvanwink5
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Post by mvanwink5 »

Lawyers make agreements in cigar smoke filled rooms because they know from experience that engineers will figure out how to make it work. Don't believe me? look at environmental regulations. Just put in some shock absorbers and springs.
Near term, cheap, dark horse fusion hits the air waves, GF - TED, LM - Announcement. The race is on.

KitemanSA
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Post by KitemanSA »

mvanwink5 wrote: ... Just put in some shock absorbers and springs.
So you want to use springs to prevent the MaGrid coils from smacking into each other under shock. Then how do you plan to keep them aligned when there is no shock? The multi-Tesla field will try to push these things apart like crazy. Either the springs will be strong enought to resist the filed, and will transmit the shock, or the springs will be soft enought to isolate the shock and the coils will spread under the field load.
Trust me, the best way is to tie the coils together and isolate the entire thing.

MSimon
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Post by MSimon »

Trust me, the best way is to tie the coils together and isolate the entire thing.
Yes.

Which got me to thinking though. Maybe the first ship will not be a warship but maybe a high speed auxiliary. A mechanized battalion mover perhaps. Two hulls and a platform - 75 kts. 1800 nM/day. Ten ships to move a division and keep it supplied.

In any case the first one is going to be land based. Designed for at most a few Gs shock load if that is a limitation at the beginning. The 10 G jobs will be third round designs. Probably.
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KitemanSA
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Post by KitemanSA »

MSimon wrote:
Trust me, the best way is to tie the coils together and isolate the entire thing.
Yes.
Which got me to thinking though. Maybe the first ship will not be a warship but maybe a high speed auxiliary. A mechanized battalion mover perhaps. Two hulls and a platform - 75 kts. 1800 nM/day. Ten ships to move a division and keep it supplied.
In any case the first one is going to be land based. Designed for at most a few Gs shock load if that is a limitation at the beginning. The 10 G jobs will be third round designs. Probably.
With a monocoque MaGrid, many problems should be solved, and shock isolation would be a piece of cake.

mvanwink5
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Post by mvanwink5 »

Sorry, I wasn't clear, only the vacuum chamber would be isolated with springs and shock absorbers and hung from above. The coils would be held ridged by being attached to the vacuum chamber walls. Ever see the steam lines that feed turbines in a power plant? They are designed to handle thermal growth and shocks from steam hammers (heaven forbid). The boilers are also suspended from hangers for thermal growth (downward).
Near term, cheap, dark horse fusion hits the air waves, GF - TED, LM - Announcement. The race is on.

KitemanSA
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Post by KitemanSA »

mvanwink5 wrote:Sorry, I wasn't clear, only the vacuum chamber would be isolated with springs and shock absorbers and hung from above. The coils would be held ridged by being attached to the vacuum chamber walls. Ever see the steam lines that feed turbines in a power plant? They are designed to handle thermal growth and shocks from steam hammers (heaven forbid). The boilers are also suspended from hangers for thermal growth (downward).
Shock mounts outside the chamber would help, but I suspect the flexibility of the chamber wall will still be a problem for keeping the coils from colliding. Keeping something the size of the chamber sufficiently rigid would be nye on impossible, IMEO.

MSimon
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Post by MSimon »

KitemanSA wrote:
mvanwink5 wrote:Sorry, I wasn't clear, only the vacuum chamber would be isolated with springs and shock absorbers and hung from above. The coils would be held ridged by being attached to the vacuum chamber walls. Ever see the steam lines that feed turbines in a power plant? They are designed to handle thermal growth and shocks from steam hammers (heaven forbid). The boilers are also suspended from hangers for thermal growth (downward).
Shock mounts outside the chamber would help, but I suspect the flexibility of the chamber wall will still be a problem for keeping the coils from colliding. Keeping something the size of the chamber sufficiently rigid would be nye on impossible, IMEO.
Boilers on warships?
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KitemanSA
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Post by KitemanSA »

MSimon wrote: Boilers on warships?
You are surprised? You, an ex-navy nuke? Why? Every nuke ship in the navy has a boiler. They call it a steam generator, but it is a boiler.

http://en.wikipedia.org/wiki/Pressurize ... tor#Design

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