WB-8 Coming

Point out news stories, on the net or in mainstream media, related to polywell fusion.

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Aero
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Postby Aero » Sat May 30, 2009 12:25 am

I expect the LHC has the start-up problem (SC Magnet start-up) in spades. The did it successfully once, has anyone looked for a published paper. Of course, with their budget, they probably could absorb the cost of a few burned power supplies. But I doubt they would plan it that way.
Aero

MSimon
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Postby MSimon » Sat May 30, 2009 1:02 am

Aero wrote:I expect the LHC has the start-up problem (SC Magnet start-up) in spades. The did it successfully once, has anyone looked for a published paper. Of course, with their budget, they probably could absorb the cost of a few burned power supplies. But I doubt they would plan it that way.


It was not just magnet power supplies. They probably lost a fair amount of He as well. All the magnets needed to be rebuilt to new specs. And I believe it wasn't a magnet failure but a failure of the supporting structure. From what I read it was inadequate to handle the shear stress.

http://public.web.cern.ch/User/QuickLin ... iplet.html

http://blogs.discovermagazine.com/cosmi ... t-failure/

pictures:

http://stephatcern.blogspot.com/2008/12 ... amage.html
Engineering is the art of making what you want from what you can get at a profit.

KitemanSA
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Re: Slightly OT...

Postby KitemanSA » Sat May 30, 2009 1:46 am

Nik wrote: Problem with SC was, as this thread states, charging the ruddy thing up. One trick seemed to be ramping in an external supply then short-circuiting the link. Fuse blows in supply, of course, of course, but current continues to circulate in SC.
The way they do it now is similar but has a more elegant way of "short circuiting the link". The place a shunt of warm SC between the feed leads and after ramping up the power, cool the shunt until it is SC. At that point, the current takes the path of least resistance and follows the shunt within a fully SC coil. Neat! AFAIK, no blown fuses, but I am not an expert.
Last edited by KitemanSA on Sat May 30, 2009 12:51 pm, edited 1 time in total.

MSimon
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Re: Slightly OT...

Postby MSimon » Sat May 30, 2009 2:41 am

KitemanSA wrote:
Nik wrote: Problem with SC was, as this thread states, charging the ruddy thing up. One trick seemed to be ramping in an external supply then short-circuiting the link. Fuse blows in supply, of course, of course, but current continues to circulate in SC.
The way they do it now is similar but has a more elegant way of "short circuiting the link". The place a shunt of warm SC between the feed leads and after ramping up the power, cool the shunt until it is SC. At that point, the current takes the path of leawst resistance and follows the shunt within a fully SC coil. Neat! AFAIK, no blown fuses, but I am not an expert.


Power supplies these days are easy to protect against short circuits. It is all done electronically. The supplies are designed to deliver "constant" current. So when the voltage rises - no load - you trip the supply to off.
Engineering is the art of making what you want from what you can get at a profit.

KitemanSA
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Postby KitemanSA » Fri Jun 05, 2009 2:31 pm

KitemanSA wrote:
MSimon wrote:
IntLibber wrote:....
Another possibility is eliminating the nubs altogether by going to supports for all coils similar to the bottom supports.
And yet another option is to eliminate the "nubs" by creating a real/real magnet system with holey X cusps.

From the views I have seen of the vacuum chamber, I suspect they could get a 1.5x to 2x WB7 scale unit in it without too much difficulty.


Ya know, having re-read this all this time later, I am not sure that I have ever seen a good picture of the WB7 chamber. I was thinking of the WB6 unit in ITS chamber when I made the 1.5 to 2x statement. Does anyone know the size of the WB7 chamber?

rnebel
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Postby rnebel » Fri Jun 05, 2009 4:28 pm

It's a 1 meter cube.

rnebel
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Postby rnebel » Fri Jun 05, 2009 4:29 pm

It's a 1 meter cube.

TallDave
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Postby TallDave » Fri Jun 05, 2009 8:46 pm

rnebel wrote:It's a 1 meter cube.


That's interesting, I see the WB-7 pic every day (it's my desktop) but the area it shows is too small to give the shape of the vessel. I always assumed it was spherical.

Are you able to share any details on WB-8 (e.g. size, magnet strength, changes to the interconnects, geometry changes, etc.)? From your earlier comments I'm guessing the emphasis will be on transport.

D Tibbets
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Postby D Tibbets » Fri Jun 05, 2009 9:47 pm

MSimon wrote:
Aero wrote:I expect the LHC has the start-up problem (SC Magnet start-up) in spades. The did it successfully once, has anyone looked for a published paper. Of course, with their budget, they probably could absorb the cost of a few burned power supplies. But I doubt they would plan it that way.


It was not just magnet power supplies. They probably lost a fair amount of He as well. All the magnets needed to be rebuilt to new specs. And I believe it wasn't a magnet failure but a failure of the supporting structure. From what I read it was inadequate to handle the shear stress.

http://public.web.cern.ch/User/QuickLin ... iplet.html

http://blogs.discovermagazine.com/cosmi ... t-failure/

pictures:

http://stephatcern.blogspot.com/2008/12 ... amage.html


I believe the first link refers to one of the detecters having a design flaw that showed up in tests ~ 1 year earlier, it was not the acellerator ring itself.

I heard that the failure last fall was due to a short in some adjacent equipment (a transformer?) or a feed through, that then heated up the cryostat enough to quench the superconducter, and the magnet then blew up in a helium steam explosion. If quenches are expected to happen and design considerations are included, I'm not sure why the magnet ruptured it's casing (along with the tube of the acellerator). Perhaps the heating from the short(?) introduced alot more heat into the cryostat than anticipated. Bad luck and/or bad design and/or out of spec equipment.


Dan Tibbets
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Tom Ligon
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Postby Tom Ligon » Fri Jun 05, 2009 11:57 pm

Dr. Bussard once described this problem as a glossed-over problem with ITER.

The problem is the huge amount of energy stored by the SC magnets. I've seen all sorts of numbers, but Dr. Bussard had gotten an estimate somewhere that supposedly worked out to on the order of one kiloton of TNT equivalent joules for all the magnets taken together. I have not confirmed that high, and have seen numbers much lower. The Wikipedia article on the ITER says the stored energy is 41 gigajoules, but one kiloton would be 4.184 e 12 joules, or about 1/100 of Doc's estimate. Still, dam-buster bomb levels.

Anyway, the idea is the magnets can store the energy only if they remain superconducting. If they warm above the critical temperature, they acquire resistance, thus lose heat to I^2 R. This rapidly increases their temperature, thus their resistance, and so forth. If the stored energy is high enough, the result can easily be explosive.

With a working tokamak, even a few tons TNT explosive power would be incredibly dangerous right up against a liquid lithium blanket contaminated with tritium.

The fix, in principle, is to have safety dumps so that a magnet approaching critical could shunt its power to some external resistive load. But if the energy is really in the kiloton range, the load better be a small ocean. We're talking tactical nuclear energy release, and I'm not sure a mere lake will handle it.

SC Polywells would probably have the same problem at lesser levels. There would be less magnets and much smaller, and probably don't require 6T fields. Also no molten lithium contaminated with T, and there will be a vacuum gap between the magrid and the walls that would mitigate a blast somewhat.

The same heated shunt used to close the circuit on the SC magnet would probably be used to shunt to the dump load. Keep in mind it takes a deliberate act to heat the shunt.
Last edited by Tom Ligon on Sat Jun 06, 2009 12:09 am, edited 1 time in total.

rnebel
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Postby rnebel » Sat Jun 06, 2009 12:09 am

That's interesting, I see the WB-7 pic every day (it's my desktop) but the area it shows is too small to give the shape of the vessel. I always assumed it was spherical.

Are you able to share any details on WB-8 (e.g. size, magnet strength, changes to the interconnects, geometry changes, etc.)? From your earlier comments I'm guessing the emphasis will be on transport.[/quote]

It's not all set yet, but the magnetic field will be much stronger than the WB-7 and so will the dynamic range.

MSimon
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Postby MSimon » Sat Jun 06, 2009 1:04 am

The same heated shunt used to close the circuit on the SC magnet would probably be used to shunt to the dump load. Keep in mind it takes a deliberate act to heat the shunt.


The amount of power it takes to keep a SC charged is so small that it is not an operational consideration. I'd keep them powered up with a diode/ resistor to absorb quench energy. In addition the circulating current idea is only important where changes in the field on the order of less that one part in 1E6 is desired. My guess is that field variations on the order of .1% over hours will not matter much. That is not hard to do. With more effort .01% is probably possible with power supplies. The one difficulty this presents is an additional load on the cryocoolers. However, if they are adequate for charging they are adequate for continuous current input.

If you go the total superconducting ring route quench energy has no place to go except to further heating the magnets. BTW micro quenches are a feature AND a bug. They are expected and the hope is that there is enough coolant to keep them from spreading.

In any SC design back up power is critical. A LOP (Loss Of Power) accident could be very expensive. Batteries for short term (seconds) until you can get your MG set(s) on line. And you need enough MG power to run the cryocoolers as well. Plus all the monitor and control eqpt. Maybe as little as a 100 KW MG set. Possibly as much as a 1 MW job. Get one that is nuke plant rated.

My project for the next few weeks is learning all I can about SC magnets. Their design, construction, operation, costs. I'm making some progress.

I'm an old hand with Nuke Power and aircraft systems. This sort of design thinking is second nature. What happens if xxx fails - for every single component. No single point of failure. The really important stuff has triple or quad redundancy.

===

Energy storage in an inductor is 1/2 LI^2. If you know the inductance (it can be calculated from coil dimensions rather easily for a Helmholtz coil). And you know the current (amp turns and required B field give that) then you know the threat level close enough. Multiply by 2 for a big safety margin. Still dumping 1 MW in less than a second in a small package is no easy task.

And even better - a mfg will probably be able to give you a spec +/- 10%. Which is engineering heaven. Close enough for engineering work is my motto.

Once we get experience we can adjust the design margins for less waste.

You know -

Optimist - glass half full
Pessimist - glass half empty
Engineer - glass too large
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MSimon
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Postby MSimon » Sat Jun 06, 2009 1:12 am

Energy estimates -

I have seen numbers for MRI coils in the 1 to 10 Henry range. Charging currents on the order of 100 A.

Energy at maximum inductance = 5 X 10,000 joules per coil. For MRI magnets. For operational coils in a working reactor it could get up into the MJ range per coil.
Engineering is the art of making what you want from what you can get at a profit.

Torulf2
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Postby Torulf2 » Sat Jun 06, 2009 5:58 pm

The emergency energy storage for the energy in the supra conductor may be another supra conductor.

This risk of explosions may make fission-fusion hybrid with super conducting magnets unsuitable.

ravingdave
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Postby ravingdave » Sat Jun 06, 2009 7:10 pm

Torulf2 wrote:The emergency energy storage for the energy in the supra conductor may be another supra conductor.

This risk of explosions may make fission-fusion hybrid with super conducting magnets unsuitable.


It is my understanding that super conductors don't like rapidly changing fields. From what I have read, superconductors are charged up VERY slowly.

I haven't read anything which leads me to believe that energy can be dumped into a superconductor rapidly.

Does anyone else know if this is possible, and if so how ?


David


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