Alpha collector geometry idea...

93143 · Post by **93143** » Wed Jul 16, 2008 5:23 pm

I forgot - we also need a trap grid, slightly negative with respect to the magrid, between the magrid and the collectors.

Another collector might also be useful, to catch any ions lost from the core (if such losses are significant). It would be outside the trap grid, but well inside the high-energy collection system, and it would be at the same potential as the magrid. Alphas would just fly past it, for the most part. And since it's so close to the coils, it would be protected better from the effects of a large fusion core (remember, even if the core filled the magrid completely, a small region right next to the far side of the coil casings would still be in shadow). If the power balance of including an ion collector is not favourable, don't include it...

Aero wrote:The minor torus diameter of the magnetic coils being the same in the two different Polywell configurations results from the assumption that the dodec coil major torus diameter was half the truncube coil major torus diameter. That results in the circumference being half also, but there are twice the number of coils so that the sum of all circumferences, in meters, are equal for the two polywell configurations. Therefore, to block an equal percentage of the Polywell’s surface, the minor torus diameters must be equal.

I think you've oversimplified your geometric assumptions. Suppose you were going from 12 to 96 coils instead of from 6 to 12. Is the occluded area from a 10 cm minor diameter still the same? Is a 10 cm minor diameter still smaller than the major diameter?

Note2: Regarding the question of shadows: The fact of small width of the coil for high transparency of the Magrid requires that the fusion core also be small in order that the Magrid cast a shadow. If the diameter of the fusion core is greater than the width of the magnets, the magrid will not cast a shadow.

The penumbra may still be wide and deep enough to use, provided the core isn't too much wider than the coils. Even if the shadow depth were nearly zero, though, I still think this type of collector geometry would be more efficient than not even trying to separate out the energies, because a significant fraction of the high-band alphas should still be collected at the correct energy.

If someone has a better idea at any point, I'll be all for it...

tombo · Post by **tombo** » Wed Jul 16, 2008 11:37 pm

If the core is large we still can't use the part of the shadow within a few gyro-radii of the coil because we need to allow space for electron recirculation.
Is it absolutely necessary to be in the shadow? Or, is it just nice?
If we are limited by the stack-up of 2-3 coolants, insulation, Dewars, etc. to Msimon's 8" thick coils, then there is a hard limit to how many faces our polyhedron can have.
Do we have any numbers about how quickly the alphas can be bent away from the shadow?
It should be relatively simple ballistics once we know the fields.

93143 · Post by **93143** » Thu Jul 17, 2008 1:16 am

tombo wrote:If the core is large we still can't use the part of the shadow within a few gyro-radii of the coil because we need to allow space for electron recirculation.

I know - I'm just saying that the penumbra is deeper near the magrid, so an ion collector might not have to deal with as much alpha flux if it were relatively close in. Other factors will probably come into play as well...

Is it absolutely necessary to be in the shadow? Or, is it just nice?

It's just nice, because if you have shadow space you can have low-band collector area which is not subject to any high-band impingement. If the shadow is wide enough, it should be possible to separate the two bands completely - but then you have to deal with more magrid coil impingement, and shielding from that has a currently unknown (if easily computed) effect on the shadow width.

Do we have any numbers about how quickly the alphas can be bent away from the shadow?
It should be relatively simple ballistics once we know the fields.

We can't grow the shadow. It would involve pulling the alphas away from the collectors, resulting in a lack of collection.

If you really wanted to, you could use an attractor spike instead of a repeller spike, but the efficiency would be awful because it would have trouble separating the bands (if you use solid insulation between potentials the alphas will hit the insulation and start building up charge on it, screwing up the potential map), and it would be adding lateral kinetic energy to the alphas. It would probably be worse than just using the outer wall at the low-band energy.

The only reason I can think of to do this is to keep alphas away from the hardpoints, and if they are exposed dielectric surfaces, you could probably count on them accumulating a positive charge, solving the problem for you.

...This could have an effect on the efficiency of the structure I've suggested - the collectors aren't just going to float in space, and they can't be connected by metal...

Aero · Post by **Aero** » Thu Jul 17, 2008 3:55 am

93143 wrote:
Aero wrote:The minor torus diameter of the magnetic coils being the same in the two different Polywell configurations results from the assumption that the dodec coil major torus diameter was half the truncube coil major torus diameter. That results in the circumference being half also, but there are twice the number of coils so that the sum of all circumferences, in meters, are equal for the two polywell configurations. Therefore, to block an equal percentage of the Polywell’s surface, the minor torus diameters must be equal.
I think you've oversimplified your geometric assumptions. Suppose you were going from 12 to 96 coils instead of from 6 to 12. Is the occluded area from a 10 cm minor diameter still the same? Is a 10 cm minor diameter still smaller than the major diameter?

As I explained in my original post, I used pictures (or drawings) as the foundation for these assumptions. I made the assumptions because that's what it looks like to me. If anyone can develop a better estimate, go for it, do the math and tell us the answer. As for a 96 coil device, has such a device been contemplated? If so, I have not seen a picture or drawing of it. I do think the 10 cm estimate is in the ball park for the truncube and dodec and provides useful ideas about the shape and extent of the shadow that do exist. That was the point of the post.

tombo · Post by **tombo** » Thu Jul 17, 2008 5:08 am

Aero,
That result is somewhat counterintuitive but if that is what the drawings and numbers show then it must be right.
I agree with 93143 that the pattern can't extend too far, as eventually the hole in the donut would shrink to zero.
The last picture on this page viewtopic.php?t=289&postdays=0&postorde ... g&start=60
shows one with 128 sides (actually only 1/4 of it) and describes a method for making an arbitrarily large number of coils. (in steps of 4x)
And yes it is a nonstandard Polywell but is has some things going for it.
Now there is a little progress on the simulation of the smallest of the series.
But, you are right, the normal designs stick with much lower numbers of coils for many reasons.
I think most people aren't thinking beyond icosahedrons.
But if some overwhelming reason pushes us to a large number of faces we have ways to do so.
I also am waiting on an estimate of the alpha source diameter.
My ideas here have not yet gelled. There are still too many worms crawling out of the can.

93143 · Post by **93143** » Thu Jul 17, 2008 6:52 am

I wasn't seriously suggesting a 96-coil magrid. I was just illustrating the fact that if you maintain the coil minor diameter while increasing the number of coils, eventually you will cover the whole polywell with metal. Therefore the result you stated is almost certainly not, strictly speaking, correct.

Using eyeball-based assumptions that give you roughly equal minor diameters between 6 and 12 coils is fine for a zeroth-order approximation, but you seemed to be trying to infer more from it than the precision of the assumptions would bear.

I'm far too tired to do the math right now. Perhaps I should have waited until tomorrow (later today?) to post...

Aero · Post by **Aero** » Thu Jul 17, 2008 6:19 pm

93143,
I'm pretty straight forward, I try to say what I mean. Please don't read inferences into my work unless I state them explicitly. If you think that my published result implies a further consequence, feel free to publish the consequence, but take the credit for yourself. Please don't try to put words into my mouth. Am I a little touchy? Well, maybe but I found out long ago that mind reading is like anti-gravity, it violates some of the rules of this universe.
Oh, I will speculate that somewhere there exists a formula for the major radius of the magnet coil for the higher order Polywells. Further, I will speculate that this formula can be directly related to the radius of the Polywell. If so, then the 96 coil question could be answered.

Aero · Post by **Aero** » Wed Jul 23, 2008 2:34 am

drmike wrote:
Aero wrote:Hi drmike,
I have been posting to a design thread (direct conversion) where we really need a value for the radius of an area within the Polywell that I have dubbed the "fusion core." I define the fusion core as that area where conditions exist for p-B11 fusion and from where alpha particles may be expected to originate. Is that the same as what you are calling r_c? Am I correct when I interpret your estimate of the value of the radius of the fusion core to be about 0.3 meters for a 100 Megawatt BFR? This would give the volume of the fusion core of 0.11 cubic meters.
Yes, it's a BOE guess which is order of magnitude close enough.

This looks like one of the answers that we need. Given the width of the magnet is 10 cm, the width of the fusion core is 60 cm and the two are centered at 150 cm apart, then the size and shape of the shadow can be calculated and it might be usefully large.

93143 · Post by **93143** » Thu Jul 24, 2008 12:01 am

IMCAC the umbra proper (assuming zero effect of the B field) starts at 10 cm wide and goes to zero at 25 cm. The penumbra also starts at 10 cm wide, expanding at about 47 cm/m (13.9 degrees from centerline in both directions). Infinitely far from the magrid, the shadow centerline sees a maximum of about 79% of full irradiation.

I just realized that my idea for pulsed mag-shielding of the coils in tune with POPS (isn't POPS MHz-range?) results in (a) a hellacious radio source, and (b) a frightening metal-fatigue environment - high-strength steel, and lots of it, is probably a requirement, unless nanotube materials are well enough understood by then... also, can you say "eddy currents"?

ravingdave · Post by **ravingdave** » Fri Jul 25, 2008 3:14 am

93143 wrote:IMCAC the umbra proper (assuming zero effect of the B field) starts at 10 cm wide and goes to zero at 25 cm. The penumbra also starts at 10 cm wide, expanding at about 47 cm/m (13.9 degrees from centerline in both directions). Infinitely far from the magrid, the shadow centerline sees a maximum of about 79% of full irradiation.

I just realized that my idea for pulsed mag-shielding of the coils in tune with POPS (isn't POPS MHz-range?) results in (a) a hellacious radio source, and (b) a frightening metal-fatigue environment - high-strength steel, and lots of it, is probably a requirement, unless nanotube materials are well enough understood by then... also, can you say "eddy currents"?

Not to mention that superconductors of large inductance (which is what the magnets are going to be in a working design) cannot be magnetically pulsed at a high frequency. Inductive reactance will limit the maximum possible flux change rate, and I just can't see it being very high.

Besides that, the method that I've heard of for charging up a superconductor simply cannot be done rapidly.

David

MSimon · Post by **MSimon** » Fri Jul 25, 2008 4:38 am

Two points:

1. I would go with the Bussard calculation of a 20% coil intercept at 100 MW size (1.5 to 2 m radius) and work back from there to outside coil diameter. Dr. B was a pretty good engineer. Every thing he has done re: the 100 MW machine looks calculated. And possible.

2. I wonder if the collector supports could be made conductive so that they become invisible (this was suggested a while back some where around here).

Note: my 8 inch estimate was based on not just heat transfer, but also slowing down neutrons in a D-D machine. If neutrons are not a problem making the coils thinner should be possible.

TallDave · Post by **TallDave** » Sun Jul 27, 2008 7:30 pm

2. I wonder if the collector supports could be made conductive so that they become invisible (this was suggested a while back some where around here).

Hmm? How does that make them invisible?

Lately I've started to worry the magrid supports will burn through due to the electron loss, weakening them till the machine flies apart due to the force of the magnetic fields. I wonder if they could be given their own weak magnetic shielding? Seems that might break the wiffleball though.

EDIT: Oops, didn't see "collector" the first time through.

93143 · Post by **93143** » Sun Jul 27, 2008 7:54 pm

The collector supports can't be made conducting because you need half a megavolt between the low-band and high-band collectors, and half a megavolt across a conductor constitutes a short circuit. You could make intermediate parts of them conducting, but they wouldn't be invisible - they'd be at some uniform electric potential, which could be useful or not depending on the value of said potential.

The magrid supports can't be made conducting for electron loss reasons, or so I gather. They're insulators so that as electrons hit them they build up a negative charge, keeping other electrons away.

ravingdave · Post by **ravingdave** » Sun Jul 27, 2008 11:06 pm

My view of the simplest design (one in which the various alphas are not seperated by energy ) is using the wall of the reactor as a collector.

Unless someone can come up with a design to efficiently seperate the various alphas by energy, we are still stuck with a collector that catches them at the lowest energy level, and higher energy alphas are simply converted to heat.

David

ravingdave · Post by **ravingdave** » Sun Jul 27, 2008 11:10 pm

I've been wondering if something like a mass spectrometer design could be used to seperate the alphas by energy into three distinct bands aimed at three distinct collectors. So far all my ideas along this line require impractically large vacum vessels, but still, the mass spectrometer seperates particles by mass, and surely a similar idea could seperate same mass particles by velocity ?

David