Heat Shield

1 12 2009

The red hot fusor grid reminds me  – I must address thermal issues from plasma, xrays and neutrons for polywell fusion without boiling the superconductor’s liquid nitrogen.

I asked for help with thermal modeling on the polywell talk forum. Good feedback.

Here is a rough draft of the superconducting magrid with a vacuum separated heat shield:

The trick is, the shield must have a gap so that you can weld the lid to the chassis. The welded magrid would have a gap in the shield along the midplane of the torus.

This gap would bring the vacuum between the heat shield and the inner superconductor holder. Well actually it would be  ~10 mToor of ionized deuterium.

This design does not include liquid water cooling. Although it’s easy to add cooling channels with the Arcam process, the real challenge is connecting fluid channels when you weld the lids onto the chassis.

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14 responses

1 12 2009
Wouter

I would advice you to not worry too much about superconducting coils until you’ve got a non-SC polywell working. You have fairly limited resources (time & money), so I would on the next step, not on a few steps ahead. I think it will be very difficult to design an insulating/cooling system of which you can be SURE it will not lead to a quench. The heat load will not be that large, but I think it will be quite tricky to make something where in no place heat leaks in. It would be a damn shame if you spend a lot of effort into this and to have it quench on the first try. And you could then carry over the lessons you learned from running the non-SC polywell over to a SC design. Keep your eye on the ball and don’t make it more complicated than it needs to be. It’s complicated enough as it is!

Don’t take me wrong, I’m not trying to put you down, underestimate your abilities or take away from your effort. I’m really trying to help you. I think it’s great what you’re trying to do here! It takes a lot of guts to quit your job and work at this. I’m just saying, there is NOTHING wrong with building a (short-)pulsed device. Hell, I might even quit my job and work on this if you/somebody demonstrates significant repeatable fusion rates in a pulsed machine! As you are finding out, you will run into a million little obstacles trying to get ANY polywell working, without the added complication of superconductors. Once you have all those bugs worked out, then start with the next step.

Good luck, cheers,
Wouter

1 12 2009
FAMULUS

Wouter, you make a really good point. I’ve though about this a lot. The strategy decisions are so important.

I’ve decided to pursue the superconducting goal for these reasons:

First an foremost because it has never been done before. This is so important. This is the scale we must aspire to. At the absolute edge of what is known. You must aim for the stars, so after all the fuck-ups and mistakes, you at least escape the earth. It must be unreasonably ambitious. It’s the nature of the project.

There are practical advantages to a superconducting magrid. You don’t have to worry about ohmic heating of the coils, you are much closer to a continuous machine. A continuous machine would open a new chapter in the research of the polywell. The path is perilous, but the possibilities justify the risk.

Copper coil operation requires high speed measurement, precise gas puffing and coil heat dissipation, which are huge challenges.

A superconducting magrid gives you a vast time horizon to work with… even if you have to pulse the main power supplies.

Having played around with the YBCO, I feel it’s doable. There are huge challenges for sure, but I feel it can be accomplished. I _want_ to build this machine.

Attempting a superconducting magrid increases the potential for failure. I feel the answer is not to reduce the scale of the ambition, but rather to increase the checkpoints along the way. Failure is the norm. Fail small and often. Comprehend, correct, proceed.

3 12 2009
Wouter

I’m not saying “don’t pursue a SC polywell”, I’m saying don’t focus on it NOW/YET. I fully agree with your last paragraph, but I see a pulsed device as a “checkpoint” for many of your other systems and operations.

But it is of course 100% your own call, so I wish you good luck on your chosen path. I’ll be reading to see if you’ll make it!

BTW, it’s probably none of my business, so feel free to not answer the following question, but how do you finance this project? Savings? As far as I can tell, you have no significant source of income. Without going into details about your finances, could you roughly indicate how long the current rate of development is sustainable for you? Are you going to look for outside investments/grants?

Cheers,
Wouter

3 12 2009
FAMULUS

I saved up money from ruby on rails consulting. I have a few months of overhead runway left. But I will need some fresh money soon. I am building a fundraiser at http://www.kickstarter.com/

We are working on the video. Trying to launch this ASAP. Incidentally, a smaller fusor project won funding this way:

1 12 2009
ike

liquid nitrogen is cheap. why dont you just circulate the nitrogen? (if this amount of cooling is necessary at all)

1 12 2009
FAMULUS

From what I understand… liquid nitrogen, although very cold, it not a good medium for gross heat removal. I’m imagining using something like Fluorinert:

http://en.wikipedia.org/wiki/Fluorinert

2 12 2009
ike

but that means you’ll need two cooling circuits, complicating the whole a lot more than just pumping the nitrogen faster. any shortcomings like low heat capacity can be compensated by pumping stronger. plus, i could imagine that the hull of the grid should hug the coils as closely as possible to limit surface area electrons can impinge on.

2 12 2009
ike

not to mention that the heat load most probably won’t be that big if you include that fancy heat shield.

3 12 2009
M. Simon

Fluorinert is ridiculously expensive. And you still have to cool it. And pump it. Pumping LN2 is not a bad way to get rid of modest amounts of heat. And its specific heat is not too bad.

If you pull a vacuum on LN2 it will cool. You can get it down to 63K. Which means a reasonable temp is 65K. That gives you a 10C delta T. From the specific heat vs power to absorb you can figure the flow rate. Also look at the equilibrium temperature vs pressure.

If you are going to a channel type system (vs pipes in pipes) pumping LN2 is your only option. LN2 will freeze the water. And parts of the shell will be too hot (far above 77K)

3 12 2009
M. Simon

And Fluorinert minimum temperature is 120K – far too warm.

3 12 2009
M. Simon

I have worked out LN2 cooling for copper coils for a machine similar to WB-6. 10KW per coil IIRC. It was doable. I can send you the spread sheet if you like. e-mail me. The heat capacity of LN2 is built in.

Maximum pressure was in the 100 to 150 psi range. You wouldn’t need as much pressure if you kept the LN2 below 77K. Ten or twenty psi might be enough to prevent boiling at hot spots.

1 12 2009
ike

btw: the nubs! the nuuuuubs!

1 12 2009
FAMULUS

what about them?

2 12 2009
ike

apparently they tend to get struck by lots of electrons when they’re that deep inside,seems like you have to move them out of the way

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