Science/Engineering/Technical forums

My last paid gig was as Varian's MRI surface coil department -- yes, department. I designed them, shopped for the parts, built them, serviced them, their packaging, and imaging phantoms. I didn't even touch a single active component in the probes themselves, but did design some test equipment like a gradient coil monitoring system.

Why am I here? I have know Doug personally since about 1996, and I am smart enough to realize that he is The Man when it comes to technology, if not personal life-extension techniques. The Fukushima meltdown got me EVEN MORE interested in radiation monitoring: there is currently a huge demand for rad monitors, and this market will not decline in my lifetime, if EVER. I am interested in the Altoid project, and I even have a small tin full of uranium-glazed pot shards to use as a test source!
My real interest here is as a grunt -- I can spend hours crunching a circuit board design into a smaller footprint. My latest design is a colloidal silver generator: USB-powered, LED-indicated, polarity-switched constant current with 100V max drive from an H-bridge with auto shutoff and pulsed stirring motor drive in a device about the size of a matchbox. I should have used an embedded uP, but it is mixed analog/digital.

Good to have you on board from Fla, Alex, though we miss you up here. I think you'll find plenty of other smart guys here (it's kinda the reason for the board to exist, so we can find each other and have good people to work with and share ideas). Welcome!

Edit:
Alex also plays that roulette we call the stock market (or is that poker?). If you want to start an "Alex's corner" thread down in trading markets I'd love to have it. We've had a couple properly cynical traders join recently (From ZeroHedge if that tells you anything), but they've not posted yet -- I'm hoping to get that area to be less "all me" if I can.

I was just wondering what the energy density of fusion in the Sun is, in terms of how that might scale to 'hot' fusion (totally random with an indefinite number of degrees of freedom) tokamak-style power generation here. From quanta I could easily google, the calculation shouldn't require more than a 4-banger calculator. I didn't think the energy density would be very high, because the sun won't run out of power for a very long time.

Of course, the calculation had already been done for me, but I was not quite prepared for how really low it is! Wikipedia, in its entry for 'Solar Core', states:

"The energy production per unit time (power) produced by fusion in the core varies with distance from the solar center. At the center of the sun, fusion power is estimated by model to be about 276.5 watts/m3, a power production density which more nearly approximates reptile metabolic heat generation than it does a thermonuclear bomb. Peak power production in the Sun's center, per volume, has been compared to the volumetric heat generated in an active compost heap. The tremendous power output of the Sun is not due to its high power per volume, but instead due to its gigantic size."

I also had thought there might be a LAYER in the sun where fusion reactions are favored. Nope. According to Wikipedia, they are most common at the very center, and get less with distance from it. How would they know this? I'll take their word for it:

"At 24% of the radius (the outer "core" by some definitions), 99% of the Sun's power has been produced. By the time 30% of the radius is reached, fusion has stopped almost entirely. "

So much for 'hot' fusion on this planet. What a scam.

To cut the sun a little bit of slack, the fuel in there isn't really all that great for fusion. If the sun was all D, or DT - it'd be another story (and we wouldn't be here). P-P fusion is very very rare (super low cross section), so other also-rare reactions are most of what goes on. As the Z of the reactants goes up, they become a lot harder to push together so they quantum-tunnel into fusion. Whilst the sun isn't big enough to support much of Bethe's CNO cycle, it's more like that kind of thing than just pushing the H's together. http://en.wikipedia.org/wiki/Stellar_nucleosynthesis
If you look up the reaction cross sections for the possibilities, it explains a lot.

Having said all that - yeah, hot fusion sucks - too many degrees of freedom to spread the input energy around, and no energy feedback without all those miles thick of super dense gas to absorb the energy of the reactions and keep the stuff hot.

Having said that - fusor fusion sucks too, and I think I'm onto at least one of the main reasons. In the absence of perturbations (collisions with background gas for example), the D's come into collisions with precisely the wrong orientation to satisfy some conservation laws that go all the way back to Pauli - so when I prove this (again...) I won't get much intellectual credit (that really goes to Pauli and friends), but ignoring those laws cuts the cross sections down by huge factors - 100's to 1000's. Doh! Obvious in hindsight...but one does have to wonder why decades of work by billions of bucks worth of PhD's never paid one bit of attention to this...spin conservation is pretty well established in reactions...and a few others like wavefunction phase. It's not easy but far from impossible to control most of this "going in" so that when you do get the fuel nuclei close, they "want" to fuse. In fact, some not-too difficult math says that alone can take us from microwatts out per watt in to actual gain! Only some minor other enhancements are needed too.

It works out that just about anything that perturbs a fusor at any of a large bandwidth of possibilities increases the reaction rate (and therefore Q) by big fat factors, as I'm about to report "officially". It's why I'm building linear accelerators right now as well - a more-complex apparatus is required to make a simpler reaction situation!

You might like to see a post I made on fusor.net, in which I mock the oft vaunted comment 'making a little Sun on Earth'.

http://www.fusor.net/board/view.php?bn= ... 1235937096

Funny one, Chris. Our "star in a jar" look more like a star, with those beams - the artist concept anyway, than the real deal, eh?