MTF Illustration

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

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MSimon
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Re: Rapid maintenance and sufficient reliability

Post by MSimon »

bwang wrote:Thanks M Simon for the reliability analysis. I knew that was key but did not take the time to quantify it.

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

MTBF is Mean Time Between Failures, which is described at the wikipedia link. The reliability of the pistons is a key aspect.

Although having a system for monitoring the pistons and system for wear and having an easy method to rapidly swap out pistons could maintain a sufficiently high operating reliability. Keeping 90% uptime throughout a year with an average of one piston or some other component replaced every day with a 15 minute hot swap would be doable. Quality and cost of components would need to be balanced against overall costs.

http://ntrs.nasa.gov/archive/nasa/casi. ... 015833.pdf

Nasa has stirling free piston cryocoolers with mean time between failure of over 500,000 hours. Aircraft engines wiith 40,000 hours of mean time between failure are common and microturbines have 14,000 hours of mean time time between failure. Twice weekly and even daily maintenance can be doable if the maintenance can be fast (using a robotic arm to pop a piston assembly out and replace it). Faulty units can be refurbished and put back into service if that keeps costs down without sacrificing overall reliability.
Note that the whole assembly has to have a 20 million hour MTBF. If there are 10 components that means 200 million hour MTBF each. If there are 100 components that means 2 billion hour MTBF each (assuming the MTBFs are all equal). Now this is strictly BOE but it gives you a rough idea.

Maintenance doesn't kill you. Unscheduled maintenance kills you.
Last edited by MSimon on Sun Dec 21, 2008 10:03 pm, edited 1 time in total.
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Skipjack
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Post by Skipjack »

There is a rocketry company called XCOR that has successfully built a ´(reusable) rocket engine using piston pumps instead of turbo pumps. Assuming that the requirements for rocket engines are not that low either, I think that it is possible to get a relyable system given enough time and engineering skills. Then of course I am still wondering why they need soo many pistons..

chrismb
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Post by chrismb »

Isn't it a curious thing?! The immediate reaction is that it is a money-sucking gimmick. But the actual report seems fairly level headed and is somewhere towards a diligent attempt at fusion.

Unfortunately that doesn't mean it is a viable fusion experiment. As such piddling little neutron rates it is difficult to really tell what is going on. And an absence of diagnostics serves to confobulate any possible conclusions.

They are even having to subtract off suspect cosmic radiation off the measurements!!!

Presumably the energy input for each 'shot' is a few 10's to 100's of kJ (just a guess) which would set it about 18 orders of magnitude below break even. This strikes me as a hurdle too big to successfully conquer.

I just don't think a [neutron emitting] fusion experiment can be considered as demonstrating any sort of fusion suitable for power applications until it can either get a good load of bubbles going in a bubble dosimeter or diagnostics that show the conditions are heading in the right direction.

The room for erroneous neutron signals is enormous. OK, they ran it with hydrogen and didn't get a couple of blips out of all the noise, but that's why electronic devices just aren't the thing to show conclusive evidence of neutrons. 7 clicks in 4 deuterium shots!! Come on.....! You're really scraping the bottom of a subjective statistical barrel with just a few clicks of radiation in an experiment.

At those neutron rates, whose to say that there isn't some sort of fusion going on in ion-solid with adsorbed/embedded deuterium, rather than 'as billed' high density plasma fusion?

Without enough neutrons to get bubbles popping up in a bubble chamber, and diagnostics showing plasma characteristics are heading in the right direction (and look reasonable to extrapolate to net-power levels) then, IMHO, it is a fusion curiosity and does not represent a 'serious' fusion experiment.

This has to apply to Polywell also, I'm afraid. Neutron bubbles or diagnostics as bare minimum, please.

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Post by MSimon »

Polywell has the advantage here in that fusors work and produce copious neutrons.

The question for Polywell is not neutrons. It is: "enough neutrons?"
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chrismb
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Post by chrismb »

MSimon wrote:Polywell has the advantage here in that fusors work and produce copious neutrons.
Sorry, did I miss a bit of connecting logic there?

A fusor works by fusing fast beam ions with lab-stationary background neutrals in a partially ionised [very low fraction] gas discharge device where most of the ionisation is through consequences to electron conduction through said gas discharge.

I was not lead to believe Polywell works like this. Presumably this type of argument could be used for ITER, which has the advantage that the Sun works?

choff
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probably dumb idea for general fusion concept

Post by choff »

I was thinking about all those pistons, why not scrap them altogether. Instead put shaped ribs inside the chamber, either attached to the walls or on a separate inner wheel assembly. Either spin the rib assembly or the whole chamber to get the molten metal up to speed, inject the fusion fuel, and then slam the brakes on hard. The molten metal would be forced through the shaped ribs in the desired compression wave towards the core. This way you have only one or two moving parts.
CHoff

bwang
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problem with spinning and then stopping

Post by bwang »

It seems to me the problem with the spinning and stopping approach of Choff is how do we reset for another shot in one second or half of a second.

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Post by blaisepascal »

chrismb wrote:A fusor works by fusing fast beam ions with lab-stationary background neutrals in a partially ionised [very low fraction] gas discharge device where most of the ionisation is through consequences to electron conduction through said gas discharge.
This was not my understanding of how fusors work. Where can I learn more about this mode of operation of fusors?

chrismb
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Post by chrismb »

blaisepascal wrote: This was not my understanding of how fusors work. Where can I learn more about this mode of operation of fusors?
The evidence and discussions are scattered around the fusor.net forum. Basically, if you plot out the reactivity curve for DD on a linear scale, you'll see it pretty much asymptotes to zero below 10keV, collision energy. If you then plot out actual neutron emissions from a fusor, that same shape of curve repeats but comes off zero at 20kV drive voltage, which shows it's fast-neutral. If it were working fast-fast, then it would come off the line at 2.5kV drive voltage. The experimental results are plotted out for various experiments, and they come up with pretty much the same numbers.

You might argue that there are some fast-fast reactions in the data uncertainty of the results, but if you think about the transit times for deuterons and electrons, bearing in mind that this is a gas discharge device, i.e. like a neon light, the electrons just peel off the cathode central grid and head straight for the anode (unlike the ions that reciprocate across the centre). This takes 1/60th of the time it would take a deuteron, but at the same time there must always be as many electrons as there are ions (there is no polarisation in the device), so the consumed current of the device, which is typically a couple of 10's of mA, will be at least 120 times the deuteron current travelling into the centre of the device. Do the back-of-envelope sums on ion current beams of 100uA or so and you get a tiny reaction rate, units to 10's per second. Do the same on a beam passing through a neutral background and it pretty much adds up to the 100,000's of neutron's/sec, and this is what is actually seen.

This is a broad-brush treatment and interpretation of the data, but I feel these issues are generally agreed upon, notwithstanding a few remaining debatable caveats.

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Post by MSimon »

chrismb wrote:
blaisepascal wrote: This was not my understanding of how fusors work. Where can I learn more about this mode of operation of fusors?
The evidence and discussions are scattered around the fusor.net forum. Basically, if you plot out the reactivity curve for DD on a linear scale, you'll see it pretty much asymptotes to zero below 10keV, collision energy. If you then plot out actual neutron emissions from a fusor, that same shape of curve repeats but comes off zero at 20kV drive voltage, which shows it's fast-neutral. If it were working fast-fast, then it would come off the line at 2.5kV drive voltage. The experimental results are plotted out for various experiments, and they come up with pretty much the same numbers.

You might argue that there are some fast-fast reactions in the data uncertainty of the results, but if you think about the transit times for deuterons and electrons, bearing in mind that this is a gas discharge device, i.e. like a neon light, the electrons just peel off the cathode central grid and head straight for the anode (unlike the ions that reciprocate across the centre). This takes 1/60th of the time it would take a deuteron, but at the same time there must always be as many electrons as there are ions (there is no polarisation in the device), so the consumed current of the device, which is typically a couple of 10's of mA, will be at least 120 times the deuteron current travelling into the centre of the device. Do the back-of-envelope sums on ion current beams of 100uA or so and you get a tiny reaction rate, units to 10's per second. Do the same on a beam passing through a neutral background and it pretty much adds up to the 100,000's of neutron's/sec, and this is what is actually seen.

This is a broad-brush treatment and interpretation of the data, but I feel these issues are generally agreed upon, notwithstanding a few remaining debatable caveats.
Image

Tom Ligon says WB-6 got neutrons at 5 KV drive.

One thing to keep in mind is that the WB-6 operates at a lower pressure than your typical fusor.

Still as you point out - evidence for how the device actually works is sparse.
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choff
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Post by choff »

bwang wrote:

It seems to me the problem with the spinning and stopping approach of Choff is how do we reset for another shot in one second or half of a second.

My explanation was sloppy. How about, a turbine drives the liquid metal towards the core, where its blocked by a second wall with a metal sleeve inside it, the sleeve acting as a valve. When the fuel in the core is ready the sleeve is shifted to open inlets to the core, the liquid metal comes charging in and crushes the fuel. Think clothes washing machine fusion instead of steampunk fusion. Three or four moving parts insteady of one or two. Still a dumb idead.
CHoff

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Post by MSimon »

The spinning is done by pumping. Vortex generation. Or water going down a drain. No rotation of the machine required.
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choff
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Post by choff »

In the General Fusion approach the spin is only required to create a space in the core to inject the fuel into. In the washing machine approach the spin would be used to provide pressure for the shockwave.
CHoff

TallDave
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Post by TallDave »

Hey, implosion works great in the only practical use for net-power fusion so far devised...

I hope they're making videos. It will be fun to watch, if nothing else.

Axil
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Post by Axil »

High all

This is my first post here and I hope that you will permit me to join this discussion of the General Fusion inc. approach. I have found a few problems in the design but they can be fixed. The first design problem is described below as follows:

Reference:

http://teachers.web.cern.ch/teachers/ar ... amics.html

<snip>

1. Lead is "transparent" to neutrons.
2. Lead is highly diffusive.

Transparency is the property of the medium in which neutrons undergo mostly elastic scattering. (Little elastic hits with a very small energy loss). Both properties means that lead provides a very small probability of absorbing neutrons while a very isotropic probability of elastic scattering.

The succession of many closely elastic scattering events gives to the neutrons a random propagation in the medium. Each neutron looses very little energy in each collision. They traverse the whole energy spectrum (1 MeV – 0.025 eV) in multiple steps: before the neutron loose most of its energy, about 1800 collisions and 60 m of random path will happen. But the total migration length is only = 1.2 m from the source. That means neutrons remain confined in a lead volume of > 4 m side. (This is a very important point because determine the size of the device).


The General Fusion approach uses lead to form the compression wave that produces D-D or D-T fusion. The neutron energy that is produced exceeds 14MeV. These high energy neutrons will penetrate many tens of meters of lead (14 * 4 = 56 meters). General Fusion does not provide a blanket to absorb those neutrons. This lack of a blanket will make their fusion reactor very dangerous. The steel of the reactor will radiate heavy gamma rays due to neutron activation and the acoustic generators will be unapproachable for maintenance.

This is a major design oversight. General Fusion will be required to build their reactor conformant with the need to add a blanket to slow and absorb fast neutrons.

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