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A Thought Experiment on Ideal Conditions

Posted: Mon Dec 03, 2018 12:33 am
by S Silvawalker
I post this to approach the commonly asked question of, "why can a fusor not break even?" in a hopefully more professional and direct thought experiment. I do not claim to have any theoretical system to attain the conditions described, all I wish to do is get a more intuitive understanding of the physics behind this concentric accelerator system when nuclear fusion occurs. There are several questions which I think it would benefit most beginners in this forum to see answers to. Firstly, the effect of the inner accelerating cathode on falling ions is to attract them towards the central negative potential well of the fusor, if there were to be a grid which was one hundred percent transparent to the falling ions and produced alpha particles, what impact would that have on the efficiency of a fusor? How would an increase to the current and thus the number of effectively accelerated ions, along with an increase in potential well depth differ from the impact they would have in a real fusor. Secondly, is there any equation that would describe the number of fusions from the velocity, density, and losses, along with factoring in, somehow, the interactions of outgoing products of fusion to heat the plasma? It is confusing to myself at least, whether or not the Lawson Criterion can be used in the case of a fusor with or without a perfectly transparent cathode, and if it can be used I would greatly appreciate an example for a fusor. Finally, what is the greatest ion loss in a fusor? Is it the falling ions colliding with the cathode or thermionically emitted electrons, bremsstrahlung radiation, metal ion contaminants from the heated cathode, or some other mechanism which I have missed? Also, on a shorter note would one say in a that a fusor would have an infinite confinement time or a non-existent one since the ion collisions can be negated in this line of thinking and the ions exist in an infinitely bound velocity space?
Thank you for your time.

P.S if anyone knows the vapor pressure of solder paste please tell me, I think I have a useless grid now since I can't clean all of it off...

Re: A Thought Experiment on Ideal Conditions

Posted: Mon Dec 10, 2018 1:02 pm
by Dennis P Brown
You have a few mis-understandings and need to learn a bit more on both fusors here (see FAQ's) and read up on the physics of fusion in our Sun.

A fusor produces essentially zero (as far as instrumentation for power goes) energy output from fusion. All the heat is created by the power supply so fusion products are not an issue at all. The reason is obvious and if you read the FAQ's you will learn why. Ditto for most the issues you ask about. For example - issues like minor contaminates isn't generally a concern for fairly average vacuum work because the plasma will burn away many of those problems. Speaking of which, that should fix your "flux" issue unless you used massive amounts. Then the issue of a physical cathode means little for the average fusor - as for alpha particles, they have no relevancy at all for a simple fusor.

Buzz words are only useful if you understand them and relate the processes to a fusor; again, there are FAQ's and many, many posts on all these subjects by people here who are really knowledgeable so take the time and search these topics yourself. You will discover a wealth of useful information. This is far better than spoon feeding a few random answers (like I just did) that don't provide enough context unless one wants to write many papers of information.

Re: A Thought Experiment on Ideal Conditions

Posted: Tue Dec 11, 2018 12:14 am
by S Silvawalker
I regret starting with that quote, it was meant to act as a hook although I think now it may have simply acted as a distraction from what I meant to say. In broader terms, the obvious reason as to why a fusor can not come anywhere near to ignition is because of heat losses caused by deuteron collisions with the inner grid and any product heat is lost to the walls of the chamber or simply leaves the system, that is established. The question was how an impossible device, similar to the ideal operating form of the polywell designed by Robert Bussard, in which the inner cathode was simply an area of negative charges would act. This question is not for the technical formation of a device but rather for a more useful set of operating laws for an inertial electrostatic fusion device.

The theory behind Dr. Robert Bussard's work, as is well known by many of those on this site some of whom had the fortune of meeting and working with him, was to use a series of magnetic mirrors to act as a confinement system for an electron cloud, and while the electrons remained in the center they would act as the cathode of the IEC system. Further more the theory perdicted and later demonstrated a "wiffle ball" effect where the electrons would be mostly excluded from the center of the cathode area by their own velocity and simply act as a negative sphere of charge allowing for the ions to only interact with each other for the most part. The logic behind this theory was that since the electron cloud was even more transperent to falling deuterons, the losses by ion impact to the inner grid would of course be zero allowing for the plasma to only radiate its heat away through particle collisions with the outer wall. The problem with polywells is that they rely on a perfect magnetic bottle and unfortunately lack the recirculation of plasmas needed to make up for their low confinement time as a magnetic system. They also lose energy to the redirection of the elctrons by the falling ions.

The summary of polywells above is to help show the similarities between those devices and this impossibe device with a perfectly transperent grid. In such a decice no deuterons could be lost to the outer wall unless by tunneling since they can never (ignoring centrifugal force) fly further out than the potencial with which they started unless they gain energy seperate from the potencial well. This would mean almost perfect recirculation and vastly reduced losses.
This can be seen if one takes into consideration the thermal capacity of a tungsten grid as is often used in neutron generating demo fusors as 24.8 J/mol K. So of course that is the major point of loss for those fusors as even the best fusors which I have seen have only reached 500,000 neutrons per second which would only generate about 2.6x10^-7 joules of total energy. But that is te point of the thought experiment. Based on some rough calculations for the number of particles involved in that system at 9.6 microns, the total number of paricles in the chamber was around 3.0x10^9, and that means a loss percentage of 99.98%. By what I have read those are grid losses and a result of the low probability of the tunneling which this type of fusion depends upon.

So the question I was asking was, based on that, what is the percentage loss to the grid and if one made a system with a virtual grid, then what would be the equation to define its output energy, both useable and non useable and could the Lawson criterion be used. This question seems important as while a fusor may never be able to break even, many seem to think that a polywell stands a chance and the mechanism for that to be possible either depends on the confinement time of that system or its lack of an inner grid.

Re: A Thought Experiment on Ideal Conditions

Posted: Tue Dec 11, 2018 5:00 am
by Richard Hull
Good fusors regularly hit the mega n/s mark. ( 2 million fusions/s +)
Grid losses in any fusor is pitiably small. There are many other losses of greater import keeping the fusor from improving. These have been discussed at some length in past posts. The grid is of no real concern, loss-wise. Due to the many losses in a simple fusor, as built here, it is an energy black hole. Still, it does fusion very inexpensively, if that alone is your goal. Improving the fusor by an order of magnitude, on the very cheap is just not possible.

Richard Hull

Re: A Thought Experiment on Ideal Conditions

Posted: Wed Dec 12, 2018 1:09 pm
by Dennis P Brown
As a FYI: The Polywell concept just isn't the answer at all for the problem with a fusor. As an aside: a very large polywell device was built not long ago, funded by the Navy, had some nice results as a virtual cathode and was abandoned because it will not ever be a usable methodology to do fusion for even minor power output; forget break even. That is many, many orders of magnitude higher than what a polywell will likely achieve. And that ignores issues like Bremsstrahlung loses. You are focusing on a topic many have looked at and frankly, has gone no where.

Your number of particles in a chamber at 9.6 x10^ -3 torr is very, very far off. There are, roughly 10^21 molecules STP per liter (about the volume for a small chamber.) At 10^ -2 torr (10 microns) or 10^-5 atm (rounding 760 to 1000 to keep things simple) that would be 10^16 particles in the chamber - appropriately. So you might want to recalculate using that value.

Again, a physical cathode has no impact at all on fusors and for reasons that Richard pointed out.

As Richard says, if you have an idea - go and build it and then come back and we will discus your results.

Re: A Thought Experiment on Ideal Conditions

Posted: Fri Dec 14, 2018 4:43 pm
by S Silvawalker
Thank you for the feedback and I will recalculate for the new values and construct a better model for this situation.