definition of electrons

Discuss how polywell fusion works; share theoretical questions and answers.

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Peter
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definition of electrons

Post by Peter »

The Polywell Wiki entry says:
http://en.wikipedia.org/wiki/Polywell
Electrons are introduced outside the "quasi-spherical" MaGrid and are accelerated into the MaGrid due to the electric field.


What exactly is meant by "electron" Are they freestanding, meaning they are not part of any atom? What is the relationship between "electron" , "fuel" and "boron" or "deuterium" For example, if deuterium fuel is used, does "electrons" refer to deuterium electrons ?

Professor Science
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Post by Professor Science »

Electrons are a fundamental particle actually, all nuclei have the same electrons. The electron has an equal but opposite charge compared to the proton and the idea is that you are able to inject an electron inside of the polywell which is acting as a magnetic bottle for the electrons. The electrons in turn attract the positively charged fuel. I'd go into more depth but I'm a bit pressed for time.

edit: Welcome to the boards/community. Don't feed the Msimon.
The pursuit of knowledge is in the best of interest of all mankind.

93143
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Post by 93143 »

Free electrons is what is meant. Just zipping around on their own.

The fuel, either deuterium or protium (light hydrogen) and boron, is intended to be fully ionized (have all the electrons ripped off, resulting in bare nuclei. Yes, this means that in the protium/boron-11 case there will be individual protons flying around). The electrons removed from the fuel are NOT the same ones that are accelerated by the magrid.

It is desirable to keep the population of neutrals (ie: actual atoms) as low as possible. Various bad things happen when neutrals get in the way of the electrons and ions.

It is not currently clear whether the fuel will be ionized before injection or after. I think after is better/more likely, but I've been surprised before.

MSimon
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Post by MSimon »

Professor Science wrote:Electrons are a fundamental particle actually, all nuclei have the same electrons. The electron has an equal but opposite charge compared to the proton and the idea is that you are able to inject an electron inside of the polywell which is acting as a magnetic bottle for the electrons. The electrons in turn attract the positively charged fuel. I'd go into more depth but I'm a bit pressed for time.

edit: Welcome to the boards/community. Don't feed the Msimon.
Too late.

I was just reading the Feynman Physics Lectures and came across a bit where Feynman asks: is the electron a fundamental particle or is it, like the proton and neutron, a composite.

And Feynman - excellent physicist that he was - says that there is no way to be absolutely sure. The best we can say is that so far it seems like it.

The thing to keep in mind is that all scientific theories are representations of the world. They are not the world.

From a practical standpoint all that is really important is: can you do engineering based on the theories.

Engineering is the art of making what you want from what you can get at a profit.
Engineering is the art of making what you want from what you can get at a profit.

Peter
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Joined: Wed Aug 27, 2008 11:23 am

Post by Peter »

Professor Science wrote:Electrons are a fundamental particle actually, all nuclei have the same electrons. The electron has an equal but opposite charge compared to the proton and the idea is that you are able to inject an electron inside of the polywell which is acting as a magnetic bottle for the electrons. The electrons in turn attract the positively charged fuel. I'd go into more depth but I'm a bit pressed for time.

edit: Welcome to the boards/community. Don't feed the Msimon.
Ok, thanks. Next question: The electrons spin around but are kept in the reactor by (magnetic?) force. Then some other stuff happens (fusion) which produces helium, which is thrown out of the reactor with so much force it can somehow produce electricity. My question is, if the electrons can't escape, why can the helium escape?

Every time I try to read on of those explanations of Polywell there are always big holes in my understanding, so I'm trying to fill some of those holes.

Professor Science
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Post by Professor Science »

So what's keeping the electrons confined is that as a charged particle moves through a magnetic field, it'll feel a force at right angles, making it want to spin in circles. This force is dependant on velocity and magnetic field strength (VxB).

Now lets say we have electrons and these helium particles (usually referred to as alpha particles, helium's have electrons associated with them) and they are going the same speed (which wouldn't quite be the case in the actual reactor, as the alpha's will be booking with MeV's of kinetic energy and the electrons will only have a few KeV's, orders of magnitude smaller)

SO, this means that they have the same forces applied to them, but because of a mass disparity of about 4 orders of magnitude, the electron will be accelerated 4 orders of magnitude harder. since, for circular motion A=V^2/R the radius for the alpha will be much much bigger than it is for the electron, which results in the alpha just leaving.

And I thought it goes without saying anytime someone makes a declarative that it's the best understanding at the time, MSimon.
The pursuit of knowledge is in the best of interest of all mankind.

MSimon
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Post by MSimon »

And I thought it goes without saying anytime someone makes a declarative that it's the best understanding at the time, MSimon.
True. However, the questioner is obviously a beginner (as he so states) so I thought it wise to make that statement.

And to get a little more complicated: In high field regimes (above about 1 Tesla) the escape of alphas is a probability function (according to Rick Nebel). However, the electrons are expected to make 100,000 passes before escaping while the alphas make 1,000 passes. Those are rough approximations and are dependent on the actual field used.
Engineering is the art of making what you want from what you can get at a profit.

ravingdave
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Post by ravingdave »

Peter wrote:
Professor Science wrote:Electrons are a fundamental particle actually, all nuclei have the same electrons. The electron has an equal but opposite charge compared to the proton and the idea is that you are able to inject an electron inside of the polywell which is acting as a magnetic bottle for the electrons. The electrons in turn attract the positively charged fuel. I'd go into more depth but I'm a bit pressed for time.

edit: Welcome to the boards/community. Don't feed the Msimon.
Ok, thanks. Next question: The electrons spin around but are kept in the reactor by (magnetic?) force. Then some other stuff happens (fusion) which produces helium, which is thrown out of the reactor with so much force it can somehow produce electricity. My question is, if the electrons can't escape, why can the helium escape?

Every time I try to read on of those explanations of Polywell there are always big holes in my understanding, so I'm trying to fill some of those holes.
The helium nuclei has a mass which is about 8,000 times heavier than an electron, and the fusion reaction gives it a velocity between 1 and 4 Megavolts.

So imagine throwing a baseball at a fence, with the fence easily stopping it. Now imagine a cement truck traveling at mach 4. (The fence wouldn't stop it.)

That's the sort of difference between an electron and a Fusion reaction helium nuclei.


:)



David

Peter
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Post by Peter »

Professor Science wrote:So what's keeping the electrons confined is that as a charged particle moves through a magnetic field, it'll feel a force at right angles, making it want to spin in circles. This force is dependant on velocity and magnetic field strength (VxB).

Now lets say we have electrons and these helium particles (usually referred to as alpha particles, helium's have electrons associated with them) and they are going the same speed (which wouldn't quite be the case in the actual reactor, as the alpha's will be booking with MeV's of kinetic energy and the electrons will only have a few KeV's, orders of magnitude smaller)

SO, this means that they have the same forces applied to them, but because of a mass disparity of about 4 orders of magnitude, the electron will be accelerated 4 orders of magnitude harder. since, for circular motion A=V^2/R the radius for the alpha will be much much bigger than it is for the electron, which results in the alpha just leaving.
When you say " they(helium atoms) have the same forces applied to them" (as the electrons) are you saying the helium is also charged and therefore it will also feel a (magnetic) force at right angles, making it want to spin in circles?

Do the helium particles also circulate 100,000 times before they escape, or does their velocity mean they never circulate at all?

Also, how does "spin in circles" turn in to "escaping" every 100,000 circulations? Its spins around 100,000 times, then all of a sudden it decides to ignore the right angle magnetic forces which caused it to spin and then suddenly to travel at something not at right angles to the magnetic forces. Because if spinning is right angles to the magnetic forces, then escaping must, by definition, be something besides a right angle.

Do you see the problem I am talking about?

93143
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Post by 93143 »

In high-energy physics, two protons and two neutrons all in a clump, emitted at high speed from a nuclear reaction, is called an alpha particle.

Slow it down and add two electrons, and you get helium. Same difference really; an alpha is just a He^{2+} ion.

Regarding "spin in circles", that's not really accurate. What happens is that the electron-rich plasma pushes the magnetic field back from the centre of the reactor due to internal induced currents, resulting in a fairly large, more or less spherical region that is basically free of magnetic fields. Inside it, charged particles move according to the electrostatic potentials - more or less radial, in fairly straight lines. When they get to the edge and encounter the magnetic field, they do a quick half-circle (well, roughly) and head back into the core. Unless they happened to hit one of the cusps in the magnetic field - the cusps act as holes in the confinement (hence the name "wiffleball"), and charged particles can leave that way. Electrons want to; fuel ions don't particularly (due to the net negative charge of the plasma). Of course, the electrons change their minds once they leave the magrid and see its massive positive charge below them; this results in what we call recirculation. The alphas are travelling far too fast to worry about a few tens of kV and will happily leave if they get too near a cusp.

In a power reactor, with the high magnetic fields currently planned, the alphas will have a gyroradius of about 5 cm, which means that the cusps look fairly wide to them and they will leave after ~1000 passes on average. This is in contrast to the much lighter electrons (which are actually somewhat faster than the alphas despite their lower energy, primarily because of their very low mass). The electron gyroradius is very small and results in much better confinement - IIRC the 100,000 passes thing was for the experimental reactor WB-6, so a power reactor should be even better...?

For lower magnetic fields such as those found in the experimental subscale machines, alphas would more or less ignore the field and stream straight out. Electrons are still confined by these lower fields, because their gyroradius is still fairly small.

Professor Science
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Post by Professor Science »

Also, I made a small goof regarding how forces are compared. Given equal kinetic energies an electron would not only be lighter, but also faster and thus more force exerted on it. So at this point in the night I can say the magnetic field confines electrons better than alpha particles in all regards.
The pursuit of knowledge is in the best of interest of all mankind.

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