Electron Temperature

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

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TheRadicalModerate
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Electron Temperature

Post by TheRadicalModerate »

I need to ask another dumb question: Why is the temperature of the electrons important to the power balance? I'd think that, in general, you'd want the electrons to have as little energy as possible, i.e. just enough to get injected into the wiffle ball and move a few gyroradii away from whatever cusp they got injected through. And yet there's a lot of talk about thermalization of the electrons being a Bad Thing, and even some talk about high electron temperature being a Good Thing. Can somebody explain the issues?

kcdodd
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Post by kcdodd »

The boltzmann distribution determines the spread of energy of particles at a single temperature. Even though you have one temperature in a system at thermal equilibrium, the individual particles have a statistical spread of energy. This is bad when the thing keeping all the electrons in the chamber has a fixed energy. As they reach thermal equilibrium some electrons gain more energy then the potential keeping them inside the chamber. The distribution at thermal equilibrium is unbound, so no matter how deep you make the well there will always be some electrons with more energy.

I think high energy just means deeper potential well to do fusion. You need to think in total energy, not just kinetic energy, because the total energy is somewhat fixed while the kinetic energy varies depending on where you're looking at. Each electron delivers a small amount of charge to the well, making the well a little deeper. Just like carrying rocks on a hill to make the hill higher. The more rocks you put up there the harder it is to put the next rock etc. So each successive electron needs more energy to put that next bit of charge on top of the pile. So they need larger total energy to make a deep well. Of course that well is a different well than the one keeping the electrons bound, they are disconnected by the magrid.
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TheRadicalModerate
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Post by TheRadicalModerate »

That makes sense--thanks.

So, if I'm understanding you correctly, we'd really like to have cold, mono-energetic electrons in the wiffle ball, but we can't, because they have to be hot to overcome the coulomb forces from the other electrons at injection and, once hot and trapped in the wiffle ball, they thermalize over time (although various folks are asserting that the thermalization is slow enough so that the confinement times are long enough to have a positive power balance).

Do I have this right?

kcdodd
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Post by kcdodd »

That's pretty much my understanding anyway. Except, I think temperature only relates to kinetic energy. So in the very center that would probably be low ("cold"). And it would be highest ("hot") right around the magrid. It's just equilibrium includes total energy. So I think when you're looking at the energy spreading its the total energy not just the kinetic energy.
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dch24
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Post by dch24 »

kcdodd wrote:So in the very center that would probably be low ("cold"). And it would be highest ("hot") right around the magrid.
I know for sure the ions (H- and B5-) are the opposite. Can somebody explain which way around the electrons are?

This is how I thought they were: (but kcdodd is making me wonder which way it is)

Cold (slow) coasting near the MaGrid where they are confined by the electric & magnetic fields, then accelerating toward the center.

Hot (fast) flying through the center, then decelerating as they approach the other side.

93143
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Post by 93143 »

The well is negative. It's natural that the electrons and ions would behave oppositely in the presence of a net charge.

The electrons were 'dropped' into the magrid from outside (the magrid is more positive than the wiffleball is negative, so it attracts electrons) and thus have high kinetic energy right near the magrid, but their speed decreases as they are found closer to the core. If the (negative) core potential relative to the magrid is 90% of the drive voltage, the electrons at the edge should have 10 times the kinetic energy of - and should thus be moving sqrt(10) times as fast as - the ones in the core.

The ions are formed at the edge, at basically zero speed. They accelerate as they fall in, up to a maximum in the core, and then decelerate on the way back out. But you knew that...

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