Magrid Vs electrically biased grid

Discuss the technical details of an "open source" community-driven design of a polywell reactor.

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Postby sd_matt » Tue Jul 20, 2010 10:22 pm


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Postby KitemanSA » Thu Jul 22, 2010 4:01 am


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Postby D Tibbets » Thu Jul 22, 2010 6:53 pm

My take on oscillation is that the escaping electron follows a field line (gyrates along it) untill it reverses due to some type of mirror reflection (bounce) or untill it follows the field line into a structure or back into the machine through another cusp. But, this discribes the situation in which the magrid casing is neutral or grounded. Any charge on the magrid will change the dynamics. eg: with a potential well of ~ 10 kV and a magrid charge of ~ +12 kV, when the escaping electron passes the magrid with the full acceleration provided by the potential well it sees the 12 kV positive potential on the grid (Gauss's law stuff) and it is quickly decelerated by (or more precisely accelerated towards) the magrid. The brakes are put on, it reverses quickly and it renters the internal magrid volume through the same cusp. What I'm unsure of is how far it will travel before this charge reverses it. Is it 1 mm, 1 cm, 10 cm... ? Also, I think that once it reverses it will gain a velocity dependent on the potential, and the distance it traveled before reversing does not change this (Gauss's law again) so, the recirculating electron that returns through the same cusp is reset at the original energy (12 kV (actually KeV but I'm sticking with volts to keep it simple)) even if they were mildly upscattered. In this regard, the inefficiency of creating the potential well (80-85% efficient) is actually an advantage as it permits the recapture (recirculation) of mildly upscattered electrons without the penalties of mild, but progressive upside thermalization.
Mildly upscattered electrons 10.1 to 11.9 kV will travel a little farther before reversing. Upscattered electrons above 12 kV would slow but not stop. These electrons would continue to follow the field line till it hit something or reentered through another cusp. The problem if it recirculates in this fashion is that it would reenter the magrid at it's residual velocity plus the mageid accelerating velocity. A limited amount of this escalation may be tolerable, but there are limits to how much of this sequential upscattering could be allowed before undesirable thermalization effects reared it's ugly head. In that regard, some limit on the efficiency of recirculation may be desired. Of course, external structures like electron guns, ion guns, supports, etc. may eat/ deflect these electrons, once they climb far enough beyond the magrid mid line, depending on their placement and shielding. Another example of the synergistic compromises that make up the Polywell.

[EDIT]
It occurs to me that if mildly upscattered electrons are reset to the drive potential with recirculation, then downscattered escaping electrons would also be reset as they traveled a short distance past the magrid midline and were reversed. The part of Gauss's law that says that a mobile charge is accelerated the same amount whether it starts a short distance or a large distance from the charged plate would dictate this.
This fits the picture that the Wiffleball is important for the obtainable density, not for energy efficiency. In fact, if Wiffleball containment is to great, the electrons may thermalize too much before escaping and exceed the capacity of the magrid potential to reset their energy. In essence the recirculation does two critical things. It conserves energy (decrease electron losses) and it serves as a voltage (energy) regulator.


Dan Tibbets
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Postby MSimon » Fri Jul 23, 2010 6:35 am

Last edited by MSimon on Fri Jul 23, 2010 4:39 pm, edited 1 time in total.

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