I would really like to bottom out this question as it overturns what I previously gathered from the various descriptions of the Polywell offered to me.
How *isn't* a Polywell an ambipolar device when its primary mode of operation is supposedly a bunch of electrons (very slightly out-populating ions) in the middle forming e-fields to pull in ions from further out? Surely 'ambipolar' means that regions of differential charge pull on each other by the e-fields thus generated between them? Is this not the very definition of ambipolarity??
rnebel wrote:If you inject ions and electrons at the same rate, you get the ambipolar result which will likely have small electrostatic potentials and the confinement likely isn't what you desire. On the other hand, if you flood the device with electrons you will get deep potential wells and the ions will be extremely well confined and at very high energy.
So, exactly how many more electrons do you need to put in than ions before it goes from 'ambipolar' to 'quasi-neutral'?
chrismb wrote:So, exactly how many more electrons do you need to put in than ions before it goes from 'ambipolar' to 'quasi-neutral'?
Chris,
Quasi-neutrality and ambipolarity are two completely different things.
If we have a million more electrons in a volume than protons but there are billions of protons, the volume is quasi-neutral. Static condition.
If I feed in a billion electrons and the same number of protons each second and leak the same billion, the charge state does not change. It is still quasi-neutral. And the equal flow of polarity means it is ambipolar.
If I feed in a billion electrons and NO protons per second, and leak the same billion electrons and NO protons, the charge state still does not change. It is still quasi-neutral, but unequal flow of charges means it is NOT ambipolar. This is the Polywell.
If I did not get that right, I hope Dr. N will correct me.
Yes, I think that's about right. Polywells have a quasineutral plasma that is not ambipolar because it runs electron-rich. That allows for well-confined ions and deep wells, and means out at the edge there will be many more electrons leaving the device than ions.
In contrast to a tokamak, where after ignition there is no energy input needed, a Polywell always needs significant drive power. I think the estimate was ~5MW for a 100MW reactor.
If my understanding of the final phase of wiffleball formation is right, the corner cusps are very small when complete. Electrons that come into the machine from the center of the coils and follow the flattened out magnetic field lines would have to make a better than 90 degree turn to make it out the corners. If the electrons have too much momentum they miss and continue spinning around inside. Only those electrons below this speed limit can make the turn and leave, but then they would have to be too slow to escape the field lines and positive attraction of the magrid. Only those fast electrons than make the bullet hitting bullet hitting instable bullseye get to leave the system. But then wouldn't a positively charged magrid still be more attractive than the wall.
Shouldn't any positively charged ions be repelled back by the positively charged magrid, especially at the funny cusps, with those low kinetic high potential electrons at the core pulling them back at the same time. If anything, I would expect the ions to be more attracted to the center of the coils where the electrons come inside towards them rather than the more distant corner cusps where any escaping electrons are moving away.
I will add one last silly idea from transistor theory that doesn't apply here but makes for a good joke. Hole flow theory, those electrons leaving the system out the corner cusps leave positive holes behind that are repellent to positive ions.
TallDave wrote:Yes, I think that's about right. Polywells have a quasineutral plasma that is not ambipolar because it runs electron-rich. That allows for well-confined ions and deep wells, and means out at the edge there will be many more electrons leaving the device than ions.
In contrast to a tokamak, where after ignition there is no energy input needed, a Polywell always needs significant drive power. I think the estimate was ~5MW for a 100MW reactor.
The power gain is only 8 with no losses at the pB11 peak. (1 MeV drive - 8 MeV fusion) at the resonance peak the gain is around 22. In theory you have to make the higher gain reactor larger to compensate for a fusion cross section of about 1/10th the peak. About twice as large in linear dimensions.
A 50 KV drive for D-D produces about the same reactivity as a 50 KV drive with pB11. The D-D machine is capable of 50% more power due to D-D giving 12.5 MeV vs a 200 Kev Drive Energy.
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