The Brillouin limit is sometime described as an absolute limit, but this is apparently not the case. Bussard addressed this (see the 2008 EMC2 patent application).
And this study exceeded this limit.
http://pop.aip.org/resource/1/phpaen/v4 ... horized=no
This might mention Brillouin limit. It does present evidence that electron thermalization may not occur in time frames of over 500 microseconds ( ~ twice the non recirculated electron lifetime in the Polywell (WB6)).
http://www.dtic.mil/dtic/tr/fulltext/u2/p012563.pdf
As far as the cusps being areas of zero magnetic strength, while this is true, it is misleading. What is important is how quickly the opposing fields build on either side of this null magnetic region (very close to infinity thin) to levels where mirror reflection occurs or adiabatic bouncing of the electrons results in a new direction where the electron is not directed deeper into the cusp geometry. I believe these are two separate mechanisms. This is illustrated in one of Bussard's early papers. In a collisionless plasma this would be much like a game of billiards where the ball bounces off of the bumpers angled towards the hole, but the ball bounces several times and is deflected away from the hole. Of course in a collisional plasma things are much more complicated.
As far as instabilities, I think a key consideration is that the Polywell is quasi spherical. This is very different from cylindrical or torus geometries. Also, instabilities may not be detrimental. I assume that POPS techniques introduces or takes advantage of potential instabilities to created advantagous conditions.
In any given cusp the field lines are in the same direction. Either inwards or outwards. This is (I think) almost totally irrelevant concerning the electron gyro motion direction. If the electron hits the field line perpendicurly, it will be deflected in a Northward direction, either in a ~1/2 gyro orbit as it adiabatically deflects or is captured on the field line. If captured, this would introduce a preferred direction for the electron, but only for it's first pass along the field line, once mirror bounced (reversed), on the field line, it's direction is in the opposite direction.. If 'cusp confinement' of electrons is ~ 60 passes (Wiffleball confinement would be even better at a few thousand passes) and this applies to these captured electrons equally to those adiabatically deflected back into the B field free central region, then the initial preferred direction along the field line would be quickly diluted to a near random directional field line average. I speculate that this would moot any concern about the initial field line direction preference. And this is before considerations of collisional interactions, and that electrons that meet and are captured onto a field line will only rarely be traveling perpendicular to the field line. Even with good central focus of the electrons- near radial motion, the B field lines are always curved away from the center, so even in a theoretical perfect radial situation there would only be very rare situation where the electron enters the B field prefectly perpendicular to the field lines. How much deviation from perpendicular entry is needed before the deflection is not in a Northward direction (if I'm keeping the charged particle direction in a magnetic field straight (N pole attracting electrons ))? In any case , I suspect that the direction of the magnetic field is ( very nearly?) irrelevant in this regard. The average electron would not show any preference for which cusp (N pole inward, or N pole outwards) they accumulate in, or escape through..
I do wonder about the free cusps verses the cusps that have an electron gun shooting new electrons through. These guns would seem to have a side effect of acting like a cusp plugging electron repellar. Is it better to have an e gun at every cusp? Is it better to have the minimum number of guns? What compromises in e gun standoff and number gives the best results? There is mention of the best e gun standoff distance in the 2008 patent application.
Parent page with lots of other pertinent papers:
http://www.askmar.com/Fusion.html
and direct link:
http://www.askmar.com/Fusion_files/Pate ... 187086.pdf
Dan Tibbets
To error is human... and I'm very human.