chrismb wrote:Why don't electrons go directly to the magrid, and neutralise/hit it?
Why don't the electrons go directly to the magnets in a Tokamak?. The same principles apply, except for some differences in the background electric field - neutral in a Tokamak and negative in the Polywell. Why don't electrons fly straight to the magnets in a focused electron gun? The magnets may be at ground, which is positively charged from the electrons perspective, or they may be at actual positive potentials to further accelerate the electron stream.
Whatever accelerates the electron to its final speed. A transverse magnetic field will turn it to perpendicular directions or reverse it at some acute angle depending on the B field gradiant compared to the gyroradius of the electron at that energy, unless the gryroradius exceeds the magnetic field thickness.
Provided the electron is turned by one gyroradius it is contained till cumulative random walk ExB drift (and a couple of other drift mechanisms that add up to the Bohm diffusion rate) allows it to work its way through the magnetic field. The greater the distance it has to travel- in terns of it's gyroradius , the longer it will be contained. It jumps one gyroradius for each coulumb collision with another electron. This is the advantage of the magnetic containment of the Polywell over the Tokamak. The electron gyroradius is much smaller than the ion gyroradius. The (reasonable?) claim that the ion containment is decoupled from magnetic containment of the much more forgiving electrons, and they are thus confined by the resulting negative space charge. This advantage of only magnetically containing the electrons (or at least minimizing the contribution of magnetic containment of the ions to the final net ion confinement efficiency). Another way to look at is to look at the speeds of the ions. Once they have climbed the potential well, if they are not reversed by the electric field, they hit the Wiffleball border- magnetic domain at relatively slow speeds. So the non upscattered ions have a corresponding small gyroradius. So they are easily containedlonger. This does have a deleterious effect on ion confluence towards the center though.
The electrons presumably do have a higher energy at the edge of the Wiffleball due to negative potential well than they would have in a neutral plasma at the same average temperature, but I assume this effect is minor compared to the advantage gained by the gyroradius advantage of the electron vs ion. I assume this advantage is proportional to the square root of the relative masses. At the same energy, the gyroradius would be ~ 1/60th of a proton. Thus only 1/60th as thick of a magnetic field would be needed to confine it for a given amount of time compared to a Tokamak. This, with some adjustments, allows for the size and/ or density advantage of the Polywell (I assume).
This resistance to the electrons flying directly to the magnets is represented by the GwB factor used in the simulations. The losses directly to unshielded surfaces (cusp losses) dominates this by several orders of magnitude. This is where WB6 demonstrated the importance of this. With conformal surfaces, minimized unshielded surfaces, and spacing that allows recirculation, Bussard, etel expressed the prediction that these cusp loss could, in principle at least, be reduced to levels comparable to ExB drift losses.
Dan Tibbets
To error is human... and I'm very human.
