KitemanSA wrote:D Tibbets wrote:If there is not a substantial B field protecting the metal surfaces of an X cusp- direct impingement of electrons would be bad. Remember the minimum allowable non shielded surface are can not exceed ~ 1 part in 10,000.
Saying electrons would not exit or hit here and yet electrons could be easily injected makes no sense.
If there is a pair of substantial B fields that protect the metal surfaces but cancel out where there is no metal, then your objection is moot.
You just described a magnetic cusp. As the opposing fiends approach the local field strength does cancel out, so assuming percise geometry, wire conductivity, etc. there will be a core parellel to the cusp axis where the B field is zero or very close to zero. This core corridor though is very this. Only the very (?) rare electron will be traveling outward directly down this corridoer exactly perpendicular to the center of the machine. The vast majority will travel on a tangent that crosses the midline of the cusp, but travels deep into the B field on either side. The average electron thus will experiance ExB drift. One can argue that this drift- gyro radius is quite small and can be ignored for the majority of the electrons. But this is not what happens. Otherwise seperating the magnets several gyro radii is required. Also, it is consistent with Nebel,s comments about the nubs being a major heat source in WB7. I don't know what adjustments were made in WB7.1- whether more wires was added in the nubs, they were moved outward (even so far as being wall standoffs).
The eletron MFP may be longer than the cusp length, and the average electron velocity perpendicular to the central cusp B field may be small, but with billions upon billions of electrons traversing the cusp per second there will be many ExB driving collisions that results in originally well behaved electrons gaining more trasverse vectors and walking deeper into the side walls of the cusp, untill a certain amount hit the magrid surface. It is unavoidable. The question is by how much it can be minimized, not if it can be avoided.
Also, keep in mind that ExB is only one form of B field diffusion/ drift. EyB (I think) drift is a collision driven movement of charged particles along a B field line laterally- not penetrating deeper but moving laterally or transversly. In a point cusp I think this movement is moot, but in a line cusp- even one that is highly modified as in the Polywell allows for electrons to slide sideways along the width of the cusp till they might hit a bridging struture like a nub (or X-cusp). In WB 6 the nubs had only 1 wire instead of 200 wire windings so they were essentially unshielded against not only the electrons directly exiting at this point but also from EyB drift electrons. Admittedly having many windings in the nub or X cuspwould help, but never as well as magically having no nubs or standoffs. Electrostatic shielding might help the standoffs as well, so long as they are well outside of the mid plane cusp structures (M.Simon once mentioned this as a reasoned/ pursued (?) application). There is a point cusp in the center of the x-cusp so that may have very little loss as it has strong B fields and small overal are. But, the problem is the other side of the metal tube that makes the connections. Here you have twice the surface are exposed to EyB drift. I'm not sure what is gained by having the nub split in two. You can only load each arm with only 1/2 the magnetic windings, while adding another point cusp that while small in loss area is still an extra cusp.
The line cusp/ equatorial cusp in the opposed biconic mirror machine is highly modified in the Polywell. Instead of one intolerable loss line cusp, you have eight cusps that act nearly like point cusps , but they still have line cusps intercepting the nubs. The close proximity of the magnets here though make for very thin and thus much improved losses, but not totally absent losses. I know you believe the X-cusp nubs makes all of the cusps true point cusps, but I am not convinced if this is true, and if so, if the penalty of more vulnerable surface area to ExB drift and accompanyingly greater electron gyro radius hurts as much as any improvement in EyB losses.
As for injecting electrons through the central null field (or very small B field areas very near the center of the cusp. Of course this would very ExB drift with excellent electron aiming, perfect radial velocities would not mirror those electrons. The problem is that achiving this condition is essentially impossible for a large number of electrons. The beam has to have some diameter, and mutual repulsion, two stream instability, etc. results in very quick dispersion of the electrons. The magnetic field can resist this/ focus the electron beam, but this is a tradoff. Collisions drives ExB drift and thus losses, and also mirroring to some extent. Add to that the problems of space charge buildup in the cusps if the electrons are not transiting this cusp region quickly, and you have the picture of the competing processes that have to be addressed to achieve the best overall compromise that not only allows for good electron and ion confinement with reasonable electron injection efficiency.
Another knob to consider is to introduce the ions deeper in the electron induced potential well. This will keep most of the ions further away from the midplane cusps and any electron buildup space charge there. This though essentially makes for a smaller machine from the ions perspective, while introducing different electron and power input dynamics. A small adaptation in this direction though may add to the best compromise arrangement.
Consider WB5. Assume the ions were introduced at a radius of 9/10ths of the radius to the magrid. The electron repellars/ e-gums attracted the ions as they approached the top of their central virtual cathode potential well and led to ion escape. If the ions were introduced at 8/10ths of the radius this effect would be lessen, but at the cost of smaller effective virtual cathode accelerating potential. You are wasting electron power. But...
Dan Tibbets
.
To error is human... and I'm very human.