Joseph Chikva wrote:D Tibbets wrote:I've been talking about looses- charged particles with with their associated KE reaching the magnet surfaces or exiting through a cusp.
And I am talking about that may be that some of those charged particles will have KE more than you estimate.
And by this reason reachable number density will be much less than calculated for condition beta=1.
Of course ions will be upscattered. Annealing is one process to limit this. Another is that as the ions speed up their time to fusion becomes closer to thei thermalizing times- the coulomb collision rate decreases, as the fusion rate increases. This would decrease the proportion of progressively upscattered ions. Also, always remember that when the ions are scattered above the potential well confinement, they are no longer electrostatically confined (actually there is some variability here depending on how deep in the potential well the ions are introduced). They will escape after a few thousand passes when they hit a cusp. This is a energy loss , but not as bad as it might seem. The key of course is how many of the well behaved and upscattering ions fuse before escaping with their upscattered energy and any energy gained from the magrid. My impression is that a few thousand passes may be all that is needed for a good likelyhood of fusion, so few of these more reactive upscattered ions would escape before fusion. So the losses while not zero, is considerably less than losses from electron escape, bremsstrulung, etc.
This is one of the advantages of the Polywell density advantage, the ion lifetime to fusion is much shorter than a Tokamak (perhaps by as much as a million, or a more conservative number of ~ 60,000 which Nebel mentioned). There is much less time for thermalization.
What, you argue that the increased density also accelerates the coulomb collisions? Well, certainly. But, if there is any significant ion confluence (focus) towards the center, many of the coulomb collisions will occur near the dense core, but, that is limited by the dwell time of the fast ions in this portion of the machine. The fast ions will have a longer Coulomb MFP, while the mean path length to fusion will be shorter. Near the edge the fusion rate among the slower ions will be less, and the Coulomb collisions will be greater- much greater as the edge is reached. In this area the coulomb collisions will be at their greatest frequency, but the average temperature will be so much lower than in the core, the unavoidable thermalization will dominate. But, the Maxwell distribution will only be a small (?) fraction of the velocity the ion will gain as it again falls down the potential well. This is 'annealing'.
Concerning electron two stream instability, it is one of at least several influences on beam cohesion and electron thermalization. Again, describing the injected electron streams as beams is a convenient simplification. Even on their first pass the electrons are dispersed by magnetic effects as they pass through the cusps. This is at least in part why only ~80-85% of the drive voltage will be transformed into a potential well. How much radial persistence the electrons maintain during their primary confinement life time is uncertain. Certainly bouncing off of the magnetic field imparts some angular momentum on each pass. Few of the electrons will pass straight through the center, or stop just short of the center and reverse. Most will glance off the central region at some angle. Some feel that the electrons will quickly assume a cloud that concentrates near the Wiffleball border. I'm uncertain, but I think the situation may be some where in between. There is a large current of new or recirculated electron which are initially within perhaps ~ 20 degrees of radial on their first pass. If this current is ~ 1000- 10,000 Amps, that would be ~ 10^22 or 10^23 new or reconditioned electrons per second.
Additionally, with ions present, their greater momentum will result in electrons being dragged towards the center.. In the Google talk Bussard mentioned that the potential well from the electrons was square, which to me means most of the electrons were accumulating near the edge. Once ions were introduced, the dragging resulted in a more parabolic well.
How can electrons near the edge accelerate ions towards the center? Easy, provided that the ions are introduced further out than most of the electrons. Gauss's law takes care of the rest.
Comparing the Polywell to flares on the Suns surface is wrong. These magnetic structures are loops that are concave towards the center. Polywells are claimed to not suffer from this MHD instability due to convex fields towards the center.
This would be a much better match to Tokamaks and their macro instabilities.
Some resorces that describes some of the stability issues are below. I leave it to you to Google them for the actual links.
Collisional Equilibration
Robert W. Bussard, EMC2
The Polywell: A Spherically Convergent
Ion Focus Concept
page 5 of this paper does not give much pertinent infromation to your two stream criticism, but it does demonstrate that such issues were not ignored.
"'Electron instability processes will undoubtedly be an
issue in this device. As the density rises, the wavelengths
for beam instabilities shrink and may play a role in the
electron behavior. If these instabilities lead to electron
thermalization, they may be unimportant to the gross
energetics. If they lead to cavitons and the production
of superthermal electrons, they can increase the elect
tron loss rate. If they lead to turbulence, which decays
by cascade into low frequency turbulence, they can
change the ion angular momentum, which we have
noted will affect the ion convergence radius."
ELECTRON TRANSIT TIME
IN CENTRAL VIRTUAL ANODE WELLSt
Some Physics considerations of the Polywell
Dan Tibbets
To error is human... and I'm very human.