chrismb wrote: D Tibbets wrote: chrismb wrote:
D Tibbets wrote: Dr Bussard mentioned in one of his papers some reasons why the Hirsch Meeks outproformed predictions by several orders of magnitude. I didn't understand his reasoning at the time.
maybe he didn't understand his reasoning either - he didn't understand fusors don't work by ion-ion collisions, an erroneous presumption on which he based his design of the Polywell. (IMHO)
I'm not sure where you get that assumption. He has mentioned both effects. Also, I,ve not heard any counterargumnets concerning ion- neutral ratios in in ion gun fed systems versus electrostatic induced ionization in regular fusers.
Which assumption? That he thought fusors worked by ion-ion reactions, or that he based his design to perform ion-ion reactions? The reality of the former is still difficult to convince many on even today (that it is mainly ion-neutrals in a fusor), the latter is implied (I thought?)
Yes to both questions. Again I'm not sure why you assume there are alot of neutrals present in a Polywell. It is a given that neutrals above ~10^19 neutrals per cubic meter ( ~ 1 micron) will shut down the machine due to arcing. That taken with the target of ~ 10^22 ions per cubic meter needed for net power production mandates that there has to be a thousand ions per neutral within the machine. I believe this means that there would be 1 million ion- ion collisions per ion neutral collision. If these ion to neutral proportions cannot be maintained the whole system is a failure from the start and presumably the physicists involved (including the reviewers) would abandon the effort. I believe Bussard's work has shown this gradient (~100-1000) is possible, and Nebel's work has confirmed it ( internal densities vs external densities). This is a presumptive argument about neutrals not contributing significantly to the fusion output in the Polywell. I'm not sure how difficult it would be to directy measure this in the current low output machines, though I again assume that the review panel was satisfied that it was not a show stoper due to my above arguments, other arguments, or actual measurements.
One reference by Univ. Wisconsin used in another thread that did some measurements of localized neutron production in a gridded fusor showed ~10% coming from the core. This means the rest was outside the grid and was probably dominated by ion- neutral collisions, but looking at it another way: 10% of the fusions were occurring in the small core volume (assume 10 cc) and 90% was occurring in the volume outside the grid ( assume 200 cc). This means that fusions per unit area was dominate in the core. While these could still be in part ion- neutral collisions I argue that the nature of the experiment suggests otherwise. In the experiment the detector saw either the core area, the grid area or a modest area outside the core (not the entire fusor volume). In the D-D fusion experiments the rate was higher in the core than in the volume around the grids, or immediately outside the grid. If the increased fusion rate was due only to the ions reaching their maximal velocity at or near the grid, then striking a neutral I would expect the counts from the unmasked tests would dominate (maximum rate and observed volume - again remember that this observed volume included the core, grid and near grid volumes. Also, the detector was seeing increasing reaction volumes as you moved out from the center. Once inside the grid the ions would be coasting or decelerating due to virtual anode formation so the fusion rate from ion- neutrals should be lower due to smaller volume and same or decreased ion velocities.
I believe there are two ways to look at this.
There were significant ion- ion fusion collisions in this area due to convergence (increased density) combined with increased collision speeds (head on ion- ion collisions ~ twice as fast as ion - neutral collisions with resultant larger fusion crossections).
Or, the increased rates were only due to the convergence of ions effectively increasing the total energetic collisions weather ion- ion or ion - neutrals. The D-He3 tests also showed probable ion- target collisions presumably from He3 embedded in the grid, but this was not seen (as much) in the D-D tests. And ion-beam collisions are is irrelivant with a virtual cathode.
The convoluted conclusions are that even in this non ion injected gridded glow discharge fusor in which there would have been a dominance of neutrals to intercept the ions and impede their confluence and direct collisions, there was evidence of significant core convergence and possibly significantly increasing core ion- ion fusion collisions. In a hand waving way, I claim that this supports my belief that ion- neutral collisions as a contribution to the fusion rate quickly becomes insignificant as the ratio of neutrals/ions drops, both because of probabilities of collisions and the magnifying (increasing only the effective ion density in the core region, while the neutrals density is not effected) benefits of any ion convergence .
In a clean fusor that is pumped down to a good vacuum and then filled with ions from an ion gun , any residual neutrals should be in a minority, perhaps by a factor of several hundred (eg: pump down to ~0.1 micron, the turn on the ion guns on and operate at ~10 microns). Any neutrals added would be from recombinations (a two way street), out gassing (that is why I stipulated a 'clean' fusor) or neutrals knocked off the walls or grid. Unless these were neutral deuterons that had been loaded/ embedded in the metal, these sputtered ions and neutrals would not be contributing to any fusion, but be poisoning the system.
Your claim that Bussard was ignorant of neutrals contributions to the system, while convenient for your position, is not supported by facts. Bussard has discussed neutrals in several contexts in the papers which I have read.
http://iec.neep.wisc.edu/usjapan/UW-IEC ... ashley.pdf
http://mr-fusion.hellblazer.com/pdfs/co ... -focus.pdf
Sorry for the length of this post.
To error is human... and I'm very human.