Outgassing

Discuss the technical details of an "open source" community-driven design of a polywell reactor.

Moderators: tonybarry, MSimon

D Tibbets
Posts: 2775
Joined: Thu Jun 26, 2008 6:52 am

Post by D Tibbets »

Wait a minute, 10_9 torr? Where does that come from? The various units used for vacuums is confusing. 760 torr = 1 atmosphere, so 10^-9 torr = ~10^-12 atmospheres. 760 microns = ~ 10^-3 atmospheres (rounded up). Fusors often operate at ~ 10 microns, and I understand that WB6 operated at ~ 0.1 microns (0.1 millitorr or ~ 10^7 atm). Even if 10^-9 atm. is the desired goal, this would be ~10^-6 torr. I think the numbers being thrown around here are off by two or three orders of magnitude. Am I even more confused than I think?

http://www.engineeringtoolbox.com/vacuu ... d_460.html

[EDIT] Adding some presumptive numbers. If the predicted density within the Wiffle ball is susposed to be ~ 10^22 charged particles / M^3. This is ~ 1 torr or ~ 0.001 atm.
If the wiffleball traping factor is ~ 1000-10,000 then the density outside the magrid would be ~ 10^18 or 19 particles per M^3. This would be ~ 0.000001 to 0.0000001 atm or ~ 10^-3 or 10^-4 torr. This agrees with what I said above. The limiting factors here are two fold as I understand it. First- to keep the neutral population low enough that beam- beam collisions dominate the fusion process. Some recent research with gridded fusors suggests that this starts to kick in at ~ 1 micron (10^-3 torr). The second concern is the prevention of arcing, which seems to need a similar level of vacuum, or perhaps a little higher.
From this I'll assume (till proven otherwise) that a good target pressure in a Pollywell (outside the magrid) would be ~ 10^-4 to 10^-5 torr, or ~ 10^-7 to 10^-8 atm.
Lower vacuums, if obtainable, might progressively increase the beam- beam fusion dominance, but at the same time the effective density of the fusion plasma would be decreasing. At some point there is a sweet spot that I'm guessing is near my estimate, or perhaps at a little lower vacuum. Somewhere in the past I think I might have heard Bussard say that 10^9 atmospheres might be ideal. That would be ~ 10^-6 torr. That is a lot different than 10^-9 torr.

Dan Tibbets
To error is human... and I'm very human.

chrismb
Posts: 3161
Joined: Sat Dec 13, 2008 6:00 pm

Post by chrismb »

This has gone around and around the houses before. I thought we were all agreed that "the theory says" Polywell needs to get into the 10^-6 to 10^-9 torr range. Personally, I still think that is way too high a density to avoid near instantaneous thermalisation, but I thought we at least agreed the 'usual' 10^-3 torr range of magnetic confinement and fusors definitely would compromise any chance of the Polywell beam-beam fantasy.

Are we gonna have to bash that around again!?

MSimon
Posts: 14334
Joined: Mon Jul 16, 2007 7:37 pm
Location: Rockford, Illinois
Contact:

Post by MSimon »

chrismb wrote:This has gone around and around the houses before. I thought we were all agreed that "the theory says" Polywell needs to get into the 10^-6 to 10^-9 torr range. Personally, I still think that is way too high a density to avoid near instantaneous thermalisation, but I thought we at least agreed the 'usual' 10^-3 torr range of magnetic confinement and fusors definitely would compromise any chance of the Polywell beam-beam fantasy.

Are we gonna have to bash that around again!?
You have something better to do?
Engineering is the art of making what you want from what you can get at a profit.

ladajo
Posts: 6258
Joined: Thu Sep 17, 2009 11:18 pm
Location: North East Coast

Post by ladajo »

Famulus just took apart his fusor to clean it up, and it is worth a look at his photos. Not a clean system. Begs the question about maintaining vacuum and controlling arcing in Polywell with a continuous run system. If y'all recall the old photos of WB6 and previous showed significant build up on the standoff's etc.
Definately a design and maintenance cycling concern. Especially if we go to standoffs for all the coils vice nubs, which is where my money remains.

D Tibbets
Posts: 2775
Joined: Thu Jun 26, 2008 6:52 am

Post by D Tibbets »

chrismb wrote:This has gone around and around the houses before. I thought we were all agreed that "the theory says" Polywell needs to get into the 10^-6 to 10^-9 torr range. Personally, I still think that is way too high a density to avoid near instantaneous thermalisation, but I thought we at least agreed the 'usual' 10^-3 torr range of magnetic confinement and fusors definitely would compromise any chance of the Polywell beam-beam fantasy.

Are we gonna have to bash that around again!?
Just read this (reread?). ChrisMB's point is valid, except he is forgetting the effect of Wiffleball trapping of the charged particles. Using ChrisMB's quote of a density of ~ 60 neutrals for each ion in a typical fusor (say at ~ 10 Microns of pressure). If you assume the neutral density drops at the same rate as the pressure, while the charged particle density stays the same (reasonable I think, as the ion density (along with the electrons) stays about the same- as evidenced by the current through the plasma staying the same in this example.
As the pressure drops to 1 Micron, the ion to neutral ratio would be 1/6. At 0.1 Micron the ratio would be 1/0.6 or ~ 1.6. This seems consistent with several experiments that showed beam- beam interactions starting to dominate at ~ 0.1 to 1 Microns.

Now consider the Polywell without the Wiffleball effect. At 0.1 Micron. the beam - beam interactions may be dominate, but only just. But, with the the Wiffleball, the ion concentration will be as much as 1000 to 10,000 times higher than the background total particle (including the neutrals) density outside the magrid. Now you have an ion to neutral ratio of over 1000/1 within the Wiffleball. Thus, it is easy to have beam- beam dominance at these background densities (outside the magrid). So the Wiffleball not only makes useful fusion rates possible, it does so at much more tolerable background vacuum levels. Because of the Wiffleball concentrating effect, the neutrals in the background vacuum (dispersed throughout the entire chamber) is limited by arcing concerns, which seems to become a problem above ~ 1 to 5 Microns ( ~10^-6 atm), and under these limits the neutrals effects on the beam- beam fusion dominance is insignificant.

Dan Tibbets
To error is human... and I'm very human.

Post Reply