Stoney3K wrote: D Tibbets wrote:
There are enough other practical neutron sources that can do the same job when powered externally and are easier to obtain (e.g. fusors), so building a Polywell for that purpose might even be overkill.
A fusor and many other approaches will make neutrons, but they produce relatively few neutrons. If a Polywell produced enough neutrons to get the job done in one day, these other approaches might take a million years. Other approaches like laser ignition is way to costly and large. A FRC, dense plasma focus, or General Fusion approach might fit the bill, especially if you are not concerned about reaching breakeven. Prevous methods required very difficult uranium enrichment, or access to spent fuel rods from fission reactors. Any of these new "cheap" methods could reduce the capital investment by a nuclear weapons seeking government by at least an order of magnitude.
A Polywell has significantly more neutrons per gram fuel that is spent, but an over-unity fusion is not a condition for a neutron producing device. If you alread have plenty of power from the grid or a generator, there is no reason you couldn't make a fusor big enough to generate enough neutrons. Remember, you don't have to generate your own power to do so.
Sure, more neutrons per unit fuel is better, but the construction cost of a Polywell versus a fusor outweigh that. For one Polywell you could build a hundred fusors with a lot less intelligent manpower.
That is what I said. Efficiency is not the issue in this application. Power is. A good amateur fusor might produce 10,000,000 neutrons per second. Hirsch's best effort with D-T fuel produced ~ 10^11 neutrons per second. If you sleep very close to Hirsch,s operating machine overnight, you might get a radiation exposure comparable to a CT scan. A machine near breakeven might be making 10^18 to 10^21 neutrons per second. That is a power (dose) ~ a billion times higher. At those levels a leathal neutron dose could kill an unprotected individual with only a few seconds if he is 1 meter from the machine. It would take perhaps 100 seconds if he was 10 meters away, or ~ 10,000 seconds if he was 100 meters away.
Typical amateur fusors might put out micro Rads of neutron radiation per min. The x-ray radiation from the high voltage is a much greater concern in these machines. And, a machine designed to maximize x-rays (like a medical x-ray machine) will do a better job in this regard.
PS: I have heard that in theory a gridded fusor could be scaled to very high powers, though never breaking even. I'm uncertain,, but I believe the size needed to reach power levels to be a good neutron killing machine would make a Tokamac look small. Assume ~ 10^16 neutrons per second is needed (~ 10,000 watts). A typical fusor is ~ 6 inches wide and might produce 10^7 neutrons per second. If the power scaling is linear based on radius cubed, the volume would have to increase 10^9 times. That would be ~ 10^3 increase in radius . That is 6 inches *1000 = 6,000 inches, or 500 feet.
[EDIT] I might add that if you use Hirsch's best effort with D-D fuel (10^9 neutrons per second) and improve it by a factor of ten, you would still need to scale the volume by 10^6, or the radius by 10^2. This would result in a 50 ft wide machine. That would be way too big to fit on a truck. Now if you managed a machine that put out only 1/1000th of the neutrons (5 ft wide) and stealthily placed int in a neighborhood for several weeks, you might be inducing some radiation sickness and a rare death (so long as the people were in the same building) before the populous became aware of the threat.
Of course you might need many megawatts of power to run the machine, even at these relatively low neutron levels.
To error is human... and I'm very human.