In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.
Subject: The physics of neutron production
Date: Mar 23, 10:00 am
Poster: Richard Hull
On Mar 23, 10:00 am, Richard Hull wrote:
It is important to understand a bit of the physics of what we are up to.
We are colliding deuteron beams in our system.
We are making deuterons by uncoupling the electron from the deuterium gas atoms (ioninzing it).
This creates a plasma within our fusion chamber.
Plasmas are electrically neuteral collections of ions (in our case deuterons) and electrons in a one to one correspondence.
We ionize by field emission of elecrons from the chamber walls and collisions of gas atoms in the inter grid area resulting from large field gradients.
The deuterons produced in the intergrid region accelerate towards the negative inner grid. They "fall" through the potential difference between the point they are created and the inner grid. (this is important- we will return to it soon.)
Fusion occurs when two deuterons collide in such a manner that the sum of the kinetic energies are able to overcome the coulombic force of repulsion between them. This occurs around 200 million degrees kelvin in a plasma which corresponds to a potential energy gradient of ~18-20KEV. (1ev=~11,000 degrees K) Thus, collisionally, we should be barely able to do fusion in our fusors with 10 kv on the system. (Two 10kev beams colliding equals 20kev.)
The problem we encounter is that with our system, we create deuterons over the entire intergrid region. Most all of our deuterons are at some potential energy below 10KV at the inner grid as they have not offically fallen from the outer wall all the way to the inner grid. At pressures of 2 microns or so, we are just barely able to warrant a mean free path for our deuterons across the chamber. Still, there are some few perfect wall originating deuterons which will crash into other crap on its way to the inner grid. This is a dead deuteron. Some few do make the journey unhindered, however! These are at fusion energy! Once these few fusion energy deuterons arrive, there is another possibility that they will not hit head on in a manner neaded to do fusion with another equally energetic cousin coming from the opposite wall!! This means that it is virtually impossible to detect neutrons at or even near the threshold of fusion potentials in our system! We must rely on quantity dicounts and wholesale numbers to do fusion.
We could avoid this and use only ion guns in our system which would warrant that 100% of all deuterons originate at the walls and are prepared to fall through the full potential. (very, very big bucks and multiplies the hassle a lot.) A dispenser cathode just outside the outer grid and biased negatively with respect to it is simpler and will be a method of suppling ionizing electrons in quantity just where they are needed the most, (outer grid area). Both of these solutions require pressures below 10E-5 torr. More vacuum hassles. However the mean free path is incredible and most all deuterons would arrive un-molested at the inner grid. The down side is the fusion density is way down now. At higher gas loads we are using quantity to over come quality.
To keep the fusor simple we use our current model.
In doing so, we are forced to use higher pressures than the elegant ion gunned systems of Farnsworth. This assures us of countless billions of deuterons on hand to take a stab at fusion. Most of them will just bang around uselessly. We must, therefore, increase their relative effectiveness or up the fusion rate by raising the voltage.
As we raise the voltage above 10kv, we, in effect, increase the effective area of gas volume in the field where fusion energy deuterons are produced!
Imagine, if you will, a thin zonal sperical layer or volume just at the outer grid where fusion energy deuerons are made at 10KV. As we raise the chamber voltage, the volume of this fusion working zone or "zone of fusion deuterons" increases dramatically. We are literally increasing the shear number of deuterons at fusion potential!
We observe that at about 15kv in a 4-6" diameter chamber we are starting to see some fusion occur. (10-1000 n/s) Keeping all things equal, a bigger chamber would not necessarily make more fusion and might even make less as our mean free path has not changed, but our distance to go for the fusion deuterons has increased with increasing chamber radius. It is no surprise that the Farnsworth team never made a chamber bigger than 10" in diameter. They knew that the shear numbers given by gas density was an asset.
At 20-25kv in amateur systems, under 6" in diameter, things are really cookin'. (10E3-10E4n/s). This is due to the larger volume of fusion energy deuteron production, good high gas densities and adequate mean free path.
At 20kv we also no longer have to hit head on to fuse! Think about it! Those few wall produced deuterons that make it are at 40KV for ideal head-ons! (absolute 100% gauranteed fusion) Thus, a 20kv deuteron hitting a 7KV deuteron smack on should fuse or hitting a 10kev deuteron slightly off center might fuse. We just have more probability of success by exceeding the bare minimum of 10kev.
The voltage determines only energy!!!!!!
We must be assured of quantity as well!
This is current and current only! One deuteron is one charged particle. One amp is one coulomb of charges and that is about 10E19 deuterons per second whistling in to the inner grid. Ten milliamps would mean about 10E17 deuterons per second.
We need both voltage and current to see the numbers go up. There is a limit though for any given geometry. As the voltge climbs higher and higher, we get less and less new fusion deuteron volume per unit volt. Also at higher currents we heat and destroy the inner grid.
There are practical engineering concerns that limit this as well such as insulation issues, etc.
The envelop of this thing is just not well known. there has been almost no engineering effort done on this thing which says that it is best to make it this way or that way. The experimental possibilities are wide open.
A 3 meter diameter fusor is just not an answer for our purposes. Keep 'em small...under 10" Bigger only gives you more gas volume (nice), but demands higher voltages and or lower pressures.
One can also see how abyssmally inefficient our amateur fusors are with probably 99.99999999% of the energy going away as waste heat. Still, we are doin' fusion and having fun.