FAQ - How to get more neutrons / fusion

[Richard Hull]

1. The number of neutrons, (which represent only one half of all fusions in most any fusor), is a function of the applied voltage. As the voltage is increased, the energy, (velocity), of the deuterons is increased. Thus, the probability of fusion, (increased cross section), for any given deuteron pair in collision is also increased. More Voltage at any given current and pressure means more fusion and, thus, more neutrons.

2. The fusor's current is very important as it controls the number of possible deuterons that might actually be in the system. For any given voltage and pressure, more current means more fusions and more neutrons.

3. The fusor chamber's deuterium pressure is also important as it determines the amount of fusion fuel actually available for ionization, (making deuterons). For any given voltage and current, more pressure means more fusion and more neutrons.

These are the big players in getting neutron numbers. It is all about balancing all of these three key parameters that actually determines the number of fusions taking place each second in a fusor.

There is no secret grid or geometrical contrivance within the simple fusor that will significantly increase neutron production. Naturally, terrible construction, a leaky vacuum fusor chamber and poor controls will always reduce the number of fusions and neutrons due to gross asymmetry, pollution of the fusion fuel and inept operational regimes.

Major modifications and additions to the simple fusor, extra grids, ion guns, etc., will naturally have a noticable effect on neutron production. To what extent, we are unsure, as there has been little definitive research done in these areas which are fertile for amateur investigation.

Note: The average fusor can be a cantankerous beast to operate smoothly. Thus, repeatability within a series of tests is often frustrating to a researcher who expects consistent results as a measure of any possible improvement in operation based on new artifice.

In closing, only the three major items enumerated above have been shown to have difinitive and easily recognized effects on fusion rates and neutron numbers in the simple fusor.

Richard Hull

[Chris Bradley]

There is a 4th dimension to producing more neutrons too - time!

Run a fusor for longer means more neutrons. This is not necessarily a trivial observation, because running for as long as possible at the limiting knife-edge of glow-discharge mode is dependent on the set-up (per Richard's additional 'note'). It can be enhanced by some forms of feed-back control (usually an imperfect human operating it, rather than an automated system). Therefore, adjusting a set-up to make it more 'stable' and easier-to-control at that max knife edge will also gain improvements to overall fusion rate, albeit not necessarily to the instantaneous fusion rate itself.

[Rich Feldman]

Good point, Chris, about getting more neutrons (as distinguished from getting neutrons at a higher rate).

Also:
Richard's three variables (voltage, current, and pressure) are anything but independent.
For a given geometry there are only 2 degrees of freedom.
We can increase all 3 variables by changing the geometry, and I bet that bigger is better.

[Chris Bradley]

We can increase all 3 variables by changing the geometry, and I bet that bigger is better.

The Paschen discharge equation says something different.

If you have experimented with negative electrodes in large chambers, you'll likely have noted how easy it is to prompt 'streamers' running all over the chamber, which drags down the applied voltage (of a soft or current-limited supply).

'Smaller' reduces the 'pd' of the Paschen equation, once you are at a lower 'pd' than the minimum.

It would seem, however, that very small does not prompt the same ion streaming behaviour as in larger chambers. On a purely empirical basis, I would say the optimum lies somewhere between 2 to 10 inches in diameter, grid-properties dependent. Perhaps not just co-incidentally, this seems to be around the size of the commercially available neutron generators, such as NSD sell.

Below 2 inches, I suspect you are sufficiently far below the 'pd' minimum that you have to increase the pressure to 'viscous flow' regime which does not seem to work as a chamber operating at the edge of the molecular flow regime.

[Richard Hull]

From what we have observed thus far, Chris seems to be correct. I would say 3-12 inches might be the optimum size range with the 6-8 inch range being the easiest system to make and maintain with normal HV supplies and amateur resources. If a larger unit than 12-inches in diameter is ever made and reported on, we are open to major operational reports confirming or giving the lie to the above generalities.

**NOTE**** Larger fusors mean large volumes of precious and expensive deuterium are needed and are, therefore, wasted!

I am currently working up a 10" diameter fusor V.....Very slowly.

Richard Hull