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#1 FAQ - At what voltage does fusion begin?

Posted: Tue Jan 20, 2015 11:13 pm
by Richard Hull
This is a loaded question with both no real definitive answer and many possible answers.

First, as amateur scientists and would-be fusioneers, we should pose the question properly and definitively........

At what "voltage" does "demonstrable, provable" "D-D fusion" begin?

Let us break it down based on that mult-part highly qualified quote.

1. "Voltage" - Fusion does not occur in a vacuum..... Err, ahh, well, yes, it kinda' does. What is meant is that fusion requires an input of energy. This energy is supplied within a fusor in the form of a potential difference in a rarified gas atmosphere under special conditions whereby, the gas atoms are ionized and due to that potential difference, the ions, (deuterons), are accelerated to fusion energy. This fusion energy is a function of how high the potential difference is. (how high the voltage is that is applied to the fusor)..... More voltage means, more probability of more fusion. This probability is all based on a well known "collisional cross section" graph for deutrons colliding with other deuterons relating a dreamed up area term (the barn) of increasing size versus the increasing applied voltage.

2. "demonstrable, provable" - Here is where the amateur catches the bullet to the brain and more people fail here. It is all about detection. Fusion has only one escapee that exits the steel chamber at a more of less detectable level and that escapee is the "fast neutron". Fast neutrons are among the rarest of free flying nuclear particles and are normally only associated with the fission and fusion processes. Thus, if we detect neutrons in and around our fusor, we are doing fusion.

Neutrons are hard little guys to detect.... Very hard.... Neutrally charged, they will not be detected by cheap nuclear instrumentation. Neutrons are only detected by secondary reactions as they interact with matter, such interactions are rare. Thus, you typically need a huge herd of neutrons to be easily and definitively detected, even with specially made neutron detectors that are costly. The best special neutron detection systems, (3He - helium3), which are the most costly and rare, can detect a proportionally smaller herd of neutrons. The first pass amateur efforts are usually at lower energies, (lower voltages) and the heard of neutrons is often abysmally small. More voltage means a bigger herd of these little beasties and easier detection. Thus, the better the detector you arm yourself with, the lower the voltage where you will be able to prove you are doing fusion. In the end, it is all about proving you have done fusion and not just saying it.

3. "D-D fusion" - Fusion is a process, whereby, two atomic nuclei are forced to fuse together against their will. The coulombic forces of two positive nuclei just will not allow them to fuse casually on a regular basis. Fusion is a probabalistic process even under ideal conditions and high energy.

Some atoms are more easily fused than others. It turns out that only a very few of the very lightest elements can undergo fusion at readily realizable energies here on earth. In a malestrom of fast moving nuclei, every now and then, two colliding nuclei will fuse via a quantum tunneling process, totally bypassing the ever resistive coulombic force. This bizzare process will not be explained or discussed here. It is a probabalistic process based on a number of factors. The energy is a major contributor and probability tables and graphs over various energies have been empirically determined. These are called fusion cross section probabilities.

Cross section??? A dreamed up and empirically determined relative area for a two particle collision at a given energy..........Look at the chart below and see how the particles virtually "swell up" at higher energies. Fusion collisions are more probable with swollen particles. On a busy one lane highway with traffic moving in all directions, you have a much higher probability of two giant moving vans colliding than two Yaris' or smart cars. Wasn't that a cute example? Actually it is an area of relative approach over which two nuclei have a high probability of quantum tunneling and fusing.

For deuterium-deuterium fusion there is a cross section graph that shows this probability increasing rather continuously as voltage is raised for 0 to over 1.5 million electron volts. The best slope or highest probability per applied electron volt is between 10kev and 100kev. You will gain almost nothing at all other than more horrid and deadly x-radiation by going over 200kev in D-D fusion. There is a viable and rapidly increasing probability above 20kev. However you are doing fusion at 5kv applied and 10kv and 20kv and 50 kv and 100kv!!!! This takes us back to proof. You will never detect fusion and prove you are doing it until you are well over 20kv applied and, even then, only with a good neutron detection scheme. Your herd of neutrons is just too small.


Now do you see? It is about a combination of voltage and how good your neutron detection capabilities are. As much effort should be devoted to neutron detection as to your fusor construction or you will never prove fusion.

Statements like, "fusion begins for the amateur at 20kv", might be taken with a grain of salt. At 20kv you will need a really nice neutron detection scheme. Some amateurs can rise to the occasion, others never will. Above 30kv, neturon detection gets cheaper and easier real fast. At 40kv applied, a properly configured, $70.00 civil defense GM counter will easily show that fusion is occuring.

In the end, it is all up to you to seek a balance and be successful at getting a lot of voltage to, and suitable detection around your fusor.

Thanks to Allen Wallace for the suggestion to put up a cross sections graph. I include this classic chart with a select three most viable candidates, which is by no means all inclusive for all fusion fuels. For those adroit enough to read the log-log data plot, we are treated to three fuel groups. You can see why those giant multi-billion dollar efforts focus on D-T fusion. We are fortunate that D-D is as good as it is, since it is the only one we can use.

Of all the fusion fuels, you get the most bang for your energy buck, the earliest, with only D-T and D-D. Man! Are we lucky or what? What is the worst light element fusion fuel? you guessed it....What the sun uses...Protium. This crap does not like to fuse at all! Great news for us, of course, as the sun would burn up like a cheap, over nitrated cigarette in no time.

Ricahrd Hull

Re: FAQ - At what voltage does fusion begin?

Posted: Tue Feb 10, 2015 3:04 am
by Peter Schmelcher
Richard the horizontal axis of the graph might improve with some clarification. My understanding is the empirical data is usually measured as beam target but authors often massage the data into a center of mass frame.

Is the keV axis of the graph beam target or beam counter direction beam?


Re: FAQ - At what voltage does fusion begin?

Posted: Tue Feb 10, 2015 8:19 pm
by Richard Hull
I have no idea whether it is beam on target or center of mass as the source did not specify, but like you note, most such charts are adjusted to center of mass, which is my working assumption. The important thing is that the general relationship to the other fuels is fixed. D-D is certainly the number two goto fuel of all of them if simple fusion is demanded at the lower energies and certainly the only one we can effectively use.

Emperically, within the standard fusor, we know that easy, useful fusion for the amateur experimenter begins at about 30kv and great experimental usefulness begins above 35 kv in the properly worked D-D gas environment. Regardless, the cross section for D-D is not stellar by any account at any reasonable voltage and current obtainable by the amateur before the system becomes a hazardous x-ray source.

In the end, D-D fusion that can do superlative neutron production for a large array of informed amateur experiments is best handled between 40 and 60kv applied. Where fusion begins for the DIY win that so many here seek can be lower provided excellent neutron detection is at hand. As a non-experimenter, non-amateur scientist DIY, this fusion effort can be expensive and frustrating leaving a lot of sad, poorer folks, failed, in its wake.

Richard Hull