FAQ - Fusor - detailed theory of operation

If you wonder how/why fusion works, or how/why the Fusor works, look here first.
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Richard Hull
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FAQ - Fusor - detailed theory of operation

Post by Richard Hull »

The simple fusor is the best example of how fusion can be done on the cheap.

Physical description........

In our example, we will take a stainless steel spherical fusor of the type so common to budding fusioneers found here.

The unit consists of an electrically conductive, vacuum tight, metallic outer shell.

Within the outer spherical shell casing is found a single, centrally located, electrically conductive spherical grid that is more or less transparent in that it is made up of a hollow, wire ball. This "central" or "inner" grid is supported by a rod-like stalk and electrically connected to the outside world, by an insulated terminal on top of the sphere. This "feedthrough" insulator is also vacuum tight.

There are other add-ons that facilitate operational control and observation, such as view ports, vacuum gauge ports and the mandatory vacuum system connection which allows the air inside to be removed. There is also a mandatory gas line which allows the introduction of a fusionable gas. This gas is always deuterium in a simple amateur fusor.

What verbiage describes the fusor best?..............

The fusor can be considered to be an " electrostatically focused,and accelerated, deuteron collider type of fusion device" relying on "inertial electrostatic confinement" to allow fusion to take place in "velocity space".

How is it viewed in terms of its fusion function?...................

The fusor, as a form of accelerator, is a closed electrical system, voltage gradient device. It demands input energy to achieve fusion. It will not self sustain or achieve "ignition" as is classically sought in an energy producing fusion reactor. (none of this desirable breed have ever existed on earth). As such, the fusor is not, nor will it ever be, a net energy producer.

What is the fusion reaction?..........

The reactions found in the deuterium-deuterium fusor (d-d) manifest themselves in two forms, each being roughly of equal probability. (50:50). These reactions are as follows:

d + d = He3 (.82mev) + n (2.45mev)
d + d = H3(1mev) + p (3mev)

Stated verbally, d-d can make a reaction occur that yields a Helium 3 atom, (stable), with a kinetic energy of .82mev and a Neutron of 2.45 mev kinetic energy. 50% of the time, d-d can also form a reaction that yields a tritium atom, (radioactive), with a kinetic energy of 1 mev and a proton of 3 mev kinetic energy.

All of these particles except the neutron will NEVER leave the fusor, but collide with other gas atoms in the device and or the metal outer shell wall. Here, their kinetic energy will be transformed into HEAT and X-rays/gamma rays. NOTE** these X-rays/gamma rays can be of massive energy, (up to 3mev!), but result in a normal, external, net x-ray current in the sub atto-ampere range and effectively be undetectable due to their large penetrating power. The neutrons will pass right through the casing as if it were not there. Thus, we say that the fusor is a "neutron producing device".

There is a third reaction possible only once every 20,000+ fusions that is not part of any real discussion of d-d "hot" fusion and it is:

d + d = He4 + gamma ray with about ~20 mev of energy distributed among the two particles.

What is the physical process and how does it happen mechanically?.........

The fusor device, first, has all the air extracted via a vacuum pump. This is much easier said than done. Much time, expense and effort is put forth in attaining this mandatory goal.

The required pressure for evacuation is a minimum of one micron or 10e-3 torr. It is far better if one can achieve lower pressures in the 10e-4 to 10e-6 torr range. Such higher vacuum levels would indicate a more professional job and represent very "clean" and well sealed fusor.

Ultimately, any vacuum achieved will be filled back up to a pressure of about 10 microns (10e-2 torr) with the reactant gas, deuterium, from which the fusion is actually derived. Getting the gas there and regulating it is another mission that must be accomplished for fusion to take place. Typical d-d pressures that can do fusion can have a very wide range 2-30 microns. 10 microns is just an average where really good fusion is normally found to occur. 2 microns will produce minimal fusion that just is not normally detectable and 30 microns, in most fusors, will produce a tremendous amount of fusion. However, this high figure is rarely achieved without melting a grid. The fabulous fusion range is 5-20 microns in a 6" spherical fusor.

We now have a fusor device that is evacuated of all air and re-pressurized to only about 1/100,000th of an atmosphere of pure deuterium gas. There is still a vacuum in the vessel, obviously, but all the gas in the vessel is a fusion ready, deuterium gas.

To make fusion happen, we must apply energy externally to the device. This energy is electrical energy. This electrical energy is applied as a very high voltage gradient across the two fusor electrical components, the outer shell and the inner grid. This potential gradient can be as low as ten kilovolts to cause fusion to commence, though over 20 kilovolts is needed to make readily detectable fusion with normal instrumentation found in amateur hands.

This application of electrical energy does two very important things.

1. It supplies the energy necessary to strip the outer shell electrons from deuterium gas atoms. This turns them into "ions" called DEUTERONS which are a naked hydrogen nucleus with one neutron and one proton in it.

2. The potential gradient established between the negative inner grid and the positive outer spherical shell forces the, now positive, deuterons created within the inter grid gas region to push away or be repelled from the positive outer shell and rush or be accelerated towards the highly negative inner grid. (Opposite charges attract, like charges repel)

It only takes a minimum gradient of 14 volts or so to ionize a deuterium atom, transforming it into a deuteron. With such a huge gradient as we apply, deuterons can be created over the entire fusion gas volume!! This process is called high-field ionization. Most of this ionization occurs near the inner grid due to the small radius wires. Deuterons created here are lost to fusion.

Due to the laws of physics and conservation of energy, the location of a particular deuteron's creation within the volume of chamber gas is of key importance in its successful fusion.


In any accelerator, the bombarding particle is accelerated by falling through a potential gradient. The ideal is to have the particle fall through the entire gradient to allow it to rise in energy to the full gradient potential. Thus a deuteron falling from its creational point at zero energy, to a target of the opposite potential of say 100,000 volts would arrive at that target with a 0.1 mev energy or 0.1 million electron volts.

In the average fusor the field gradient is spherical. This is fantastic, for it allows not just a beam of deuterons to collide with the target (other deuterons) but deuterons from anywhere in the vessel to fall into a central point. This is ideally where they are at a maximum velocity and can collide with each other summing their velocities and quadrupling the collisonal kinetic energy (1/2mv^2).

In reality, and as deuterons are created all over the gaseous fill region, a 20kv fusor would have deuterons of 4kev colliding with deuterons of 10kev and of 1kev colliding with 18kev, etc, etc.

What's wrong with this?

CROSS SECTION..........................

There is a term called collisonal cross section and this relates to the probabilities of two identical particles actually being able to collide and do fusion. Without delving into the specifics, in general, higher energy particles colliding have more probability of fusing than lower energy particles. The graph of energy versus cross section are all non-linear and some are very bizarre. Most cross sectional data is gathered empirically from experiment.

There exists a well known chart for the cross section of the d-d collisional fusion reaction. As the deuterons rise in energy there is an ever increasing probability of fusion up to nearly 3 mev. where it rolls off again towards zero. All such collisional fusion in the amateur fusor is accomplished via a quantum tunneling process not to be explained here.

It turns out that in a practical situation, for easy neutron detection, (the normal signature heralding fusion in a fusor), the fusor needs to have an applied gradient potential of about 20kv. The bulk of the fusions demand head on, near full energy collisions of deuterons. Inspite of currents of 10e14 to 10e16 deuterons/second, only about 10,000 d-d fusions occur per second at this voltage. This makes the fusor a poor energy conversion device, (electrical energy to fusion energy). BUT, It is cheap and easy to fabricate judged by any other fusion energy standards around today.

What happened to all those other deuterons in the current?............

They just fall back to their creation point, for they can go no farther than their zero energy point. Thus, they turn around and are re-accelerated back for another go around. The thrifty fusor can reuse some of its old deuterons! Unfortunately, at the operational pressures in an amateur fusor, the very density of deuterons that make the device so attractive, also limits the "mean free path" of any particular deuteron to about 6 inches, though many do go farther. As such, there is some "re-circulation" but even a very lucky deuteron would rarely get a third pass in an 6"-8" diameter amateur fusor. Some deuterons will not even be able to complete one pass!!

Those deuterons that do not fuse and do not recirculate (99.99999 percent of the total deuteron count), just recombine with electrons and become fast neutrals losing the energy they have slamming into the walls of the vessel, thereby, heating it a small amount.

There is also a degree of beam-on-target fusion due to fast deuterium neutrals entering the chamber wall's metallic surface and fusing with other fast neutral deuterium atoms impacting them in the wall. The fusor has many fusion reactions occurring all over the place.

Fusion process......................

All fusion relies on quantum tunneling as both deuterons are positively charged. They can never touch and fuse mechanically or electrostatically in a simple fusor! Quantum tunneling is a bit complicated to explain, but suffice it to say that the energy of the approaching particles often gets the two particles within range of the nuclear strong force which takes over from coulombic repulsion and they fuse.

What about all those electrons we stripped off way back at the beginning of this FAQ?.....

These represent the greatest loss and maximal heating component of the fusor shell. These are accelerated just like the deuteron, but towards the shell wall!! They slam into it producing x-rays and accounting for the bulk of the heating of outer shell wall. NOTE**** at voltages above 30 kv applied these electron generated 'wall x-rays' will start to "shine" through the shell along with the neutrons, creating a new and very serious radiation hazard that must be shielded against. That's right X-Rays! In most all fusors these are the real worry for the operator long before the neutron level gets serious. Blessedly, the actual "neutron run" is on the order of 5-10 minutes limiting one's exposure time. Read up on this in the radiation FAQs.

So there you have the rough workings of the amateur fusor.

If you have gathered the fusor is a terribly inefficient fusion engine, then you have listened well. If you think it can be improved, then have at it in a hands on mode and keep us informed.

The one positve note is that there is no currently available d-d fusion system or engine that can out perform, watt for watt out of the wall outlet, and dollar for dollar out of pocket, the simple Farnsworth fusor!

Given that we are limited to d-d fusion for the most part, as amateurs, and that materials limit other aspects of amateur fusion, the best you might hope to do is about 2 million plus fusions per second in a 6-8" fusor with 40 - 50kv applied to the device. Multiple order of magnitude mechanically related improvements are just not possible beyond raising the potential gradient and creating a deadly x-ray radiation hazard.

It would also be very instructive to look at the "mean free path" FAQ in the vacuum forum for more data on how the fusor works.



This section added 12/14/04 relates to the rather high loss of created deuterons close to the inner grid.........RH

The fusor, as we build it, gets its ionization through field emission and the subsequent collisions of electrons with neutrals. So, where we find the most electrons is where most of the ionization takes place. The electrons and therefore the ions will be created at the highest density points of high field gradient within the fusor. The highest gradient is just at the inner grid wires. (Tiny radius wire= high field zone)

This literally means that virtually zero ions created in the area of greatest ionization stand any chance of fusing. This is all lost energy. While ions are created throughout the volume of the fusor, the bulk of ionization occurs at a place within the volume where the deuterons can do little good.

The best fusor is a gunned fusor or a fusor with an ionizing grid. very close to the shell. While still grossly inefficient, a very much larger percentage of ions created per unit input energy will stand a better CHANCE at fusion.

With the inner grid still in place, we would still have the high losses of ionization near it. That would not go away. It might turn out that we would apply about double the energy to ionize, (one price at the inner grid and another creating them where we want them), but at least half would now be doing decent ion production in the proper region of the fusor.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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