The essential nature of the fusor is a method for creating a large negative space charge in the center of some geometry, usually spherical, although not necessarily so. The grids described by Zarchon are the most common way to do this, and by far the simplest, but not necessarily the best way to go about it. Prof Bussard has posited in several patents and scientific papers that it is possible to create the same effect by confining a group of electrons in the center of the fusor device, with magnetic fields. This has the enormous advantage that you do not lose energy to collisions between fuel ions and the grids. Bussard is known to be working on fusor research with Tom Ligon, but they are keeping it quiet.

The nature of the fusor is, as has been noted by several theorists, to shift fusion from the thermal domain to the velocity domain, i.e. instead of heating some fusion fuel to a sufficiently high temperature and hoping it fuses, you accelerate and focus a population of fuel ions at a single region, fostering head-on collisions and therefore more fusion events per unit input energy, at least in theory. This means you can use fuels other than the really easy ones like deuterium and tritium.

The most interesting alternative fuel combinations are helium-3/deuterium and boron-11/proton. Both are aneutronic, meaning they do not spit out neutrons to make the reactor structure radioactive. The He3 reaction is by far the easier to do; however, the reaction products have widely divergent energies, which means that the energy has to be extracted with a thermal blanket and a heat engine. This is where the boron-11 combination becomes really attractive, if it can be done. All of its energy is released as three alpha particles in a narrow range of energies. Since alpha particles are all positively charged, it is a relatively simple matter to extract the energy directly to electricity.

The fusor has been commercialized. Prof. Miley, of UIUC (IIRC -- feel free to correct me) has teamed up with DaimlerChrysler Aerospace to produce a scientific neutron source based on the fusor concept.

There are people doing hydrogen fusion in their garages.

Hydrogen fusion is a nuclear reaction, as opposed to igniting a balloon full of hydrogen, which is a chemical reaction, or just popping a balloon full of hydrogen and letting the gas escape, which is a physical process.

A nuclear reaction means you've done something to the nucleus of an atom. It's alchemy. Atoms that come out of the process are no longer the same stuff you started with.

There are a bunch of different recipes for hydrogen fusion, but the one used most frequently by hobbyists is this one: fuse two deuterium nuclei together and get a result of one helium nucleus and one free neutron. Home fusion people know they have achieved garage fusion when they can detect free neutrons coming out of their machines.

Now, there are all sorts of interesting engineering difficulties in building a home fusion reactor. You have to have a good vacuum chamber. You have to have a high voltage source. You need to get your hands on deuterium. And you need a way to detect flying neutrons, which is not usually done outside major laboratories because the typical citizen isn't exposed to any meaningful flux of disassociated neutrons nor is there a need to detect such a condition.

But these challenges are within the solution reach of the typical high school student. In fact, high school students have created hydrogen fusion reactors for their local science fairs. So if you are older than a high school student and maybe have equivalent or greater education, and equivalent or greater means, then hydrogen fusion is within your reach.

Hydrogen fusion is what makes the sun go, by the way.

Scientists in expensive labs have been doing hydrogen fusion for some decades. For a long while there was the idea that it could be a cheap energy source. If you could fuse all the hydrogen in a liter of water you could supply the electric demands of a major city for a month. When I say, "fuse the hydrogen", what I am saying is that you would take all the H2O molecules of the water, split off the two hydrogens from every molecule to get elemental hydrogen, then smash the resulting hydrogen atoms together. The result will be helium atoms, and extra energy. That extra energy is what would power the city. You'd have lots of helium for party balloons or blimps to boot.

Of course, as nobody is fusing tap water to fuel cities, you can surmise the existence of some quirky detail that prevents large-scale production of energy that way. There is. We can fuse hydrogen plenty good, but it requires more energy to split the H2O (or other) molecules out of the water, and then fuse the hydrogen, than you'd ever get from the energy released by the reaction. At least that's true the way we've been going about it.

The fusion reactions you can do in your garage fit into the category of "inefficient". You have to put more energy in than comes out, so there's absolutely no possibility of any kind of nuclear explosion going on there - that is, unless you can get more power than a hydrogen bomb from your wall socket. That's probably a good thing or home fusion enthusiasts would have accidentally annihilated the world a thousand times over by now.

The garage fusion device everyone builds is called a Farnsworth-Hirsch fusor. You may be familiar with the name Farnsworth if you've been interested in electronics or generally haven't been living in a cave wearing mammoth-skin jackets for the last seventy years. Philo T Farnsworth invented many things, among them, television.

He also invented a device that would electrostatically contain a plasma and allow fusion. Robert Hirsch refined that original design and made it infinitely more practical to build. Hence their names are stapled on device together.

A home fusor is simple to imagine. Create a vacuum. A bell jar with a pump will do. You can find this sort of thing in a high school physics lab. In the bell jar you place two electrodes. These electrodes are curved wires, or circles arranged to form a very open sphere, or usually imperfect approximations thereof. One is inside the other. You connect those spherical electrodes to a high voltage source - like the rectified output of the sort of neon sign transformer you use to run a tesla coil. You pump out most of the air in the bell jar - get it down to the sort of vacuum that exists in outer space, introduce a little deuterium gas, turn on the voltage, and voila. Fusion.

This whole process looks like magic. You tube videos of the process look like they came right out of the sort of sci fi movie where the mad scientist starts his machine and then warps the entire universe onto a catsup-encrusted melmac seat in a McDonald's on Mars.

What you see is this: A glowing ball of plasma forms inside the spheres. The glowing ball emits jets of radiation.

If they've got fusion, x-rays come streaming out. So do neutrons.

By the way, neither of these beams of things is particularly good for you, so most fusor builders situate a camera near the reactor to send an image to a TV screen some distance away so you don't have to be irradiated with cancer causing beams by your own machine.

If you don't introduce the deuterium to the bell jar, and just let the electrodes sit in the rarefied air when you turn on the voltage, you see the same light show. You just don't get the radiation. People call this a "Demo" Fusor, because it looks very cool. In fact, you can't tell the difference with your eyeballs whether the thing is making fusion reactions or not. So you get all the wow factor of the fusion reaction, without all the actual radiation.

Imagine the joy of your own mushroom cloud, without any of the ensuing destruction or cancer.

Most normal people might ask why someone would want to build a hydrogen fusion device in one's garage. There is no fame or fortune that can come of it, except in that if you can prove you've generated beams of neutrons you can join one website's "Neutron Club". You have to put in more energy than you get out, so you're not going to lower your electric bill doing it.

Then why?

At this point, if you have enough interest to try such a thing you need no further encouragement or justification from me. Garage fusion awaits you.

And if you're still puzzled as to why people are motivated to such feats of dangerous experimental hoopla, there is no shame. Nobody actually knows, but I suggest we can all make ourselves feel better under the weight of this knowledge by clicking away from here and reading a peaceful political blog or perhaps going outside, turning on the hose, and watering the geraniums.

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