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2 Basic fusion 3 Fusor fusion 4 Fusor as a neutron source 5 References 6 External links |
The fusor was originally conceived by Philo Farnsworth, the man who is largely responsible for television. In the early 1930s he investigated a number of vacuum tube designs for use in television, and found one that led to an interesting effect. In this design, which he called the multipactor, electrons moving from one electrode to another were stopped in mid-flight with the proper application of a high-frequency magnetic field. The charge would then accumulate in the center of the tube, leading to high amplication. Unfortunately it also led to huge amounts of erosion on the electrodes when the electrons eventually hit them, and today the multipactor effect is generally considered a problem to be avoided at all costs.
What particularly interested Farnsworth about the device was its ability to focus electrons at a particular point. In the early days of controlled fusion experiments in the 1950s one of the biggest problems was to keep the heated fuel from hitting the walls of the container, if this were allowed to happen the fuel would rapidly cool off, leading to a huge loss of power. Farnsworth reasoned that he could build an electrostatic confinement system in which the "walls" of the reactor were electrons or ions being held in place by the multipactor. Fuel could then be "injected" through the wall, and once inside they would be unable to escape. He called this concept a virtual electrode, and the system as a whole the fusor.
His original fusor designs were based on cylindrical arrangments of electrodes, like the original multipactors. Fuel was ionized and then fired from small accelerators through holes in the outer (physical) electrodes. Once through the hole they were accelerated towards the inner reaction area at high velocity. Electrostatic pressure from the positively charge electrodes would keep the fuel as a whole off of the walls of the camner, and impacts from new ions would keep the hottest plasma in the center. He referred to as inertial electrostatic confinement, a term that continues to be used to this day.
Various models of the fusor were constructed in the early 1960s. Unlike the original conception, these models used a spherical reaction area but were otherwise similar. Farnsworth ran a fairly "open" lab, and several of the lab techs also built their own fusor designs. Although generally successful the fusor had a problem being scaled up, since the fuel was delivered via accelerators, the amount of fuel that could be used in the reaction was quite low.
Things changed dramatically with the arrival of Robert Hirsch at the lab. He proposed an entirely new way of building a fusor without the ion guns or multipactor electrodes. Instead the system was constructed as two similar spherical electrodes, one inside the other, all inside a larger container filled with a dilute fuel gas. In this system the guns were no longer needed, and corona discharge around the outer electrodes was enough to provide a source of ions. Once ionized the gas would be drawn towards the inner (negativily charged) electrode, which they would pass by and into the central reaction area.
The overall system ended up being similar to Farnsworth's original fusor design in concept, but used a real electrode in the center. Ions would collect near this electrode, forming a shell of positive charge that new ions from outside the shell would penetrate due to their high speed. Once inside the shell they would experience an additional force keeping them inside, with the cooler ones collecting into the shell itself. It is this later design, properly called the Hirsch-Meeks Fusor, that continues to be experimented with today.
New fusors based on Hirsch's design were first constructed in the later 1960s. Even the first test models demonstrated that the design was a "winner", and soon they were producing production rates of up a billion per second, and has been reported to have observed rates of up to a trillion per second.
All of this work had taken place at the Farnsworth Television labs, which had been purchased in 1949 by ITT with plans of becoming the next RCA. In 1961 ITT placed Harold Geneen in charge as CEO. Geneen decided that ITT was not going to be a telephone/electronics company any more, and instituted a policy of rapidly buying up companies of any sort. Soon ITT's main lines of business were insurance, Sheraton Hotels, Wonderbread and Avis Rent-a-Car. In one particularly busy month they purchased 20 different companies, all of them unrelated. It didn't matter what the companies did, as long as they turned a profit.
A fusion research project didn't. In 1965 the board of directors started asking Geneen to sell off the Farnsworth division, but he had his 1966 budget approved with funding until the middle of 1967. Further funding was refused, and that ended ITT's experiments with fusion. The team then turned to the AEC, then in charge of fusion research funding, and provided them with a demonstration device mounted on a serving cart that produced more fusion than any existing "classical" device. The observers were startled, but even by this point all available funding had been locked up by large research projects who resisted any funds being allocated to "new" systems, no matter how promising.
Farnsworth then moved to Brigham Young University and tried to hire on most of his original lab from ITT into a new company. The company started operations in 1968, but after failing to secure several million dollars in seed capital, by 1970 they had burned through all of Farnsworth's savings. The IRS seized their assets in February 1971, and in March Farnsworth suffered a bout of pneumonia and died. The fusor effectively died along with him.
In the early 1980s the round of "big machines" had demonstrated themselves to be no more practical than the earlier generations, and a number of physicists started looking at alternative designs. George Miley at the University of Illinois picked up on the fusor, and re-introduced it into the field. The fusor has remained a popular device since then, and has even become a successful commercial neutron source.
Controlled fusion attempts to cause ions to fuse by forcing them together at high energies. The lowest energy reaction occurs in a mix of deuterium and tritium, when the ions have to have a combined energy of about 4 keV (kilo-electron volts). Temperature is the average kinetic energy per unit volume, so any energy measure can be converted into a temperature with the conversion ratio of 1 eV = 11604.45 K. In this case the D-T fusion threshold temperature is about 45 million degrees Celsius.
In order to make such a reaction practical, some significant fraction of the expensive fuel used must undergo fusion and generate power. This rate varies with temperature, and the total number of fusion events with the amount of time that the fuel is held at a particular temperature. This relation is known as the Lawson Criterion, and contains a Catch-22 – as the temperature of the fuel is increased it becomes increasingly difficult to "contain" it for the needed amount of time.
In traditional designs, this is achieved by slowly heating a plasma fuel that is being contained by magnets. This approach has proven to be very difficult to achieve in practice, as the fuel tends to "leak out" of the reaction area too fast to heat it to the required temperatures. Increasingly complex systems have been introduced to quickly heat the plasma, but these detract from the usefulness of the design for a practical generator.
The fusor attempts to avoid heating problems by adding the required energy directly to the ions. Whereas 45 million degrees sounds impressive (and is), it is important to remember that it corresponds to about 4 keV, the energy that an electron would gain by being accelerated between two electrodes charged to 4 kV. In the grand scheme of things 4 keV is a very minor amount of energy – it is commonly found in such devices as neon lights and televisions.
In the original fusor design, several small particle accelerators, essentially TV tubes with the ends cut off, provided a small amount of this energy. Once the ions entered the reaction chamber they found themselves being pushed towards the center by the charge on the electrodes, which was charged to about 80 kV.
In the Hirsch version the basic mechanism consists of two concentric spherical grid electrodes in a vacuum chamber containing a very dilute fuel gas. Depending on the design, the inner electrode is negative and thus accelerates ions toward the center of the chamber, or alternately the inner electrode is positive and accelerates electrons towards the center. Most research has focused on ion acceleration: the ions, being heavier, are much easier to focus and give a consistent energy.
In theory the fusor is perhaps the most promising form of fusion reactor studied. Energy is added to the fuel directly through acceleration, as opposed to the various indirect means required in a Tokamak or similar magnetically confined systems. Better yet, since the fusor is accelerating the ions (or electrons) directly, the range of velocities (or temperatures) is quite narrow. This means that most of the ions have enough energy to undergo fusion, whereas in a magnetically confined system it is typically only the "hottest" ions that can. Finally, failed collisions scatter inside the reaction area, heating other ions around them, thereby returning some of the energy to the reaction.
Another advantage to the fusor is that any ion can be accelerated easily, not just the "low temperature" mixes like D-T. This makes the fusor particularly useful when running on other potential fusion fuels with much higher threshold temperatures. One of the most attractive such combinations is the proton - boron-11 reaction, which uses cheap natural isotopes, produces only helium, and produces neither neutrons nor gamma rays. This is a very clean reaction that would dramatically reduce waste when decommissioning a plant, and there is considerable interest in such aneutronic fuels.
Nothing in fusion is ever easy however. In the fusor a number of problems conspire to rob energy from the ions as they move towards the reaction area. One problem is the presence of "cooler" unionized particles of gas in the system, which can collide with the ions and cool them. Another problem is the presence of the inner electrodes, since ions often hit them and spray the reaction area with high-mass ions which soak up considerable energy from the surrounding fuel through collisions and then radiate the heat away as X-rays. This problem plagues traditional fusion designs as well, where it is known as sputtering.
A more serious concern was first outlined in 1994. In his doctoral thesis for MIT, Todd Rider did a theoretical study of all non-equilibrium fusion systems, of which the fusor is one of many. He demonstrated that all such systems will leak energy at a rapid rate due to Bremsstrahlung, radiation produced when electrons in the plasma hit other electrons or ions at a cooler temperature and suddenly decelerate. The problem is not as pronounced in a hot plasma because the range of temperatures, and thus the magnitude of the deceleration, is much less.
In most of the systems that he studied, the energy radiated away from the system was greater than the energy of the fusion itself. Unless a significant amount of energy from this radiation, namely X-rays, was captured, the system would never "break even". The problem is dependent on the mass of the fuel ions, so D-T and D-D fuels still provide net energy, but many of the more interesting aneutronic fuels appear to be impossible to use as an energy source.
Regardless of its eventual use as an energy source, the fusor has already been proven extremely useful as a neutron source. Fluxes well in excess of most radiological sources can be made from a machine that easily sits on a benchtop, and can be turned off at the flick of a switch. Commercial fusors are now produced by a number of companies, including such industrial giants as DaimlerChrysler.
Industrial might is not required to build a fusor however, and small demonstration fusors that achieve fusion (but not break-even!) can and have been constructed by amateurs, including high-school students for science projects. Each electrode is spot-welded from hoops of stainless-steel wire (often welding rod) at right angles. The fusor's electrode dimensions are not very critical. The outer electrode can range from beach-ball to baseball size, and the inner from baseball to ping-pong ball size. Usually such projects use the high-voltage transformer from a neon sign, and high voltage rectifier from a hobby shop. Spark plug wires carry the power, with spark plugs to pass it into the vacuum chamber. Deuterium is available in lecturer bottles and is not a controlled nuclear material. Neutrons can be sensed by measuring induced radioactivity in aluminium foil after moderating the neutrons with wax or plastic, or a plastic neutron luminescent material can be used with a photodetector. The major expense is the vacuum pump. Note that the voltages are dangerous (though less dangerous than a TV), and neutron emissions do present some hazard. The X-ray emissions are less than those of a color TV since the voltages are less.
History
Basic fusion
Fusor fusion
Fusor as a neutron source
References
External links