Farnsworth-hirsch Fusor

1968 ]]

The Farnsworth–Hirsch Fusor, or simply '''fusor''', is an apparatus designed by Philo T. Farnsworth to create Nuclear Fusion . It has also been developed in various incarnations by researchers including Elmore, Tuck, and Watson, and more lately by Robert W. Bussard . Unlike most controlled fusion systems, which slowly heat a Magnetically Confined Plasma , the fusor injects "high temperature" Ion s directly into a reaction chamber, thereby avoiding a considerable amount of complexity. The approach is known as Inertial Electrostatic Confinement .

When the Farnsworth–Hirsch Fusor was first introduced to the fusion research world in the late 1960s, it was the first device that could clearly demonstrate the production of any fusion reactions at all. Hopes at the time were high that it could be quickly developed into a Practical Power Source . However, as with other fusion experiments, development into a generator has proven difficult, and recent research suggests that there are fundamental barriers to net energy production from such a system. Nevertheless, the fusor has since become a practical Neutron source and is produced commercially for this role. It has been assembled in low-power forms by hobbyists.

HISTORY

Invention

The fusor was originally conceived by Philo Farnsworth , the man who developed the Vacuum Tube Television display. In the early 1930s he investigated a number of vacuum tube designs for use in television (called CRT s), and found one that led to an interesting effect. In this design, which he called the '' Multipactor '', Electron s 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 amplification. Unfortunately it also led to huge amounts of erosion on the Electrode s when the Electron s 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 Electron s at a particular point. One of the biggest problems in Fusion Research is to keep the hot fuel from hitting the walls of the container. If this is allowed to happen, the fuel cannot be kept hot enough for the Fusion Reaction to occur. Farnsworth reasoned that he could build an Electrostatic Confinement system in which the "wall" Field s of the reactor were electrons or Ion s being held in place by the ''multipactor''. Fuel could then be injected through the wall, and once inside it would be unable to escape. He called this concept a virtual Electrode , and the system as a whole the ''fusor''.

Design

His original fusor designs were based on cylindrical arrangements 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 charged electrodes would keep the fuel as a whole off of the walls of the chamber, and impacts from new ions would keep the hottest plasma in the center. He referred to this 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.

Robert Hirsch

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 (negatively 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.

Work at Farnsworth Television labs

New fusors based on Hirsch's design were first constructed in the later 1960s. Even the first test models demonstrated that the design was effective; soon they were showing production rates of up to a billion per second, and rates of up to a trillion per second have been reported.

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 no longer going to be a telephone/electronics company, 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 was not regarded as immediately profitable. 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 the timing was bad; Hirsch himself had recently revealed the great progress being made by the Soviets using the Tokamak . In response to this surprising development, the AEC decided to concentrate funding on large tokamak projects, and reduce backing for alternative concepts.

Work at Brigham Young University

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.

Recent developments

In the early 1980s, disappointed by the slow progress on "big machines", a number of physicists took a fresh look at alternative designs. George Miley at the University Of Illinois picked up on the fusor and re-introduced it into the field. A low but steady interest in the fusor has remained since then. An important development was the successful commercial introduction of a fusor-based neutron source.

On fusion process that is 100,000 times more efficient than previous designs. However, the company's funding has run out, and Bussard is looking for additional funding to continue development, which he claims will result in full-scale net-power systems.

THE FUSOR AS A POWER SOURCE

Basic fusion

Nuclear Fusion refers to reactions in which light Nuclei are combined to become heavier nuclei. Several such reactions release energy that can, in principle, be harnessed to provide Fusion Power . The lowest energy reaction occurs in a mix of Deuterium and Tritium , when the ions have to have a temperature of at least 4 keV ( Kiloelectronvolt s), equivalent to about 45 million Kelvin s. At such temperatures, the fuel atoms are ionized and constitute a Plasma . In a practical fusion power plant, fusion reactions have to occur fast enough to make up for energy losses. The rate of reaction varies with the temperature and the density of the fuel and the loss rate is characterized by the energy confinement time τ_E. The minimum conditions required are expressed in the Lawson Criterion . In the Most Successful Approach , the necessary conditions are approached by heating a plasma contained by magnetic fields. This has proven to be very difficult in practice. The complexity of the systems applied detracts from the usefulness of the design for a practical generator.

Fusor fusion

In the original fusor design, several small Particle Accelerator s, essentially TV tubes with the ends cut off, inject ions at a relatively low voltage into a Vacuum chamber. In the Hirsch version of the fusor, the ions are produced by ionizing a dilute gas in the chamber. In either version there are two concentric Electrode s, the inner one being charged negatively with respect to the outer one to about 80 kV. Once the ions enter the region between the electrodes, they are accelerated towards the center.

In the fusor, the ions are accelerated to several keV by the electrodes, so heating as such is not necessary (as long as the ions fuse before losing their energy by any process). Whereas 45 megakelvins is a very high temperature by any standard, the corresponding voltage is only 4 kV, a level commonly found in such devices as Neon Light s and Television s. To the extent that the ions remain at their initial energy, the energy can be tuned to take advantage of the peak of the reaction cross section or to avoid disadvantageous (for example neutron-producing) reactions that might occur at higher energies.

The ease with which the ion energy can be increased appears to be particularly useful when "high Temperature" Fusion Reactions are considered, such as Proton - Boron -11, which has plentiful fuel, requires no radioactive Tritium , and produces no neutrons in the primary reaction. Although the potential advantages make such reactions very popular, other considerations seem to rule out the possibility. See Aneutronic Fusion for a detailing of the problems involved.

Power density

Because a potential well cannot simultaneously trap both ions and electrons, there must be some regions of Charge Accumulation , which will result in an upper limit on the achievable density. The corresponding upper limit on the power density, even assuming D-T fuel will be several watts per cubic meter. Such a system would be completely unsuited for power production. The same consideration leads to a required confinement time of several hours. (See Inertial Electrostatic Confinement for details.)

Thermalization of the ion velocities

When they first fall into the center of the fusor, the ions will all have the same energy, but the velocity distribution will rapidly approach a Maxwell-Boltzmann Distribution . This would occur through simple Coulomb Collision s in a matter of milliseconds, but beam-beam instabilities will occur orders of magnitude faster still. In comparison, any given ion will require a few minutes before undergoing a fusion reaction, so that the monoenergetic picture of the fusor, at least for power production, is not appropriate. One consequence of the thermalization is that some of the ions will gain enough energy to leave the potential well, taking their energy with them, without having undergone a fusion reaction.

This problem is discussed in detail in ''Fundamental limitations on plasma fusion systems not in thermodynamic equilibrium'' , where the author concludes that "Systems which attempt to maintain highly non-Maxwellian particle velocity distributions without explicit means of keeping the particles non-Maxwellian despite Coulomb collisions (e.g. inertial-electrostatic confinement and migma) ... cannot operate without having to circulate a prohibitively large amount of power in comparison with the fusion power".[http://theses.mit.edu/Dienst/UI/2.0/ShowPage/0018.mit.theses%2f1995-130?npages=306&format=inline&page=195]

Electrodes

There are a number of unsolved, and possibly unsolvable, problem with the electrodes in a fusor power system. To begin with, the electrodes cannot influence the potential within themselves, so that the fusion plasma will be in more or less direct contact with the inner electrode, resulting in contamination of the plasma and destruction of the electrode. Also, the transparency of the electrode will have to be unbelievably good since an ion will have to pass through it on the order of 10¹⁰ times before undergoing a fusion reaction. (See Inertial Confinement Fusion .)

Bremsstrahlung

Another serious concern is Bremsstrahlung (German for "stopping radiation"). In ''Fundamental limitations on plasma fusion systems not in thermodynamic equilibrium'' it is shown that these losses will be prohibitive for any fuel other than D-T (or possibly D-D or D-He3).

FUSOR AS A NEUTRON SOURCE

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