Northeastern Illinois University
Society of Physics Students
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Northeastern Illinois University
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| Next Meeting: To Be Determined |
As indicated in several places on this website, the purpose of the SPS is to allow us to practice our physics and interpret it to those in our community. To that end, we will be building a fusor, the details of which will be explained below. It’s our hope that this experiment will allow us to hone our lab skills and provide a valuable opportunity to exercise lab problem solving. Experiments don’t often run the way they’re supposed to, so we’re looking forward to encountering some challenging, yet surmountable, hurdles. In the following sections we hope to provide an understanding of what a fusor is and the physics involved in its operation.
Philo Farnsworth was the mind behind the first fusor design. He is also known for developing some components that allow a television to operate, namely the vacuum tube display. During his television research he discovered a vacuum tube design that would allow for electron focusing when a high frequency magnetic field was introduced to it. Taking advantage of this concept he designed a pair of cylindrical electrodes that were given a high positive potential. When light ions were accelerated and fired into the electrodes, they would focus and ideally fuse. The charge on the electrodes would not only serve to focus the ions, but also keep them from interacting with the walls of the container. His name for this system was inertial electrostatic confinement.
Unfortunately, this design for the Farnsworth fusor could not be scaled up with ease. Enter Robert Hirsch. Hirsch determined that fusion could be achieved with much less fuel and power use if the ions were created inside the device. Two concentric spherical electrodes were designed; one containing a high negative potential, and the other kept at zero potential. If the electric field between the electrodes were high enough, the potential gradient would cause coronal discharges (described later), which would create ions. These ions would be accelerated toward the center where they would collide and possibly fuse. The center negative electrode would then serve to contain the plasma in the center of the device. In principle, this design was very similar to Farnsworth’s original fusor, however it later became known as the Hirsch-Meeks fusor.
This is the basic layout of most fusors. Some of the components included here are optional, and this schematic should only serve as a guide to what a fusion project could entail. Note that there are four areas of focus: current supply, gas delivery, vacuum and pressure maintenance, and detection.
At this point, let’s discuss briefly what’s happening when the fusor is activated. First of all, a vacuum is created in the device to limit unwanted collisions, and remove any dielectric material from between the electrodes. The high voltage power supply delivers a high negative voltage to the center electrode (the small criss-crossed one). The outer electrode is connected to ground and kept at zero potential. This in turn creates a very strong electric field between the two grids. That field is important, as it will come into play during ion creation. Once the field is established a very small amount of Deuterium is injected into the fusor. In the electric field the Deuterium atoms become polarized and very close to the outer electrode electrons leave some Deuterium atoms. This creates a very small current near the surface of the electrode (coronal discharge). These Deuterium atoms are now ionized and influenced by the negative potential at the center of the fusor.
Like all positive ions in a field, these will be accelerated toward the negative center. In some cases, they may hit the inner grid, but in most cases they will coast through and reach the center of the device where they can hit other ions. Hopefully, there will be enough head-on collisions to allow several Deuterium ions to fuse into Helium. Assuming fusion takes place, the next challenge is to find the proof. Visual inspection of the reaction is not enough to determine if fusion occurred because the plasma itself is too bright, and the number of atoms that actually fuse will be quite low. Detection of the reaction’s byproducts is the key to determining if fusion succeeded. A discussion of the reaction involved is below, but the two possible resultants are neutrons and Helium-3. Neutrons are difficult to detect because they do not interact with much due to their neutrality. Helium-3 would be difficult to detect due to the low number of fusion events producing a low quantity of Helium-3. Considering these products, it’s important to keep in mind the effect they will have on the laboratory surroundings. Please make safety a number one priority when conducting this experiment. The safety concerns for our experiment are discussed in another section.
Fusion is the binding of two atoms that forms a more massive atom. When light elements fuse the energy of the resulting atom is always less than the energy of the two independent atoms that formed it. This results not only in the more massive atom, but also a release of energy in some form. In large-scale fusors the goal would be to harness that release of energy and use it to produce electrical current. Since our fusor will have a low number of fusion events, the extra energy created will not be noticed and the amount we’re putting into the device will greatly outweigh the fusion energy produced. Let’s look at the Deuterium cycle for fusion, assuming that ALL types of fusion can take place in the device:
This combines to:
Remember our assumption was that all fusion could occur in the device; in truth it will not be the case. Our fusor will not have enough energy to allow Deuterium to fuse with Tritium, or Deuterium to fuse with Helium-3. Imposing that restriction, we’re left with only two possible reactions:
The reaction involving Tritium probably won’t happen in our case. Energy that is slightly beyond the capability of this device would be required for Tritium production, however there still is a very small possibility it could happen. That leaves only Neutrons and Helium-3 as likely products.
