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B Fusion Theory

  1. May 21, 2016 #1
    << Disclaimer by PF -- You may need to get government permits to operate a reactor that produces neutrons and radiation, and this could potentially be a dangerous project >>

    Hello,

    I recently finished building a functioning IEC fusion reactor and posted a video of its construction and operation to YouTube. Here is the link if you are interested:

    .

    I received some requests to make a separate video, which explains how my reactor functions and does fusion as well as more information on how I built it. I am also preparing to present this project to my AP Physics class. I have almost finished writing a script that I will narrate while showing various diagrams, graphs, and videos. I would be very appreciative if someone could review my script for factual errors, possibly over-confusing statements, or missing information. I want this to make sense to laypeople.

    Here is what I have:

    "Nuclear fusion is the process of two lighter atoms combining to form a heavier atom. In this process, large amounts of energy are released in accordance with Einstein’s famous equation: E=mc^2. Unlike nuclear fission, fusion requires a lot of energy to initiate. The sun uses its gravity to squeeze atoms together in its core, but on Earth we lack the technology required to recreate such extreme conditions. Instead, scientists must find other ways to initiate nuclear fusion. For amateur scientists, the easiest is a method called inertial electrostatic confinement or IEC for short. IEC fusion reactors use strong electric fields to heat atoms to fusion conditions. Temperatures in these reactors usually exceed several hundred million degrees Kelvin, which makes the reactor core the hottest place in our known universe according to scientists’ current understanding. Now you may have some questions right now such as “how is that possible?” or “is that even safe?” or “can this be done in my bedroom?” Don’t worry, all of those questions will soon be answered.

    A standard IEC fusion reactor consists of two vacuum pumps, a mechanical vacuum pump and an oil diffusion pump, a gate valve, a main chamber, an inner conductor also called the grid, a power supply, a pressure gauge, and a lecture bottle of deuterium gas with a regulator and needle valve. First, the mechanical vacuum pump is turned on, which evacuates the entire system to a pressure of about one 100 thousandth of an atmosphere. For reference, one atmosphere is the air pressure at sea level. Then the oil diffusion pump is turned on, which takes almost all of the remaining air out of the vacuum system. Interestingly, diffusion pumps have no moving parts. They remove air by shooting scalding streams of oil at the air molecules. The oil then falls to the base of the diffusion pump where the mechanical pump can then easily remove the trapped air from the system. After the diffusion pump is finished pumping down the chamber, the pressure is equivalent to around six trillionths of an atmosphere.

    Then deuterium gas is leaked into the chamber until the pressure rises to about six millionths of an atmosphere. This ensures that the contents of the chamber is only deuterium. Now you may be wondering why deuterium gas is so important. Deuterium is the special name for hydrogen atoms that have one extra neutron in their nucleus. This makes deuterium atoms less stable than regular hydrogen atoms, which means that deuterium atoms can fuse with less energy. Maintaining a constant deuterium pressure is difficult, as both the deuterium leak rate into the chamber and the rate of gas flow out of the chamber must be the same. The needle valve controls the flow of deuterium into the chamber and the gate valve is able to throttle the diffusion pump, limiting its ability to remove gas from the system. After the main vacuum chamber contains a constant pressure of deuterium gas, the power supply can be turned on.

    When the power supply is active, the grid becomes charged to a very high negative electric potential. The charged grid generates a strong electric field. This electric field is so strong, it is able to knock electrons off some of the deuterium atoms. The now free electrons go on to knock more electrons off other deuterium atoms. This process is called ionization and results in the generation of a deuterium plasma. Plasma is the fourth state of matter that consists of ions, charged particles such as atomic nuclei and electrons. A deuterium plasma is basically a mess of positively charged deuterium nuclei, also called deuterons, and free electrons. The positively charged deuterons are attracted to the negatively charged grid. The specific geometry of the grid causes the generated electric field to have weak spots. This can cause beams of plasma to shoot out from the grid’s interior. The deuterons accelerate to very high velocities following a Maxwell-Boltzmann velocity distribution. This means that only some of the deuterons have enough energy to undergo fusion. The grid is bombarded with deuterons, which causes it to glow white hot as the kinetic energy of the particles is transferred into heat energy. Some of the deuterons do not collide with the grid and have a small chance to fuse with each other.

    However, there is something that is not adding up. If two deuterons are both positively charged, don’t they repel each other? The answer is yes. The coulomb repulsion force between them is actually so powerful, they would require an infinite amount of energy to meet. So then why does the sun shine? The answer lies in the strong nuclear force and quantum tunneling.

    The specifics of strong force interactions between subatomic particles can get a little confusing, so here is a brief explanation of why the strong force is so important. Consider a helium atom. Its nucleus is composed of two protons and two neutrons. As I mentioned before, positively charged particles repel each other. The two protons in the helium nucleus want to get as far away from each other as possible, but for some reason they are attracted. This can be explained by the strong force. The strong force is incredibly powerful, but its range is very limited. For two protons to experience an attraction due to the strong force, they must be within just several protons width of each other. However, once they are within that range, it becomes nearly impossible to split them apart. The distance at which the attractive strong force overpowers the repulsive Coulomb force is called the coulomb barrier. In order to overcome that energy barrier, two positively charged particles must approach each other with very high kinetic energies. If they are not moving fast enough, they will simply be repelled without experiencing any attraction due to the strong force.

    In order to solidify this concept, consider a marble on a frictionless track. The track has two very steep hills and a valley between them. Your goal is to find a way to get the marble trapped in the valley. The only tool you have to do this is a spring launcher that can launch the marble at a specified speed. However, after several attempts you will realize that this task is impossible. If you give the marble enough energy so that it can go over the first hill, it will also have enough energy to continue along the path and then go over the second hill. The marble will have to lose energy while in the valley in order to become trapped. You can probably guess that I am not really talking about marbles. Think of the marble as a speeding deuteron and the bottom of the valley as deuteron that is fixed in place. The hills represent both the repulsive Coulomb force and the attractive strong force. As the deuteron approaches the other, the Coulomb force increases just like the slope of the hill. The peak of the hill represents the coulomb barrier. If a deuteron has enough energy to overcome the coulomb barrier, it accelerates towards the other deuteron due to strong force interactions. This is represented by the valley between the two hills. However, just like in the marble example, a deuteron that enters the attractive realm of the strong force can actually escape and pass the coulomb barrier on the other side. This means that in order to accomplish fusion, the deuterons have to perform a nearly perfect collision.

    Now you may think that this solves the deuteron fusion problem, but for our specific case involving an IEC fusion reactor built in my bedroom, it does not. The kinetic energies of the deuterons are simply not high enough to breach the coulomb barrier. Now you may be wondering why I went through that long explanation of the coulomb barrier if it is not even the mechanism that allows fusion to occur. Well, we’ll get back to that in a moment, but right now I need to explain quantum tunneling.

    I will start out by saying that quantum tunneling counters our classical physics based intuition. Subatomic particles such as electrons and atomic nuclei don’t have a defined position until they are observed. These particles’ probabilistic nature can be described by wave function. The probability of a particle being observed at a specific location is dependent on the wave function of that particle. For example, let’s allow the wave function of the speeding deuteron in our previous example to be interpreted as a probability cloud. In this example, the probability cloud is essentially a mass of points, where each point represents a possible location of the deuteron after it has been observed. The points are denser near the center of this cloud as there is a high probability of finding the particle near that location. Going back to our marble on a track example, let’s apply the phenomenon of quantum tunneling.

    Let’s say that the spring launcher gave the marble just enough energy to reach three quarters of the height of the first hill. At the three quarter mark, the marble changes direction and rolls back down the hill as is does not have enough energy to reach the peak. This is consistent with classical physics and does not account for quantum tunneling. However, if quantum tunneling is considered, this seemingly simple test can yield a different and surprising result. Let’s freeze the moment that the marble reaches the maximum height on the hill. If we replace the marble with a probability cloud, you will notice that there are points in the cloud that exist on the other side of the hill. This means that there is a chance that the marble may be found inside the valley if observed. When the marble tunnels through the barrier it does not gain or lose energy, so it cannot escape the valley as it lacks the required energy. The marble then oscillates between the two hills. It is important to point out that if the marble was given more initial energy and was able to climb higher on the hill, its probability cloud would have more of its points in the valley, which means that its probability of tunneling would be higher. But, marbles do not tunnel through barriers. I am really discussing the behavior of two deuterons. The marble tunneling through the hill is an analogy of a deuteron tunneling though the coulomb barrier. When a deuteron tunnels past the coulomb barrier, it becomes trapped by the energy well created by the attractive strong force. The two deuterons then oscillate around each other until one of them releases energy and they fuse together.

    Consider two deuterons that are about to fuse. In order for the fusion to occur, energy must be released. In deuteron-deuteron fusion, there are two possible results that can occur. Either a neutron is ejected leaving a helium-3 atom, or a proton is ejected leaving a tritium atom. This ejection of mass allows one of the oscillating particles to lose energy, which leads to the synthesis of new atoms and the generation of energy."

    Thanks,
    Kuba
     
    Last edited by a moderator: May 26, 2016
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  3. May 21, 2016 #2

    Drakkith

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    Nice work! I always wanted to build an IEC Fusion Reactor, but never did. Overall I think your explanation is just fine for an AP Physics class. There's just one thing I wanted to comment on.

    Technically I think the reason deuterium fuses easier than hydrogen-1 (protium) is because deuterium has two possible stable/metastable bound states after fusing, helium-3 + ejected neutron, and tritium + ejected proton. Without the weak force, Protium can only fuse into Helium-2, which is so unstable that it immediately decays back into two protons. During protium fusion in stars, the weak force turns one of the protons into a neutron, which then fuses with the remaining proton to form deuterium. Unfortunately the chance that the weak force turns a proton into a neutron during a collision event is vanishingly small. So small that it takes, on average, about 1 billion years for any particular proton to fuse with another proton inside the Sun.

    Now, I wouldn't put all that in your explanation. I'd simply say that having that neutron makes deuterium much easier to fuse than regular hydrogen. If anyone cares enough to ask, then you can explain why.
     
  4. May 22, 2016 #3

    mfb

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    Deuterium is completely stable, there is nothing unstable about it.
    10 billion years, otherwise we would not exist. That has to be compared to the fusion timescale for deuterium, which is a few seconds.

    For man-made fusion, p+p fusion is basically nonexistent.
     
  5. May 22, 2016 #4

    Drakkith

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    Can't find my reference again, but I remember it said the half life of a proton was about 1 billion years. Maybe I should have said that instead of saying the "average lifetime".
     
  6. May 22, 2016 #5

    mfb

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    Well, we still have many protons around after 5 billion years and we'll have them around for 5 billion years more, and the sun is not fully convective (the core has 1/3 the total mass). The rate depends on the position in the core, it might be higher in the center.
     
  7. May 27, 2016 #6
    I thought the lifetime of the proton was on the order of 1033?
     
  8. May 27, 2016 #7

    mfb

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    That is the lower limit (!) for proton decay. "Lifetime" in the context discussed here is "how long does the proton exist before it fuses?"
     
  9. Jun 5, 2016 #8
    So if I understand correctly from reading , most of the fusion in a IEC fusor comes from tunneling and only a very small portion is due to direct straight 180 degrees apart collisions? So some of the ions get a good enough angle and velocity to overcome the Coulomb barrier by force?

    If I recall the highest loss in the fusor comes from the way it works , the heated plasma being a good conductor mostly shorts the power supply + to the negatively charged center grid and so most of the energy from the PSU is short circuited? Like a thermal diode of sorts.
    Maybe one should include this in the description you are aiming for , atleast I would love to hear about its weaknesses if i were to hear a lecture.

    P.S. Nice work!
     
  10. Jun 5, 2016 #9

    mfb

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    No design would have any relevant fusion rate without tunneling.
     
  11. Jul 17, 2017 #10
    There was a vial of liquid, but I couldn't very well see what it was. Are you preforming D-D or D-H fusions using the D/H in water as a target? Or is all the fusion resultant from the particles within the vacuum chamber?
     
  12. Jul 17, 2017 #11
    The vial is a neutron bubble dosimeter. It contains superheated fluid droplets suspended in a gel. When a fast neutron strikes the liquid droplets, they vaporize to form visible bubbles. The number of bubbles generated is proportional to the neutron dose experienced by the detector. D-D fusion is occurring inside the reactor. Calculations are described here: http://www.irradiantdesigns.com/mit/#fusion-reactor

    Here is a better video of the whole process:
     
  13. Jul 18, 2017 #12
    @kubaanglin: The video actually provides a clearer picture and your article is amazing, Way to go!!
     
  14. Jul 18, 2017 #13
    I have goals pretty close...ok, almost exactly the same... as yours. Im in high school, researching this stuff like a madman, with the goal of fusion. Ive been looking more into beam-target fusion using a cyclotron, as one can achieve an optimally powerful beam with just a few inch radius cyclotron!
    It would be an amazing help to get some advice. Ive got some detailed sketches, I think now I just need a list of parts and some cash to magically appear .
    Thanks!
     
  15. Jul 18, 2017 #14

    mfb

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    Based on your other threads, I don't think you should try to assemble anything that leads to fusion. You'll need a lot more knowledge to do it safely.
    The difficult part is not to get fusion. The difficult part is not to harm yourself in the process.
     
  16. Jul 18, 2017 #15
    I've got four whole years if I want this done by highschool. That's plenty of knowledge and more given how quickly I can learn. I also have people that can guide me along so as to not create too big of an accidental explosion. I understand this is not an easy, or cheap task. Im willing to do what it takes: Weekend job, classes, etc. As I said, its a goal, not a want.
    I don't think my threads show a lack of knowledge. They are clarifying questions that are much better answered by a human than by a google search.
     
  17. Jul 18, 2017 #16

    mfb

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    There are many risks, but explosions are not part of it.
    Figuring out what you don't know yet is the first step then.
     
  18. Jul 18, 2017 #17
    Exactly.
    And as for the mention of explosions and cash magically appearing: curse my sarcastic sense of humor.
     
  19. Jul 20, 2017 #18
    @Aidan Davis

    I would recommend joining the fusor forum. We are a group of amateur and professional scientists interested in fusion technology and IEC fusion reactor construction/research. Richard Hull has written many FAQs regarding reactor construction and theory. If there is information you need but cannot find on the forum, feel free to ask me.

    If you have limited experience with high voltage systems or radiation safety, I would recommend you contact someone qualified who can supervise your project. With the voltages used in this project, there are no second chances. Make one unlucky mistake, and you will die.

    Another thing: this project will be expensive. Acquiring funding was a huge part of the project. Expect to spend at least several thousand dollars. I spent $6k. Some materials are very difficult to source and are highly dependant on availability. Making a fully functional UHV system from pieces scavenged on eBay is no simple task.

    Again, If this is your first time with high voltage systems, I advise that you don't proceed with this project. There are many newcomers to the fusor forum with ambitions like yours, but very few (less than 20 in the entire world) have succeeded.

    One last piece of advice. Ask yourself why you want to complete this project. What is the real reason? For me, it was because I wanted to learn by doing. I discovered a passion for fusion energy and wished to pursue it. I aspire to become a fusion scientist. This project pushed my patience and scientific aptitude to their limits. I nearly gave up quite a few times during the year-long process. If you are doing this project solely for the glory, don't. You will lose interest when things don't go as planned. While the forums are a great resource, you will need to find other sources of information. I read through many pages of scientific literature, contacted many professors in relevant departments (specifically at UC Berkeley and MIT's nuclear engineering departments), and even made friends with the engineer who designed my -40 kV power supply.

    I am rambling at this point and will just summarize. This project is incredibly difficult and dangerous, especially for an inexperienced high school student. Please seek proper guidance if I cannot deter your ambitions.

    I am also in the process of writing a guide on how to build a fusion reactor. I am almost finished with the theory section and then will detail the construction. Here is what I have so far: http://imgur.com/a/uSflp (work in progress)
     
  20. Jul 20, 2017 #19
    Thanks!
    Im doing this because it seems like an amazing learning experience. I can learn from many fields all at the same time, and not to mention the skills of gathering money and experienced people to learn from. With my background in maths, this felt like a physical continuation of that.
     
  21. Jul 20, 2017 #20

    Vanadium 50

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    Can you post where we can send your parents the condolence cards once you electrocute yourself? I'd even chip in a couple bucks for some flowers.

    Someone who thinks they know more than they do is very dangerous. "This project is incredibly difficult and dangerous, especially for an inexperienced high school student" is right on the money.
     
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