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Fusion Energy from a DD Reaction – Just the Facts

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Fusion Energy from a DD Reaction – Just the Facts Fusion energy from a D-D reaction – just the facts By Richard Hull The reductions below are incredibly easy to follow by any 8th grade student who has passed math. However, the units and scientific notation used, to be fully understood, might require a bright student who has taken and retained his high school physics. D-D fusion is a 50:50 reaction as seen in the diagram. For each fusion only one of these branched reactions can take place. Example: If we detect one million, (1e6), neutrons per second in our neutron counting effort during fusion, this means that two million fusions have occurred in that second. From this we know that we fused two million different D-D reactions in that second or we produced 2e6 fusions per second. Of these two million reactions, only one million produced a reaction that produced a neutron. This still means that in one second we produced 1e6 neutrons, 1e6 protons, 1e6 tritons, 1e6 3He, (helium 3), nuclei. Side note: You will notice what appears to be a helium 4, (4He) nucleus formed in the center of the diagram. While this may be, in fact, the case, the reaction is so rapid and energetic that in most every instance, it instantly decays, (billionths of a second), into the fusion ash or debris seen to the right. You probably noticed that I said, “in most every instance”. It seems that on extremely rare D-D fusions, a stable helium 4 nucleus is formed with a huge release of energy. No precise rate is known for these very rare events, but it is generally agreed that for every 25,000 D-D fusions, only one stable 4He nucleus is formed. Thus, in a million D-D fusions we might expect only about 40 helium 4 atoms to be formed. Let us pose a question: How many joules of energy did we really produce should all 4 particles be absorbed and turned into joule heat. (Crash into an ideal absorber) It is to be remembered that this is all the energy available from 2 million fusions/second. We have 1e6 D-D reactions in the neutron branch, each of which, totals 3.27 meV We have 1e6 D-D reactions in the proton branch, each of which, totals 4.03 meV Converting to energy by ergs for each branch 1.6e-6 ergs/meV X 3.27 = 5.2e-6 ergs/neutron branch fusion 1.6e-6 ergs/meV X 4.03 = 1.3e-5 ergs/proton branch fusion We produced 1e6 of each of these separate reactions in a second (joule energy is per second) Thus: 1e6 X 5.2e-6 = 5.2 ergs of neutron branch energy in a second 1e6 X 1.3e-5 = 13 ergs of proton branch energy in a second Total energy produced from 2 million D-D fusions per second is thus: 18.2 ergs. There are 1e7 ergs per joule 18.2/10,000,000 = 1.8e-6 This means for a continuous 1million neutrons per second on our neutron counters, the fusion energy produced by 2 million D-D fusions, produced a grand total of: 1.8 microwatts or true fusion energy As this performance in a common fusor might require an average of 40kv X 10 ma or 400 watts of energy input, the pure energy output to input ratio is: 1.8-6/400 = 4.6e-9 now 1 / 4.6e-9 = 2.18e8 This means we have had to put in ~220 million times more energy that we get out in fusion energy. Now, you need not do such calculations constantly The above may be simply scaled for energy output only. Let us say we are getting 2e12 neutrons/sec or 6 orders of magnitude better than a fabulous Amateur fusor. (improved to 1 million times higher output) we would get 1e6 X 1.8e-6 = 1.8 watts of fusion energy output. Just for grins, let us say the input energy scaled to the same degree. (an assumption, of course) To get 1.8 watts out of the fusor we would have to put in: 1e6 X 4e2 = 4e8 watts we would have to feed 400 million watts into the fusor to get 1.8 watts of fusion energy out. You can see there is no path to useful fusion energy production in any fusor while doing fusion. For those new to fusion Where does all that energy that I put in go if I only get out microwatts of fusion for 400 watts put in? It goes where all energy loses go in such situations……Waste Heat! All the electrons slam into the walls producing heat. (the biggest loss). All the accelerated high-speed deuterons that do not fuse, ( virtually 100%), turn into high speed neutrals and crash into the walls. (Almost all the remaining energy). X-rays, lots of them, make up the rest of the energy loss. The grid in the center heats as well, (electron and deuteron collisions), this transfers by conduction to the insulator and the wall. (oh, the wall is the fusor reaction chamber’s electrically grounded metal shell or exterior.) This is usually made of stainless steel and with 400 watts fed in, it heats to the point that you can burn your hand. One might look at the amateur fusor as an efficient space heater. An explanation of the fusion process (simplified) Energy is energy you can’t pull it from space. At least not by any process we know of. Fusion is a quantum tunneling process. As such, it is probabilistic by its very nature. These probabilities are like throwing dice. Every throw will not produce “box cars”. You will have to figure the odds on the number of throws that will be needed to repeat a box car throw. This involves statistics. As noted, fusion is probabilistic. However, we need not involve ourselves with statistics. It has all been worked out for us. It turns out that in nuclear physics, empirical experiments have derived a probability chart for us. It is called a “fusion cross section chart” This chart is a battle between two throws of a dice for fusion to take place just once! It was found that as a particle, (deuteron or deuterium nucleus), travels faster, (gains more kinetic energy), the fusion rate, (probability), goes up. By imagining that the more energy a deuteron has, its “cross section”, (area increases), much like a balloon swelling up. This makes, it more likely to crash into and fuse with another deuteron of equal energy. The unit used to describe cross section is the “barn”. The barn is equal to an imaginary circular area of 1e-24 square centimeters. It is to be remembered that the cross section is a mime of probability. As such, even if the “fusion energy” deuterons approach dead on, Quantum uncertainty rules, based on whether the natural coulombic repulsion of the positively charged nuclei can be overcome and allow them to fuse. These are the dice thrown in fusion. You will not fuse every time two deuterons meet. This fact alone keeps stars burning for billions of years. If every hydrogen atom fused that met in the core of a star, it would burn its atmospheric fuel up in a short time period. The universe would consume itself almost instantly! Thank goodness fusion is such a rare an event. This fact makes us happy we are still here due to the rules of physics. However, it makes us sad in that fusion is so difficult and nearly impossible to do here on earth in a controlled manner. The chart seen below will help explain the realities of fusion. On the x-axis, we plot the matching energy of two particles colliding in a fusion friendly environment, (perfect vacuum), versus the y axis cross section that the deuterons have attained at the given energy seen on the x-axis. From the graph, we see that the green line represents the D-D fusion cross section that is possible within the amateur fusor. All the other fusion fuels on the chart are intrinsically impossible for the amateur to work with. The best reaction is D-T, but the amateur cannot secure the NRC license to obtain it legally as Tritium is highly radioactive. Helium 3 gas can be had, but at tremendous cost. However, it doesn’t begin to perform better than D-D until you apply an acceleration voltage in excess of 100,000 volts! The P-B11 reaction involves the corrosive and lethal Boron Tri-fluoride gas and, then, only beats D-D at 300,000 volts applied. D-D fusion is marginally possible at 10,000 volts applied. However, fusion is only detectable by amateur means at around 20,000 volts. Most great amateur fusors work at or above 40,000 volts. As seen in the chart, there is little to be gained in D-D fusion after about 90,000 volts applied. The cross section is leveling off. Moving into the physics of the near impossible related to fusion. The reason fusion is so pitifully difficult is due to the quantum limitations placed on matter versus the coulombic forces. This probabilistic process is called “Quantum Tunneling”. You might ask what if we just threw the quantum dice more often? We can do this by putting more nuclei in the same space, thus increasing the number of possible fusions in a specified time period within the denser plasma. Fusion has a special criterion related to doing it successfully.
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