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19

The Sun fuses protons, and this is a very slow process because there is no bound state of two protons. Hydrogen bombs fuse deuterium and tritium, and this is much, much faster because there is a bound state of these nucleides. You might like to have a look at: How much faster is the fusion we make on earth compared to the fusion that happens in the sun? ...

11

If they didn't release energy, they wouldn't happen. The alternative, nuclear reactions that require energy, clearly need said amount of energy, which has to come from somewhere, e.g. kinetic energy involved in the collision of two nuclei (even ones that release energy usually have a "barrier" and some amount of initial kinetic energy is needed to overcome ...

8

Elements heavier than iron are produced mainly by neutron-capture inside stars, although there are other more minor contributors (cosmic ray spallation, radioactive decay). Neutron capture can occur rapidly (the r-process) and occurs mostly inside supernova explosions (though other mechanisms such as merging neutron stars have been mooted). The free ...

8

I think the colloquial term for that type of plot is "spaghetti diagram" because you have a bunch of lines running across it. It's really the mass fraction as a function of interior mass. From our stellar structure equations, we have that $$\frac{dm}{dr}=4\pi r^2\rho,$$ which is derived from the mass-continuity equation, so you can relate the radius, $r$ ...

4

I think you already know the answer... Pop III stars, by definition, are born from primordial gas that is basically Hydrogen, Helium with trace amounts of deuterium, tritium, lithium and beryllium; they initially contain almost no C, N, or O. Therefore the primary fusion in massive Pop III stars has to be (well, initially the deuterium is burned but this is ...

4

Very high temperature, on the order of 10 keV (100 million degrees Kelvin), is needed for fusion reactions to start to happen at appreciable rates. However, in magnetic fusion devices (tokamak, stellarator) the transport of heat across the plasma (mainly due to plasma turbulence) causes heat losses. Making the system larger allows increasing the heating ...

4

Very interesting question! In chemistry you spend lots of time discussing exothermic and endothermic reactions: when you put your reagents together, sometimes the reaction heats things up, and sometimes the reaction cools things down. Nuclear reactions are very different, in that essentially all spontaneous reactions studied in laboratories are exothermic. ...

4

The final stage of nucleosynthesis at the core of a massive star involves the production of iron-peak elements, mostly determined by competition between alpha capture and photodisintegration. The starting material is mostly Si28 and weak processes are unable to significantly alter the n/p ratio from unity on short enough timescales. Thus the expected outcome ...

4

Transmuting chemically significant quantities of one element to another using nuclear reactions is not cost effective for any naturally occurring element. Nuclear physics is the end of alchemy. Two examples I happen have off the top of my head: the "Fat Man" and "Little Boy" nuclear weapons deployed in the second world war each involved about $10^{24}$ ...

3

After doing some more research I found the answer to my question. The method I proposed was actually one of the first methods for hydrogen-boron fusion that was tested. It's called "fixed/solid target proton-boron-11 fusion". Experimentation very quickly showed that the method could not work because of two big problems: As #dmckee already commented above, ...

3

A very nice question about a common misconception in books on astrophysics (I've made the same mistake in a comment here). According to M.P. Fewell, the origin of this misconception lies in the theory of stellar nucleosynthesis and the abundance of the elements. While other nuclei have higher binding energy per nucleon, $^{56}\mathrm{Fe}$ is more abundant ...

3

The important argument for this discussion is the Bethe Weizsäcker formula, which describes the binding energy of nuclei. I will try to give a cursory overview of the most important aspects. Not only heavy elements show fission and fusion. All elements up to iron-56 (one of the nuclei with the highest binding energy per nucleon) can create energy in ...

3

The sun gets its energy from the pp-chain. The first step is the two protons forming the diproton (Helium-2): $$\,^1_1H+\,^1_1H\to\,^2_2He+\gamma$$ where the $\gamma$ is the photon (of energy about half an MeV). This quickly $\beta^+$-decays into a deuterium by converting a proton into a neutron: $$\,^2_2He\to\,^2_1D+e^++\nu_e$$ where $e^+$ is the ...

3

There isn't exactly a mathematical relationship, but there is a physical one. It is the gravitational compression that causes the increase in temperature in the core of the gas cloud that becomes a star. When the temperature reaches a critical value (in the millions of Kelvin range), hydrogen fusion can occur. This is because the temperature is great enough ...

3

Oh, but we do! I'm assuming you mean using the fields to simply collide particles with each other, right? Then that's already being done. For example, take this neat little machine: http://en.wikipedia.org/wiki/Fusor This one runs on the exact same principle you described (though I'm not quite familiar with the inner workings of the LHC). For energy ...

3

I personally doubt that the Compact Fusion Reactor as presented by Lockheed Martin last week can work, but I haven't seen enough information to be certain. And to some extent, you never know until you try. (As I understand it, they only have a very early prototype, I mean try as in a full scale prototype.) What I think I can say with certainty, is that it ...

3

$^{56}Ni$ is produced in silicon-fusion stars. The fusion process doesn't "stop" at $Fe$. Several A=56 nuclides show up. See the Wiki-pedia article on :Silicon burning. Also, Introductory Nuclear Physics by Krane, Chapter 19, Section 4.

3

The main problem with boron (relative to 3He) is that the atomic number is high. This means that the plasma must run at a considerably higher temperature, about a factor of 10, in order to overcome the Coulomb barrier. Higher temperature means faster electrons in the plasma. Faster electrons means more radiation when the electrons "hit" the walls. This ...

3

The amount of energy liberated per gram of material per second in the fusion reactions depends on the density, the mass fraction (hydrogen, $X$, helium, $Y$, and all others $Z$) and temperature: $$\epsilon = \epsilon(\rho,X, Y, Z, T)$$ Typically we express the energy generation rate as a power law, $$\epsilon\propto\rho^\alpha T^\delta.$$ though the ...

3

Since your plasma is in a vacuum environment, the only way for it to loose energy is by radiation (conduction transfer through the magnets are neglected). You have thus to consider which bodies are surrounding your plasma and which radiative model is the best ton consider for them. I guess you can consider a black body with the simple Stefan equation. The ...

3

I wonder if your number 48% comes from the typical Carnot efficiency of a heat engine - see for example a detailed description at http://www.visionofearth.org/industry/fusion/how-do-we-turn-nuclear-fusion-energy-into-electricity/ When you want to use heat to create electricity, you typically convert the heat into motion (for example by rotating a turbine, ...

3

Some rough estimates (you can dig up more accurate numbers): The oceans contain about 321 million cubic miles of water (source: http://oceanservice.noaa.gov/facts/oceanwater.html), or 3.5e20 U.S. gal. 1 gal seawater contains roughly enough deuterium to provide the same energy as 300 gal of gasoline (maybe slightly less - that's the part for your homework!), ...

3

A brief history of what science thought about the sun can be found here . It is reasonable that once thermodynamics advanced to the point of measuring and calculating energies the discrepancy between heat output of the sun and the age of the earth had to be explained. They tried with gravitation, but until the discovery of nuclear energy and E=m*c^2 it ...

3

The simple answer is No. Fusion happens at nuclear energies between particles to be fused, i.e. MeVs, because it is at the framework of nuclear bound states. LHC particles start with energies of TeV, so particle particle interactions are way over any nuclear bound state levels. Even if one accelerates deuterium nuclei the phase space is way over the ...

2

No, because the LHC puts too much energy into its particles for them to fuse. While we need enough energy to fuse particles, too much will stop it from happening.

2

You question isn't specific enough; it needs a little work to clarify the fusion setup. For example, what fuel type are you talking about fusing? Is there confinement, so that this is a thermal fusion reaction, or would just one fusion reaction be sufficient? For example, the temperature required to overcome the Coulomb barrier for deuterium-tritium ...

2

The motivation for pursuing fusion is clear, but there are currently several main physics and engineering challenges: Confinement time: An operational reactor requires a long energy confinement time, $\tau_E$. An empirical scaling law for confinement time has been found to depend on the size of the tokamak as $\tau_E \propto R^{2.04} a^{1.04}$, where $R$ ...

2

There is a lighter nuclide which undergoes fission: $^8Be$. It fissions to two $^4He$ nuclei ($\alpha$ particles) with a lifetime on the order of $10^{-17}$ s. The binding energy per nucleon is much less for the beryllium than for the two $\alpha$s. It's important to note that $^8Be$ is an important link in the triple-alpha fusion process in older stars ...

2

I am addressing this part of the question: Also why do only neutrons show fission/fusion and why can't electrons preform fission/fusion? Nuclei with a large number of neutrons are unstable . It so happens for some of them that an extra neutron in a specific low energy range can be caught when impinging on that nucleus , but the resultant new isotope ...

2

Jupiter will never (not on any timescale like the lifetime of the Sun anyway) accrete enough mass to begin hydrogen fusion. It would need to accrete 12 times its current mass to undergo a brief period of fusing its interior deuterium and to accrete more than 70 times its current mass to attain a central temperature high enough to sustain hydrogen (pp chain) ...

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