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Natural Science Forum / Physics / Research / September 2007Fusion chain reaction?
Hi, I was wondering what would happen if a roughly 10 MeV proton or neutron was fired into a large dense target consisting of a mixture of deuterium and tritium - let's say several cubic metres of gas held at high pressure but low temperature so that it is almost at the density of liquid water. I imagine that the target is large and dense enough so that the projectile particle is guaranteed to scatter off a deuterium or tritium atom rather than pass straight through the target. Would the high energy particle transfer enough kinetic energy to a number of deuterium or tritium atoms so that they scatter with a high kinetic energy and fuse with other deuterium/tritium atoms and thus produce tritium, helium-3, helium-4 and further protons and neutrons with varying kinetic energies? Could one end up with a self-sustaining chain reaction occurring at a relatively low temperature? This would be different from the standard fusion schemes in which the whole of the fuel is heated up to a temperature high enough to produce fusion reactions. John > Hi, > [quoted text clipped - 19 lines] > whole of the fuel is heated up to a temperature high enough to produce > fusion reactions. Light element fusion requires the product of density, time, and temperature exceed a critical value. Fusion spontaneously disassembles without propagation (second order reaction) unless actively confined - implosion, gravitation, external EM fields, possibly inertia. Deuterium is nobody's idea of inertial confinement - including deuterated polyethylene. You only have a few shakes (tens of nanoseconds) to get the job done before the mass thermally expands, reaction rate dropping as the inverse square of concentration. You could dump in muons. That works on paper and nowhere else.  SignatureUncle Al http://www.mazepath.com/uncleal/ (Toxic URL! Unsafe for children and most mammals) http://www.mazepath.com/uncleal/lajos.htm#a2 >>> Light element fusion requires the product of density, time, and > temperature exceed a critical value. Fusion spontaneously [quoted text clipped - 6 lines] > > You could dump in muons. That works on paper and nowhere else. As I remember that doesn't work on paper. The muon's half-life is too short. > >>> Light element fusion requires the product of density, time, and > > temperature exceed a critical value. Fusion spontaneously [quoted text clipped - 8 lines] > > > As I remember that doesn't work on paper. The muon's half-life is too short. It's long enough for producing many muon-catalyzed fusion events. The problem is producing muons cheaply enough. When some clever soul invents a way to produce muons for a few hundred MeV apiece the proces will become quite practical. Jan >> > You could dump in muons. That works on paper and nowhere else. >> > [quoted text clipped - 6 lines] > for a few hundred MeV apiece the proces > will become quite practical. I may be wrong and my information was based on my knowledge from 1977 when I did a literature search on "exotic" atoms where a muon would replace an electron in a D-T or D-D molecule that would draw the two nuclei closer together so that the wavefunctions of the nuclei would overlap and cause fusion. I knew at that time "hot" muons could induce the fusion because the of time dilation. But I don't think thermallized muons have a lifetime long even to cause many fusion events. If there has been more data since 1975 on the topic, it would be interesting to see what it is and how it impacted the prior conclusions. > Light element fusion requires the product of density, time, and > temperature exceed a critical value. Fusion spontaneously [quoted text clipped - 6 lines] > > You could dump in muons. That works on paper and nowhere else. It does work in a bubble chamber. In fact, muon catalysed fusion was discovered experimentally, instead of predicted on a piece of paper. The only thing that's wrong with it is that muons are too expensive. Best, Jan Colliding a beam of deuterium ions into a target loaded with tritum is a common way to produce a burst of high energy neutrons via D-T fusion (there are applications for that). The shortcoming of this approach is that net nuclear energy gain is impossible this way because the fusion cross section is relatively small, and not every collision results in one. Once the D starts bouncing around inside the target, momentum transfer to the T's due to collisions causes the D to loose energy too fast for the stray fusion reaction to make up for it, on average. This is why the target must be very hot. If the target is hot enough (10 keV or more), and in thermal equilbrium, collisions no longer result in an average loss of energy, and the energy is effectively confined within the plasma. If it's hot enough and confined long enough (the Lawson Criterion), net energy gain is achieved. > Colliding a beam of deuterium ions into a target loaded with tritum is > a common way to produce a burst of high energy neutrons via D-T fusion [quoted text clipped - 4 lines] > transfer to the T's due to collisions causes the D to loose energy too > fast for the stray fusion reaction to make up for it, on average. Maybe the problem is that the deuterium ions are loosing their energy by scattering off other more highly charged nuclei in the target rather than the tritium. Perhaps if one only used pure tritium for the target then the scattering cross-section would be comparable to the fusion cross- section. John |
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