Angular vs Linear momentum in reaction cross-sections

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james6742
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Real name: James Thrush

Angular vs Linear momentum in reaction cross-sections

Post by james6742 »

Is anyone aware of research or theories regarding fusion reaction cross-sections where a significant portion of the kinetic energy is delivered via angular momentum vs. linear momentum? That is, imagining we can "spin-up" the individual atoms (either on-axis or off-axis), imparting significant angular momentum before impacting a target or plasma? My guess is that the cross section would go down as there would be less linear velocity to combat Coulomb repulsion (if we keep total energy constant), but perhaps there would be significant changes in the charge distribution of the nuclei that would lead to some other effects? Just wondering if this has ever been studied before.
Doug Browning
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Re: Angular vs Linear momentum in reaction cross-sections

Post by Doug Browning »

I think those would generally be called nuclear isomers with high spin states. You'll have to hit a nucleus with something first just to make one. Most are very short lived, like nanoseconds, but there are some long lived exceptions. Might get a little "pancaking" from the high spin for heavy nuclei, doubt if it helps cross sections materially. Running two high spin state nuclei into each other might make the final desired reaction product unstable as well. And correct me if I'm wrong, but I don't think deuterium has any stable isomers.
RealBorg
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Re: Angular vs Linear momentum in reaction cross-sections

Post by RealBorg »

I think he is not referring to the quantum mechanical spin of the nucleus (which has nothing to do with angular momentum) but to the angular momentum of the atoms and molecules as used in microwave ovens and MRTs. Interesting thing is, that the deuteron could already be polarized in our setup...

tom
Dan Tibbets
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Re: Angular vs Linear momentum in reaction cross-sections

Post by Dan Tibbets »

Umm... perhaps he is referring to a cyclotron like flight of ions, then hitting a target. In such a case the actual impacts would effectively be linear beam- target impacts.
In a rotating plasma like a tokamak the ions are spinning around the torus in one direction. As such the ions would never collide in a perfect situation, they are all traveling parellel to each other. This ignores centrifugal forces, etc. Actually, in a tokamak there is a dominate angular momentum to the bulk plasma, but I suspect that the unavoidable collisions due to various disturbances , turbulence, etc. results in many collisions between ions. This is presumably so prevalent that the plasma can be considered as a ball of randomly moving ions, the fusions would be predominately beam- back ground or beam- target type. Beam - beam collisions would be small due to almost all of the ions having a common direction of travel, at least on the large scale. It is like a moving gas. It might be flowing down a pipe, but the side to side collisions would be essentially the same as a a gas that is not flowing down a pipe. It would still exert the same pressure against the confining walls, whether solid or magnetic. This is of course subject to all kinds of qualifying modifications.

I do not think you could achieve much beam- beam collisions in a plasma with much angular momentum dominate in one direction, unless you had two counter rotating beams with an intercept point. Alternately, a sphere of plasma with little angular momentum, but mostly radial in and out motions oscillating back and forth could have most of the collisions of the beam - beam type. This probably happens in a fusor in an ideal world, but in actual operation is probably a very small percentage of collisions unless certain conditions are met.

Dan Tibbets
james6742
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Re: Angular vs Linear momentum in reaction cross-sections

Post by james6742 »

Very interesting answers everyone, thank you! I was specifically thinking of a beam-beam type collision where the ions in the beams have been spun either along the beam axis or along a perpendicular axis so that they contain significant kev in their angular momentum. Not sure if it would really affect the reaction cross section or not, just curious if anyone had heard of it being tried.
Dan Tibbets
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Re: Angular vs Linear momentum in reaction cross-sections

Post by Dan Tibbets »

Certainly beam - beam interactions have been done with circulating beams fro cyclotrons that are focused to an intesectng target region. I think the problem with applying this to fusion is the density issue. For fusion to be useful the the intersecting beams have to be dense relative to the beams in accelerators. This complicates things like bean cohesion and focus. The already poor efficiency of the beam generation is worsened. Combine this with the poor fusion cross section relative to the Coulomb collision cross section results in further losses versus fusion. You need to somehow recirculate the beams many times so that there are multiple passes for possible fusion, and do so with minimal increased confinement losses. Intersecting beams could work, but the issue remains one mostly of containment. A tokamak, with it's achievable density and beam background dominate fusion needs hundreds of seconds of confinement time to break even. A Polywell, which May operate in a spherical beam- beam dominated method, and with it's achievable density may need only a small fraction of a second confinement times. It is primarily the efforts to achieve the required confinement times that is the issue. Beam- Beam instead of of Beam- Background helps , but to a modest extent, especially in a thermalized plasma. It may have more significance to Bremsstruhlung losses and performance in a monoenergetic plasma.

Dan Tibbets
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