Anyone Build a Neutral Beam Gun

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
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Donald McKinley
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Anyone Build a Neutral Beam Gun

Post by Donald McKinley » Sun Aug 26, 2012 5:56 am

Has anyone done any work with making a neutral beam gun (D of course and low total power) for use in your fusor or other device? I know it would be quite a bit more involved than an ion beam.

Did you have any success?

Don M

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Chris Bradley
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Re: Anyone Build a Neutral Beam Gun

Post by Chris Bradley » Sun Aug 26, 2012 8:34 am

Hello Don. Neutral beams are very much a feature of hot magnetic plasma fusion, only.

The only advantage [that is to say, the reason] of their use is that they can penetrate a magnetic field freely. Once within a hot plasma, the fast neutrals will undergo ionising collisions. In colliding they will then dump energy into the plasma, thus heating it.

A fusor contains a very low ionisation ratio, low temperature plasma. Most of what's in there is neutral gas. A fusor works by its electric field, which is the only difference between it and an empty bottle! Anything that is not charged is not within the control of the fusor, because it cannot contain hot particles by any other means. So the formation of fast neutrals are essentially a pure energy loss for a fusor and are to be avoided.

In my device, like in a fusor, fast neutrals are formed and lost. I can now see this is a real problem for the particular operating principles of my device because it appears to be that it's really pretty good at making fast neutrals!!! Ooops... Scientifically interesting maybe, but bad news for fusion rate!

Dan Tibbets
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Re: Anyone Build a Neutral Beam Gun

Post by Dan Tibbets » Mon Aug 27, 2012 11:15 pm

That raises a question that I am unsure on. I know that in cuspless confinement machines like Tokamaks, neutral beams are injected to heat the plasma. But are these high energy beams made up of neutral gas without ions or are they neutral net charge beams of both positive ions and electrons. If neutral gas, how are they accelerated and collimated. Or is it a net neutral plasma beam that is dense enough that the close coupling causes the beam to act almost as a neutral gas, if that is even possible. I would think the density would have to be very high for the coupling to dominate and allow the beam to hold together, even briefly.

What ever the composition of the neutral beam, I assume it has to be very hot, perhaps 10-20 KeV if it is used to heat an already warm Tokamak plasma.

Dan Tibbets

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Chris Bradley
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Re: Anyone Build a Neutral Beam Gun

Post by Chris Bradley » Mon Aug 27, 2012 11:27 pm

They are formed by accelerating and collimating negative ions (hence why Culham has just started a new project looking at efficient -ve ion generators) then stripping the extra electrons by passing the formed beam through a foil window as it enters the main vacuum vessel. (I believe the ITER design does away with the window that strips the electrons, I guess by using a plasma window.) So the neutral beam is monatomic gas particles, if you will, going very fast in a beam.

In JET's DT runs with NBI, energy range was/is multi-MeV and multi-MW. The neutral beams alone generate a few additional MW of fusion power (in addition to their injection energy, but not break-even) once they plough into the plasma and undergo collisions (which, as far as I can tell, is added to the total fusion power output, rather than treated as heating input - another example of where statistics mean exactly what the person who wrote them wanted them to mean?).

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Donald McKinley
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Re: Anyone Build a Neutral Beam Gun

Post by Donald McKinley » Mon Aug 27, 2012 11:54 pm

If it weren't for the acceleration/containment problems of neutrals, it would seem from a novice point of view that neutrals should fuse with lower energy input because of reduced Coulomb repulsion enabling a closer approach of the atoms before a steep rise in repulsion.

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Re: Anyone Build a Neutral Beam Gun

Post by Frank Sanns » Tue Aug 28, 2012 2:53 am

Using neutral particle beams does not mean that the nucleus has become neutral. It means that the entire atom is neutral. This is of no help for fusion as atoms are huge and the distance between the nucleus and the electron(s) is many, many orders of magnitude larger than the diameter of the nucleus. It is the nucleus-nucleus repulsion that makes fusion so difficult.

Frank Sanns

Donald McKinley wrote:
> If it weren't for the acceleration/containment problems of neutrals, it would seem from a novice point of view that neutrals should fuse with lower energy input because of reduced Coulomb repulsion enabling a closer approach of the atoms before a steep rise in repulsion.

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