Bremsstrahlung, X-rays, Coulomb collisions and Fast Neutrals. (a 'semi-FAQ')

It may be difficult to separate "theory" from "application," but let''s see if this helps facilitate the discussion.
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Chris Bradley
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Bremsstrahlung, X-rays, Coulomb collisions and Fast Neutrals. (a 'semi-FAQ')

Post by Chris Bradley » Fri Mar 01, 2013 7:06 pm

JamesParkin wrote:
> I thought thermalisation was the killer not Brunstrang radiation. There seem to be many confilicting opinions and theories on this forum.

This is a public forum directed towards experimentation, so opinions often get expressed without any great detailed dissection and discussion possible over them. Then those opinions seem to get repeated by others.

So, let's be clear on these terms, and where they appear in a fusor... discussions welcomed on the following...

'Bremsstrahlung' is the emission of electro-magnetic energy when a charged particle is subjected to an acceleration. The power of the emission is proportional to the square of the rate of acceleration and the square of its charge. (For relativistic particles, an additional Lorenz term also applies.) As the acceleration of more massive particles is less than that of lighter particles for the same forces then it is the lighter particles that tend to emit the greatest bremsstrahlung.

X-ray emission by electrons hitting a target is, I believe [please correct this if in error] a form of bremsstrahlung in which interaction with the atoms of the target cause decelerations/accelerations that cause EM to be emitted. It is not usually referred to as bremsstrahlung radiation because the precise processes are different to thermal bremsstrahlung or cyclotron radiation and may involve not so much the deceleration of the incoming electron but the acceleration of other electrons.

Cyclotron radiation is a form of bremsstrahlung specifically related to the centripetal acceleration of a charged particle in a magnetic field. Again, this is a very specific form that is not usually referred to as 'bremsstrahlung' but simply as cyclotron radiation.

'Bremsstrahlung' is therefore generally only used in reference to the direct translation of that word; 'braking radiation' - that is to say, when a particle comes under the influence of forces causing its sudden deceleration in its trajectory, and typically when the deceleration is caused by interactions with other charged particles.

Thermal bremsstrahlung is the typical usage for the term and refers to particles being decelerated within a thermal plasma. A plasma contains ions and electrons (as well as neutrals - more on that later). As the electrons are much lighter than the ions they will accelerate much more than the ions. In a deuterium plasma, for example, the ions are some 3,500 times heavier than the electrons, so an electron decelerating against another electron would be the source of 12 million times more bremsstrahlung radiation power than an ion decelerating against another ion.

A fusor contains very little plasma and, particle-by-particle, is mostly neutral gas. There is no such thing as a fully ionised plasma (at least, with the 'non-exotic' conventional physics talked about in fusion), only degrees of ionisation. A thermo-nuclear plasma is, in general, mostly fully ionised and because it is a thermal distribution it suffers from bremsstrahlung radiation losses because all the plasma is being subjected to this form of energy loss as it is all under constant thermal agitation.

A fusor contains a small fraction of ions and electrons at any one time. A fully ionised non-magnetic plasma would be a very good electrical conductor, whereas the glow-discharge regime we want to operate in is, intentionally, pushed to be 'just conducting'. If it were otherwise, the plasma would screen out the electrical fields, and the whole thing wouldn't work. So generally, in a fusor there is very little thermal bremsstrahlung because all the electrons, the main source of bremsstrahlung, are going in one direction only and are not colliding with each other. Only X-ray emissions at the shell (and then only if one were to talk about such processes as 'bremsstrahlung').

In a fusor, for electrons, the main loss mechanism is by bombarding the shell of the fusor. The actual power conversion of this loss process to X-rays can be calculated by empirical equations available in an internet search. It is in the range of something like sub-to-one percent conversion efficiency, depending on the electron energies and shell material. The rest is from thermal losses through Coulomb collisions.

Coulomb collisions are the dominant processes for energy loss in a fusor. These might lead to deflections of ions from other ions or gas molecules that they cause to become ionised in the same collision, or electrons deflected by other electrons, or electrons in gas molecules, or shell molecules. Energy from electrons into the shell is also lost in the process of creating 'sputtered' electrons and ions, in which the atoms of the shell are ejected, wholly or partially, from the incoming high energy electron. Sputtered electrons will fall back towards the -ve bias of the shell, whereas ions may be liberated and accelerated into the fusor, and contaminate it. The sputtered electrons might also go on to ionise the local neutral gas near to the shell, thereby creating ions right at the top of the electric field potential, which is a good addition to the distribution of ion energies available for fusion.

Fast neutrals are formed sometimes during ionising collisions between ions and background gas molecules. In this case, the fast ion dislodges an electron from the gas molecule but instead of then being deflected by the now bare ion instead it teams up with the freed electron. It still has the momentum of the fast ion but is now neutral so is no longer under the influence of the electric field in the fusor. Unless this re-ionises before it hits the shell, as it continues on its trajectory, its energy will be lost and will likely become an 'embedded' deuterium atom within the shell surface.

Coulomb collisions therefore lead to a progressive loss of energy of the ions, by collision after collision, with background or occasionally other ions, until there is nothing left of its kinetic energy. This is 'thermalisation'. Whereas fast ion loss is an immediate, complete loss of the ion's kinetic energy when it reaches the shell.

Coulomb collision losses will heat up the internal background gas, 'thermalisation', whereas fast neutrals and electrons into the shell will, obviously, heat up the shell (which is generally not referred to as thermalisation, but simply 'heating' of the shell).

This is not intended to be a comprehensive description of all process that go on in a plasma. Plasma behaviour is a multi-threaded process of interactions, all playing their part in harmony (or disharmony, if you like!). The above is to provide a basic description of a few applicable, common terms and where one might find such processes in a fusor.

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Dennis P Brown
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Re: Bremsstrahlung, X-rays, Coulomb collisions and Fast Neutrals. (a 'semi-FAQ')

Post by Dennis P Brown » Fri Mar 01, 2013 8:00 pm

Very good post on this subject of Bremsstrahlung radiation by charge particles but one issue remains that needs to be addressed; and why this subject was originally mentioned in that thread.

In the proposed "Polywell" fusor, where supposedly the electrodes do not interact enough with the excited gas to cause enough loss to prevent non-economical fusion (and a problem with fusors that some have tried to solve in other ways), losses by Bremsstrahlung would then dominate. This 'braking radiation' would then be the primary problem and prevent net power from any such fusor device.

So, if any one believes they now have an improved fusor design that avoids the primary loss of the 'ionized' fuels being de-excited by collisions with the walls, neutrals, and electrodes, then this issue will prevent viable fusion. While any such new idea is still worth doing, they need to realize that from an economical view point, the fusor would still not work for power generation.

Bottom line - the basic fusor suffers from a number of mechanisms that prevent net fusion energy production.

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Re: Bremsstrahlung, X-rays, Coulomb collisions and Fast Neutrals. (a 'semi-FAQ')

Post by Chris Bradley » Fri Mar 01, 2013 8:06 pm

Bremsstrahlung is a significant loss mechanism for thermal plasmas. A device that confines by 'long distance' electric fields (viz. by control electrodes) will not operate a thermal plasma, because the plasma will screen those electric field.

Unless these IEC devices, which are essentially accelerator devices, can be made to operate with a thermal plasma then 'free-field bremsstrahlung' (as it is specifically called when applied to thermal plasma) is a non-issue. It is by the thermalised motion of the electrons that you'd get electron-electron collisions, whereas in an accelerator they are not in thermal motion but instead are generally all heading the same way.

If there was a significant 'off-beam' deflection of electrons in a fusor then I would think otherwise; that is to say, if electrons travelling along the 'beamlines' were being significantly deflected by ions travelling the other way (that is, electrons deflected out of the beamlines), then we would tend to see more bremsstrahlung from the effects of electron trajectories being bend around the ions' electric fields. But I don't think there is much in the way of observation that supports the view that electrons are being significantly deflected by ions in any great number, else the glow plasma would be more diffused, whereas in reality in the highest performing fusors there is almost no visible glow outside the beamlines.

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Re: Bremsstrahlung, X-rays, Coulomb collisions and Fast Neutrals. (a 'semi-FAQ')

Post by George Dowell » Sat Mar 02, 2013 12:24 am


never mind.
George Dowell

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Re: Bremsstrahlung, X-rays, Coulomb collisions and Fast Neutrals. (a 'semi-FAQ')

Post by John Taylor » Sat Mar 02, 2013 12:58 am

Chris,
In the 7th paragraph, I believe you have electrons and ions transposed when you state that electrons are 3500 times more massive than the ions. Otherwise a very good and informative write-up.

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Re: Bremsstrahlung, X-rays, Coulomb collisions and Fast Neutrals. (a 'semi-FAQ')

Post by Chris Bradley » Sat Mar 02, 2013 7:53 am

Quite so! Thanks. Correction now edited in.

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