Talk-Polywell.org

Greetings all,
This is a first cut at the beginnings of a draft of a glossary. Please note that I am obviously not trying to turn out PhDs here, just trying to give newbies a taste of the meaning.

If I have gotten anything grotesquely wrong, please let me know. I'm sure you won't hesitate to make other comments either!

Glossary

• *Ambipolar A condition where the flow rate of positive and negative charges across a containment boundary is equal. A Polywell is non-ambipolar.
  *Anneal A process, claimed to be inherent in Polywell systems, that helps keep the ions almost mono-energetic. Two processes have been suggested to account for annealing. The first posits that thermalization at the low energy high potential edge of the potential well will help even out the thermalization the occurs at the high energy, low potential center of the well. The second posits that klystronstyle bunching will keep the ions mono-energetic.
  * Debye length (Radius) In plasma physics, the Debye Length (also called Debye radius), named after the Dutch physicist and physical chemist Peter Debye, is the scale over which mobile charge carriers (e.g. electrons) screen out electric fields in plasmas and other conductors.
  *Gyroradius The gyroradius (also known as radius of gyration, Larmor radius or cyclotron radius) defines the radius of the circular motion of a charged particle in the presence of a uniform magnetic field.
  *IEC Inertial Electrostatic Confinement
  *Larmor Radius See Gyroradius
  *LTE. Local Thermodynamic Equilibrium
  *MaGrid Magnetically Protected Grid.
  *Maxwell (ian)(ize) Most fusion power efforts use plasmas that have a broad spread of kinetic energies similar to a hot gas. The curve used to describe the spread is known as the Maxwell-Boltzmann distribution. It is similar to a bell curve around an average energy level which is the temperature. The width of the bell (the standard deviation) is also a function of temperature. The plasma in a Polywell is often claimed to be non-Maxwellian. In point of fact, the distribution approximates Maxwellian, but with the very small standard deviation of energies typical of the cold edge of the well. The ions then gain a uniform additional energy by falling into the potential well. Annealing helps keep the distribution small.
  *non- Since Polywell fusion is different than most other types of fusion, there is an urge to distinguish it from them. If they are “Maxwellian” Polywell is non-Maxwellian.
  *Quasi-neutral A condition where the state of charge within a containment boundary or other specified volume is ALMOST, but not quite, neutral. A Polywell is quasi-neutral.
  *pB&j A term initiated by a regular poster to the Talk-Polywell forum meaning “proton-Boron and joules”, referring to the proton Boron11 fusion reaction.
  *Thermalize See Maxwellianize.

I took a real WAG at the Maxwellian part, but it kind of makes sense, ya know? I mean, if it is going to "thermalize" it will thermalize at both the center AND the edge of the well. The thermalization at the edge...

Good stuff.

At this point in time it might also be good to bring in klystron bunching as a possible annealing process. At least until we know more.

Technically the distribution is not Maxwellian if it operates as described, because you have the nearly-Maxwellian low-temperature distribution superimposed on a two-stream distribution (one going in, one going out), which is very different from the Maxwellian, both mathematically and in terms of distance from homogeneous thermodynamic equilibrium. This distribution is probably partially relaxed (ie: a compromise between this and a high-temperature Maxwellian) in a power reactor.

http://www.google.com/search?hl=en&safe ... on&defl=en

Distribution of velocities in a gas at equilibrium at a certain temperature.

In physics, thermalisation (in American English thermalization) is the process of particles reaching thermal equilibrium through mutual interaction.

en.wikipedia.org/wiki/Thermalize

Please propose specific wording changes.

Thank you.

MSimon, I will take a cut at adding the Klystron bunching, but Dave I really don't understand the nuances of Maxwellian, so I hope you will help out here.

Hrm, I think it basically means the temperatures of the various particles are arranged in a bell curve around an average temperature, as opposed to all being the same temperature (or two temperatures, or anything other than a Maxwellian distribution).

You could link this in the glossary:

http://en.wikipedia.org/wiki/Maxwell-Bo ... stribution

This is one reason why aneutronic fusion would be so much harder to accomplish in a tokamak: even assuming you can get the average temperature up to p-B11 fusion energies, the tail of the temperature distribution includes wide exposure to cross-sections of other (neutron-generating) reactions you don't want.

TallDave wrote:Hrm, I think it basically means the temperatures of the various particles are arranged in a bell curve around an average temperature, as opposed to all being the same temperature (or two temperatures, or anything other than a Maxwellian distribution).

You could link this in the glossary:

http://en.wikipedia.org/wiki/Maxwell-Bo ... stribution

?? I did! Click on the (ian) part of Maxwell(ian). I guess I just got too clever! And ok, I guess I should describe it a bit more. Normal (bell curve) distribution around an average which is the temperature. Is it truly a normal distribution? Looks somewhat skewed in the wiki-plot, even ignoring the relativistic effects.

Check out the edited version above. (The (ian) click no longer works!)

Is it truly a normal distribution?

I think so, except for relativistic effects. If you look at the graphs, the non-relativistic ones are all bell curves of varying steepness.

The Maxwellian in velocity space is a normal distribution around ZERO. In all three axes (u, v, and w). The width of the distribution is proportional to temperature.

A "drifting Maxwellian" is a normal distribution around a nonzero average velocity (eg: a cylinder of gas on a moving train). In the acceleration region of a Polywell, neglecting partial relaxation due to high-energy collisions, you have two drifting Maxwellians going in opposite directions.

The cross section for collisions goes down as relative velocity increases, so this situation doesn't instantly self-destruct, because the superimposed distributions preferentially collide with themselves rather than each other. If the superimposed-drifting-Maxwellians state were the only one in the system, this would only delay the inevitable...

The Maxwellian in speed is a skewed distribution, because it has to pass through the origin - what, after all, are the odds of a particle being completely at rest in a gas at a finite temperature?


The way the entry is formulated at the moment strikes me as... clunky. I'm not sure what to do about it right now...

TallDave wrote:

Is it truly a normal distribution?

I think so, except for relativistic effects. If you look at the graphs, the non-relativistic ones are all bell curves of varying steepness.

Actually it is more of a Poisson distribution.

So, aside from the "Maxwellian" definition, are we mostly happy with the others? If so, I will insert them in the FAQwiki.

MSimon wrote:

TallDave wrote:

Is it truly a normal distribution?

I think so, except for relativistic effects. If you look at the graphs, the non-relativistic ones are all bell curves of varying steepness.

Actually it is more of a Poisson distribution.

True. I was using "bell curve" in the sense of "to describe, at least approximately, any variable that tends to cluster around the mean." Except, of course, for relativistic effects.

93143 wrote:The Maxwellian in velocity space is a normal distribution around ZERO. In all three axes (u, v, and w). The width of the distribution is proportional to temperature.

That does make a very elegant picture.

Ok, another cut at this term.

• *Maxwell(ian)(ize) Most fusion power efforts use plasmas that have a broad spread of kinetic energies similar to a hot gas. The curve used to describe the spread is known as the Maxwell-Boltzmann distribution. It is similar to a bell curve around an average particle speed which is related to the temperature and particle mass. The width of the curve (the standard deviation) is also a function of temperature and particle mass. Collisions between particles tend to drive the speed distribution toward that curve. If the particles have a narrower distribution, they tend to spread. If they have a wider distribution, they tend to narrow.
  
  The plasma in a Polywell is often claimed to be non-Maxwellian. One hypothesis suggests that in fact, the distribution approximates Maxwellian, but only at the cold edge. At that point, it has the very small standard deviation of energies typical of cold plasma. The ions then gain a uniform additional energy by falling into the potential well. Any thermalization (spreading) that happens near the center is counter-acted by thermalization (narrowing) near the edges. This is called annealing.