Talk:Magnetic mirror

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I've never seen a clear explanation for this effect. I expected wikipedia would have one. -- Euyyn

Better now? Art Carlson 2005 June 30 09:38 (UTC)

There are 2 misleading statements in the given explanation. It is not just a function of particle direction vs magnetic angle. It should be made clear that total particle velocity plays a critical role in determining whether reflection will occur or not; faster particles have a better chance of escaping even when they ARE NOT moving parallel to their local field lines, and all particles moving parallel to their field lines WILL ESCAPE. <== Surely this is undeniable. It is the escape of fast particles which renders the solenoidal approach to magnetic mirror fusion so very difficult because, by preferentially allowing the hottest particles to escape, it destroys the very desirable Maxwellian velocity distribution which had been counted on to produce ions at velocities (and hence temperatures) far above the average of the already very high bulk temperature. Also, since the rate of fusion reactions goes up faster than temperature, it is very desirable to have at least a portion of the fusion plasma particles at a temperature higher than the minimum temperature required for overcoming the Coulomb repulsion between nuclei, and thus eligible for creating "hot fusion" reactions. (Particle velocity and particle Energy can both be expressed in Electron Volts, and since 1 eV = 11600 Kelvins, velocity and temperature are closely related.) For a more detailed explanation, see http://farside.ph.utexas.edu/teaching/plasma/lectures/node21.html about which I note that they are for some reason discussing attempts to maintain a magnetic mirror fusion device at a high, flat single temperature rather than the normal curved Maxwellian temperature distribution.

Also, Art, please respond to Shawn's comments. 24.136.234.65 10:48, 3 October 2005 (UTC) Essen (updated 19 Nov 2005 by Essen)

You are mistaken. The Web site you mention clearly derives a loss cone criterion that is independent of the total velocity, just as I did in this article. You also seem to have trouble keeping the concepts of velocity and temperature straight. --Art Carlson 19:46, 3 October 2005 (UTC)

Art, this article is very interesting and well written. I have tried to research this topic more broadly, but the explanations I have found use vector calculus extensively, which is not my strength. Your equation is compact, simple and elegant, easily within the grasp of an educated lay reader. I understand the general idea, but can you clarify some questions for me without resorting to vector calculus?

I like the given equation, but since no units are specified and no caveats are listed, I worry that it may possibly be a bit misleading. Can it really be true that a proton travelling at say, an overall velocity of 3,000,000 meters per second, is equally well reflected and has the same Critical Angle (splitting out the vectors for V-perpendicular and V-parallel) for reflection by two different magnetic mirrors, one of 1 nanotesla --> 10 nanotesla and the other a mirror of 1 gigatesla --> 10 gigatesla ??? At the very least this would seem to imply radically different "thicknesses" for the two mirrors, but does it also imply something about the requirements of other magnetic properties of the mirrors, such as their total magnetic flux? It also seems implicit in all these discussions that the magnetic field must extend far enough so that the particle under consideration does not stray outside the volume of the identified magnetic system.

Also, does the equation you gave apply equally well to all collections of charged particles, even plasmas: charged, uncharged and with charge unequally distributed? (I realize that the latter case could introduce some additional complications.) And if a particle is relativistic, wouldn't it be better to speak of momentum rather than velocity? How does plasma density affect the whole question of mirror requirements? What happens if a plasma carries a magnetic field of its own?

Please give a mirror design example for a proton with velocity, momentum, accelerations, energy, B-field strengths, etc.

This is a very interesting topic. I think that by illustrating the physical meaning and implications of magnetic mirrors, you could significantly enhance the value of an already very well-written article.

24.27.61.121 21:01, 16 October 2005 (UTC) ShawnM

[edit] November 19

Please add new comments at the bottom rather than editing older comments. It's confusing. I do deny that total particle velocity plays any role in determining whether reflection will occur or not in an ideal mirror. If you will state your question more clearly, I will be happy to respond. --Art Carlson 17:12, 20 November 2005 (UTC)

I believe that magnetic mirror calculations generally assume that the gyro radius is small compared to the size of the machine, which makes total velocity irrelevant. Paul Studier 09:20, 14 January 2007 (UTC)