# Circular Beam of Electrons

Beam of electrons moving in a circle, due to the presence of a magnetic field. Purple light is emitted along the electron path, due to the electrons colliding with gas molecules in the bulb.
(Photo: Marcin Bialek)

Oh, how I love you guys. In our recent post A Fiery Dance on the Sun, Neatoramanaut PlasmaGryphon kindly took the time to explain to us the physics behind solar flares. In the explanation, there was a link to Wikipedia article on Lorentz force, where I found this fascinating image of a circular beam of electrons in a Teltron tube. Neat, huh? (Thanks PlasmaGryphon!)

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This is a pretty standard first or second year physics undergraduate experiment, where the charge-to-mass ratio of an electron can be measured. See, for example:
http://www.ic.sunysb.edu/Class/phy122ps/labs/dokuwiki/doku.php?id=phy124summer:lab_8
http://www.clemson.edu/ces/phoenix/labs/cupol/eoverm/index.html
http://physics.nyu.edu/~physlab/Eng_PhysIII/eng_physIII.html
http://www.physics.upenn.edu/undergraduate/undergraduate-physics-labs/experiments/electron-motion-magnetic-field
It's actually quite elegant, and was quite similar to the work that J.J. Thomson did to show that cathode rays were in fact charged particles.
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Mirror reflection of a stronger electron beam in a magnetic field which converges to the right. Note that the guiding center (axis of spiral) of the reflected beam does not coincide with that of the incident. This is due to the gradient and curvature drift in a nonuniform field.

This loopy one above from Prof. Reiner Stenzel's page on single particle motion in electric and magnetic fields is fascinating! Thanks for the link, PlasmaGryphon!
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I didn't know those things had an actual name. I had seen examples of them in science catalogs for years, although they are expensive enough to typically be found only in university demo collections. And at that level, someone in the department might end up making their own custom one.

In particular, I had come across this page which shows a range of motions possible if you had a giant version of one of those tubes. I've used one of the images near the end in talks before as an example of a magnetic mirror motion, a topic important to some fusion plasmas and to behavior of plasmas near the Earth too. Unfortunately that seems to be another topic that could use a better intro level explanation somewhere on the web (assuming there isn't one I haven't found yet), although one could easily spend a whole hour long lecture trying to explain all of the motions seen in just the second to last image on that one page.
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