simple question about a cyclotron

[bwanaaa]

Although the wiki entry is clear, I do not understand how the mechanism of a cyclotron induces a unidirectional electron flow. It would seem that an alternating electric field between the two Dees would cause electrons to shuttle back and forth. I guess the same question applies to an electric motor - but in that case, the momentum of the armature keeps it spinning in the same direction for a bit while the field changes (although it is a magnetic field in the case of the motor, the idea is the same). The magnets in a cyclotron only serve to confine the particle beam. So my interpretation is that the momentum of the electron is what keeps it moving across the gap while the electric field is changing. OK, so why do all the electrons move in one direction around the circle? Why are there not two currents moving clockwise and anti clockwise?

I answered and understood the questions here:
http://webphysics.davidson.edu/physlet_resources/bu_semester2/c13_cyclotron.html

There is no explanation of how all the electrons 'decide' to go around in one direction.

And which direction is that? the same as the water circling the drain due to coriolis acceleration?

 

[PottedMeat]

not sure i understand what you mean.


one D is + in the cycle, the other -, so the electron is attracted to the pos. in the D the magnetic field keeps the electron in a half circle path. when it exits the D the field is reversed and it accelerates towards the other D

 

[harrkev]

The electric field DOES just cause them to go back and forth, if that was all that it did. There is also a magnetic field too. Moving electrons experience a sideways force as they move through a field, which causes them to go in a circle.

 

[bwanaaa]

Each D has two 'arms' attached to its flat edge. Let's label the points where the arms (which are really just parts of the same semicircle) meet the edge - A and B. on top of the Dees is a giant north pole and under the Dees is a giant south pole.When one D is negatively charged, the electrons move from it across the gap to the other D. In moving, they generate a magnetic field. At A and B the magnetic fields are the Same because they are moving in the same direction . The north of the field generated by the moving electron points ahead of the direction of travel and south points behind.Both electrons are then deflected downward. When the electric fields in the Dees alternate, the electrons move in concert the other way. How do you get electrons to be emitted frm A and absorbed at B? There is no asymetry in the forces to favor circulation in one direction.

 

[serpretetsky]

here is a crude drawing I made to try to explain it.
One note, there is a terrible error; if that were an electron, it would circle the OTHER direction. I didn't want to change it once I finished, just imagine it's a proton instead, and that the electri field charge applied in the second frame is reversed, and then we're groovy. It doesn't change the understanding of concepts either way.

 

[bwanaaa]

tnx. I cannot believe I screwed up the right hand rule. I kept thinking the electrons wanted to fly perpendicular to the magnet and jump out of the Dees.

So to confine the electrons to the Dees, one would need a progressively stronger field towards the circumference of the dees. The design I am thinking of is an array if magnets instead of one big one. the weakest magnets in the center (with the same pole facing towards the Dee) Stronger magnets would be arrayed in the more peripheral regions. So as the electrons accelerate and move away from the center, the stronger fields then confine them.