On 15/02/2012 3:33 AM, Bill Sloman wrote:> On Feb 15, 4:01 am, Jamie M<jmor...@shaw.ca> wrote: >> Hi, >> >> How does a solenoid act to focus a beam of electrons that are passed >> axially through the coil? > > Don't know. The lenses in an electron microscope were lumps of high > purity soft iron (nickel plated to stop them rusting) that > concentrated the magnetic field generated by the solenoid inside that > - carefully shaped - lump of soft iron > > http://www.microscopy.ethz.ch/lens.htm > > The lens shape shown has little to do with reality. This reference > shows a schematic approximation (on page 10) which comes a little > closer to reality > > http://web.utk.edu/~prack/MSE%20300/SEM.pdf >Hi, Thanks for all the replies! That is a lot of interesting information, the only reason I can see why the solenoid coil will act asymmetrically (different electron beam angles on the input and output of the coil) is perhaps because of the inherent charge of the electron causing "space charge" gradients? That picture on page10: http://web.utk.edu/~prack/MSE%20300/SEM.pdf of the electron beam focus being proportional to solenoid current shows the asymmetry of the input and output of the solenoid. Is that diagram correct so that you can focus a beam to a position beyond the solenoid? cheers, Jamie> > -- > Bill Sloman, Nijmegen > > -- > Bill Sloman, Nijmegen >
electron beam through solenoid coil
Started by ●February 14, 2012
Reply by ●February 15, 20122012-02-15
Reply by ●February 15, 20122012-02-15
On 14/02/2012 7:36 PM, bitrex wrote:> On 2/14/2012 10:01 PM, Jamie M wrote: >> Hi, >> >> How does a solenoid act to focus a beam of electrons that are passed >> axially through the coil? >> >> cheers, >> Jamie > > Hi Jamie, > > Your question made me curious about this effect that seems pretty > non-intuitive given what we know about the magnetic field inside a > solenoid and the Lorentz force, etc. A lot of stuff seems to be behind > paywalls but I did find this reference on Google Books that seems to be > relevant: > http://books.google.com/books?id=pw4KalLZZ7gC&lpg=PA281&ots=sKJ8yOU-K8&dq=Busch%E2%80%99s%20theorem&pg=PA275#v=onepage&q=Busch%E2%80%99s%20theorem&f=false > > > The interaction between the magnetic field and the space charge electric > field generated by the beam is apparently pretty interesting. You'll > have to know some electromagnetic physics to follow it, but the math > doesn't look too bad.Hi, Also apparently if the electron is considered as an electromagnetic wave, then using maxwell's equations gives the correct results: http://en.wikipedia.org/wiki/Electron_optics "As electrons can exhibit non-particle (wave-like) effects such as diffraction, a full analysis of electron paths can be obtained by solving Maxwell's equation�however in many situations, the particle interpretation may provide a sufficient approximation with great reduction in complexity." cheers, Jamie
Reply by ●February 15, 20122012-02-15
Jan Panteltje wrote:> On a sunny day (Tue, 14 Feb 2012 19:01:38 -0800) it happened Jamie M > <jmorken@shaw.ca> wrote in <jhf76n$r8u$1@speranza.aioe.org>: > > >>Hi, >> >>How does a solenoid act to focus a beam of electrons that are passed >>axially through the coil? >> >>cheers, >>Jamie > > > That is the focus setup in the old vidicon system. > The electrons start spiraling in ever smaller circles, and if the field > strength is just right the focal point of the spiral is at the target. > This is because electrons like to move sideways in a magnetic field, > ever going sideways creates a spiral in your beam. > > In the old vidicon deflection system the deflection coils were located > inside a bigger focus coil. > Changing the focus current (in the big coil) would also rotate the picture..You know, it's strange how this thread comes in as I am also working on a magnetic project that involves some research similar to what is being discussed here. Blowing off the dust from old notes and references that hasn't see the light of day in years, along with new material adding to it. :) Jamie
Reply by ●February 16, 20122012-02-16
On Feb 16, 1:33=A0am, Jamie M <jmor...@shaw.ca> wrote:> On 15/02/2012 3:33 AM,BillSlomanwrote: > > > > > > > > > > > On Feb 15, 4:01 am, Jamie M<jmor...@shaw.ca> =A0wrote: > >> Hi, > > >> How does a solenoid act to focus a beam of electrons that are passed > >> axially through the coil? > > > Don't know. The lenses in an electron microscope were lumps of high > > purity soft iron (nickel plated to stop them rusting) that > > concentrated the magnetic field generated by the solenoid inside that > > - carefully shaped - lump of soft iron > > >http://www.microscopy.ethz.ch/lens.htm > > > The lens shape shown has little to do with reality. This reference > > shows a schematic approximation (on page 10) which comes a little > > closer to reality > > >http://web.utk.edu/~prack/MSE%20300/SEM.pdf > > Hi, > > Thanks for all the replies! =A0That is a lot of interesting information, > the only reason I can see why the solenoid coil will act asymmetrically > (different electron beam angles on the input and output of the coil) > is perhaps because of the inherent charge of the electron causing > "space charge" gradients? =A0That picture on page10: > > http://web.utk.edu/~prack/MSE%20300/SEM.pdf > > of the electron beam focus being proportional to solenoid current shows > the asymmetry of the input and output of the solenoid. =A0Is that diagram > correct so that you can focus a beam to a position beyond the solenoid?The focal point of an electron beam emerging from the final lens of an normal scanning electron microscope is below the lens - usually a centimetre or so below the lens - well away from the peak of the magnetic field doing the focusing. The Mulvey lens is an immersion lens and doesn't work this way, but despite it's charms it isn't all that practical. -- Bill Sloman, Nijmegen
