Quadrupole focusing

[r_c_edgar]

Here's another interesting lens idea that I'll throw out there: quadrupole ion lenses.

The setup is a quadrupole (electric or magnetic, either works) with the plane of the quadrupole perpendicular to the direction of the ion beam.

Quadrupoles have stronger focusing than cylindrical Einzel lenses, or pretty much any other non-gridded lens particularly at medium to high energies. They have the disadvantage that they focus only in one plane at a time. To get uniform radial focusing, you need a doublet (or, for an astigmatic lens, a triplet).

The construction difficulties in a quadrupole lens are more severe than the simpler cylindrical lenses as well.

One idea I've had is to use a single electric quadrupole, but drive it at rf frequencies... It'll act like a whole series of electrostatic quadrupoles, but without the need to physically construct lots of them.

This is actually the avenue I have been pursuing for a while, a cylindrical colliding beam device using quadrupoles (magnetic and rf electric) for ion beam focusing / radial confinement, with axial recirculation (ala reflex ion gun).

--Ryan

[DaveC]

Of course, the quadrupole "lens" is the heart of the mass selective filter used in quadrupole Mass Spectrometers. Not sure if you had this had in mind, here, but with a DC bias and the proper AC frequency, one could make the filter work, to pass only Deuterons. Thus eliminating other gases from the output. Might help to boost efficiency.

Could... also be tuned for He...to make an internal detector for fusion by-products.

Interesting idea, Rayn.

Dave Cooper

[DaveC]

oops... Ryan...

Dave Cooper

[guest]

I think in practice it is not possible to drive the quadrupole winding with a RF frequency...
you can use a dc controled signal to drive them.
Seadat. Varnaseri

[3l]

Hi Ryan:

Yeah I've heard of it ...RFQ Accelerators.
They use a magnetic field to make the particles go in a circle
while driving them with a quadrapole array.
They array is spaced so that at the entrance is a small circle that goes into a larger circle. (like a megaphone or cone)
As the particle gains energy the bigger the radius of the circle it makes with a fixed field so that slanting the electrodes makes a free space for the particle and positions them for the next acceleration. I will post a picture of one. The ones I familiar with pre accelerate before RFQ usually. The limit of course is the radius of the RFQ . It has the same characteristics as the cyclotron, just picture it as a four dee cyclotron.
All the formulas for the cyclotron will apply.
I had considered using one as an ion generator.
Electrons bend easier than protons or deuterons.
of course that is acceleration .
focusing simply reverses the process.
All you would need is four insulators.
Four rods
Four insulative supports
A Magnetic Field (stacks of Neo iron magnets)
A phase shifter (usually Tubes) 90 degree shifts
An rf generator usually 450 mhz.

Happy Fusoring!
Larry Leins
Fusor Tech