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Radio Frequency Quadrupole Linac Question

  1. Aug 9, 2010 #1
    (Sorry if this is off-topic for this group. There's really no group that addresses accelerators in the Physics Forums)

    I've looked at many examples of RFQ linear accelerators. Most of them share the same characteristics:
    1. So-called vane electrodes
    2. Opposite-facing electrodes are energized with a common RF signal
    3. Adjacent pair of opposite-facing electrodes are energized with RF + pi or 180 degree out of phase RF signal

    However, from all the images and diagrams I've seen, it looks like all four electrodes have continuity with each other. That's the way it appears but it doesn't make sense. What reinforces this idea is often the electrodes are machined from solid pieces of copper then brazed together apparently forming a monolithic unit.

    Can anyone shed light on what's going on here? If so I would appreciate it! Thanks--Marlin
  2. jcsd
  3. Aug 9, 2010 #2


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    Linacs accelerate particles through "slow wave" structures consisting of RF/microwave waveguides. The RFQ modulates the RF field alternately in two planes to achieve both acceleration and focusing at the same time. These are analogs of ordinary transmission lines, but at high frequencies it is possible to propagate along structures that defy simple intuition.

    To understand the complex RFQ structures, you should first learn something about how microwaves propagate down basic waveguides. This is a topic covered by hundreds of texts on E&M, microwaves and microwave electronics. You'll need to understand Maxwell's equations at a sophomore/junior level for it to make much sense. There are non-mathematical websites that will give you a flavor, though probably not much understanding:

    http://www.microwaves101.com/encyclopedia/waveguide.cfm" [Broken]
    http://en.wikipedia.org/wiki/Waveguide" [Broken]
    Last edited by a moderator: May 4, 2017
  4. Aug 9, 2010 #3
    Thanks marcusl. I'll check out that first link which sort of looks like a pretty good waveguide primer and then see what I can learn about Maxwell's equations.

  5. Aug 16, 2010 #4
    I had another question regarding this type of linear accelerator. Virtually all of the existing designs are made of nearly solid copper. I did an estimate on one design based on the volume of material and the 5 meter long machine would probably require around 2 tons of copper. And this is for a machine that could benefit from being mobile (contraband detector)

    Why solid OFHC Copper? Why not a fairly thin shell of copper and use the open interior spaces for coolant. Or perhaps better yet build the electrodes out of a plastic such as fiberglass and use copper plating on the fiberglass and again use the interior spaces for channeling coolant. With a high frequency RF generator you will have a certain fairly shallow skin depth that even when multiplied by 5 is easily within the realm of some form or another of copper plating/coating technology.


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