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B Is my Cockcroft-Walton accelerator design correct?

  1. Jun 19, 2017 #1
    I am building a Cockcroft-Walton accelerator for a science fair project, so how does this design sound?

    Starting from the top, there is a polished steel terminal, like a VDG terminal. Inside here is the proton source and power supply to the source (the source is a hydrogen discharge tube) . The top terminal is connected to the output (positive), of a Cockcroft-Walton voltage multiplier. From the top terminal comes the accelerator tube. It is a glass (probably borosilicate) pipe.

    Three polished copper drift tubes are fitted within it. The highest tube is connected to the spherical terminal at the top. The middle tube is connected to the middle of a six-stage CW voltage multiplier. This is half the top terminal voltage (still positive). The bottom tube is connected to the start of the multiplier , ground. The proton beam enters a machined aluminium chamber and strikes a target. This creates gamma rays which can be detected. If lithium is struck with the beam, alpha particles are created which escape through a mica window in the target chamber and are detected by a end-window Geiger counter.

    Should I place external corona needles connected to the drift tubes, outside the accelerator tube, to equalise potential over the tube, or is my current design fine? Should a seal the proton source with a mica window or us the design from http://info.ifpan.edu.pl/firststep/aw-works/fsII/alt/altineller.pdf. If I don't seal the source, won't the hydrogen just escape down the acceleration tube?

    Please improve upon it and tell me what I'm doing wrong.
    Thanks so much,
    Last edited by a moderator: Jun 19, 2017
  2. jcsd
  3. Jun 19, 2017 #2
    Just for starters. Do you have sketches/drawing of your device? What is your goal or purpose for this device? What is you design terminal voltage? What radiation safety procedure do you intend to implement? An needless to say I presume you are well aware of the HV safety issues both for users and spectators.
  4. Jun 19, 2017 #3
    Yes, I do have the sketches and drawings.
    My goal is to create a proton beam for the bombardment of targets.
    The terminal is charged from a Cockcroft-Walton multiplier, if that's what you mean.
    The target chamber is shielded with lead, except for the window. I will also make use of dosimeters.
    And yes, I'm aware of HV safety.
    Thanks for replying, so do you think it will work?
  5. Jun 19, 2017 #4


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    He is asking what the output voltage of your multiplier is...
  6. Jun 19, 2017 #5
    Oh, sorry. I don't know exactly but it's probably about 72Kv. I got that from a rough estimate, I haven't got a high-voltage multimeter probe yet, so can't measure exactly.
  7. Jun 19, 2017 #6
    Going on just your description it is difficult to say how successful you will be in obtaining a usable beam.
    Getting a usable beam may be more difficult than you expect. How do you expect to evaluate its performance? Beam current for example and are you measuring what you think you are

    At 72 KV I wouldn't hold my breath on seeing any nuclear reaction products. the coulomb barrier will be too high most of your beam to penetrate to a distance which will result in a reaction.

    I should add that you might want to check for soft x-ray too say from back streaming electrons flying up toward the HV terminal.
  8. Jun 19, 2017 #7
    Thanks for the help! What is the value of the Coulomb barrier, so I can design a higher voltage multiplier? What could I do to create a usable beam?
  9. Jun 19, 2017 #8
    You should be able to compute a reasonable approximation by determining from Coulomb's law and assume that the target nucleus' surface and projectile surface just touch. the work (energy required) you must do to bring them that close.

    As far as useable beams are concerned there may be a lot of trial and error. But factors that are important are purity of the gas you use , the method you use to extract the ions from the source and how you focus the beam coming out of the source and down the beam tube. you don't want the beam hitting anything after it extracted. Also the beam tube vacuum is important the longer the tube the lower the vacuum should be.
  10. Jun 19, 2017 #9
    Ok thanks, and very finally, the acceleration method is correct though, with the beam accelerated down lower potentials to ground ?
  11. Jun 19, 2017 #10
    Yes, An one additional point. In trying to experimenting with and optimizing the ion source you might construct a setup specifically designed for this instead of putting it on the CW machine and measuring current at the end of the beam tube.
  12. Jun 20, 2017 #11
    Ok, thanks a lot !
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