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Help with designing an electron accelerating mechanism

  1. Jul 21, 2012 #1
    ion accelerating source setup

    I'm currently building a rectified, filtered power supply for use as an ion accelerator.
    The setup is attached as a JPEG.

    I am wondering how/where I can add resistors to the arrangement to make it so that when the dielectric breakdown voltage is between the anode and cathode, minimal current flows through that branch, essentially preventing breakdown from actually happening.
    Or in addition to resistors, are there other ways to limit the current?

    This is all in a ~50 militorr chamber, btw

    Thanks in advance!
     

    Attached Files:

  2. jcsd
  3. Jul 22, 2012 #2
    Hi!

    I'm trying to design a mechanism for accelerating, then de-accelerating an electron beam.
    I've attached the circuit design. The peak to peak ripple voltage is negligible, and the donut looking things are parallel plates with small holes in them (for the electron beam).

    I'm having trouble in the bottom circuit. I want to make it so that the left(first) parallel plates accelerate the electrons to some energy, say "X" eV, whereas the right(second) parallel plates de-accelerate the electrons by some energy smaller than "X," for example "X-Y" eV.

    Would the battery in the second circuit work? How would it work if placed in the circuit like currently so?

    Will a battery discharge if the positive terminal is attached to ground, but the negative terminal isn't attached to anything? I thought it would, but today I hooked my ammeter in between a AA battery and the ground socket on mains and no current flowed..
     

    Attached Files:

  4. Jul 24, 2012 #3

    berkeman

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    Staff: Mentor

    What are you thinking will let the electrons leave the cathode? Are you going to use thermionic emission, or photoelectric emission, or some high voltage arcing emission (using the Paschen curve to calibrate the voltage with your vacuum level)?
     
  5. Jul 24, 2012 #4

    davenn

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    Science Advisor
    Gold Member

    no, as there needs to be a completed circuit and you dont have that because the negative terminal isnt connected to anything

    Dave
     
  6. Jul 24, 2012 #5
    Re: ion accelerating source setup

    You cannot use anything to prevent breakdown. It is pretty much what it is. Before breakdown, you don't have current flowing, you have the E field across the gap regardless what resistance in series. It's the E field that break the dielectric.

    You improve the stand off voltage by careful design on the dimension of the C and A. You make sure the components are absolutely clean, you make sure the creepage path is in check.

    From working with HV in mass spectrometer in high vacuum environment. Dielectric break down is not the major problem. Mostly is creepage problem.

    BTW, in your drawing, the cap between the C and A will make things much much worst when an arc occur. The capacitor serve as a charge reservoir and once you get the arc going, it avalanche and really burn. You want to minimize capacitance across. We put a series resistor to limit the current. BUT it is not good to put the resistor out side and then connect to the vacuum chamber by a coax. The capacitance of the coax is enough to do great damage. The 100pF of the coax is enough to fuse metal!!! The resistor has to be inside the chamber connecting right to the A or C.

    When we first build any e-gun or other HV components for HV in vacuum, we high pot it, let it arc, let it burn up all the sharp edges and dirt. The arcing would die down after a while.
     
  7. Jul 24, 2012 #6
    I want to use thermionic emissions from a separate circuit. Would a Mini Maglite's incandescent bulb with .4 amps running through it (normal operating current, i.e. connected to 2x AA batteries) work? My vacuum pump can pull down to 15E-3 torr (15 microns); will that relatively high pressure cause the filament to oxidize quickly, within seconds?
    And how can I calculate the current flowing from the hot filament?

    But after the breakdown voltage is reached, wouldn't the "huge resistor" (500 Mohms) in my second diagram effectively limit the current flowing into the arc to the point that it is negligible? All current would instead go through the resistor shunted with the capacitor (10 Mohm)

    Can you please explain how creepage will affect my setup and how I can avoid it?
     
  8. Jul 25, 2012 #7
    Creepage is the current travel along the surface of the insulator. Even if you have insulator separating the two pole, any dirt, moisture, imperfect vacuum will cause a conductive surface beyond certain voltage. While the normal good insulator can achieve 400V/mil, creepage is a lot lower. I don't remember the number, it is more like 50V/mil or less. For example if you have a hv wire feed going through a cage into the high voltage circuit. For 20KV, the spec for CE called for almost 4" of insulation from the open end to the part of the insulation that is safe to touch. This mean in extreme condition, the arc can travel on the surface of the insulation up to 4" back. So we have to design in such a way that there is no way anyone can touch the cable closer than 4" from the open end.

    That's the reason you see those Fisher HV connector, the male part is like 3" long metal shaft sticking into the female receptacle. Creepage is a much more serious problem, in the vacuum, you support the lens with ceramic standoff, the surface of the ceramic forms the creepage surface. Any dirt etc. will promote a creepage path.

    Worry more on the creepage, insulation breakdown is usually not a problem. For good vacuum, you almost get 400V/mil, you don't need a lot of distance to standoff HV. Go dig up more info on creepage, if not for anything, the CE requirement have section describing this. This is usually much harder to conform.
     
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