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In Need of a 50-kV Electron Gun

Discussion in 'General Electronics' started by Murat Ozer, Jun 16, 2004.

  1. Murat Ozer

    Murat Ozer Guest

    Dear colleagues:
    In a posting in 2002, I had asked for information for a 50-kV electron
    gun. Somebody from Seattle, Washington had contacted me and offered me
    his old electron gun. Unfortunately, I seem to have lost his e-mail
    address. If you are that person, will you please contact me if you
    still have the electron gun?

    Regards,

    Murat Ozer
    e-mail:
     
  2. What's the part number, and manufacture???


    Jerry G.

    =======
     
  3. Robert Baer

    Robert Baer Guest

    Steal one from a color CRT...
     
  4. Rich Grise

    Rich Grise Guest

    They're only 25kV. He'd need two in series.

    Cheers!
    Rich
     
  5. A color TV has three of those, so what is the problem :)
     
  6. Marlboro

    Marlboro Guest

    He may kill himself twice
     
  7. James Sweet

    James Sweet Guest


    You can't wire electron guns in series! They need only be grounded, just
    charge the target to 50kV.
     
  8. Robert Baer

    Robert Baer Guest

    Nope! The CRTs run from about 20KV to about 30KV, and that is the
    *anode* voltage; not relevant to the electron gun at all.
     
  9. Robert Baer

    Robert Baer Guest

    Aye-Yup!
     
  10. Bill Sloman

    Bill Sloman Guest

    The problem with high-voltage electron guns is flash-over. The 50kV
    between the cathode and the anode that is accelerating the electrons
    that you want to produce at the cathode can accelerate other charge
    carriers, and if the dynamic are wrong the curent in this unintended
    sneak path can rise very rapidly to create an arc.

    This process is called a flash-over, and can discharge the capacitance
    of the cable carrying the 50kV current in a few nanoseconds.

    When I was first working on electron microscopes at Cambridge
    Instruments, we used a cheap-skate cable termination at the electron
    gun that wasn't properly coaxial, and we'd get hundreds of amps of
    current running around the ground connections during a flashover,
    which had a tendency to blow up sensitive inputs if you didn't protect
    them with catching diodes.

    When my boss finally got his way and we went over to proper coaxial
    connectors at both ends of the high voltage cable, the ground currents
    during a flashover confined themselves to the ground return (screen)
    of that cable and the electron microscopes became a lot more reliable.

    The sources of the unintended sneak paths were interesting and
    various. One of the more interesting problems turned out to be organic
    vapour contamination in the "high vacuum" in the region of the
    electron gun. Basically, it is very difficult to avoid having a single
    molecule layer of lubricating oil on every metal surface inside an
    electron microscope, and if this shows up in the electron gun the film
    is exposed to 50kV X-rays, which knocks off hydrogen atoms, converting
    the film into an electrically conducting graphite-like layer of
    carbon, which promply adsorbs own its single molecule layer of
    lubricating oil. In areas of high electric field, the intially smooth
    surface develops sharp points, which in turn develop into sharp
    needles, where the tip is a single hydrocarbon molecule, which
    eventually becomes a cold field-emission tip emitting electrons in
    competition with the intended cathode.

    As the field emission current builds up, the needle stops being cold,
    and becomes a heated emitter, emitting a lot more current, before
    breaking off and hitting the anode with loads of energy where it
    generates a load of positive ions, who travel back to the cathode,
    accelerated by the 50kV field ....

    Very high voltage electron guns - 100kV and higher, are always
    "bakeable" with a private vacuum pump which allows you to generate an
    "ultra-high vacuum" in the electron gun, which is to say a vacuum high
    enough to prevent the single-molecule thick layer of lubricating oil
    from building up. IIRR physicists worked out the technique to allow
    them to work with bare metal surfaces back in the 1950's, but it's
    adaption to commercial electron microscopes was still problematic in
    the 1980's when I was working for Cambridge Instruments.
     
  11. Bill Sloman

    Bill Sloman Guest

    The problem with high-voltage electron guns is flash-over. The 50kV
    between the cathode and the anode that is accelerating the electrons
    that you want to produce at the cathode can accelerate other charge
    carriers, and if the dynamic are wrong the curent in this unintended
    sneak path can rise very rapidly to create an arc.

    This process is called a flash-over, and can discharge the capacitance
    of the cable carrying the 50kV current in a few nanoseconds.

    When I was first working on electron microscopes at Cambridge
    Instruments, we used a cheap-skate cable termination at the electron
    gun that wasn't properly coaxial, and we'd get hundreds of amps of
    current running around the ground connections during a flashover,
    which had a tendency to blow up sensitive inputs if you didn't protect
    them with catching diodes.

    When my boss finally got his way and we went over to proper coaxial
    connectors at both ends of the high voltage cable, the ground currents
    during a flashover confined themselves to the ground return (screen)
    of that cable and the electron microscopes became a lot more reliable.

    The sources of the unintended sneak paths were interesting and
    various. One of the more interesting problems turned out to be organic
    vapour contamination in the "high vacuum" in the region of the
    electron gun. Basically, it is very difficult to avoid having a single
    molecule layer of lubricating oil on every metal surface inside an
    electron microscope, and if this shows up in the electron gun the film
    is exposed to 50kV X-rays, which knocks off hydrogen atoms, converting
    the film into an electrically conducting graphite-like layer of
    carbon, which promply adsorbs own its single molecule layer of
    lubricating oil. In areas of high electric field, the intially smooth
    surface develops sharp points, which in turn develop into sharp
    needles, where the tip is a single hydrocarbon molecule, which
    eventually becomes a cold field-emission tip emitting electrons in
    competition with the intended cathode.

    As the field emission current builds up, the needle stops being cold,
    and becomes a heated emitter, emitting a lot more current, before
    breaking off and hitting the anode with loads of energy where it
    generates a load of positive ions, who travel back to the cathode,
    accelerated by the 50kV field ....

    Very high voltage electron guns - 100kV and higher, are always
    "bakeable" with a private vacuum pump which allows you to generate an
    "ultra-high vacuum" in the electron gun, which is to say a vacuum high
    enough to prevent the single-molecule thick layer of lubricating oil
    from building up. IIRR physicists worked out the technique to allow
    them to work with bare metal surfaces back in the 1950's, but it's
    adaption to commercial electron microscopes was still problematic in
    the 1980's when I was working for Cambridge Instruments.
     
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