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Thread: Trying to revive an old argon laser

  1. #11
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    Before trying to refill this laser I looked through a book about russian vacuum tube technology, especially the section about vacuum pumping and outgassing. That is strange, but as for small tubes, or big transmitting ones, or CRTs, or low-grade magnetrons there are used vacuum stations with oil diffusion pumps. Advanced stations with turbo pumps or ion ones were used only for very special tubes like radar travelling wave tubes. The book is edited in 1992. Also the authors recommend to begin the cathode activation at vacuum from at least 10-2 Torr to 5*10-5 Torr at final stages of cathode degassing, as it is shown on the first screenshot. I used this book as a reference for my attempts. Here are some proofpics. So, what is the difference between an argon laser tube and a big vacuum tube, like a high-power thyratron that is filled with argon and has a heated cathode too ?
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    Last edited by Laserbuilder; 08-18-2018 at 00:32.

  2. #12
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    Its not a difference between NATO Cathode and Russian cathode.I had some time to think about this last night. Its difference between depleted cathode exposed to air while hot forming irreversible tungsten compound. Gas lasers use the later as the plasma current tears conventional tungsten cathodes apart.


    There are a couple of active compounds used in cathodes world wide. One activates fine at diffusion vacuum and can reactivate a few times. Other uses tungsten as sponge to hold a mixture which slowly forms an active layer about 100 micron thick on surface that slowly depletes and exposure to air when hot ruins it forever.

    As soon as I can later today I will scan you internal laser factory document explaining why you need higher vacuum for Dispenser cathode. Blame the Dutch, it was their idea.

    Steve
    Last edited by mixedgas; 08-18-2018 at 07:24.
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  3. #13
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    BTW, I remembered that I've got a similar NOS laser tube, that is broken. But it is brand-new. Is it worth to try to replace the cathode?

  4. #14
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    Take a look a US Patent 3,928,820

    then 3,646,476: Pulsed Gas Ion Laser

    New Scandate or Dispenser cathodes are more then 250$ here, and you still need a higher vacuum and a tube oven and window stem ovens, probably a new NEG type getter.


    Steve
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    you still need a higher vacuum
    I'm thinking about replacing my diffusion pump oil with "silicon" one, like DC704 or DC705 type and fill my cold trap with LN2. Than I'll possibly hit 5*10-6 Torr. Any custom oven is possibly not a problem.

    Looking through old books I realise that I can pump out and refill He-Ne, He-Cd, Cu-vapor, CuBr, CO2, N2 and maybe some other lasers but I didn't think that it will be so difficult with an ion one.

  6. #16
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    Ions are easily poisoned. In 20 years of doing this I've encountered more the a few Argons and one Krypton (as well as repumped two myself) that even at the correct tube voltage, with proper pressure, with good windows, that would not lase no matter what even with a good plasma.

    Hydrogen, Oxygen, Nitrogen traces will take down an Ion. Hydrogen and Oxygen in minute traces will poison a HENE.
    ~
    Pros pump the tubes with an RGA attached at all times for this reason.
    ~
    Steve
    Last edited by mixedgas; 08-22-2018 at 07:55.
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    Hydrogen and Oxygen in minute traces will poison a HENE.
    Right. And this is what the getter is for. Even badly leaked he-nes which glowed violet could be revived after getter activation. At least 5 he-ne tubes I managed to make lasing again using only the getter. Maybe I'll try replacing the cathode in this tube with one from the brand-new broken tube, when I improve my vacuum and buy 99.999% pure Argon.

  8. #18
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    Quote Originally Posted by mixedgas View Post
    ~
    Pros pump the tubes with an RGA attached at all times for this reason.
    ~
    Steve
    +1
    They also double as leak detectors

  9. #19
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    Many Modern tubes no longer have getters. The sealing and bakeout is that good.

    Steve
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  10. #20
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    Find a Russian Paper:
    ~
    Kuchennko, Yavorsiky
    "A Study of the Operation of Sintered Cathodes under Gas Discharge Conditions"
    Radio Engineering and Electronic Physics, Volume 10, 1965, Pgs 629-634
    ~
    Mentioned in an American Paper on lifetime problems of Ion Lasers
    Covers poisoning of Russian Sintered Cathodes
    `
    I cannot access it.
    `
    Steve
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