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    User name R. Michaels

    Log entry time 16:27:44 on April 12, 2010

    Entry number 316930


    keyword=about O-rings

    A failed O-ring at the beam-exit flange on the target chamber caused
    the downtime of PREX.  Today it was replaced by Ed Folts and the techs.
    Since I get a lot of questions about O-rings, here is my best shot
    at answering.  If anyone knows better, feel free to follow up.
    Q: Is that the only leak ?  What about the rectangular O-rings ?
    A: It's thought likely to be the only leak.  We'll find out after
    we pump down, probably by Wednesday Apr 14.
    Q: What happens if we need to replace the other O-rings, e.g. the
    rectangular ones on the sieve box ?
    A: First the techs may try to "goop" it with silicon, a standard
    practice.  Or try to torque the bolts.  But if it's leaking, then
    replacing those O-rings is potentially a major downtime (~1 week 
    plus whatever time to wait for suitable RADCON).  This is why we
    didn't simply replace all O-rings.
    Q: Will the situation improve or are we doomed to cook another O-ring ?
    A: We put back the massive tungsten shielding that had been in during
    HAPPEX-2.  This happens to shield the O-ring in question, the one at
    the smallest angle.  Taking it out was a mistake, I think.  We've recently
    estimated the power that was going into this region and I believe
    we were heating the O-ring and/or irradiating it before but will
    not now.  I don't know how to estimate the lifetime of an O-ring, but
    it seems likely that the situation will be much improved and similar
    to HAPPEX-2.
    Q: Why not use metal O-rings, etc ?
    A: My understanding is that, first of all, for all "regular" expts that
    don't use Septum magnets, they always had metal conflat seals which 
    are robust.  For the Septum connection an O-ring was necessary 
    and the metal O-rings were foreseen and the design called for them; 
    but they did not work because the target flange is aluminum and 
    the groove was wrong (too deep or too shallow, not sure which).
    The point is, metal O-rings WERE thought about and do NOT work.