[Bteam] yet another pathlength issue

[Michael Tiefenback]

Arne Freyberger wrote:
>   Today, Curtis and Oren came into my office and asked what the 
> implications of a elements shifting in the east arc due to tying in the 
> Hall-D line.   They asked if motion of 1cm would be an issue!!!
>
> We need to provide a requirement for the allowed motion of the East Arc 
> during the 6 month down.    This is mostly a pathlength issue.  How much 
> excess capacity is there at 5.6GeV for the odd-Dogs + Arc-offset?  
> Naively I think 1cm is too much, but what is the right number?  Anyone 
> want this hot potato?
>
> Arne
>
>   
OK.  I just found this question. 

My take:  a uniform "splay" of the recirculation arc end positions at 
the East end by 1 cm can be turned by repositioning of elements into a 1 
cm increase in the straight section at the arc midpoint.  Angular 
changes are small and can be accommodated in the E region tunnel.  Small 
variations in the bend angles using the horizontal correctors can make 
the result less than this exactly 1 cm increment to overall path, so 1 
cm is an upper bound on what one -must- tolerate.  The 1 cm out of 
6550-odd 20-cm wavelengths is equivalent to about 7-1/2 parts per 
million in path, and can be compensated by lowering the machine 
frequency by 11 kHz.  This is within the band that the RF and 
diagnostics are -supposed- to work, but eats up all of the margin we've 
tried to demonstrate and preserve, and is outside the physical 
tolerances we must respect in the upgrade layout.  However, as a holding 
action prior to a full re-survey and re-location of the elements during 
the 12 month down, it appears to be a tolerable "bump in the road."

What could possibly drive such a question at this late date?  CASA has 
been pressed to provide much finer precision in our answers than that, 
and already a long time ago.  Maybe we need to string some piano wire 
around the magnet stands and mount and monitor some strain gauges to 
record changes in overall positioning of elements.  If, as I suspect 
from conversations I've had, it is driven by suspicions about gross 
earth movement associated with Hall D construction and de-watering, then 
some monuments ought to be selected and referenced to the survey 
references now for later comparison.

All comments and insightful questions welcome.  Or even just 
thought-provoking questions.  We may need some more thought.

Michael Tiefenback