Log entry time 04:32:45 on August12,2005

Entry number 151522

This entry is a followup to: 151337

Posted here are a couple of RHWP scans done during some opportunistic beam 
studies when the accelerator was recovering from CHL trip.

The first attachment is for the IHWP OUT and you can see that Dx = 0 at about 
107.25 degrees while Dy = 0 at approximately 108.75 degrees.
Compared to other recent compromises made with the RHWP, this one seems very 
reasonable. The Aq at approximately 108 degrees is roughly -350 ppm and notice 
that correcting it at this position suggests that the position differences will 
move closer to zero. It may even be reasonable to move the RHWP to 106 or 106.5 
just to keep the the position differences positive for the OUT state. A 
previous log entry I made showed them to be negative.

Looking at the second attachment (IHWP IN), the RHWP position is a bit of a 
tougher compromise in this region, but the idea remains the same. At 110 
degrees, the Aq is about -250 ppm, Dx is about 0.4 um and Dy looks fairly close 
to zero. Looking at the top plot of Aq, correcting the -250 ppm would involve 
moving the position differences downward (to the left) moving Dx closer to zero 
and possibly moving Dy a bit negative. But, in any case, if Dx in both 
waveplate states can be kept small and positive, they may cancel a bit between 
IHWP states. Now, they have opposite sign (based on a previous log entry of Dx 
and Dy in the injector), and after applying a sign flip to the IHWP state, 
there is no cancellation. Also, in the log entry I refer to, I mentioned a 
dominace in x by a steering effect. That is incorrect. The plots suggest the 
dominance of some birefringence effect in x which does not cancel with IHWP 
state. That was my mistake.

But, this suggestion could possibly end up creating a problem in y that we now 
have in x. Dy could be positive in in IHWP OUT but negative in IHWP IN not 
allowing any cancellation. In this case, moving the RHWP angle for the IHWP IN 
state higher (111 or even 112) may remove this possiblity (Dx and Dy all 
positive for both IHWP states). In any case, it seems like some movements 
around this region could at least give us a situation with the source that we 
understand, clear dominance of a steering effect as evidenced by cancellation 
between IHWP states. The game you play of course is in this attempt, do you 
have to make either Dx or Dy unacceptably large.