[BTeam] Injector Alignment

[Michael Tiefenback]

I would opine that the dZ offset is negligible for us, but that the amount of offset the beam shows post-cryounit is much larger than 1.2 mm.  The quoted 1.2 mm is _not_ the issue with which we struggle, and we have to buck the beam steering strongly (in my recollection) between cryounit and the first viewer downstream.  The _partially-corrected-by-steering-magnet_ offset is itself more than 1.2 mm.  The hole size on the linac viewers is, as I recall, 3 mm (1.5 mm radius) and what we tolerate as partly corrected is yet more than this.

Or so it seems to me.  The blame has been placed on a transverse kick generated by RF fields in the cryounit, as I recall.  I was not party to those determinations, and cannot guarantee more than that is what I think I remember.  That is why I suggested to Reza that we take this opportunity to move the cryounit transversely, presumably by the few-mm amount which was estimated in times past.  Even partial correction would seem advisable.

For the consideration of this august assembly....

Michael

________________________________
From: BTeam <bteam-bounces at jlab.org> on behalf of Todd Satogata <satogata at jlab.org>
Sent: Friday, October 2, 2020 08:01
To: BTeam <bteam at jlab.org>
Subject: [BTeam] Injector Alignment

(Email forwarded from Reza bounce; originally to Joe Grames and
Chris Curtis with CC: to Chris Gould, Matt Poelker, BTeam, Will
Oren, Camille Ginsburg, Alicia Hofler, and Yan Wang at 07:15 Fri
Oct 2 2020.)

I had a good meeting yesterday (Thursday Oct 01-20) with Curtis,
Gould, and Grames on Injector Alignment. The decision is to leave
things as is.

What does it mean?

(Chris Curtis please correct me if I misstate something)

According to the latest surveys (2008 & 2012) there is an offset of
about 1.2 mm in X and 1.5 mm in Z between injector and North Linac
coordinates. Everything before girder 0L03 is aligned to injector
coordinates and everything from 0L03 and higher is aligned to NL
coordinates.

This situation causes almost no problem for us in operating or
modeling of the Injector. Why? Because there is a long drift right
before 0L03. A 1.2 mm X offset becomes unnoticeable in a long
drift. (Z offset is even more insignificant)

Now, if we attempt to eliminate this coordinates' difference during
the Phase 1 of injector upgrade, this 1.2 mm offset will show up next
year right at the choppers where the elements are much closer to each
other and 1.2 mm offset will all of a sudden become significant.

The other point to consider is that there is no guarantee that the
North Linac won't move farther with respect to the injector in
future. It may continue in the same direction or reverse course. Since
moving all injector back and forth is not practical, my vote is to
take this offset at a location which is least disruptive, and that
would be 0L03 girder. In other words don't change anything.

The only problem remains is what Chris Curtis brought up and that is
potential for confusion when we align elements in the injector. We
should keep clear documentation that 0L03 is the breakpoint and make
sure there is no ambiguity in the sign of 1.2 mm.

Chris, if you or other experts in alignment group start a tech note to
document this, I would be happy to contribute to any part that falls
under my expertise.

I have added more recipients for the email. Please let us know if we
are overlooking something important.

Reza,

Reza Kazimi
kazimi at jlab.org

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