None.
Items of Discussion
-----------------
- Steve Benson discussed the twisted quad alignment
problem. The bulk of the slides follow:
- Subj: Twisted quads summary
TWISTED QUAD ISSUE
In trying to align the double quadrupole girders, a potential problem has
come to my attention. First some background.
- The QB and QG quadrupoles are made up of stacks of precision stamped
laminations which define the pole surface and the reference surfaces. Four
grooves exist in the middle of the four laminations.
- Each quadrupole has an "aisle side" and a "wall side" which are
determined by the location of the plug strip to which the electrical
connections are made. The plug strip is on the wall side and should
therefore always be on the outside of the accelerator ring. Note that the
CEBAF linac quads were installed backwards and do not follow this
convention but that all the quadrupoles in the FEL will follow this
convention.
- The quads are measured upside-down while clamped in a fixture which
contacts the middle of the reference groove on the plug strip side and two
points near the ends of the reference groove opposite the plug strip.
- The alignment fixture mounts on the same three points as the magnet
measurement fixture. The position and angle of the QB or QG is adjusted
and measured by measuring the position and angle of the alignment fixture.
There may be some errors in transferring the fixture position to the magnet
position.
LET'S DO THE TWIST
- Though the laminations are precisely made they are not well constrained
in all directions. They can flex and slew with respect to each other.
They are welded to bars at eight points which constrains the motion
somewhat but also creates a twist in each quadrant of the magnet.
- Due to the magnet clamps and the good lamination tolerances each two
reference grooves should occupy locations in parallel planes. There can be
a rotation in each plane however. Given these constraints it is easy to
show that the parallelepiped which is formed by the four reference grooves
will be distorted so that it is twisted. Each "face" defined by four
adjacent grooves will be twisted with respect to the opposite face.
- If the support for the quadrupole mirrors the alignment and magnet
measurement fixture, the roll and pitch of the quad should be set correctly
by setting the roll and pitch of the fixture. The vertical axis of the
quadrupole will be an approximately linear function of the distance along
the quadrupole, i.e. there will be a screw dislocation in the quadrupole.
- The quads are quite elastic and so the twist cannot be easily taken out
by trying to straighten them out. According to W. Oren, straightening out
laminated magnets can be done by twisting them past their elastic
deformation limit, but they return to their original twist after the field
is cycled.
THE "PROBLEM?"
- The screw error will not affect the beam as long as the quadrupole can
be considered as a thin lens. The skew quad moment at the entrance of the
quad will cancel the skew quad at the exit. It is clear that for a thick
lens the entrance and exit do not cancel. Is this a problem?
- (Note: Dave Douglas answers that it is only a problem for a twist of
greater than 10 mrad.)
- The yaw angle of the quadrupole is set using the yaw of the alignment
fixture. This means that the top of the quad will be properly aligned but
that half of the twist will still be present in the quadrupole axis. The
amount of this twist is not known at this time. The pitch angle is set
with respect to the aisle side of the magnet. This means that there will
be a pitch error as well, approximately equal to the yaw error. Could this
be a problem? (Note that this error is present in all the main machine
quads and that the measured field is about an axis which is also off in yaw
and pitch).
- It would be nice to align double quad girders by making the two quads
parallel using a mandrill. This will "misalign" the quads as far as yaw
and ptich goes. Is this a problem? One could also align the bottoms of the
quads and reverse the yaw error. What might be the result of this?
- Some of the comments from the conversation follow(apologies for errors
stuff was flying pretty fast):
- D. Douglas - What is the tolerance for the magnetic center?
- S. Benson - +/- 100 um for the quads around the wiggler.
- D. Douglas - What drives that spec?
- S. Benson - By constraining the quad alignment to set a fixed,
known axis through the wiggler, Steve hopes to cut the
number of search variables for the FEL from 12 to 4. This
should have a major impact on "Turn-on" times and will be
worth the effort in the long run.
f the field measured corresponds to the axis aligned to, and it's
adequate, why change it? By using different alignment system, may
be able to improve the field integral without changing the magnet.
But is this really a problem for the bulk of the FEL quads?
We don't really know.
Actions- D. Douglas to try simulating the alignment errors Steve is
describing. Once we have some analytic answers, we will schedule a
meeting for Dave, Steve, J. Karn, C.L. Bohn, G. Neil, W. Oren and
Leigh to decide how alignment should be done.
New Issues
----------
- G. Neil moved to change the meeting location to the site
and I said I'd take care of it - the meeting will be in
89 next week.
Action Items
------------
- D. Douglas modeling. S.Benson to schedule meeting after
he hears from Dave.
Action Priority Items (assigned)
----------------------------------
A-15 1 Commissioning Procedure Legg, Krafft 5/16/96
A-19 0 Web Documentation weekly
A-24 1,2 Beam loss Bohn
A-30 1 Burnthrough/Conflat Expansion Denard, Kloppel
A-34 2 Consistent Linac phasing plan Yunn
A-42 2 2nd gun for long term tests Bohn 10/1/96
A-44 1,2 Start Rui on emittance code Bohn 8/13/96
Action Items (pending)
------------
P-05 1 Devise fine-tuning procedure for buncher gradient Krafft/Kehne
P-08 1,2 How do we change energy? What is the energy range? Benson
P-19 2 Consistent linac/bunching phase sets Yunn
P-20 1 Lock plans (who in particular) Johannes?
P-21 1 Save/restore plans Benesch
P-25 1,2 RF control sensitivities/ Merminga
Microphonics Diff for FEL/
Pathlength effect
P-29 2 FEL/linac interaction Merminga
P-30 2 Tracking including realistic wiggler fields Douglas
Agenda for Next Meeting
-----------------------
Item Person Responsible Time
---- ------------------ ----
* Review agenda/corrections to minutes Legg 5 min.
* Action item list update Legg 20 min
* Hapek device update Jordan/Piot 10 min
* New issues all 10 min
* Agenda for next meeting All 5 min.