Page History

...

For SpinQuest it will be difficult to get the positioning of the target
perfectly aligned with the beam.  This is because all fiducials that we
can get while the system is warm and open will change once we are running
with liquid helium.  We have X, Y positioning measured from averaged
vertex reconstruction but this takes at least 1 month of data to produce,
and this will only give the mean with a very large variance.  This means
we need to get the information we need from the survey and the final set
of surveys will need to contain some cold target information.  The
precision in the target cell to beamline positioning needs to be good to
mitigate large absolute errors while running.   This is estimated to be on
the sub-millimeter level (based on simulations being confirmed).  There
are several factors that can lead to false asymmetries relating to this
precision.  There can be a bias produced in the detector if there the beam
is off center.  There can be less polarized scattering if the beam is not
aligned and missing part of the target (beam profile dependent, also being
checked).  There can be greater scattering off the aluminum ladder on one
side as compared to the other.  Most of these manifest from X being off
but if Y is off over 1 mm then the same issues start to manifest with the
ladder as well but this may not result in false asymmetry and only result
in additional heat load to the coils.

During our discussions with the survey crew it was suggested to install
transparent windows so we could do an optical survey on the magnet and
target cells while cold.  This would be great to do but I do not think
this is possible at FNAL.  I don't believe we would ever pass a safety
review to do that.  The next best option is to do surveys using liquid
nitrogen.  We could calculate the level of contraction for both LN2 and
LHe.  We can then measure the contraction during an open system survey
with LN2 in the magnet and in a nose with a window on it.  The set of
survey would go like this:

1.) Survey on doc with fully open system (no nose, no nitrogen shield, no
beam windows).  Survey the magnet aperture, the target insert along X
only, and the outside fiducials (can and top flange).  The primary goal of
this survey is to generate fiducials on the can and top flange to help
align when going into position in the cave.  This part of the survey should

be able to be measured with error at less than 0.15 mm.  The fiducial on the can is meant

to use to align the top and bottom to our best precision. The Thetolerance drawingsof indicateathe changedrawings ofis 40.5 mm.

Use the upleveling whenpads cold.to level Thethe tolerancemagnet ofon thesethe drawingsdoc isbefore 0survey.5 mm.Use  This survey

can be use to check the variation when leveled and re-leveled or if the magnet

is plumb not the top flange is slightly off level.  Working  the leveling pads to level the magnet on the doc for

this survey will have the advantage of space unlike in the cave.  We can also

flip the insert and check target cell up stream and down stream differences.before survey.

2.) Survey in cave using LN2 in nose and magnet.  This would be a partly
open survey with a special nose put on with transparent windows on either
side for optical survey.  The nitrogen shield and beam widows on the
vacuum are left off for this.  This survey in the cave will be of the
magnet aperture, the target insert along X, Y, Z, and the outside
fiducials (can and top flange and reference locations in the cave). 
Crosshairs will be use in the front and back of the target cell to help
determine degree of twisting in the cell.  Cross hairs in the can windows
can also be use to set the rotational alignment of the can.  The target
insert carbon fiber is not expected to contract very much but the aluminum
ladder can.  The primary goal of this survey is to compare to calculations
of contraction and measure positioning of target cell with respects to
magnet aperture and also with respect to the beamline.  There should be a
hard stop setup on the actuator that positions the top target exactly in
the beam line (to test repeatability of the string potentiometer and gears).

...

reasonable repeatability.  This repeatability can also be tested during this survey.

The drawings indicate a change of 4.5 mm up when cold.

3.) Put in hard stops and re-survey.  This will be adding a shim to the
hard stop that already exists.  Here we can have the goal of .25 mm target
centering in X,Y and measure a set of cells using cross hairs in Z.

4.) An X-ray survey while cold would be the next step if possible.  We may
not needed it if we achieve good results from the first 3.  It may also be
possible to use phosphine graph paper to check beam position on a special
target insert without depending on the magnet or nose having LHe in them.

...