[Sane-analysis] Follow-up on tracking through the reversed target field

[O. A. Rondon]

Hi,

Here is a follow-up on my plots of tracks through the target field, only
for 4.7 GeV, which has the most bending. I have added the 1 cm nominal
raster envelope, showing where it would intercept the cup. Note also,
the interesting focusing of the raster by the field after it passes the
target center.
http://twist.phys.virginia.edu/~or/sane/analysis/target_4.7-Merged.PDF

But, as Mark points out, we tried centering the target on the beam,
which does result in the full raster clearing the cup, in spite of
passing through at an angle, as it is shown on page 3. The clearance is
even better for 5.9 GeV, due to the stiffer beam.

So a reversed field cannot be ruled out from looking at tracking. The
target group should confirm that the vertical adjustments of the target
movement did compensate for the ~ 3 mm upward shift from the surveyed
center, and report on any indications of radiation damage that may have
been observed on the upstream bottoms or downstream tops of any cups or
lid rims. It would also help to know how such reversal might have
happened (PS connections?, coils mounted backwards?...)

I have thought of another way to check the field direction using the HMS
lowest momentum inelastic electron data. There are 6 runs with p_HMS 1.6
GeV/c, HMS theta 18 deg. for the 4.7 GeV 80 deg. part 1 period. The
xptar distributions for these rather low E' electrons should probably
look considerably different depending on field sign, since the scattered
electron-field angle will be either 98 deg or -82 deg., leading to
almost maximum deflection.

I encourage everyone to look at these and other low momentum HMS perp
data to see if the issue can be confirmed. We absolutely need to resolve
this issue, which has been brought to a sharp focus by Whit.

Cheers,

Oscar








-------- Original Message --------
Subject: Re: [Sane-analysis] Minutes of April 13 Analysis Meeting
Date: Thu, 14 Apr 2011 09:50:16 -0400
From: O. A. Rondon <or at virginia.edu>
To: SANE Analysis <sane-analysis at jlab.org>
References: <4DA628ED.5020707 at jlab.org> <web-584279 at tetra.mail.virginia.edu>

On Wed, 13 Apr 2011 18:51:25 -0400
  Narbe Kalantarians <narbe at jlab.org> wrote:
> > _*Whit:*_ Looked into scenario of if the target field was reversed.
> > http://quarks.temple.edu/~whit/SANE/talks/sane_magnet_reversed.pdf

Here are the results of my standalone code to track through the target
field, for 4.7 and 5.9 GeV. Whit is right that the beam center is
shifted up only a few mm at the target center.
http://twist.phys.virginia.edu/~or/sane/analysis/targeta.ps
http://twist.phys.virginia.edu/~or/sane/analysis/targetb.ps

The lines going exactly through the center are straight line projections
at the same starting angles.

But the 1 to 1.1 cm raster envelope hits the top of the cup for at least
the downstream half of the cup for 4.7 GeV, and for much of the same
region for 5.9 GeV. The target group should confirm if evidence for
radiation damage to the top of the cups was seen when the inserts were
removed.

In particular, the cups of insert C which were used without reloading
during the last part of the 4.7 perp data and the early part of the 5.9
parallel would have been more likely to show damage signs when removed
during the rotation from 80 to 180. Given the very shallow angle of
incidence on the cup's top, the damage would have been very substantial.

To make the plots I used vertical downward offsets at z =-150 cm from
the target center that correspond approximately to the numbers in the
chicane settings table. Keep in mind that those settings are for the
energies indicated on the table, but for the plots I used the actual
energies of the run. The offsets I used are -7.07 cm for 4.723 GeV
(table is -6.6 cm) and -5.7 cm for 5.892 (table is -5.6), which put the
bent tracks almost exactly at the target center.
http://hallcweb.jlab.org/experiments/sane/rondon/chicane.pdf

I used the vertical angles on the chicane table, although the exact
angles would be slightly different, at about the millidegree level.

The total deflection for 4.7 GeV is 5.8 degrees, which projects to 3 m
above the standard hall dump. In summary, it still seems unlikely the
field was reversed.

Cheers,

Oscar



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