Deterioration of the skew quadrupole moment in Tevatron dipoles over time Page: 3 of 3
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veloped with age a skew quadrupole component, how big
would it need to be to explain the effects that are currently
seen?" When the authors conferred and discovered that
they were taking about numbers that matched quite well,
they and the others involved agreed to report the results
and propose further studies.
The original calculations from the production era were
repeated, affirming the linear relationship between dis-
placement and skew quadrupole error term, ai = k y,
where k is 10.6 units/mm. The magnets had been de-
signed to allow adjustment of the coil position while the
magnet was cold by adding and removing shims from the
cryostat suspension system. During production this feature
was used as a standard procedure to compensate for fabri-
cation tolerances and mechanical differences between the
top and bottom coil. Each of the almost 1000 dipoles (in-
cluding spares) built for the Tevatron was measured cold
and, if necessary, adjusted to remove the skew (and nor-
mal) quadrupole components and remeasured.
FURTHER CONFIRMATIONS
Once the Tevatron dipole magnets were considered to
be the main source of the coupling, further verifications
were performed using beam measurements as well as mag-
net measurements. It was determined that during Run I
skew quadrupoles were removed from the main corrector
circuit in the vicinities of the two major detectors. Thus,
while the distribution of a1 was essentially uniform around
the ring, its correction was no longer. This led to a residual
vertical dispersion wave which was measured and correctly
interpreted, verifying our understanding. [4]
The lifts of an additional sample of 66 magnets were
measured in the tunnel during a subsequent short mainte-
nance period, confirming the original distribution. Similar
changes in the coil position were measured in spare dipoles
that had not been in service. Several spare dipoles were
magnetically measured [5], confirming that the long term
change in lift was reflected in the expected change in skew
quadrupole.
With the recognition that reshimming the dipoles to re-
move the skew quadrupole was possible, more magnet
studies were conducted and detailed plans were laid.[6]
The position of the magnet cryostat and the coil inside is
determined by two bolts through the bottom of the yoke
45 degrees from vertical that form a cradle at 9 stations
down the length of the 6-meter magnet. These align with
small G-11 blocks inside the cryostat that directly support
the collared coil assembly, the blocks that we believe have
deformed. Brass shims between the bolt and the cryo-
stat allow reproducible adjustments of the position. Spring
loaded cartridges ("smart bolts"), penetrating from the top,
hold the cryostat in position against the lower bolts as the
system contracts during cool down.
During production the results of shimming could be veri-
fied immediately. Measuring the lifts of each magnet, com-
paring them with the production values, and reshimmingaccordingly would have been fraught with possibilities for
error. Examination of the production data quickly led us
to the conclusion that it was not sufficiently reliable on a
magnet-by-magnet basis to warrant the effort of individual
treatment. Instead, it was decided to treat all magnets as
though they had changed by the same amount. A standard
shim change was agreed upon.
REMEDIATIONS AND PRESENT STATUS
With 774 dipole magnets in the Tevatron, and with 18
shim locations per magnet, it was clear that to fix the
problem completely would take significant tunnel access
time. Any piece-wise repair of the problem had to be per-
formed with the skew quadrupole corrector system always
in mind. Thus, patterns of magnet repairs were chosen
which were consistent with the corrector system layout and
which could be carried out in tolerable amounts of time.
The sensitivity of the operation led us to augment
our usual quality assurance procedures with more formal
analyses.[7] The sheer volume of measurements, 18 lift
measurements for each magnet before and after the reshim-
ming, required extensive above ground monitoring in addi-
tion to the teams of technicians in the tunnel. Although
the job in the tunnel is tedious, requiring the technicians to
crawl over the magnets in tight quarters, a crew of three can
comfortably reshim two magnets per day.
In the fall of 2003, during the annual accelerator shut-
down, 106 dipoles were reshimmed in the neighborhood
of the intersection regions. Another 12 magnets were
reshimmed in March 2004 during a one-week shutdown.
With the worth of the project demonstrated through im-
proved accelerator performance, the fall 2004 shutdown
saw a concentrated effort to reshim 412 more magnets. We
plan to reshim the remaining 244 magnets during the next
major shutdown, currently planned for the late fall of 2005.
REFERENCES
[1] J.T. Volk, et al., "Tevatron Alignment Issues 2003-2004,"
these proceedings.
[2] J. Holt, et al., "Calculating Luminosity for a Coupled Teva-
tron Lattice," PAC95 (1996); N. Gelfand, "Coupling in the
Tevatron," PAC95 (1996).
[3] M.J. Syphers, "Strong Systematic Steering Correction in Re-
gions of the Tevatron," Fermilab Internal Report Beams-doc-
491 (2003).
[4] M.J. Syphers, et al., "Observations of Strong Transverse Cou-
pling in the Tevatron," these proceedings.
[5] G. Ambrosio et al., "Results of an Investigation of the Skew
Quadrupole Issue in Tevatron Dipoles," Fermilab Internal Re-
port TD Note 03-045 (2003).
[6] D.J. Harding, et al., "Restoring the Skew Quadrupole Mo-
ment in Tevatron Dipoles," these proceedings.
[7] J.N. Blowers, et al., "Reshimming of Tevatron Dipoles;
A Process-Quality and Lessons-Learned Perspective," these
proceedings.
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Syphers, M. J. & Harding, D. J. Deterioration of the skew quadrupole moment in Tevatron dipoles over time, article, May 1, 2005; Batavia, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc1411757/m1/3/: accessed June 11, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.