The Super conducting Magnets of the ILC Beam Delivery System Page: 2 of 4
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THE SUPERCONDUCTING MAGNETS OF
THE ILC BEAM DELIVERY SYSTEM*
B. Parker#, M. Anerella, J. Escallier, P. He, A. Jain, A. Marone, BNL, Upton, NY, 11973, U.S.A.
Y. Nosochkov and A. Seryi, SLAC, Menlo Park, CA 94025, U.S.A.Abstract
The ILC Beam Delivery System (BDS) uses a variety
of superconducting magnets to maximize luminosity and
minimize background. Compact final focus quadrupoles
with multifunction correction coils focus incoming beams
to few nanometer spot sizes while focusing outgoing
disrupted beams into a separate extraction beam line.
Anti-solenoids mitigate effects from overlapping focusing
and the detector solenoid field. Far from the interaction
point (IP) strong octupoles help minimize IP backgrounds.
A low-field but very large aperture dipole is integrated
with the detector solenoid to reduce backgrounds from
beamstrahlung pairs generated at the IP. Physics
requirements and magnetic design solutions for the BDS
superconducting magnets are reviewed in this paper.
COMPACT IR MAGNETS FOR 14 MR
In the ILC 14 mr crossing angle layout incoming and
extraction beams are focused in independently. With a
distance (denoted L*) of the first magnet, QDO, to the IP
of 3.5 m, the beam separation at QDO is 49 mm. This
small separation is accommodated via compact coil
windings produced using BNL's direct wind technology
[1]; however, even with compact coils care is taken so
that external field does not impact the extracted beam
passing just outside QDO.
We reduce QDO's external field with a weak active
shield coil of opposite polarity that runs in series with the
main coil windings. With the shield energized, magnetic
flux passes between the inner and outer coil structure
rather than spreading outside the coil package. The shield
coil reduces the gradient inside QDO but this impact is
minimized by keeping the inner coil as radially compact
and pushing the shield coil radius out as far as the
crossing angle geometry allows. A shielded prototype
coil, that fit in an existing small dewar was wound and
successfully tested [2]. The actual QDO coil design has
extra space between the main and shield coils for He-II
cooling and trim current taps for shield fine tuning.
For optics flexibility the QDO coil pack has dipole,
skew-dipole and skew-quadrupole correction windings.
Next to QDO is another coil package with octupole,
sextupole, skew-sextupole, dipole and skew-dipole
windings. These correction coils have negligible impact at
the extraction line and are not actively shielded.
'This manuscript has been authored by Brookhaven Science Associates,
LLC under Contract No. DE-AC02-98CH1-886 with U.S. Department
of Energy. The United States Government retains, and the publisher, by
accepting article for publication, acknowledges, a world-wide license to
publish or reproduce published form of this manuscript, or allow others
to do so, for the United states Government purposes.
"parker@bnl.govAlongside the incoming beam line is the first extraction
line quadrupole, QDEX1, which is similar to QDO, with
active shielding and correction coils. But QDEX1 has a
larger clear aperture to minimize energy deposition from
the outgoing beam. At present three QDO L*s, 3.5, 4.0
and 4.5 m, are studied and different QDEX1 designs exist
for three different extraction line starting points [3].
Further from the IP there is a second set of magnets, the
QF1 grouping, with properties similar to those of the QDO
grouping. The most significant difference is that QF1 is
well outside the detector solenoid and has sufficient
transverse separation from the extraction beam, for it to
have a magnetic yoke for passive external field shielding
and thus no active shield coil is needed. QF1 has its own
associated sextupole, octupole etc. coil package and
neighboring extraction line quadrupole QFEX1.
The QDO and QF1 magnet groupings are housed in two
independent cryostats in order to facilitate the push-pull
scenario where two experiments take turns sharing beam
time at a single IP. Swapping detectors at a single IP
saves considerable project cost for conventional facilities
and requires fewer beam line technical components, but
adds other complications. In order to make the
changeover as rapid as possible the IR magnets and
experimental detector are to be kept cold during a
switchover. The experiments have QDO magnet groupings
in cryostats that move with the detectors but they share a
fixed QF1 magnet grouping that remains in place.
Between QDO and QF1 is warm beam pipe with vacuum
valves and pump out ports to make natural break points.
FORCE NEUTRAL ANTI-SOLENOID
In addition to the magnets described so far, the ILC IR
optics uses anti-solenoids to locally correct deleterious
optics effects due to QDO focusing overlapping the
detector solenoidal field [4]. If left uncompensated, this
overlap leads to an effective beam size increase at the IP
and reduces luminosity. To be effective the anti-solenoid
does not have to completely cancel the detector field
overlapping QDO; in fact it is shorter than QDO and only
modifies the field profile near one end.
A simple anti-solenoid coil would experience a strong,
multi-ton, repulsion from the main detector solenoid and
is not suitable for integration into the QDO cryostat. But
integrating a large anti-solenoid coil into each detector is
also quite challenging. Instead we use a force neutral anti-
solenoid scheme. The repulsive force experienced by an
isolated anti-solenoid coil is related both to the field it
produces on axis and its cross sectional area (aperture). It
is possible to arrange to have two overlapping coils of
opposite polarity but different areas in a way that their
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Parker, B.; Anerella, M.; Escallier, J.; He, P.; Jain, A.; Marone, A. et al. The Super conducting Magnets of the ILC Beam Delivery System, article, September 28, 2007; Menlo Park, California. (https://digital.library.unt.edu/ark:/67531/metadc890047/m1/2/: accessed April 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.