PERFORMANCE SUMMARY OF THE HELICAL MAGNETS FOR RHIC*
E. Willen, M. Anerella, J. Escallier, G. Ganetis, A. Ghosh, R. Gupta, M. Harrison, A. Jain, W.
MacKay, A. Marone, J. Muratore, S. Plate, R. Thomas, P. Wanderer and KC Wu, BNL, Upton, NY
M. Okamura, RIKEN, Japan
A series of four Snake and eight Rotator
superconducting helical magnet assemblies has been built
and installed in RHIC to control the polarization of
protons during acceleration and storage in that machine.
Each of these assemblies consists of four 2.4 m long
dipole magnets in each of which the field rotates through
360 degrees along the magnet’s length. The magnets were
made by winding one millimeter diameter
superconducting 7-strand cable into slots milled into
thick-walled aluminum tubes. The magnets produce 4
Tesla field at a current of 320 amperes and are quench-
protected with 0.050 ohm resistors placed across the
winding in each slot. A total of 48 of these 2.4 m magnets
has been built, tested and installed. This paper
summarizes their quench performance as well as their
field uniformity, of which the integral field is the most
critical. All magnets reached the required operating field
level of 4 T, and the integral field of the magnets was
generally about half of the maximum permissible level of
0.050 Tesla meters.
Magnets to control proton spin were required in RHIC
to enable a program of spin physics using polarized
proton collisions . These magnets precess the proton
spin from up to down and back again on each orbit around
the ring, thereby avoiding depolarizing resonances during
acceleration. They also precess the proton spin at each of
two experimental detectors from vertical to longitudinal
for the study of such oriented collisions. The magnets
were built over several years and are now completely
installed and are operational beginning with the 2003
RHIC running period.
To achieve the required spin rotations, helical magnets
were developed in which the dipole field rotates through
360° in a distance of 2.4 m. Helical magnets offered a
more compact and efficient design than could be achieved
with a combination of rotated dipole magnets. Precessing
the proton spin without a net deflection of the orbit was
achieved by combining four of the helical magnets into
one long cryostatted device with different field strength,
helix direction, and helix orientation in each device,
depending on its task. They are called “Snakes” because
of the serpentine particle trajectory through the device.
The available space in the RHIC lattice determined the
overall length and therefore the required field of the
magnets. The coil inner diameter is a large 100 mm to
*Work supported in part by the U.S. Department of Energy.
allow adequate space for the beam trajectory (the RHIC
dipoles have a coil aperture of 80 mm). Four Snakes were
required, two in each ring, to avoid depolarizing
resonances, and eight Snakes, also called Rotators, were
required to orient the spin at the detectors. Thus, a total of
48, 2.4 m long helical magnets was required. A low
operating current was needed to ease the cryogenic load
from the numerous power leads to these magnets, which
are spread around the ring.
The design has been described in previous papers [2-4].
Briefly, the coils are made by placing conductor, a
Kapton-wrapped cable made of seven, twisted 0.33 mm
NbTi superconducting wires, into slots milled into
aluminum cylinders. These cylinders with their windings
are later overwrapped
fiberglass/epoxy, then machined to a precise diameter.
Each magnet has an inner coil with seven slots and an
outer coil with nine slots. The two coils are assembled
into a laminated yoke made of one piece laminations to
make a single, 2.4 m long helical dipole. End plates are
added and electrical connections including quench
protection resistors across each winding are made. Four of
these helical dipoles are assembled into a stainless steel
shell to complete the helium enclosure and the support
structure for the final cryostatted magnet, which operates
with forced flow helium at 4.5 K. A few parameters of the
dipole magnet are given in Table I.
with a wet layup of
Table I: Selected parameters of the helical dipole.
Number of turns
Stored energy @ 4 T
Diameter of yoke
Num. of strands in cable
Cu to non-Cu ratio
The coils are constructed by manually placing the
Kapton-wrapped cable into the slots in an orderly array.
The sides of the slots have been previously insulated with
Kapton for good electrical insulation. A layer of prepreg
0-7803-7739-9 ©2003 IEEE164
Proceedings of the 2003 Particle Accelerator Conference