Performance summary of the helical magnets for RHIC
ABSTRACT 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.
- SourceAvailable from: bnl.gov
Article: Exotic Magnets for Accelerators[Show abstract] [Hide abstract]
ABSTRACT: Over the last few years, several novel magnet designs have been introduced to meet the requirements of new, high performance accelerators and beam lines. For example, the FAIR project at GSI requires superconducting magnets ramped at high rates (~4 T/s) in order to achieve the design intensity. Magnets for the RIA and FAIR projects and for the next generation of LHC interaction regions will need to withstand high doses of radiation. Helical magnets are required to maintain and control the polarization of high energy protons at RHIC. In other cases, novel magnets have been designed in response to limited budgets and space. For example, it is planned to use combined function superconducting magnets for the 50 GeV proton transport line at J-PARC to satisfy both budget and performance requirements. Novel coil winding methods have been developed for short, large aperture magnets such as those used in the insertion region upgrade at BEPC. This paper will highlight the novel features of these exotic magnetsIEEE Transactions on Applied Superconductivity 07/2006; · 1.20 Impact Factor
Article: Future accelerator magnet needs[Show abstract] [Hide abstract]
ABSTRACT: Superconducting magnet technology is continually evolving in order to meet the demanding needs of new accelerators and to provide necessary upgrades for existing machines. A variety of designs are now under development, including high fields and gradients, rapid cycling and novel coil configurations. This paper presents a summary of R&D programs in the EU, Japan and the USA. A performance comparison between NbTi and Nb<sub>3</sub>Sn along with fabrication and cost issues are also discussed.IEEE Transactions on Applied Superconductivity 07/2005; · 1.20 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: The fast reduction of the six-dimensional phase space of muon beams is an essential requirement for muon colliders and also of great importance for neutrino factories based on accelerated muon beams. Ionization cooling, where all momentum components are degraded by an energy absorbing material and only the longitudinal momentum is restored by rf cavities, provides a means to quickly reduce transverse beam sizes. However, the beam energy spread cannot be reduced by this method unless the longitudinal emittance can be transformed or exchanged into the transverse emittance. Emittance exchange plans until now have been accomplished by using magnets to disperse the beam along the face of a wedge-shaped absorber such that higher momentum particles pass through thicker parts of the absorber and thus suffer larger ionization energy loss. In the scheme advocated in this paper, a special magnetic channel designed such that higher momentum corresponds to a longer path length, and therefore larger ionization energy loss, provides the desired emittance exchange in a homogeneous absorber without special edge shaping. Normal-conducting rf cavities imbedded in the magnetic field regenerate the energy lost in the absorber. One very attractive example of a cooling channel based on this principle uses a series of high-gradient rf cavities filled with dense hydrogen gas, where the cavities are in a magnetic channel composed of a solenoidal field with superimposed helical transverse dipole and quadrupole fields. In this scheme, the energy loss, the rf energy regeneration, the emittance exchange, and the transverse cooling happen simultaneously. The theory of this helical channel is described in some detail to support the analytical prediction of almost a factor of 106 reduction in six-dimensional phase space volume in a channel about 56 m long. Equations describing the particle beam dynamics are derived and beam stability conditions are explored. Equations describing six-dimensional cooling in this channel are also derived, including explicit expressions for cooling decrements and equilibrium emittances.Review of Modern Physics 01/2005; 8(4). · 44.98 Impact Factor
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
material (B-stage epoxy/fiberglass cloth) is placed
between each layer. When all the turns are in place, press
plates are placed on top of the windings and the coil is
wrapped with tensioned Kevlar cord, then placed in an
oven where the epoxy is cured while the coil rotates. The
turns move radially inward as the epoxy softens and flows
in this curing operation. The end result is a compact
matrix of cable turns, Kapton, fiberglass and epoxy filling
the slot. A photograph of a coil is shown in Fig. 1.
Figure 1: Photograph of a helical coil with conductors.
The thickness of the prepreg material, initially 0.25 mm
but finally 0.075 mm after compression, at first keeps the
turns at a larger radial position. As the turns are heated
and moved radially inward by the tensioned Kevlar, extra
cable length is generated because of the helix along the
length and because of the crossover from side to side in
the ends. This extra length is accommodated by providing
greater slot width in the ends. The extra space, too large to
be completely filled with the prepreg epoxy, is later filled
with ceramic-loaded epoxy.
Numerous electrical tests during construction ensured
that most construction faults were caught. An occasional
turn-to-turn short or coil-to-ground fault occurred in the
construction program caused by misplaced turns,
generally from cable buckling while curing. Abbreviated
warm measurements verified that all windings were
connected in the proper sequence. All the 48 magnets
were cryogenically tested for quench performance and
field quality. Six magnets developed faults during or after
cryogenic testing and required disassembly and rewinding
of a particular winding in the coil. Detailed field
measurements for each magnet are available in various
formats at Brookhaven.
Most of the 48 magnets required some training to reach
the desired 4 T field (320 A). Fig. 2 shows the distribution
of first quenches and Fig. 3 shows the number of
quenches before the current exceeded 320 A. Nine
magnets required no training, 28 required three or less,
and 11 required more than three. The training is attributed
to a paucity of epoxy in the windings, revealed as voids in
those windings that were on occasion taken apart or
replaced to correct faults. This was a problem particularly
in the ends where slots were wider. Several of the early
coils required excessive training (up to 18 quenches) but
this problem improved as experience in placing and
compressing the cable into the slots was gained by the
technical staff. Virtually all the training occurred in the
inner coils in one of the windings away from the
midplane, the region of highest Lorentz forces and
therefore as expected. In those magnets with thermal
cycles, the magnets for the most part remembered their
training, except for several of the early magnets where
some retraining was necessary. This is shown in Fig. 4.
180220260300 340 380
Number of Magnets
320 A (4 Tesla)
Initial Quench Current
48 helical dipoles
Figure 2: Distribution of initial quench currents.
Number of Quenches
Number of Magnets
Number of Quenches Before Exceeding 320 A (4 Tesla)
48 helical dipoles
Figure 3: Number of required quenches.
101 104, #1104, #2 202 410415
Quench Current, A
Quench Current Just Before and Just After Thermal Cycle
320 A (4 Tesla)
Figure 4: Retraining after thermal cycle.
An important requirement for the helical magnets was
that the integral field be small or zero in order to avoid net
deflections of the beam. The specification was that the
field integral be less than 0.025 T·m. This was measured
with a long, rotating coil with a radius of 25 mm that
extended the full length of the magnet including any
fringe field at the ends. A typical measurement is shown
in Fig. 5 where the maximum of the residual is 0.018 T·m,
well within the required limit. This was typical for all the
magnets; none had a value greater than 50% of the limit.
Proceedings of the 2003 Particle Accelerator Conference
Int. Dipole Field (T.m)
50% of tolerance
Figure 5: Example of measured integral field. The
structure in the curves is due to limitations in the
uniformity of the measuring coil.
In a helical magnet, all field multipoles (harmonics)
oscillate with axial position. Over the length of the
magnet, the effect of these multipoles on the particle
trajectory tends to cancel, so the requirements on field
quality are less stringent than for normal accelerator
magnets: the local ∆|B|/|B| should be kept reasonably
below 1%. A rotating coil to measure the harmonics
should be as short as possible in order to avoid signal
cancellation. The measuring coil used to measure these
magnets was only 51 mm long, about 2% of the
wavelength of the helix. It had a radius of 34.2 mm, and
the reference radius used to express the harmonics is 31
mm. A total of 56 turns in the tangential winding of the
coil ensured adequate signal strength for good
measurements. Flux cancellation because of the helical
field reduces the dipole field by 0.25% for the dipole term
and by 2.7% for the 30-pole term, an effect corrected in
the reported results.
Measurements at many currents were typically made at
a single location in the center of the magnet and as a
function of axial position at several currents . The axial
variations of the quadrupole and the sextupole
components are shown in Fig. 6. The asymmetry in the
quadrupole component along the length of the magnet
varies magnet to magnet but the data shown are typical
and are believed to result from a combination of coil
asymmetries inside the iron yoke and the centering
technique used. The variation of the sextupole amplitude
with current results from iron saturation in the yoke.
The normal sextupole field as a function of current is
shown in Fig. 7 (left) for most of the magnets on a single
plot. The right side shows the same data normalized to
zero at 102 A on the up ramp. The harmonics are
expressed in a coordinate system where the y-axis
coincides with the dipole field direction. This allowed
harmonic is quite consistent over all the magnets.
The following conclusions can be drawn:
There is considerable magnet-to-magnet variation in
the harmonics resulting from geometric differences in the
Z-scans in small axial increments show significant
axial variation in the lower order harmonics.
All harmonics are within safe limits for machine
Note: A number of these papers are available at the BNL
 Design Manual: Polarized Proton Collider at RHIC,
M. Syphers, Editor, BNL, July, 1998.
 E. Willen, R. Gupta, A. Jain, E. Kelly, G. Morgan, J.
Muratore, R. Thomas, A Helical Magnet Design for
RHIC, PAC97, Vancouver, May, 1997.
 E. Willen, E. Kelly, M. Anerella, J. Escallier, G.
Ganetis, A. Ghosh, A. Jain, A. Marone, G. Morgan, J.
Muratore, A. Prodell, P. Wanderer, Construction of
Helical Magnets for RHIC, PAC99, New York,
 E. Willen, M. Anerella, J. Escallier, G. Ganetis, A.
Ghosh, A. Jain, E. Kelly, G. Morgan, J. Muratore, A.
Prodell, P. Wanderer, Performance of Helical
Magnets for RHIC, MT16, Ponte Vedra, FL,
 A. Jain, G. Ganetis, W. Louie, A. Marone, J,
Muratore, R. Thomas, P. Wanderer, E. Willen,
Measurements of Field Quality in Helical Dipoles for
RHIC, Proc. 12th International Magnet Measurement
Workshop, October, 2002.
The current dependence of the harmonics has small
-1.6 -1.2 -0.8 -0.4 0.00.4 0.81.2 1.6
Total Quadrupole (unit wrt body TF)
-1.6 -1.2 -0.8 -0.4 0.0 0.4 0.8 1.2 1.6
Total Sextupole (unit wrt body TF)
Unnormalized (left) and normalized
sextupole harmonic for the magnets.
Normal Sextupole (units at 31 mm)
Change in Normal Sextupole (units at 31 mm)
Figure 6: Example of measured quadrupole and
sextupole fields in a typical magnet.
Proceedings of the 2003 Particle Accelerator Conference