ATLAS superconducting solenoid on-surface test

Abstract

The ATLAS detector is presently under construction as one of the five LHC experiment set-ups. It relies on a sophisticated magnet system for the momentum measurement of charged particle tracks. The superconducting solenoid is at the center of the detector, the magnet system part nearest to the proton-proton collision point. It is designed for a 2 Tesla strong axial magnetic field at the collision point, while its thin-walled construction of 0.66 radiation lengths avoids degradation of energy measurements in the outer calorimeters. The solenoid and calorimeter have been integrated in their common cryostat, cooled down and tested on-surface. We review the on-surface set-up and report the performance test results.

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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 15, NO. 2, JUNE 2005
1283
ATLAS Superconducting Solenoid On-Surface Test
R. J. M. Y. Ruber, Y. Makida, L. Deront, Y. Doi, T. Haruyama, F. Haug, M. Kawai, T. Kondo, Y. Kondo, S. Mizumaki,
G. Olesen, O. V. Pavlov, M. Pezzetti, O. Pirotte, E. Sbrissa, H. H. J. ten Kate, and A. Yamamoto
Abstract—The ATLAS detector is presently under construction
as one of the five LHC experiment set-ups. It relies on a sophisti-
cated magnet system for the momentum measurement of charged
particletracks.Thesuperconductingsolenoidisatthecenterofthe
detector, the magnet system part nearest to the proton-proton col-
lision point. It is designed for a 2 Tesla strong axial magnetic field
at the collision point, while its thin-walled construction of 0.66 ra-
diation lengths avoids degradation of energy measurements in the
outer calorimeters. The solenoid and calorimeter have been inte-
grated in their common cryostat, cooled down and tested on-sur-
face. We review the on-surface set-up and report the performance
test results.
Index Terms—Detector, magnet, quench results.
I. INTRODUCTION
T
sions at the Large Hadron Collider (LHC) at CERN. The de-
tector consists of four main parts: inner detector trackers, elec-
tromagnetic calorimeter, hadron calorimeter and muon spec-
trometer. The superconducting solenoid is the central part of
a large magnet system. It provides the magnetic field for the
crucial momentum measurements of charged particle tracks in
the inner detector. It is also of vital importance that the magnet
does not disturb the energy measurements of particles in the
calorimeter located outside the magnet. A solution was found
in a thin-walled superconducting solenoid, indirectly cooled by
circulationoftwo-phasehelium.Thesolenoidsharesitscryostat
with the electro-magnetic calorimeter, which, filled with liquid
argon, serves also as thermal shield for the solenoid. With this
arrangement the transparency for particles passing the solenoid
intothecalorimeteris0.66
(radiationlengths).Itsmajorfea-
tures are described elsewhere [1], [2]. The main parameters are
listed in Table I.
The solenoid system is being developed in collaboration be-
tween KEK, CERN and Toshiba. After fabrication in Japan all
equipment was shipped to CERN where it has been integrated
with the barrel cryostat. The realization has been challenging
due to its tight integration with the calorimeter. Proximity
cryogenics and chimney were successfully commissioned last
year [3].
HE ATLAS collaboration is preparing a general purpose
detector set-up for experiments with proton-proton colli-
Manuscript received October 4, 2004.
R.J.M.Y.Ruber,L.Deront,F.Haug,G.Olesen,O.V.Pavlov,M.Pezzetti,O.
Pirotte, E. Sbrissa, and H. H. J. ten Kate are with the CERN, CH-1211 Geneva
23, Switzerland (e-mail: ruber@cern.ch).
Y. Makida, Y. Doi, T. Haruyama, M. Kawai, T. Kondo, Y. Kondo and A. Ya-
mamoto are with the KEK, Tsukuba, 305-0801 Japan.
S. Mizumaki is with the Toshiba Co., Tsurumi, Yokohama, 230-0045 Japan.
Digital Object Identifier 10.1109/TASC.2005.849571
TABLE I
MAIN PARAMETERS OF THE SUPERCONDUCTING SOLENOID
Fig. 1.Layout of the on-surface test set-up.
Fig. 2. Cross-section of the barrel cryostat with solenoid and calorimeter.
II. ON-SURFACE INSTALLATION
Fig. 1 presents the on-surface test set-up while in Fig. 2 is
shown a detailed cross section of the barrel cryostat with sole-
noid and calorimeter. The solenoid coil is mounted on the inner
warmvesselofthecryostatby23triangularglassfibersupports.
The side of the coil toward the chimney is fixed while the op-
posite side can slide in axial direction [4]. After installation of
the calorimeter wheels into the cryostat, the solenoid and inner
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1284IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 15, NO. 2, JUNE 2005
Fig. 3.The on-surface test set-up.
warmvesselwhereslided intopositionguidedbya railssystem.
Then the superconducting bus-lines, consisting of a double pair
of conductors, and the solenoid and thermal shield cooling lines
were built-up in the bulkhead of the cryostat. After passing a
feedthrough at an 11.25 angle from the cryostat top, a joint
was made to the chimney interconnecting solenoid and prox-
imity cryogenics consisting of a control dewar and valve unit.
The complete on-surface test set-up is shown in Fig. 3.
For the installation in the final ATLAS detector set-up, the
proximity cryogenics will be located on top of the cryostat.
Due to height limitations for the on-surface test set-up it was
mounted to the side of the cryostat. Hence the interconnecting
chimney was mounted in a horizontal position instead of verti-
cally. For this the chimney was extended by 5.4 m in the hor-
izontal direction; this is the extension chimney as marked in
Fig. 1.
III. MONITORING AND SAFETY SYSTEM
The solenoid system is equipped with more thantwo hundred
sensors to monitor its thermal, electrical and mechanical be-
havior.Inadditiontherearefiveelectricalheatersforquenchini-
tiation and protection on the coil and another five on the super-
conducting bus-line inside the cryostat and extension chimney.
The magnet safety system (MSS) used in the test was largely
the same as the system to be used in the final ATLAS under-
ground installation. The MSS monitors voltage signals, temper-
atures and superconducting quench detectors (SQD) onthe coil,
thebus-linesin thechimneyand thecurrentleads. Thesesignals
are used to indicate a failure and then trigger a slow dump (SD)
or a quench condition of the magnet system and then trigger
a fast dump (FD). In slow dump the MSS will switch off the
magnet power supply and open the main breakers. In fast dump
the MSS will react the same way, but fire the five quench pro-
tection heaters (QPHT) of the coil as well.
In addition MSS was monitoring the signal of pick-up coils
installed at both ends of the solenoid. This signal was not yet
used in signalling a fault condition, but the pick-up coil signal
is fast and reliable in indicating a quench of the solenoid. The
pick-up coil signal is subtracted from the total solenoid voltage.
Fig. 4.Peak temperature of the coil winding after a quench.
This way the signal is neither di/dt sensitive during ramping nor
does it react to external stray fields.
The monitoring was split in a fast and slow system. The slow
monitoring was done by the magnet control system (MCS),
a standardized control system for all LHC experiments. Like
MSS, the MCS was largely similar to the final underground
installation. Also the solenoid on-surface test provided the first
operational test of its design. For the fast monitoring a data
acquisition system with a FIFO memory was used. It consisted
of a 16 bit 200 kHz ADC configured to monitor 64 channels at
2 kHz. It was triggered by an MSS FD signal storing 10 s worth
of pre-trigger data and 120 s post-trigger.
IV. PERFORMANCE TEST
The two main purposes of the on-surface test where to verify
operation of the solenoid at nominal field and study the intrinsic
safety of the design. In the ATLAS detector set-up the nominal
operational field strength of the solenoid is 2.0 T at a current
of 7.6 kA. In the air-core on-surface test set-up, a current of
8.0 kA is required to reach the 2.0 T field strength. The max-
imum applied test current was 8.136 kA, close to the limit of
thepowersystem.Previously,during thereceptiontestinJapan,
the coil was excited up to 8.4 kA to prove it has sufficient oper-
ating margin. The test repeated here has aimed at validating the
system and verifying the stability of the solenoid after transport
and installation in the cryostat.
A. Quench Protection and Normal Zone Propagation
Quench initiation was done with one of the five electric
heaters on the coil winding, while the remaining four heaters
were connected as quench protection heaters (QPHT) to MSS
for a smooth temperature distribution after the quench. Quench
initiation in the chimney was done by a heater in the middle of
the extension chimney while all of the five coil heaters were
connected as QPHT to MSS. Four quenches were initiated
in the coil, at 4, 6 and 7.6 kA, and two in the chimney, both
6 kA, while there were also two training quenches at 7.9 and
8.1 kA. Fig. 4 presents the peak coil winding temperature after
a quench. It combines the data from the factory test [5] and the
present on-surface test. The chimney bus-line temperature went

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RUBER et al.: ATLAS SUPERCONDUCTING SOLENOID ON-SURFACE TEST 1285
Fig. 5.
the coil. ?
Measured voltage during a quench. V101 and V105 are voltage taps at
and ?
are the differential and bridge voltages from MSS.
Fig. 6.Layout of the main instrumentation of the coil and chimney.
up to a maximum of 21 K both during a quench of the coil or a
direct quench of the chimney itself af 6 kA.
There is a rapid reaction of coil and MSS to the firing of
a heater. Within 0.2 s after the end C heater is fired (with 9.5
W during 8.4 s, at 7.6 kA), the voltage of the coil starts rising.
MSS has a 1 s filter on the input signals, and signals a quench
detection in 1.59 s after the firing of the heater. The two su-
perconducting quench detectors (SQD) which run along the top
and bottom of the coil signal a complete transition to normal in
3 s after MSS triggered the fast dump (FD) and fired the quench
protection heaters (QPHT), or 5 s after starting the quench ini-
tiation. The current was down to zero in approximately 70 s.
In Fig. 5 the measured voltages are plotted versus the time.
The location of the voltage taps on the coil is shown in Fig. 6.
Theyareinstalledateachquarterofthecoillength.MSSdetects
a quench at
and then opens the electric circuit breaker
(CB) and fires the QPHT’s. The differential voltage
a pick-up coil with the total coil voltage is plotted as well as
the bridge voltage
between voltage taps 101, 107 and
109.Thefastresponseofthedifferentialvoltagesignalisclearly
visible.
The normal zone propagation velocity can be estimated from
thesignalsofthedifferentvoltagetaps.Inthechimneyalongitu-
dinal propagation velocity of 2.4
corresponds to the estimated velocity with an uniform current
distribution over the aluminum stabilizer. The rapid reaction of
MSStoquenchdetection,anditsfiringoftheQPHTs,prevented
a study of the normal zone propagation velocity in the coil.
of
0.6 m/s was measured. This
Fig. 7. SQD signals during a quench in the chimney.
Fig. 7 displays the evolution of the SQD signals during a
heater induced quench of the chimney. After the MSS detects
the quench, the QPHT at the coil are fired. The SQD signals
indicate a temporary recooling of the chimney by the cold gas
front produced during the coil quench.
The gradient between the hot spot temperature of the coil
winding and support cylinder is typically in the order of 25 K
after a quench at 8 kA. The temperature gradient along the coil
is around 35 K from the initial quench location to the opposite
end of the coil.
B. Heat Load
Afterclosure of thecryostatthe calorimeterwas cooleddown
first. It reached a final temperature of 89
with liquid argon, which is used as particle detection medium.
The temperature of the solenoid did not decrease more than 20
degrees: from a room temperature of 292 K to 275
284
1 K for the thermal shield.
The static thermal load on the solenoid can be estimated from
the temperature gradient during warm-up. In the first phase of
the warm-up the thermal shield and detector temperatures were
constant (78.5
0.1 K and 89
cates a heat load on the coil of 7.8
the 10.5 W static thermal load measured during the factory test
[6]. An improvement of the present installation is that between
solenoid and outer thermal shield (the calorimeter) the amount
of multilayer superinsulation (MLI) has almost doubled.
Cooling mass flow stop studies were done at currents of
4 kA. It was possible to complete a slow ramp down (SD) of the
solenoid in 41 minutes before a temperature rise of the magnet
system triggered MSS into a FD. In the final underground in-
stallation a different run-down unit will be used that enables a
SD from 7.6 kA to zero in less than half an hour.
Cooling of the current leads worked well and ice build-up at
the top of the leads was prevented by a fan forcing an ambient
air flow along the top of the leads.
1 K after filling
1 K and
1 K respectively). This indi-
0.9 W. This is better than
C. Mechanical Behavior
The coil position is monitored by potentiometers mounted in
axial and circumferential direction at both ends of the coil. At

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1286IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 15, NO. 2, JUNE 2005
Fig. 8.Position measurement during excitation.
Fig. 9.Support cylinder strain during excitation.
end C the supports are allowed to slide freely in axial direction
whilethesupportmovementatend Ais limitedto
to the chimney connection. The measured axial displacement
duringcooldownaveraged
to 0.39% of the 5698.2
0.8 mm coil length. The coil length
(winding including end flanges) was surveyed before insertion
in the cryostat. It confirms a 1.8 mm permanent shrinkage after
the factory test of the coil.
Strain gauges were mounted at the top and bottom of the sup-
port cylinder at similar locations as for the voltage taps as indi-
cated in Fig. 6. Measured strain is approximately 700
both axial and circumferential direction during excitation to 8.1
kA, see Fig. 9. This circumferential strain level indicates a hoop
0.5mmdue
,whichcorresponds
in
stress of 51 MPa as expected from the design values [1]. As an-
ticipated, the measured strain is highest in the center of the coil
and decreases toward the ends of the coil. Near the end flanges
the circumferential strain is 250
of 18 MPa.
Fig. 8 presents the position sensor read-out versus the square
of the excitation current. Axial position sensors indicate a
change of
at 8.1 kA, circumferential sensors
indicate 0.3
0.2 mm. This agrees well to the measured strain
as
would give a
strain
as measured at the center would give
a
if it was uniform over the coil length and
at the end flanges gives
suggesting a hoop stress
. And the axial
.
V. CONCLUSIONS
The on-surface test verified the operation of the ATLAS su-
perconducting solenoid after transport to CERN and integration
inthebarrelcryostat.Detailedstudiesofthequenchingbehavior
have confirmed safe operation at currents up to 7% above nom-
inal. The magnet safety system has been validated for use in all
LHC experiments. Its combination of superconducting quench
detectors, pick-up coils and quench protection heaters makes an
excellent protection system which also limits the peak temper-
ature after quench. Installation in the 100 meters deep detector
cavern will now be started.
ACKNOWLEDGMENT
The authors would like to thank their colleagues from KEK,
Toshiba, CERN and ATLAS for their extensive efforts during
the integration and test. R. Ruber is grateful to Uppsala Univer-
sity for supporting his work.
REFERENCES
[1] A. Yamamoto et al., “Design and development of the ATLAS central
solenoidmagnet,”IEEETrans.Appl.Superconduct.,vol.9,p.852,1999.
[2]
, “Progress in ATLAS central solenoid magnet,” IEEE Trans. Appl.
Superconduct., vol. 10, p. 353, 2000.
[3] R. J. M. Y. Ruber et al., “On-surface integration and test of the ATLAS
central solenoid and its proximity cryogenics,” IEEE Trans. Appl. Su-
perconduct., vol. 14, p. 500, 2004.
[4] S. Mizumaki et al., “Fabrication and mechanical performance of the
ATLAS central solenoid,” IEEE Trans. Appl. Superconduct., vol. 12, p.
416, 2002.
[5] Y. Makida et al., “Quench protection and safety of the ATLAS central
solenoid,” IEEE Trans. Appl. Superconduct., vol. 12, p. 407, 2002.
[6] Y. Doi et al., “Performance of a proximity cryogenic system for the
ATLAS central solenoid magnet,” in Advances in Cryogenic Engi-
neering: Proceedings of the Cryogenic Engineering Conference—CEC
Volume 47, S. Breon et al., Eds., American Institute of Physics, 2002,
p. 47.