Post on 20-May-2020
Performance of Superconducting Magnet Prototypes
for LCLS-II Linear Accelerator
Vladimir Kashikhin, Nikolai Andreev, Joseph DiMarco, Alexander Makarov, Michael Tartaglia, George Velev
Abstract— The new LCLS-II Linear Superconducting
Accelerator at SLAC needs superconducting magnet packages
installed inside SCRF Cryomodules to focus and steer an electron
beam. Two magnet prototypes were built and successfully tested
at Fermilab. Magnets have an iron dominated configuration,
quadrupole and dipole NbTi superconducting coils, and splittable
in the vertical plane configuration. Magnets inside the
Cryomodule are conductively cooled through pure Al heat sinks.
Both magnets performance was verified by magnetic
measurements at room temperature, and during cold tests in
liquid helium. Test results including magnetic measurements are
discussed. Special attention was given to the magnet performance
at low currents where the iron yoke and the superconductor
hysteresis effects have large influence. Both magnet prototypes
were accepted for the installation in FNAL and JLAB prototype
Cryomodules.
Index Terms—Accelerator, Cryomodule, Linac, Magnet,
Superconducting, Conduction cooling.
I. INTRODUCTION
HE new Linear Superconducting Accelerator LCLS-II [1]
needs superconducting magnet packages installed inside
Cryomodules which are based on the superconducting radio
frequency technology (SCRF). Many different magnet
packages were built and successfully tested for Linear
Accelerators [2] – [4]. The first large scale Superconducting
Linear Accelerator XFEL [3] used superconducting magnets
cooled by a liquid helium bath [5].
In recent years a more advanced approach was developed
based on magnets with conduction cooling [6] – [12]. In order
to avoid combined installation of magnets and SCRF cavities
in a very clean room, these magnets are made splittable in the
vertical plane. This allows the magnet installation after the
cavity string is assembled, and the inner volume is sealed to
avoid contaminations cavity surfaces.
The magnet (see Fig. 1) is cooled through pure aluminum
heat sinks thermally attached to the 2 K, 5 K, and 50 K
cryomodule cooling pipes. In these magnets the main
quadrupole field is formed by four iron poles, and four
racetrack type superconducting coils. A vertical and horizontal
dipole are wound on top of each quadrupole coil to steer an
electron beam to the quadrupole field center. The magnet
design and fabrication described in [12].
Manuscript received September 1, 2016. This work was supported in part
by Fermi Research Alliance, LLC, under contract No. DE-AC02-07CH11359
with the U.S. Department of Energy. V. Kashikhin#, N. Andreev, J. DiMarco, A. Makarov, M. Tartaglia, G.
Velev are with the Fermi National Accelerator Laboratory, Batavia, IL 60510,
USA (corresponding author# phone: 630-840-2899; fax: 630-840-6766; email: kash@fnal.gov).
Fig. 1. Magnet inside the FNAL SCRF Prototype Cryomodule.
II. MAGNET PACKAGE MAIN PARAMETERS
Two magnets were fabricated to be installed in prototype
cryomodules at FNAL and JLAB. The magnet package cross-
section is shown in Fig. 2. This is a rather short 322 mm long
iron dominated magnet with relatively large 90 mm aperture.
Fig. 2. The magnet package cross-section.
The magnet parameters for the LCLS-II Cryomodules are
shown in Table 1. At the accelerator front end the quadrupole
integrated gradient is very low, only 0.05 T. Therefore the iron
core remnant and hysteresis effects could spoil the field
quality and reproducibility required by the accelerator. Both
prototype magnets were tested at FNAL Test Stand 3 in a
helium bath cooling mode, and underwent a series of quench
performance and magnetic characterization measurements
described next.
T
FERMILAB-CONF-616-TD ACCEPTED
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TABLE I
LCLS-II MAGNET PACKAGE PARAMETERS
Parameter Unit
s
Value
Integrated peak gradient at 10 GeV T 2.0
Integrated peak gradient at 0.4 GeV T 0.05 Clear bore aperture mm ≥78
Ferromagnetic pole tip bore diameter mm 90
Effective length mm 230 Peak quadrupole gradient T/m 8.67
Quadrupole field non-linearity at 10 mm diameter
Quadrupole field reproducibility
%
%
≤1.0
≤1.0 Quadrupole magnet DC inductance H 0.66
Number of superconducting coil packages 4
Number of superconducting sections in the coil 3 Number of turns in the quadrupole section 426
Number of turns in dipole sections 39
Peak operating current A ≤20 NbTi superconductor diameter mm 0.5
Superconductor filament size µm 3.7 Dipole correctors integrated strength T-m 0.005
Max magnetic center offset in Cryomodule mm ≤0.5
Magnet physical length mm 340 Magnet width/height mm 322/220
Quantity required 35
III. MAGNET PACKAGE ELECTRICAL AND QUENCH
PERFORMANCE TESTS
The first magnet prototype SPQA01 was cold tested in
October 2015 in Test Stand 3. Fig. 3 shows an overview and
close-up of the magnet and top plate assembly ready to install
in the helium dewar, with a 30 mm warm bore tube mounted
through and centered in the magnet aperture for magnetic
measurements. Warm electrical checks of the assembly and
instrumentation were performed prior to cool down, and
repeated when cold. Instrumentation on this magnet consists
of one cernox RTD and three silicon diodes, all mounted on
the inner coil or outer iron surfaces. The RTD resistance was
verified to be consistent with calibration values at room
temperature and in the 4.3 K helium bath. The silicon diode
voltages were measured cold with 10 µA excitation and were
consistent with the standard voltage response at that
temperature. After low current magnetic measurements were
completed (up to 10 A), the quench performance was tested:
the quadrupole, vertical dipole, then horizontal dipole were
individually ramped at 0.5 A/s to 30 A, with no quenches. All
three circuits were then powered simultaneously at 30 A for
several minutes with no quench, before ramping down to
0 A. High current magnetic measurements were then
completed, again with no quenches.
The second magnet prototype SPQA02 test began in
November, 2015 and the second thermal cycle was
completed in December. Warm and cold electrical tests were
performed as with SPQA01; an additional cold hipot test of
high voltage insulation integrity was made between the outer
(horizontal) dipole winding and the heater, which was missed
on SPQA01 (but passed warm, following the cold test).
A quench performance test was made in the first thermal
cycle, again following the low current magnetic
measurements, and all three coils were ramped to 30 A
individually and collectively held there without any
quenches. No quenches occurred during any of the magnetic
measurement ramps in either thermal cycle.
Fig. 3. SPQA01 magnet assembly ready for installation in the Stand 3
dewar for cold testing in 4.5 K liquid helium bath.
IV. MAGNET PACKAGE MAGNETIC MEASUREMENTS
The magnet package magnetic measurements were
performed by rotational coils. The rotational coil system
utilizes a PC Board design [13] and provides a measurement
accuracy of ~1 unit (10-4). The probe rotates in an anti-cryostat
(warm bore tube) placed within the magnet aperture as the
assembly is suspended in the LHe vessel. The probe radius is
limited by the ~30 mm inner diameter of the warm bore. The
PCB is 1 m long and extends beyond both ends of the magnet.
However, owing to the magnet and warm bore position in the
cryostat, the probe was not centered in the magnet, and only
extended out the far (lead) end by about 100 mm; ~200 mm
short of capturing the full end field. The board was a spare
from a previous project [8], in a temporary fixture, and as
such, the probe (and the acquisition system) were not
optimized for these measurements. All harmonics are reported
here at a reference radius of 10 mm.
The field strength of the quadrupole was measured over
three cycles at different currents. For the low field bi-polar
measurements, a bipolar 10 A Kepco power supply was used.
For current from 10 A to 30 A, a unipolar Lambda power
supply was used. The measurements match the 0.125 T/A
design value well. The hysteresis width at 1 A shows that the
change in the transfer function (TF) at lower current is about
± 5 % for both magnets.
Fig. 4. Quadrupole SPQA02 integrated gradient transfer function.
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The field strength of the horizontal and vertical dipole
correctors measured over 3 cycles at different currents are
shown in Fig. 5 for SOQA01 - again for both Kepco and
Lambda power supplies. The measurements are close to their
design values of 0.28 mT-m/A.
Fig. 5. Strength TF for the horizontal dipole corrector of SPQA01.
The hysteresis width at 1 A shows that the change in TF at
lower current is about ±7 % for both magnets. It should be
noted that when current goes closer to zero we are
approaching a singularity point where the relative field
distortions could be discontinuously large. But at the same
time the absolute field value is very low and comparable with
the Earth and fringe fields around the magnet.
All dominant quadrupole integrated field harmonics were
measured. The largest harmonics are below 0.1 %, except at
the lowest current measured of 0.4 A, where they are still less
than 0.5 % for SPQA01 and 0.25 % for SPQA02, including
any persistent current or magnetization contributions.
Nevertheless, tests showed rather high hysteresis effects at
low magnet currents caused by the iron yoke made from
AISI 1006 low carbon steel. A special cold test program was
developed to investigate the field reproducibility effects.
V. SPQA03 MAGNET COLD TEST
The first production magnet SPQA03 was fabricated at
FNAL with the intent to make more comprehensive cold
studies to establish the magnet operational scenarios for the
accelerator and verify reproducibility of magnetic conditions.
For reducing the remnant and hysteresis field effects,
degaussing and standardization procedures were developed.
For degaussing the following current drive formula was used:
I
where k, τ, m are coefficients that define the peak current, the
current amplitude decay, and the cycle time period. On the
base of this formula at k=64, τ=20, m=400 was programmed
the degaussing cycle for the regulated power supply shown in
Fig. 6.
Fig. 6. Power supply current variation during degaussing.
The quadrupole unipolar cycling of current after the initial
degaussing is shown in Fig. 7. It confirms the previous result
of 5 % variations in the quadrupole transfer function (TF)
during current ramp up and down. So, these variations are
above the specified value of 1 %.
Fig.7. Quadrupole TF variations for the unipolar current cycling in the range
of 0.4 A – 18 A.
At the same time TF hysteresis loops are very reproducible
(see Fig. 8) when the current went up or down. In this case the
reproducibility is better than 0.5 %, and meets specification.
Fig. 8. Quadrupole TF variations for different current ramps.
Fig. 9 and Fig. 10 show expected linear dependence of the
quadrupole magnetic center displacement for different
combinations of the horizontal and vertical dipole corrector
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currents. It should be noted that dx and dy displacements are
fully decoupled.
Fig. 9. Quadrupole magnetic center displacement at different Horizontal
Dipole Corrector currents.
Fig. 10. Quadrupole magnetic center displacement at different Vertical Dipole
Corrector currents.
The integrated magnetic field quality was also investigated.
The measured magnetic field quadrupole harmonics are less
than 5 units at the reference radius of 10 mm. When the
quadrupole field is combined with the dipole corrector all
harmonics are also less than 5 units except the sextupole
which has a maximum of 110 units at current 20 A. It should
be noted that the maximum needed dipole corrector strength is
reached at 20 % of the quadruple current, in order to
compensate a possible 0.5 mm quadrupole magnetic center
shift caused by magnet installation accuracy and thermal
effects. In this case the sextupole field component will be two
times lower.
During accelerator operations each magnet will operate at its
fixed nominal operating current. Because some SCRF cavities
might be turned off, a 20 % quadrupole strength adjustment
might be needed. Fig. 11 and Fig. 12 show that 20 % magnet
strength change causes less than 0.4 % TF variations.
One of the main results of this test is that after an initial
degaussing the field increases in a very reproducible 0.5 %
way to the nominal operational value. Subsequent 20 % field
adjustments up or down result in 0.4 % magnetic field
reproducibility.
Fig. 11. Quadrupole TF change for 20 % current variation in the range of 0.4 A – 0.5 A.
Fig. 12. Quadrupole TF change for 20 % current variation in the range of
4 A – 5 A.
VI. CONCLUSION
Two prototypes of the splittable conduction cooled magnet
packages, and the first production magnet were thoroughly
tested and showed a good performance. Prototype magnets are
now installed in FNAL and JLAB cryomodules. The magnet
package combines a quadrupole with orthogonal dipole
correctors. During cold tests the following features were
observed and verified:
- The field quality and reproducibility are acceptable.
- The field geometric harmonics are low and meet the
specification.
- Both magnets were successfully excited to 30 A
without quench (20 A is the peak operating current).
The successfully completed tests validated the magnet
design and fabrication for use in the LCLS-II SCRF
cryomodule.
ACKNOWLEDGMENT
The authors would like to thank Prof. Akira Yamamoto
(KEK), Chris Adolphsen, and Paul Emma (SLAC) for very
useful discussions. We are very grateful to the SLAC team for
providing and commissioning a regulated power supply, and
to all FNAL Technical Division personnel involved in the
design, fabrication and tests of these magnets.
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