CERN-ACC-2020-0020
16/09/2020
CERN – EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH
APPLYING SUPERCONDUCTING MAGNET TECHNOLOGY FOR
HIGH-EFFICIENCY KLYSTRONS IN PARTICLE ACCELERATOR
RF SYSTEMS
A. Yamamoto
2
, S. Michizono
2
, W. Wuensch
1
, I. Syratchev
1
, G. Mcmonagle
1
, N. Catalan Lasheras
1
,
S. Calatroni
1
, S. Stapnes
1
, H. Watanabe
3
, H. Tanaka
3
, S. Kido
3
, T. Koga
3
, Y. Koga
3
and K. Takeuchi
3
1
CERN, Geneva, Switzerland
2
KEK, Tsukuba, Japan
3
Hitachi, Tokyo, Japan
Abstract
An MgB
2
superconducting solenoid magnet has been developed for electron beam focusing in
X-band (12 GHz) klystrons for particle accelerator RF systems, to provide a central field of 0.8 T at 57 A
and at ≥ 20 K. It has successfully realized significant AC-plug power saving in one order of magnitude
compared with that for a conventional Cu solenoid magnet. The large-scale application may be expected
for the Compact Linear Collider (CLIC) project proposed as a future accelerator candidate at CERN. It
requires ~ 5,000 klystrons, and the MgB
2
magnet application will realize significant AC-plug power
saving. This paper describes progress in a prototype MgB
2
superconducting solenoid magnet development
and discusses the future prospect.
Presented at the Magnet Technology Conference, Vancouver, Canada, 23-27 Sep 2019
Geneva, Switzerland
September 2020
CLIC – Note – 1159
> Wed-Af-Po3.15-08<
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1
Applying Superconducting Magnet Technology for
High-efficiency Klystrons in Particle Accelerator RF Systems
A. Yamamoto, S. Michizono, W. Wuensch, I. Syratchev, G. Mcmonagle, N. Catalan Lasheras, S. Calatroni,
S. Stapnes, H. Watanabe, H. Tanaka, S. Kido, T. Koga, Y. Koga, and K. Takeuchi
Abstract— An MgB
2
superconducting solenoid magnet has been
developed for electron beam focusing in X-band (12 GHz) klys-
trons for particle accelerator RF systems, to provide a central field
of 0.8 T at 57 A and at ≥ 20 K. It has successfully realized signifi-
cant AC-plug power saving in one order of magnitude compared
with that for a conventional Cu solenoid magnet. The large-scale
application may be expected for the Compact Linear Collider
(CLIC) project proposed as a future accelerator candidate at
CERN. It requires ~ 5,000 klystrons, and the MgB
2
magnet appli-
cation will realize significant AC-plug power saving. This paper
describes progress in a prototype MgB
2
superconducting solenoid
magnet development and discusses the future prospect.
Index Terms— MgB
2
superconducting solenoid magnet, Klys-
tron, Particle accelerators, RF power system, and CLIC.
I. INTRODUCTION
HE Compact Linear Collider, CLIC, has been proposed
as a future electron-positron collider accelerator candidate
at CERN [1]. It is planned to be built in staging at a center-of-
mass collision energy of 380 GeV, CLIC-380, as the general
layout shown in Fig. 1, and to be extend up to 3 TeV. A klys-
tron-based RF power system in the CLIC-380 staging has been
investigated as illustrated in Fig. 2. It requires X-band (12
GHz, 50 MW) klystrons and the high efficiency in the AC-
plug power consumption is a critically issue. The klystron
consisting of an electron-beam accelerator for RF power am-
plification requires the beam focusing with solenoidal magnet-
ic field. A conventional Cu solenoid magnet consumes an AC
plug-power of ~ 20 kW, to be resulted in ~100 MW consump-
tion with ~ 5,000 klystrons. The Application of MgB
2
super-
conducting solenoid magnets at an operational temperature of
~ 20 K may contribute to the significant power saving with
one order of magnitude, even including the cryogenics opera-
tion power [2]. A prototype MgB
2
superconducting solenoid
magnet with a solenoidal field of 0.8 T has been developed,
Manuscript receipt and acceptance dates will be inserted here. This work has
been supported by a commissioned research program between CERN and KEK.
(Corresponding author: Akira Yamamoto.)
A. Yamamoto is with High Energy Accelerator Research Organization (KEK),
Tsukuba, 305-0801, Japan and also with European Organization for Nuclear Re-
search (CERN), Geneva, 1211, Switzerland (e-mail: akira.yamamoto@kek.jp).
S. Michizono is with KEK.
W. Wuench, I. Syratchev, G. Mcmonagle, N. Calaran-Lasheras, S. Calatroni,
and S. Stepnes are with CERN.
H. Watanabe, H. Tanaka, S. Kido, T. Koga, Y. Koga, and K. Takeuchi are
with Hitachi Co. Ltd., Hitachi, Japan.
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
and it has successfully demonstrated stable and safe operation
at ≥ 20 K, with achieving significant AC plug-power saving
down to < 3 kW, corresponding down to ~15 % of that for the
Cu solenoid. The progress is described in this paper.
II. MGB
2
MAGNET DEVELOPMENT
A. General Design
The prototype MgB
2
solenoid magnet was designed to
demonstrate significant power saving in electron-beam focus-
ing for the 12 GHz, 50 MW klystron expected in the CLIC-
380 staging option [2]. Fig. 3 shows (a) the 12 GHz, 50 MW
klystron and the electron beam accelerating structure, (b) the
T
Fig. 1. A schematic layout of the Compact Linear Collider (CLIC) with Klys-
tron RF system, proposed as a future electron-positron linear collider.
Fig. 2. A concept of pairing klystrons for supplying RF power to CLIC380.
2
2
klystron assembled with traditional, conventional Cu solenoid
coils, and (c) the klystron assembled with a pair of supercon-
ducting solenoid coils developed [3, 4]. Table I summarizes
main parameters of the superconducting solenoid magnet in-
cluding the cryostat and cryo-cooler. A central field of 0.8 T is
provided in a warm bore diameter of 0.25 m, at an operation
current of 57.1 A, and an operation temperature of ≥ 20 K, by
using MgB
2
superconductor, with an AC plug-power of
≤ 3 kW mainly required for a cryocooler to cool a pair of cur-
rent leads [5]. The magnet power supply consumption is rela-
tively small (~ ≤100 W mainly for power-supply cable con-
sumption). The magnet structure including magnetic flux re-
turn yoke is compatibly designed to the existing conventional
Cu solenoid magnet structure, enabling to be replaced.
B. Superconductor
The superconducting coil operation at higher temperature is
an important requirement to improve thermal efficiency, and
therefore MgB
2
is an optimum material to be operated at ≥ 20
K [6]. Fig. 4 shows superconductor performance of the MgB
2
conductor [7]. A continuous length of 8 km of the MgB
2
con-
ductor was successfully fabricated and a part of 5.6 km has
been used to this solenoid coil. The superconductor is insulat-
ed with glass-braid prior to the coil winding, for enabling the
wind-and-react process during the coil fabrication.
C. Superconducting Coil
The superconducting solenoid coil design consists of a pair-
solenoids with axial gaps at the middle and both ends facing to
the iron return-yoke, providing solenoidal focusing field for
electron beam acceleration in the klystron. The coil design pa-
rameters are optimized to realize a self-protected coil without
requiring an active quench protection system for realizing the
simple magnet operation, based on simulation study in the de-
sign phase and on experimental study by using a small test coil
prior to the prototype coil fabrication [8].
Each half coil using the MgB
2
conductor on the coil bobbin
made of stainless steel. It consists of 152 turns and 16 layers
per coil. Thin, 0.2 mm thick Cu sheets (half cylinder shells)
are embedded between coil layers to enhance conducting cool-
ing power and quench propagation velocity along the coil axi-
al direction. The coil is heat-treated at 600 C for 6 hours in ar-
gon gas after the coil winding and is impregnated with epoxy-
resin in vacuum to complete the electrical insulation.
D. Assembly into Cryostat and Cooling with Cryocooler
The superconducting coil is installed into a cryostat consist-
ing of an inner warm bore tube made of stainless steel, an out-
er cylinder, and axial end-plate structure made of iron for
magnetic flux return-yoke function. A cryocooler is applied
for conduction cooling of the coil via Cu thermal link. A set of
current leads are thermally anchored at the 1
st
stage of the cry-
ocooler. No thermal, radiation shield plate is placed between
the coil and the cryostat except for multilayer superinsulation.
A cross section of the magnet assembly is shown in Fig. 5.
(a) (b) (c)
Fig. 3. (a) 12 GHz Klystron (main RF part), (b) Klystron assembled with
conventional Cu solenoid, and (c) Klystrons assembled with a superconduct-
ing solenoid magnet.
TABLE I
MAIN PARAMETERS OF THE MGB
2
SUPERCONDUCTING SOLENOID.
Elements
Parameters
Superconductor
Material
MgB
2
/Cu /Fe/Monel
®
Strand bare diameter
0.67 mm
Insulation on strand (thickness)
Glass-braid (80 µm)
SC solenoid coil
Inner diameter and Length
0.34 and 0.30 m
Central field, and max. field in coil
0.8 T and 1.06 T
Current
57.1 A
Inductance
7.23 H
Stored energy
11.8 kJ
Cold mass (coil, Cu-insert, Bobbin)
71 (14 + 25 + 32) kg
Heat-treatment
600 C x 6 hours
Insulation after heat-treatment
Epoxy-resin impregnated
Cryostat
Warm ID, Iron-yoke OD, and Hight
0.25, 0.63, and 0.52 m
Cryo-cooler
Cooling capacity
4 W at 20 K, 13.5 W at 80K
AC plug-power
≤ 3 kW
Fig. 4. MgB
2
superconductor characteristics and operational point expected.
3
3
III. PERFORMANCE
A. Cooldown
The MgB
2
superconducting coil was conductively cooled
down by using the cryo-cooler, in one week, as the cooldown
trend shown in Fig. 6. The temperature distribution in the coil
was kept within 50K, contributed by axial thermal conduction
enhancement with Cu sheets placed along the coil axis as de-
scribed in the previous section.
B. Magnet Excitation
The magnet was charged by using a compact solid-state
power supply with a bypass diode at the output terminal to
provide the current loop secured for the superconducting coil
during discharge, as shown in Fig. 7. In the nominal operation,
the magnet was charged with a ramp rate of 0.2 A/s, reaching
the design current of 57.1 A in 4 min., and passively dis-
charged according to voltage drop through power supplying
cable resistance ( ~ 30 mW) and a by-pass diode at the power-
supply output, within 5 min. No active quench protection sys-
tem was required because of the self-protected coil design and
fabrication, as details described in a separate report [8]. The
very simple operation has been realized, as similar as the con-
ventional Cu-based coil operation.
C. Magnetic Field Profile confirmed
The magnetic field profile along the solenoid axis was
measured by using a hall-probe and the result well reproduced
the design profile required for the electron beam focusing
along the Klystron, as shown in Fig. 8. It is consistent with the
original Cu solenoid design given by SLAC [3, 4, 8].
D. Safe Operation confirmed at ≤ T
cs
The safe and stable operation of the MgB
2
superconducting
coil up to the current sharing temperature (T
cs
) at 29 K was
demonstrated, by gradually raising the coil temperature by us-
ing a heater attached at the 2
nd
-stage cold-head of the cryo-
cooler. Fig. 9 shows the trend of the coil temperature increase
at a coil current of 57.1 A. The critical temperature for current
sharing (T
cs
) because of the super-to-normal transition was
confirmed at 29 K consistently with the conductor perfor-
mance test result shown in Fig. 4 [7, 8]. After a passive power
supply trip-off due to the power supply voltage limit, the max-
imum coil temperature of 40 K was observed on the insulated
inner surface of the coil with epoxy-resin impregnation. It was
a result to indicate the peak coil-temperature safely kept suffi-
ciently lower than a safety guideline of 140 K, as discussed in
the next section, even in case of an emergency such as stop of
the cryocooler due to AC power outage. The test demonstrated
the MgB
2
superconducting solenoid magnet with the self-
Fig. 6. The initial cooldown characteristics of the MgB
2
solenoid magnet.
Fig. 5. A cross section of the MgB
2
superconducting solenoid magnet con-
ductively cooled by using a cryocooler. Axial end plates and outer cylinder
of the cryostat are functioning as magnetic flux return yoke. A set of pole
tips are attached at both end plates for magnetic field shaping.
Fig. 8. A typical solenoid magnet excitation characteristics.
Fig. 7. A typical solenoid magnet excitation characteristics.
4
4
protected coil design to be safely operated by using a simple
circuit system as similar as that for the conventional magnet.
The re-cooldown time of the superconducting coil by using
the cryocooler after the quench was also observed, as shown in
Fig. 9 (b). It took ~1.5 hours for enabling the re-excitation at
the coil temperature below 25 K. It sounded practical.
E. Quench Safety
The quench safety was evaluated through a combined effort
of simulation study with two stages of local heater quench
tests using a dedicated, small test coil and the prototype coil,
prior to the assembly into the cryostat, with numbers of volt-
age taps and temperature sensors to monitor the quench behav-
ior and to confirm the safety. The results are discussed in an-
other report and a major result was summarized that the peak
temperature in the coil after the localized quench be kept be-
low 140 K in the prototype magnet [8]. Then, the prototype
coil was assembled with the cryostat with limited numbers of
diagnostics for safety and reliability reasons in long-term pro-
totype magnet operation combined with the klystron.
IV. DISCUSSION
A. General Performance and Energy Saving
The MgB
2
superconducting magnet for Klystron electron
beam focusing has been successfully demonstrated with the
general performance of 0.8 T at 57.1 A, at T
cs
= 29K, at an AC
plug-power consumption of < 3 kW, realizing to save the AC
plug power down to one seventh (nearly one order of magni-
tude). Comparisons of Cu, NbTi, MgB
2
, and HTS (GdB-
CO/EuBCO) are summarized in Table II from a viewpoint of
AC-plug power saving [8-10]. The simplified operation with
no requirement for an active quench protection system is also
an important feature and advantage. We have demonstrated
that the MgB
2
superconducting magnet is a very stable and
cost-effective approach in good balance of the operational AC
power efficiency and the cost-effective conductor availability.
V. SUMMARY
The MgB
2
superconducting solenoid magnet for the klys-
tron beam focusing has been successfully developed. The gen-
eral performance of the central field of 0.8 T, below the T
cs
of
29 K is demonstrated. The stable operation at a temperature
range of 20 ~ 25 K has been ensured at the AC-plug power of
< 3 kW with the power saving nearly in one order of magni-
tude, compared with the conventional Cu solenoid. It may ex-
tend to a large-scale application such as CLIC-380, enabling
an overall power saving of ~100 MW for 5,000 klystron oper-
ation. Based on this development experience, the application
of the HTS conductor will be an expected direction for further
power saving, in future, assuming the higher operational tem-
perature in a range of 65 – 70 K, if cost-effective HTS conduc-
tor may be realized.
ACKNOWLEDGMENT
This development has been conducted under the CERN and
KEK cooperation agreement. The authors would thank various
advices on the X-band Klystron given by Dr. Jeffery Nelson of
SLAC and by Mr. Oliver Sablic of CPI Co. Ltd., and kind ad-
vices on the HTS conductor given by Dr. Masanori Daibo of
Fujikura Co. Ltd.
REFERENCES
[1] CLIC 2018 Summary Report, CERN-2018-005-M, and CLIC Project
Implementation Plan, CERN-2018-010-M, 2018.
[2] A. Yamamoto, “Status of superconducting solenoid development for X-
band, 50 MW klystrons”, CLIC workshop 2018, online available:
https://indico.cern.ch/event/656356/contributions/2848532/
[3] F. Peauger et al., “A 12 GeV PS power source for the CLIC study” Proc.
IPAC’10, THPEB053, 2010, online available:
http://www.jacow.org/Main/Proceedings?sel=IPAC
[4] J. Nelson, Private communication, SLAC X-band klystron design.
[5] SHI Cryocooler, “SHI-CH204/Zephyr”, information, online available:
http://www.shicryogenics.com//wp-
content/uploads/2012/11/Cryocooler-Product-Catalogue.pdf
[6] J. Nagamatsu, N. Nakagawa, Y. Zenitani and J. Akimitsu,
“Superconductivity at 39 K in magnesium diboride,” Nature 410 Mar.
2001, 63-64.
[7] H. Tanaka et al., “Performance of MgB2 superconductor development
for high efficiency Klystron applications”, presented in this conference.
[8] H. Watanabe et al., “Development of a Prototype MgB2 Superconduct-
ing Solenoid Magnet for High-efficiency Klystron Applications,” pre-
sented in this conference.
[9] S. Yokoyama, et al., “Cryogen Free Conduction Cooled NbTi Super-
conducting Magnet for a X-band Klystron,” IEEE Trans. Appl. Super-
cond., 32, no. 4, Jul. 1996, 2633-2636.
[10] H. Daibo, Private communication on FUJIKURA-HTS GdBCO.
TABLE II
COMPARISONS OF CU/MGB
2
/HTS COILS AND POWER CONSUMPTIONS
Coil material
unit
Cu
NbTi
MgB
2
HTS
(GdBCO)
Coil
Central field
T
0.6
0.8
0.8
0.8
Current
A
2 x 300
57
57
57
Voltage
V
35
0
0
0
Power
KW
20
0
0
0
Cooling
Cooling method
WC
CC
CC
CC
Temp
K
300
4.5
~ 25
~ 65
Capacity
W
n/a
1
4
3
AC-power @ RT
kW
n/a
6
< 3
< 2
Total Power
kW
>20
6
< 3
< 2
Reference
[3]
[9]
[8]
[10]
WC: Water cooling, CC: Cryo-cooler cooling:
(a) (b)
Fig. 9. Trends of (a) the coil temperature increase and the normal transition
at T
cs
, and (b) re-cooldown in 1.5 hours for enabling the coil excitation.