MOPOY025
Proceedings of IPAC2016, Busan, Korea
ELECTROMAGNETIC DESIGN OF β = 0.13, f = 325 MHZ HALF-WAVE
RESONATOR FOR FUTURE HIGH POWER, HIGH INTENSITY PROTON
DRIVER AT KEK
Gunn Tae Park∗ , Eiji Kako, Yukinori Kobayashi, Tadashi Koseki, Tomofumi Maruta,
Shinichiro Michizono, Fujio Naito, Hirotaka Nakai, Kensei Umemori, and Seiya Yamaguchi
KEK, High Energy Accelerator Research Organization, Tsukuba, Japan
Abstract
At KEK, a proposal is being prepared for a new linacbased proton driver that can accelerate the proton beam up
to 9 GeV with 9 MW beam power and 100 mA peak current.
In this report, we present the study on the front end design
of the linac, which will accelerate the beam to 1.2 GeV:
The baseline layout, the acceleration energy structure, RF
characteristics of components, cryomodule configurations,
and the detailed design of half-wave resonator 1.
INTRODUCTION
A new multi-MW proton driver with 100 mA peak current
is being planned for neutrino physics at KEK [1]. A proposed set of beam parameters for neutrino physics is listed
in Table 1.
Unit
Value
Energy
(Peak) current
Power
Pulse length
Repetition rate
GeV
mA
MW
ms
Hz
9
100
9
1
10
BASELINE LAYOUT
The front end of the linac consists of electron cyclotron resonator (ECR) ion source, low energy beam transport (LEBT),
radiofrequency quadrupole (RFQ), medium energy beam
transport (MEBT), 3 types of low-beta superconducting cavities (half-wave resonator 1, half-wave resonator 2, single
spoke resonator), and 2 types of 5-cell elliptical cavities
(medium beta elliptical cavity (MBE) and high beta elliptical cavity (HBE)) as shown in Fig. 2.
Table 1: Beam Parameters
Beam parameters
The driver is being considered to be located at KEKB tunnel, whose view is shown in Fig. 1. The driver will use four
200 m long straight sections of the tunnel for a linac. While
the rest of straight section will be accelerated with β ∼ 1 by
TESLA-type 9-cell elliptical cavities, the front end of the
linac that uses the 1st straight section is expected to accelerate the proton beam up to 1.2 GeV using superconducting
low-beta cavities.
This report is summary of the studies done to design the
front end of the linac: baseline layout, choice of accelerators
and their specifications, and the design of the first accelerator
in the beamline, the superconducting half-wave resonator 1
(HWR1).
'()*(#+'(,-)#)&(.-%/'-(01(234(45/%#6!
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4th straight
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9 GeV!
20
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section
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'-F' (C(?89?(Z$LL$L-5G(
Quadrupole
!=0.93 1.3 GHz
Bending
9-cell ILC cavity
G%&5$ (C(?89[(Z$S'E$G'=(
RF cavity
Eacc=26 MV/m
m
+
∗
gunnpark@post.kek.jp
SSR
0.46
!=0.67
326 MeV
MBE
0.65
!=0.9
1200 MeV
HBE
0.81
f = 650 MHz
Figure 2: Layout of the front-end baseline. The blue and the
grey are low energy and high energy linac, respectively.
Sunday, April 24, 2016
1st straight
section
1.2 GeV!
1.2 GeV
"!
Figure 1: The tunnel configuration of the proton driver
Wednesday, April 27, 2016
HWR2
0.24
!=0.51
153 MeV
The base frequency is determined by that of the RFQ,
which is set at 325 MHz. The detailed specifications of the
components of the front end is listed in Table 2.
9 GeV!
0
20
HWR1
0.13
!=0.28
39.1 MeV
9-cell ILC cavity
Eacc=26 MV/m
m
]!
RFQ
!=0.15
10.7 MeV
f = 325 MHz
9 GeV
Y)(\5L%)K5!
)0'-(C((
'E(J(?87(M(J(7(R(>(!"#$"
A5/%B'F(C(
%=+# (
(
I#+(F#-5%=+#
ECR
0
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0
20
m
Quadrupole
Bending
RF cavity
20
9-cell ILC cavity
-'&#(C(7??(LM(NO"$F'P(
Eacc=26 MV/m
0
Copyright © 2016 CC-BY-3.0 and by the respective authors
)*(5AA'$'-5B)&(C(
E(%&(7F#(F#-5%=+#8(
3rd straight
E(%&(D&G(F#-5%=+#8(
section6.2 GeV!
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GeV
! (>(78?!
@(J(D(>(@8?(:'E( !=1.0 1.3 GHz=
!=0.1
5.0 MeV
HALF-WAVE RESONATOR 1
The half-wave resonator 1 is optimized for β = 0.13 with
frequency f = 325 MHz. High intensity beam current requires a large bore radius of Rbor e = 20 mm. Because
relatively large frequency bandwidth is expected with a high
beam loading, the helium pressure fluctuation is not expected
to impact the tuning and the operating temperature is set to
be 4.2 K.
ISBN 978-3-95450-147-2
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Proceedings of IPAC2016, Busan, Korea
MOPOY025
Table 2: RF Parameters of Linac Components
HWR1
HWR2
SSR
MBE
HBE
325
325
325
650
650
ncell
2
2
2
5
5
βo pt
0.13
0.24
0.46
0.61
0.76
βin
0.1
0.15
0.28
0.51
0.67
βout
0.15
0.28
0.51
0.67
0.9
Vacc (MV)
0.7
2.1
5.3
10.2
15.4
φ s (rad)
−30
−30
−27
−27
−27
G (Ω)
40
73
117
192
236
E p /Eacc
6.9
4.8
4.1
2.5
2.4
B p /Eacc
14.2
6.2
7.9
4.6
4.4
Pbeam (kW)
53
182
473
909
1373
ncav
10
20
30
20
72
5
4
5
4
The unit for B p /Eacc is mT/(MV/m).
3
f (MHz)
ncav /cm
Hcav
Rtube
g
Rbore
d
Rcb
Rct
Figure 3: The sectional view of the HWR1.
Wednesday, April 20, 2016
Electromagnetic Design
Electromagnetic design of the cavity was optimized for
figures of merit including high cavity voltage V0 , low peak
magnetic field B p , and high geometrical factor G. The
thresholds for the peak electric and magnetic fields are set
to be E p = 35 MV/m and B p = 120 mT respectively. The
structure of the resonator was modified from the simple coaxial geometry. The re-entrant nose was introduced to keep
the gap g and center of the gap-to center of the gap distance
d while increasing the outer radius of cavity. The drift tube
was shaped into the ring for minimal axial asymmetry of
the accelerating field. This is important specially with our
plan to use superconducting solenoid as a focusing element,
which has only axis-symmetric component. Finally, the
center conductor was tapered to reduce the peak magnetic
field.
The dimensional parameters of the cavity were optimized
by using 3D FEA code CST-MWS, following the standard
procedure [2], [3]. The optimized HWR1 is shown in Fig. 3.
Table 3: RF Figures of Merit of the HWR1
Figures of merit Value Figures of merit Value
f [MHz]
325
E p [MV/m]
βo p
0.13 B p [mT]
V0 [MV]
0.78 E p /Eacc
Eacc [MV/m]
5.1
B p /Eacc
R/Q0
384
P0 [W]
TT F
0.78 G [Ω]
T [K]
4.2
Q0
The unit for B p /Eacc is [mT/(mV/m)].
Rout
35
72
6.9
14.1
0.4
40.2
3.3 × 109
The corresponding figures of merit are listed in Table 3.
In the Table, Eacc is obtained from dividing Vacc by the effective length L e f f = βo p λ. In Fig. 4, the electromagnetic
field distribution is shown. The electric field is dominant
near beam axis (See Fig 4(a)), while the magnetic fields
are uniformly distributed over the center conductor (See
Fig. 4(b)). In Fig. 5, the transverse field asymmetry, defined
as Ey (y = 3.5mm) − Ez (z = 3.5mm) with the expected rms
beam size being 3.5 mm, is plotted. The maximum asymmetry is only 4 × 104 V/m, less than 1% of the accelerating field
gradient. With most of power accounted by beam power in
heavy beam loading, the generator power available at cavity
is about 52 KW and the loaded quality factor Q L ∼ 3 × 104 ,
leading to the bandwidth of 10 kHz.
Multipaction
The multipaction was studied with the simulation by using
CST-PS. In the simulation, we used SEY (secondary electron
yield) of the niobium with wet treatment as shown in Fig. 6.
The experimental measurement of SEY of the niobium with
the standard surface treatment at KEK at Ke = 1 keV with
DC current [4] indicates that hδi ∼ 1.7 and closely resembles
the one with wet treatment. Two major multipactions are
predicted. One is at low accelerating gradient, taking place
inside the drift tube. (See Fig. 8) The other is near operating
accelerating gradient, taking place at the top and bottom
toroids. The trajectories of the electrons are cyclotronic with
centrifugal force fed by magnetic field, leading to 2-point
multipaction. The voltage bandwidth for the multipaction is
wide spread as shown in Fig. This is familiar multipaction
to the HWR, as reported in [5]. The effort to avoid the
multipaction is underway.
CONCLUSION
The front end design of the linac for the multi MW proton
driver at KEK is done determining on acceleration sections,
superconducting cavities, and their specifications. The opti-
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A08 Linear Accelerators
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Parameters
MOPOY025
Proceedings of IPAC2016, Busan, Korea
(a) The trajectory of electrons during multipaction
2.5
Multipaction band :
2~10.7 kV
2
!
1.5
(a) The electric field
1
Monday, April 25, 2016
0.5
0
0
5
10
V
15
(kV)
20
25
acc
(b) Multipaction voltage band
April 23, 2016
Figure 7: The multipaction of the HWR1 near beam port.
Student Version of MATLAB
Saturday, April 23, 2016
(b) The magnetic field
(a) The trajectory of electrons during multipaction
Figure 4: The electromagnetic field distribution of the HWR.
3.5
Multipaction band :
0.35 ~1.1 MV
3
4
x 10
number ratio
1
April 23, 2016
0
E−field (V/m)
2
operation voltage
1.5
Wednesday, April 27, 2016
−1
Copyright © 2016 CC-BY-3.0 and by the respective authors
2.5
1
−2
0.5
0
−3
0.2
0.4
0.6
0.8
1
voltage ratio
1.2
1.4
1.6
1.8
−4
(b) Multipaction voltage band
−5
Figure 8: The multipaction of the HWR1 near beam port. In
(b), number ratio is number of electrons after 12.3 ns to that
after 3.1 ns. Voltage ratio is voltage to Vacc . Operational
voltage includes the synchronous phase term.
−6
−200
−100
0
distance(mm)
100
200
Student Version of MATLAB
Figure 5: The transverse field asymmetry.
Wednesday, April 27, 2016
Ar
Student Version of MATLAB
mized electromagnetic design and multipaction study of the
β = 0.13, f = 325 MHz half-wave resonator is presented.
REFERENCES
[1] T. Maruta et al., “Design study on the 9 GeV proton linac at
KEKB tunnel for the next generation neutrino experiment",
JPS Conf. Proc. vol. 8, p. 011013, 2015.
[2] B. Mustapha et al., “Electro-magnetic optimization of a halfwave resonator", in Proc. of SRF’11, Chicago, IL, USA, paper
MOPO044.
Figure 6: The SEY curves of niobium with various surface
treatments.
[3] S. Miller et al., “Design for manufacture of superconducting
half wave cavities”, in Proc. of SRF’11, Chicago, IL, USA,
paper MOPO051.
Wednesday, May 4, 2016
ISBN 978-3-95450-147-2
04 Hadron Accelerators
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A08 Linear Accelerators
Proceedings of IPAC2016, Busan, Korea
[5] Z. Zheng et al., “Multipacting simulation and analysis for the
FRIB superconducting resonators using TRACK3P”, THP092,
Proceedings of LINAC2010, Tsukuba, Japan.
04 Hadron Accelerators
ISBN 978-3-95450-147-2
A08 Linear Accelerators
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Copyright © 2016 CC-BY-3.0 and by the respective authors
[4] R. Noer et al., “Secondary electron yield of Nb RF cavity
surfaces", Proceedings of the 10th Workshop on RF superconductivity2001, Tsukuba, Japan.
MOPOY025