Abstract The European Spallation Source (ESS) is a 5 MW, 2.5 MeV long pulse proton machine. It represents a big jump in power compare to the existing spallation facilities. The design phase is well under way, with the delivery of a Conceptual Design Report published in the beginning of 2012, and a Technical Design Report in December 2012. Why and how the 5 MW goal influences the parameter choices will be described

C Darve

D Mcginnis

Ess Ab

H Danared

M Lindroos

S Molloy

Sweden Lund

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THE ESS LINAC DESIGN
M. Lindroos∗, S. Molloy, D. McGinnis, C. Darve, H. Danared, ESS AB, Lund, Sweden
and the ESS Accelerator Collaboration
Abstract
The European Spallation Source (ESS) is a 5 MW, 2.5
MeV long pulse proton machine. It represents a big jump
in power compare to the existing spallation facilities. The
design phase is well under way, with the delivery of a Con-
ceptual Design Report published in the beginning of 2012,
and a Technical Design Report in December 2012. Why
and how the 5 MW goal influences the parameter choices
will be described.
INTRODUCTION
Spallation is a nuclear process in which neutrons of dif-
ferent energies are emitted in several stages following the
bombardment of heavy nuclei with highly energetic par-
ticles. The spallation process is the most practical and
feasible way of producing neutrons for a reasonable effort
(or cost) of the neutron source cooling system. Spallation
sources come in at least three types: short pulse sources
(a few µs), long pulse sources (a few ms) and continuous
sources. The future European Spallation Source (ESS) will
be a long pulse source and the first spallation source with a
time average neutron flux as high as that of the most intense
research reactors.
The highest power spallation source currently in opera-
tion – the Spallation Neutron Source (SNS) in Oak Ridge
– combines a full energy SC linear accelerator with an ac-
cumulator ring to provide very high intensity short pulses
of neutrons to the instruments. The European Spallation
(ESS) source will provide even higher intensities, but is
developing instruments able to use longer linac pulses di-
rectly for spallation, avoiding the need for a costly and
performance-limiting accumulator ring [1].
The obvious advantage of a linac is that beam passes
only once through the accelerating structures, enabling it
to accelerate a high current beam with a minimum of con-
straints. The current limit is mainly set by space charge
effects at low energy, as well as the power that can be de-
livered to the beam in each accelerating cavity at medium
and high energies, and by beam losses.
The spallation cross section for protons on heavy nuclei
increases as a function of proton energy up to several tens
of GeV [2]. Nonetheless it is generally agreed that a ki-
netic proton energy between 1-3 GeV is optimal for practi-
cal target and moderator designs, and in order to keep the
shielding requirements reasonable.
The ESS has the ambitious goal of becoming a sustain-
able research facility with zero release of carbon dioxide.
This will be achieved through a combination of actions, but
∗E-mail: mats.lindroos@esss.se
with the linac being the most energy hungry part of ESS,
the energy efficient design of the RF power sources and
the cryogenics systems and high-Q cavities are important
issues.
THE ESS BASELINE
The ESS accelerator high level requirements are to pro-
vide a 2.86 ms long proton pulse at 2.5 GeV at repetiton
rate of 14 Hz, with 5 MW of average beam power on tar-
get. The configuration of the current, May 2012 Baseline
linac is shown schematically in Fig. 1, and selected linac
parameters are listed in Tab. 1 [3]. The warm linac has con-
tributions from INFN Catania, CEA Saclay, ESS-Bilbao
and INFN Legnaro, the superconducting cavities and their
cryomodules are designed at IPN Orsay and CEA Saclay,
and the HEBT will come from ISA in Aarhus. The 50-mA
proton beam is produced in a pulsed microwave-discharge
source on a platform at 75 kV. A low-energy beam trans-
port, LEBT, with two solenoid magnets as focusing ele-
ments brings the beam to the entrance of the RFQ. The
LEBT has a chopper that cuts away the beam while the
proton pulses from the ion source stabilize, preventing a
beam with off-nominal parameters from being accelerated
in the RFQ and lost at high energy. The 4-vane RFQ accel-
erates the beam to 3 MeV with small losses and a minimal
emittance growth. It is designed specifically for ESS but it
is based on the IPHI RFQ at Saclay. The RF frequency
of the RFQ and the warm linac is 352.21 MHz. After
the RFQ there is a medium-energy beam transport, MEBT,
with three buncher cavities and 10 quadrupole magnets.
The MEBT has several different functions: it has optics to
match and steer the beam from the RFQ into the drift-tube
linac, it has a comprehensive set of beam-instrumentation
devices, it has a chopper which acts faster than the LEBT
chopper since space-charge neutralization is not an issue in
the MEBT, and it allows collimation of the transverse par-
ticle distribution. A drift-tube linac, DTL, with four tanks
takes the beam from 3 MeV to 79 MeV. It has a FODO
structure with permanent-magnet quadrupoles. Every sec-
ond drift tube is empty or used for steering magnets and
beam-position monitors. The superconducting linac has
three types of cavities: double-spoke resonators, five-cell
medium-beta elliptical cavities and five-cell high-beta el-
liptical cavities. The May 2012 linac has 14 spoke cry-
omodules with two double-spoke resonators in each, and
between the cryomodules there are warm quadrupole dou-
blets. The spoke resonators operate at 352.21 MHz like
the warm linac, but then there is a frequency doubling to
the 704.42 MHz of the elliptical cavities. There are 15
medium-beta cryomodules with four cavities in each and
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02 Proton and Ion Accelerators and Applications
2A Proton Linac Projects
Figure 1: A block diagram of the ESS linac design. The Radio Frequency Quadrupole (RFQ) and Drift Tube Linac (DTL)
are normal conducting while the spoke resonator and low beta and high beta elliptical cavities are superconducting
Table 1: The ESS RF parameters
Length Input energy Frequency Geometric β No of sections Temp
m MeV MHz K
LEBT 2.1
RFQ 5.0 75 ×10−3 352.2 1 RT
MEBT 3.5
DTL 32.5 3 352.2 3 RT
Spoke 58.6 79 352.2 0.50 (optimal) 14 (2c) ≈ 2
Medium β 113.9 201 704.4 0.67 15 (4c) ≈ 2
High β 227.9 623 704.4 0.92 30 (4c) ≈ 2
HEBT 100 2500
quadrupole doublets between, and there are 30 high-beta
cryomodules with four cavities in each and quadrupole
doublets between every second cryomodule. All acceler-
ating structures will be powered by klystrons, except the
spoke resonators where tetrodes will be used. With one
klystron per elliptical cavity plus a few for the warm linac,
there will be close to 200 large klystrons and almost 100
modulators since one modulator will drive two klystrons.
The density of components in the klystron building would
become too high if these were to be positioned linearly. In-
stead they will be located in groups of eight klystrons and
four modulators across the klystron building. After the last
cryomodule there is 100 m of tunnel where additional cry-
omodules can be installed for an energy upgrade. Then
the beam is brought from the tunnel to the spallation target
at the surface through two vertical bends and an expansion
section. Quadrupole and octupole magnets are used to blow
the beam up onto the desired profile of the proton-beam
window and the target window.
SUPERCONDUCTING LINAC
The superconducting part of the linac consists of three
families of SC cavities. The beam dynamics has been dis-
cussed in the previous section. We will here give some
insight to the specific issues relating the superconducting
technology for a high power linac like ESS.
Cryomodules
The cryomodules are installed in a ≈ 400 m long ESS
cold linac section. There are three families of cryomod-
ules: the spoke resonator cryomodules, the medium- and
the high-beta elliptical cavity cryomodules. The cryomod-
ules are composed of the cavity packages, the supporting
system, the alignment system, the thermal and magnetic
shieldings, the internal cryogenic piping and the diagnos-
tic instrumentation. Each elliptical cryomodule contains
four superconducting cavity packages, which include cav-
ities (or resonator), fundamental power coupler, cold tun-
ing system, its helium tank and the RF pick-up coil. Each
spoke cryomodule contains two spoke resonator packages.
The main functions of the cryomodules are to provide the
positioning and alignment of the cavity package w.r.t. the
beam axis, and to reduce the thermal losses to the 2 K he-
lium tank. The cryomodules have the capacity 1) to trans-
fer energy from RF system to the beam, 2) to confine the
protons longitudinally and 3) to steer the protons longitu-
dinally. To lower the construction and operating risk, the
ESS cold linac take advantage of the de-velopment done
on the cryomodules design and the procedures developed
at the Spallation Neutron Source (SNS), Desy (X-FEL) and
CEA for module integration. In an effort to standardize the
solution and to lower the technical unknown/risk, the ESS
has contracted the same institute (IPNO) to design the cry-
omodule of both the spoke and the elliptical cavities. The
design of the cryomodules and its jumper connection are in
compliance with the European Pressure Equipment Direc-
tive 97/23/EC and is driven by the ALARA principle.
Tab. 2 gives the length of the individual cryomodule
types and the length of different SC sections. Warm
quadrupole doublets and beam instrumentation systems are
installed in between the cryomodules to correct and to
monitor the beam profile. Fig. 2 shows the conceptual de-
Proceedings of LINAC2012, Tel-Aviv, Israel TH2A01
02 Proton and Ion Accelerators and Applications
2A Proton Linac Projects
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Table 2: The ESS Cryomodule parameters
Section Cryomodule length Cavities count
per section[m]
Spoke ≈ 2.9 28
Medium-beta ≈ 6.7 60
High-beta ≈ 6.7 120
Figure 2: Spoke cavity cryomdoule for ESS
sign of the spoke resonator cryomodule and Fig. 3 shows
the conceptual design of the RF elliptical cavities cryomod-
ule. A space-frame is used to position and align the ellipti-
cal cryomodules.
Cavities
The three families of SC cavities intended for use in the
ESS linac are:
1. Double spoke cavities operating at 352.21 MHz
2. Medium-β elliptical cavities operating at 704.42 MHz
3. High-β elliptical cavities operating at 704.42 MHz
Double spoke cavities The proton beam will be ac-
celerated from 79 MeV to 201 MeV by 28 spoke cavities.
These will be installed in 14 cryomodules (i.e. 2 cavities
per cryomodule), operating at 2 K.
It should be noted that ESS will be the world’s first ac-
celerator to make use of spoke cavities.
Figure 3: Elliptical cavity cryomdoule for ESS
Figure 4: A cross-section through a spoke cavity design for
ESS.
Figure 4 shows a cross-sectional view through a spoke
cavity designed for use in ESS.
These cavities are designed to have an optimal beam
velocity, βopt = 0.50, and a peak accelerating gradient,
Eacc = 8 MV/m.
The design of these cavities is driven by some quite con-
servative limits on the maximum surface fields – Bpeak ≤
70 mT, Epeak ≤ 35 MV/m – in order to minimise the risk
associated with this new technology.
The expected quality factor, Q0 ≥ 2× 10
9, at the nomi-
nal gradient.
Medium-β cavities Following the spokes, the proton
beam is then accelerated from 201 MeV to 623 MeV by
60 SC elliptical cavities. This is the first section to use the
higher accelerator frequency of 704.42 MHz
These cavities contain five resonant cells operating in
the π-mode, and they are designed with a geometric beta,
βG = 0.67.
At the nominal gradient, Eacc = 15 MV/m, the cavity
surface will be treated such that a quality factor, Q0 ≥ 5×
10
9, is achieved.
High-β cavities The final section of the ESS linac con-
sists of 120 high-β cavities that are used to accelerate
the protons from the 623 MeV with which they exit the
medium-β section, to the final energy of 2.5 GeV.
Figure 5: A cross-section through a high-β elliptical cavity
installed within its cryo-vessel.
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02 Proton and Ion Accelerators and Applications
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As with the medium-β section, these are also π-mode
cavities with five resonant cells, however their geometric
beta, βG = 0.92 in the current baseline.
At the nominal gradient, Eacc = 18 MV/m, the cavity
surface will be treated such that a quality factor, Q0 ≥ 6×
10
9, is achieved.
RF SOURCES
Because the vast majority of accelerating structures in
the ESS linac are superconducting, the dominant feature of
the ESS RF system is the number of RF stations. To han-
dle variations in cavity coupling, Lorentz detuning, there
is one RF power amplifier per superconducting resonator;
totaling over 208 individual RF stations. The core of each
RF station is an RF power amplifier which is typically a
klystron. For the high beta section of the linac consisting
of 120 stations, the peak power required for the cavities is
900 kW. Even though the loaded Q of the cavities is quite
low (7 × 105) to compensate for heavy beam loading, the
Lorentz detuning for ESS can be quite severe. Without any
Lorentz-detuning compensation, the resonant frequency of
the cavities could shift up to 50 degrees of RF phase during
the beam pulse. In addition, the beam pulse for the ESS
linac is very long (2.86 ms) which is about a factor of three
longer than the mechanical response time of the cavity to
the Lorentz detuning. To save on RF power overhead, all
superconducting cavities will be equipped with fast piezo-
electric tuners to negate the Lorentz detuning of the cavi-
ties. The klystrons are operated with 30 % below the max-
imum saturated power so that variations in the RF system
such as modulator droop, cavity coupling, residual Lorentz
detuning, and power loss in the waveguide distribution sys-
tem can be compensated. Also part of the overhead is used
for stable regulation of the feedback loop. The target effi-
ciency of the klystrons at maximum saturated power is 60
% which will give an operational efficiency of about 45 %.
The required maximum saturated power for the klystrons is
1.2 MW. The klystron is energized with a modulator. For
stability, reliability and economic reasons, the klystrons
will be powered in the pulsed cathode configuration. The
modulators must supply a 3.5 ms long pulse at a rate of
14 Hz. The cathode voltage is for each klystron is about
100kV with a peak current of about 20 Amperes. For a
modulator efficiency of 90 % and a power factor of 0.9, 120
kVA will be required for each klystron. For economical and
space reasons, it has been decide to power two klystrons
per modulator which limits the required power per modu-
lator to 240 kVA. To handle the long pulse length of 3.5 ms,
the modulators will implement solid-state switches at rela-
tively moderate voltages and the output voltage pulse will
be stepped up by a pulse transformer or equivalent technol-
ogy. The modulator topology will not be imposed by ESS
but functional specifications will be given to vendors. The
regulation of each RF system will be done independently
with the low level RF system. Using modern digital tech-
nology, most of the regulation will be done with adaptive
feed-foward algorithms. Feedback regulation will play a
secondary role to the adaptive feed-forward so the required
power overhead can be minimized. With the large num-
ber of RF systems, it will be un-economical for the high
power waveguide of each RF system to have its own pen-
etration to the tunnel. ESS will use a stub concept for dis-
tributing 16 RF systems into the tunnel as shown in Figure
6. The stubs will provide access to additional conventional
services such as water and power as well. In addition, the
radiation shielding issues are best handled with the stub
concept. ESS is also considering the use of Inductive Out-
put Tubes (IOTs) in place of klystrons for the high beta
section of the linac. IOT shows the promise of high effi-
ciency and much lower capital cost. IOTs can be thought
of as a cross between klystrons and tetrodes. As in tetrodes,
IOTs employ a grid to control electron flow from cathode
to collector. These grids are very robust with the advent
of pyrolithic carbon technology. Since IOTs are gridded
tubes, no pulsed modulation system is needed to energize
the cathode. This eliminates costly high voltages switches
in the power supply for the IOT. Also a gridded tube can
be run in deep class C that can produce very high efficien-
cies (less than 70 %). Also IOTs do not exhibit the severe
saturation effect of klystrons. This permits the operation of
IOTs at the maximum rated power in feedback loops which
increases the efficiency of the RF system significantly. It is
estimated that 3-4 MW of power consumption can be saved
if klystrons are replaced by IOTs. Like klystrons, IOTs use
a resonant output cavity to efficiently couple power from
the electron beam at high frequencies. However the gain
of an IOT is much lower than a klystron because an IOT
uses a single output cavity while a klystron may employ 4-
5 stages of cavities. With the advent of low cost solid stage
RF amplifiers as pre-drivers, the high gain provided by
multi-cavity klystrons is not required. Because of this low
gain, IOTs require cathode voltages on the order of 35kV
as compared to 100 kV for high gain klystrons. The lower
cathode voltage also reduces the cost of the power supply
that energizes the IOT. Current estimates for a power sup-
ply for an IOT is 60 % the cost of a klystron modulator
of comparable power. Currently there are no commercially
available IOTs for the ESS power range. However there
has been very good progress made in higher order mode
multi-beam IOTs in which power levels of 1 MW have been
achieved [4]
Figure 6: The ESS RF sources in stub lay-out
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02 Proton and Ion Accelerators and Applications
2A Proton Linac Projects
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DISCUSSION
The ESS accelerator design update is approaching com-
pletion. Work is being done in all areas from ion-source,
thorough accelerating structures to the target. We have
here reported on the work on RF, cavities and cryomod-
ules but major progress has also been made for the normal
conducting magnets, the Low, Medium and High energy
beam transsport, controls and instrumentation. The design
update work for the accelerator and for all other areas of the
project will enable ESS to enter construction in early 2013
provided the negotiations between ESS member states can
complete in time.
ACKNOWLEDGMENT
We wish to thank all our colleagues at ESS Acceleraror
division and in the ESS accelerator collaboration.
REFERENCES
[1] F. Mezei,“Comparison of neutron efficiency of reactor and
pulsed source instruments”, Proc. of ICANS-XII (Abingdon
1993) (RAL Report No. 94-025), I-137 and F. Mezei,“The
raison dłtre of long pulse spallation source”, J. Neutron Re-
search 6 (1997) 3-32.
[2] K. van der Meer et al., Nucl. Instr. and Meth. in Phys. Res. B
217 (2004) 202220
[3] H. Danared, Proc. IPAC 2012, p. 2073.
[4] Progress in CPI Microwave Tube Development, Wright, Ed-
ward L.; Bohlen, Heinz, AIP Conference Proceedings;2006,
Vol. 807 Issue 1, p145
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02 Proton and Ion Accelerators and Applications
2A Proton Linac Projects


THE ESS LINAC DESIGN

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THE ESS LINAC DESIGN

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