ESS SC Linac Design Overview
Robin Ferdinand
DSM/DAPNIA/SACM
CEA-Saclay
bldg. 124, 91191 GIF-sur-Yvette cedex, FRANCE
Abstract. The ESS project (European Spallation Source) aims to produce high power beams for condensed matter
studies. One option consists in having both H+ and H" beams. They have to be accelerated and guided to the different
Spallation targets. Beam power up to 5 MW at 50 Hz plus 5 MW at 16.2/3 Hz, will be delivered on respectively the
Short Pulse Target Station and the Long Pulse Target Station. Two 50 mA H" branches are funnelled with one 100 mA
H+ beam at around 20 MeV. A chopper line between two RPQs and DTLs constitutes the H" front end. The H+ front end
is composed of one RFQ and one DTL. After the funnel, the two species are transported through the same linac up to
1.334 GeV. This common part is composed of a SDTL and a CCL from 20 to 185 MeV and followed by a
Superconducting Linac (SCL) to reach the final energy. Up to recently (ESS Volume IQ 1996), a NC version of the
linac had been extensively studied and constituted the reference option. However, given the fantastic progresses made on
SC technology and the many potential advantages, a SC alternative was developed. We concentrate on this option.
INTRODUCTION
The European Spallation Source (ESS) [1] Project
represents the culmination of a decade of intense
research, development and design by Europe's leading
scientists. The single goal is to provide Europe with
the world's most powerful neutron facility. Some 20
laboratories, universities and research organizations
plus a large number of scientists from all over Europe,
have joined together to develop the science case and
the technical design, and to work on the planning and
the realization of the ESS project.
The ESS will go one step further compared to the
sources under construction in the US and Japan. Not
only will its power be higher, which translates directly
into more intensity, but the ESS Council has also
opted for a source with two complementary targets, a
50 Hz Short Pulse (SP) and a 16 2/3 Hz Long Pulse
(LP) target (both target stations being considered with
equal priority).
Beam power
Linac beam energy
Linac average current
Linac peak current
Linac repetition rate
Linac beam pulse
duration
Chopping efficiency
Linac beam duty cycle
10 MW
1.334 GeV
SP
3.75mA
114mA
50 Hz
2 x 0.48 ms
(gap=0.1ms)
70%
4.8%
LP
3.75mA
90mA
16 2/3 Hz
2.5ms
no chopping
3.3%
With the latest development, the SC linac option
shows the potential to provide a clearly superior
solution to the old reference NC solution. A Technical
Advisory Committee (TAC) comprising experts from
a large number of major facilities all over the world
has expressed its confidence in the proposed technical
design and the ability of the ESS partners to build it. A
dominant requirement is to minimise the uncontrolled
beam losses to be below 1 Watt per meter and to
localise the controlled beam losses in carefully
designed serviceable collimators or beam dumps.
Early in the linac design we had also to consider
the most efficient and reliable option compatible with
the ambitious goals.
SC LINAC OVERVIEW
The 5MW Short Pulse (SP) is obtained by
compressing two 480us, 114mA-peak H" pulses in two
600ns, 62.5A-peak proton pulses in two rings. H" ions
are used rather than protons for injection efficiency
reasons. Since H" sources are for the moment not able
to produce 100 mA within all the other requirements
(emittance, repetition rate, life-time...), two 50mA
sources are used and both beams are funnelled at upper
energy (-20 MeV). The 2.5ms-100mA Long Pulse
(LP) is directly sent to the target. Both H" ions and
protons can be used. The simpler solution consists in
using H" because the SP is already H". Nevertheless,
the source capability of producing the LP is doubtful.
A fallback solution consists in using protons. The main
advantage is the already existing ECR proton sources
(SILffl, LEDA) fulfilling all the requirements. The
main difficulty is to demonstrate that it is possible to
transport two high quality beams with different
charges and current in the same linac.
This proton line is inserted at the funnel cavity
input end. The first step of the beam dynamics studies
consists in showing that the beam will fulfil the ESS
requirements (losses <lW/m -short and long pulses-,
&r, norm, rms<0.5 7C.mm.mrad, 8L, norm, rms<2.3 7C.mm.mrad
-SP only-). Linac sections have been matched and
CP642, High Intensity and High Brightness Hadron Beams: 20th ICFA Advanced Beam Dynamics Workshop on
High Intensity and High Brightness Hadron Beams, edited by W. Chou, Y. Mori, D. Neuffer, and J.-F. Ostiguy
© 2002 American Institute of Physics 0-7354-0097-0/02/$ 19.00
93
multi-particle simulations were done from the first
RFQ input to the linac exit with the Saclay code
package TOUTATIS-TRACEWIN-PARTRAN.
rings
rings and
and single
single turn
turn fast
fast extraction
extraction afterwards.
afterwards. A
A fast
fast
chopper
is
inserted
after
RFQ1
and
must
chop
chopper is inserted after RFQ1 and must chop the
the
beam
beam cleanly
cleanly between
between the
the RFQ
RFQ bunches
bunches (2.84
(2.84 ns)
ns) so
so
as
as not
not to
to generate
generate beam
beam loss
loss on
on the
the downstream
downstream linac
linac
structures.
structures. The
The emittance
emittance growth
growth is
is kept
kept below
below 3%
3%
with
with aa great
great beam
beam separation
separation at
at the
the beam
beam stop
stop location.
location.
Figure 11 :: Reference (left)
(left) and
and fallback
fallback solution
solution (right)
(right) of
of the
the
low energy Room Temperature Linac.
DTLs
DTLs
DTL
DTL tanks
tanks are
are used
used to
to increase
increase the
the energy
energy of
of the
the
beams
beams from
from 55 to
to 21.9
21.9 MeV
MeV where
where they
they can
can be
be
funnelled
funnelled much
much more
more easily.
easily. The
The ESS-DTL
ESS-DTL design
design is
is
based
program. A
based on
on the
the IPHI
IPffl R&D
R&D program.
A power
power of
of 11 MW
MW
(1.3
(1.3 MW
MW klystron)
klystron) per
per tank
tank has
has been
been chosen.
chosen. The
The use
use
of
conventional
electromagnetic
quadrupoles
of conventional electromagnetic quadrupoles may
may
allow
allow restricting
restricting the
the halo
halo development.
development. They
They preserve
preserve
the
beam
quality
but
at
the
expense
of
the beam quality but at the expense of lower
lower shunt
shunt
impedance
impedance (bigger
(bigger tubes).
tubes). AFD
AFD focussing
focussing scheme
scheme is
is
chosen.
chosen. The
The phase
phase advance
advance per
per meter
meter is
is conserved
conserved
between
The
between RFQ
RFQ output
output and
and DTL
DTL input.
input.
The H
H" peak
peak
+
beam
peak current
beam current
current is
is 57
57 mA
mA while
while the
the H
If- peak
current is
is
90
90 mA.
mA. It
It+ requires
requires 22 tanks
tanks for
for the
the H
H" lines
lines and
and 33 tanks
tanks
for
the If
H line.
line.
for the
Front Ends
The front
front ends
ends (i.e.
The
(i.e. at
at low
low energies
energies below
below 20
20 MeV)
MeV)
state-of-the-art performances
of the accelerator
require
from H~
H– ion sources, choppers and funnel branches.
from
Between 20 MeV and ~200 MeV, normal conducting
accelerating cavities are
are used.
Sources and
and RFQs
RFQs
Sources
In order
order to
to meet
meet the
the SP
SP and
and LP
LP ESS
ESS beam
beam power
power
In
specifications, much
much is
is required
required from
specifications,
from the
the ion
ion source(s).
source(s).
All the
the best
best performance
performance levels
levels achieved
achieved in
in the
the world
world
All
are simultaneously
simultaneously required.
required. This
This is
is considered
be
are
considered to
to be
challenging.
Recently,
development
strategies
have
challenging. Recently, development strategies have
been drawn
drawn up
up on
on the
the basis
basis of
of expert
expert opinion.
opinion. On
been
On the
the
bases
of
present
ion
source
operational
experience
bases of present ion source operational experience and
and
of development
development programmes
programmes already
of
already launched,
launched, it
it is
is
likely that
that H"
H- ion
ion sources
likely
sources will
will be
be produced
produced to
to meet
meet
ESS short
and long
long pulse
pulse specifications
within 2–3
ESS
short and
specifications within
2-3
years (ISIS
(ISIS Penning
Penning source
source (RAL),
volume ion
ion source
years
(RAL),
volume
source
at lAP-Frankfurt,
IAP-Frankfurt, ECR
ECR H"
H- source
at CEA-Saclay).
CEA-Saclay).
at
source at
As the
the first
first accelerating
accelerating components,
components, the
the next
next
As
RFQs play
play aa significant
significant role
role in
in determining
RFQs
determining the
the quality
quality
of the
the beam
beam in
in the
the rest
rest of
of the
the accelerator.
A great
of
accelerator. A
great care
carewas
taken
in
their
design.
RFQ1
accelerates
the H"
H
was taken in their design. RFQ1 accelerates the
from
60keV
to
2MeV,
allowing
the
insertion
of
the
from 60keV to 2MeV, allowing the insertion of the
chopper line.
line. 2MeV
2MeV is
is chosen
chosen to
to avoid
avoid activation
activation in
in
chopper
RFQ1
and
the
chopper
line.
RFQ2
accelerates
particles
RFQ1 and the chopper line. RFQ2 accelerates particles
from 2MeV
2MeV to
to 5MeV,
5MeV, lowest
lowest energy
energy compatible
compatible with
with
from
high duty
duty cycle
cycle EM
EM quadrupole
quadrupole in
in the
the DTLs.
DTLs. For
For the
the
high
proton branch,
branch, RFQS
RFQ3 accelerates
accelerates protons
protons from
from 95keV
95keV
proton
to 5MeV.
5MeV. RFQ1
RFQ1 minimizes
minimizes the
the transverse
transverse emittance
emittance
to
growth, with
with large
large acceptance
acceptance (0.3
(0.3 7i.mm.mrad)
π.mm.mrad) and
and
growth,
maximizes transmission
transmission (>99%).
(>99%). RFQ2
RFQ2 reshapes
reshapes the
the
maximizes
beam,
minimizes
the
emittance
growth
and
gives
the
beam, minimizes the emittance growth and gives the
same zero
zero current
current phase
phase advance
advance at
at exit
exit as
as RFQS,
RFQ3, with
with
same
100%
theoretical
transmission.
RFQ3
minimizes
the
100% theoretical transmission. RFQS minimizes the
losses
above
2.16
MeV.
The
frequency
choice
of
losses above 2.16 MeV. The frequency choice of
352.2 MHz
MHz is
is consistent
consistent with
with present
present LEDA
LEDA and
and IPHI
IPHI
352.2
experiences [2,
[2, 3].
3]. At
At this
this frequency,
frequency, four-vane
four-vane
experiences
geometries are
are used.
used.
geometries
Funnel
Funnel
The
The funnel
funnel line
line is
is one
one of
of the
the most
most complicated
complicated
sections
of
the
linac.
The
energy
at
sections of the linac. The energy at which
which the
the funnel
funnel
line
operates
is
a
compromise
between
lower
line operates is a compromise between lower beam
beam
rigidity at
rigidity
at low
low energy
energy and
and lower
lower space-charge
space-charge effects
effects
along
beam transport
transport at
high energy.
along the
the beam
at high
energy. The
The
compromise for
ESS sets
compromise
for ESS
sets the
the funnel
funnel energy
energy at
at
21.9MeV.
21.9MeV. The
The main
main difficulty
difficulty is
is to
to transport
transport the
the beam
beam
without emittance
prevent halo
without
emittance growth
growth and
and to
to prevent
halo
development
(obtained
by
a
smooth
change
development (obtained by a smooth change of
of the
the
focusing force
between structures).
A
big
effort
is
focusing
force between
structures).
A
big
effort
is
+
made
to
insert
the
H
beam
between
the
two
H
lines.
made to insert the If beam between the two H" lines.
This is
made possible
possible by
development of
This
is made
by the
the development
of aa new
new
deflecting
cavity
allowing
±
5
degrees
deflecting
deflecting cavity allowing ± 5 degrees deflecting angle
angle
[4],
± 10.9°
being obtained
obtained with
[4], ±
10.9° being
with quadrupoles.
quadrupoles.
SDTL to
90.5MeV, CCL
SDTL
to 90.5MeV,
CCL to
to 185MeV
185MeV
The frequency
frequency is
is now
now equal
equal to
to 704
704 MHz.
MHz. At
At
The
20
MeV,
a
SDTL
was
preferred
for
its
doublet
period,
20 MeV, a SDTL was preferred for its doublet period,
more convenient
convenient for
for the
the matching
matching with
with CCL
CCL and
and SCL
SCL
more
cavities.
SDTLs
require
less
matching
than
DTLs
cavities. SDTLs require less matching than DTLs and
and
are less
less complex
complex than
than CCDTLs.
They have
have higher
higher
are
CCDTLs. They
shunt impedance.
impedance. Above
Above 100
shunt
100 MeV,
MeV, the
the effective
effective shunt
shunt
impedance
of
the
SDTL
structure
decreases
rapidly. A
A
impedance of the SDTL structure decreases rapidly.
switch
is
made
to
the
Coupled
Cavity
Linac
(CCL)
switch is made to the Coupled Cavity Linac (CCL)
structure, more
more efficient
efficient at
at high
high energy.
energy. A
A CCL
structure,
CCL is
is used
used
from
90
MeV
to
185
MeV.
from 90 MeV to 185 MeV.
Since the
the If
H+ bunch
bunch repetition
repetition is
is 352.2
352.2 MHz
MHz and
and the
the
Since
H
beam
bunch
repetition
is
704.4
MHz,
the
proton
H" beam bunch repetition is 704.4MHz, the proton
beam charge
charge per
per bunch
bunch is
is 1.58
bigger than
than that
that of
of the
the
beam
1.58 bigger
Chopper section
section
Chopper
Time structure
structure is
is required
required in
in the
the H~
H– beam
beam to
to enable
enable
Time
low loss
loss multi-turn
multi-turn injection
injection into
into aa pair
pair of
of accumulator
accumulator
low
-
94
with
emittance
to
with rms
rms transverse
transverse
emittance of
of 0.3
0.3 π.mm.mrad
7C.mm.mrad to
relax
the
H
sources.
Most
of
the
emittance
growth
is
relax the H" sources. Most of the emittance growth is
visible
in
the
funnel
line.
This
is
due
to
the
nonvisible in the funnel line. This is due to the nonperfectly
depend
perfectly smooth
smooth line
line and
and the
the deviation,
deviation, which
which depend
on
final
on the
the particle
particle phase.
phase. Nevertheless,
Nevertheless, the
the final
emittances
are
much
smaller
than
the
required
ones.
emittances are much smaller than the required ones.
HH"- beam.
of the linac common to both beams
beam. The
The part
part
- of the linac common to both beams
isis matched
for
H
.
matched for H".However,
However, since
since the
the phase
phase advance
advance
per
per meter
meter and
and the
the focusing
focusing lattice
lattice are
are kept
kept continuous
continuous
along
along the
the linac,
linac, the
the proton
proton beam
beam is
is almost
almost matched
matched in
in
the
linac
(current
independent
design).
the linac (current independent design).
Superconducting
Superconducting structure
structure
In
from
In order
order to
to evaluate
evaluate the
the beam
beam losses
losses arising
arising from
the
beam
dynamics
in
space
charge
in
a
real
linac,
the beam dynamics in space charge in a real linac,
typical
typical static
static and
and dynamics
dynamics errors
errors on
on linac
linac element
element
have
been
defined
and
simulated.
The
errors are
are
have been defined and simulated. The errors
different
from
one
section
to
the
other,
taking
into
different from one section to the other, taking into
account
More than
than 1000
account the
the element
element specification.
specification. More
1000
linacs
with
100
000
particles
were
simulated
linacs with 100 000 particles were simulated with
with no
no
losses
the
losses after
after the
the RFQs.
RFQs. -5The
The minimum
minimum ratio
ratio between
between the
bore
and
level is
is about
about 1.15
in NC
NC linac
linac
bore radius
radius
and the
the 10
10"5 level
1.15 in
+
for
H
and
2.55
in
SC
linac
for
H
.
The
correction
for H+ and 2.55 in SC linac for H". The correction
scheme
scheme allows
allows the
the transport
transport of
of both
both particles
particles type
type with
with
aa rms
rms residual
residual orbit
orbit << 200µm
200pm (beam
(beam size
size <
< 15
15 mm).
mm).
ESS
ESS linac
linac fulfils
fulfils its
its requirements.
requirements. Estimated
Estimated beam
beam
losses
are
lower
than
1
W/m,
especially
losses are lower than 1 W/m, especially at
at -high
high energy.
energy.
Most
from the
the H"
H stripping
stripping in
in
Most of
of the
the losses
losses are
are coming
coming from
the
the residual
residual gas.
gas.
Below
Below ~200
~200 MeV,
MeV, the
the energy
energy gain
gain per
per metre
metre of
of
real
estate
of
SC
cavities
is
actually
lower
real estate of SC cavities is actually lower than
than
2MeV/m,
2MeV/m, aa gradient
gradient which
which can
can be
be easily
easily reached
reached by
by
warm
NC
structures.
Furthermore,
the
design
warm NC structures. Furthermore, the design of
of SC
SC
cavities
cavities for
for beta<0.6
beta<0.6 isis complicated
complicated by
by stiffness
stiffness and
and
microphonics
microphonics issues.
issues. The
The transition
transition energy
energy between
between
NC
NC and
and SC
SC structures
structures is
is then
then fixed
fixed at
at 185
185 MeV.
MeV. The
The
different
cavity
types
and
transition
energies
different cavity types and transition energies are
are
obtained
obtained from
from length
length and
and cost
cost optimisation.
optimisation. As
As aa
result,
result, two
two5-cell
5-cell cavity
cavity families
families with
with geometric
geometric “beta”
"beta"
values
of
0.66
and
0.85
are
well
values of 0.66 and 0.85 are well suited
suited for
for the
the
acceleration
acceleration between
between 185
185MeV
MeV and
and 1330
1330 MeV.
MeV. The
The
length
length of
of the
the SCL
SCL is
is 290m,
290m, with
with limited
limited maximum
maximum
peak
peak field
field (Bp
(Bp == 50
50 mT
mT -about
-about Ep
Ep == 27.5
27.5 MV/m),
MV/m), and
and
reasonable
reasonable power
power coupler
coupler capability
capability (2
(2 couplers
couplers per
per
cavity
cavity each
each of
of 800kW
800kW max).
max). From
From the
the beam
beam dynamic
dynamic
point
point of
of view,
view, the
the constrain
constrain of
of keeping
keeping the
the phase
phase
advance
per
cell
below
90°,
has
been
fulfilled
advance per cell below 90°, has been Mfilled in
in order
order
to
toavoid
avoid structure
structure instability.
instability.
ACKNOWLEDGMENTS
ACKNOWLEDGMENTS
The
The ESS
ESS project
project acknowledges
acknowledges all
all the
the European
European
teams
involved
in
the
linac
design.
It
also
thanks the
the
teams involved in the linac design. It also thanks
SAC
and
TAC
committees,
the
very
open
and
SAC and TAC committees, the very open and
mutually
mutually beneficial
beneficial collaboration
collaboration with
with institutions
institutions and
and
colleagues
working
on
the
US
and
Japanese
spallation
colleagues working on the US and Japanese spallation
source
European
source projects,
projects, financial
financial support
support from
from the
the European
Commission
Commission through
through the
the neutron
neutron round-table.
round-table.
Field
Field control
control –- energy
energy stability
stability
The
one
klystron
per
cavity
scheme,
The one klystron per cavity scheme, each
each cavity
cavity
with
its
own
feedback/feed
forward
RF
control
with its own feedback/feed forward RF control system
system
allows
allows great
great flexibility
flexibility and
and simplest
simplest operation
operation
procedure.
Lorentz
forces
were
procedure. Lorentz forces were carefully
carefully studied
studied
(welding
(welding rings
rings between
between cells)
cells) and
and microphonics
microphonics were
were
simulated.
simulated. The
The maximum
maximum deviations
deviations remain
remain small
small
(±0.2MeV
(±0.2MeV and
and ±0.5°).
±0.5°). Random
Random errors
errors of
of uniform
uniform
distribution,
distribution, simultaneous
simultaneous cavity
cavity field
field errors
errors (1%
(1%
amplitude,
amplitude, 1°
1° phase)
phase) lead
lead to
to aa rms
rms energy
energy fluctuation
fluctuation
of
of 0.8
0.8 MeV
MeV and
and phase
phase fluctuation
fluctuation of
of 1.2°
1.2° at
at linac
linac exit
exit
(10
(10000
000 simulations).
simulations).
REFERENCES
REFERENCES
1.
1. The
The ESS
ESS project,
project, Vol
Vol III,
m, Technical
Technical report,
report, May
May 2002.
2002.
http://www.ess-europe.de
http://www.ess-europe.de
2.
2. H.V.
H.V. Smith.
Smith, and
and J.
J. D.
D. Schneider,
Schneider, "Status
"Status report
report on
on the
the
low-energy
low-energy demonstration
demonstration accelerator
accelerator (LEDA)",
(LEDA)",
proceedings
proceedings of
of Linac
Linac 2000,
2000, Monterey,
Monterey, USA,
USA, p581
p581
3.
3. P-Y.
P-Y. Beauvais
Beauvais et
et al,
al, "Status
"Status report
report on
on the
the Saclay
Saclay highhighintensity
intensity proton
proton injector
injector project
project (IPHI)",
(IPHI)", Proceedings
Proceedings of
of
EPAC
EPAC 2000,
2000, Vienna,
Vienna, Austria,
Austria, p283.
p283.
4.
Line",
4. N.
N. Pichoff
Pichoff et
et al.,
al., "The
"The ESS-CONCERT
ESS-CONCERT Funnel
Funnel Line",
Proceedings
p390
Proceedings of
of PAC
PAC 2001,
2001, Chicago,
Chicago, USA,
USA, p390
END-TO-END
END-TO-END SIMULATIONS
SIMULATIONS
Linac
Linac sections
sections were
were matched
matched and
and multi-particle
multi-particle
simulations
were
done
from
the
first
simulations were done from the first RFQ
RFQ input
input to
to the
the
linac
exit.
The
input
distribution
is
a
4D
water-bag
linac exit. The input distribution is a 4D water-bag
Figure
Figure 2.
2. ESS
ESS SC
SC Linac
Linac scheme.
scheme.
95