Status of
of the
the SNS
SNS H"
H- Ion
Ion Source
Sourceand
and Low-Energy
Low-EnergyBeam
Beam
Transport System*
Transport
R. Keller,®
Keller,@ R.
R. W.
W. Thomae,®
Thomae,@ M.
M. P.
P. Stockli,*
Stockli,# and
andR.
R.F.
F.Welton*
Welton#
R.
@
@
E.
Lawrence Berkeley
Berkeley National
National Laboratory,
Laboratory,##Oak
OakRidge
RidgeNational
NationalLaboratory
Laboratory
E. O. Lawrence
-
Abstract. The
The ion
ion source
source and
and Low-Energy
Low-Energy Transport
Transport (LEBT)
(LEBT) system
system that
that will
will provide
provide H"
H ion
ion beams
beams to
to the
the Spallation
Spallation
Abstract.
Neutron Source
Source (SNS)**
(SNS)** Front
Front End
End and
and the
the accelerator
acceleratorchain
chainhave
havebeen
beendeveloped
developedinto
intoaamature
matureunit
unitthat
thatwill
willsatisfy
satisfythe
the
Neutron
operational needs
needs through
through the
the commissioning
commissioning and
and early
earlyoperating
operatingphases
phases of
of SNS.
SNS.The
Theion
ion source
sourcewas
wasderived
derivedfrom
fromthe
the
operational
SSC ion
ion source,
source, and
and many
many of
of its
its original
original features
features have
have been
been improved
improved toto achieve
achievereliable
reliableoperation
operationatat6%
6%duty
dutyfactor,
factor,
SSC
producing beam
beam currents
currents up
up to
to the
the 50-mA
50-mA range.
range. The
The LEBT
LEBT utilizes
utilizes purely
purelyelectrostatic
electrostaticfocusing
focusingand
andincludes
includes static
static
producing
beam-steering elements
elements and
and aa pre-chopper.
pre-chopper. This
This paper
paper discusses
discussesthe
thelatest
latest design
designfeatures
featuresof
ofthe
theion
ion source
sourceand
andLEBT
LEBTas
as
beam-steering
well as
as some
some future
future improvements,
improvements, gives
gives performance
performance data
data for
for the
the integrated
integrated system,
system, and
and reports
reports on
on commissioning
commissioningrerewell
sults obtained
obtained with
with the
the SNS
SNS RFQ
RFQ accelerator.
accelerator.
sults
INTRODUCTION
INTRODUCTION
Berkeley Lab
Lab has
has just
just completed
completed building
building the
the linac
linac
Berkeley
injector (Front
(Front End,
End, FE)
FE) for
for the
the Spallation
Spallation Neutron
Neutron
injector
Source project
project (SNS)
(SNS) and
and isis commissioning
commissioning the
the entire
entire
Source
system.
The
main
subsystems
are
the
H
ion-source,
system. The main subsystems are the H" ion-source,
the low-energy
low-energy beam-transport
beam-transport system
system (LEBT),
(LEBT), the
the
the
2.5-MeV radio-frequency
radio-frequency quadrupole
quadrupole (RFQ)
(RFQ) acceleracceler2.5-MeV
ator, and
and the
the medium-energy
medium-energy beam-transport
beam-transport system
system
ator,
(MEBT). Ion
Ion source
source and
and LEBT
LEBT are
are the
the subject
subject of
of this
this
(MEBT).
paper; their
their task
task is
is to
to create
create aa 65-keV,
65-keV, 38-mA
38-mA ion
ion
paper;
beam, to
to match
match and
and steer
steer itit into
into the
the RFQ,
RFQ, and
and totopreprebeam,
chop itit into
into mini-pulses
mini-pulses of
of about
about 600
600 ns
ns duration.
duration. The
The
chop
nominal
duty
factor
is
6%,
with
1-ms
macro-pulse
nominal duty factor is 6%, with 1-ms macro-pulse
length and
and 60-Hz
60-Hz repetition
repetition rate.
rate.
length
be injected
injected into
into the
the Linac
Linac while
whileassuming
assumingaa 20%
20%beam
beam
be
loss in
in the
the RFQ.
RFQ. Actual
Actual RFQ
RFQ transmission
transmissionresults
results [5],
[5],
loss
quoted
below,
indicate
that
a
LEBT
output
current
of
quoted below, indicate that a LEBT output current of
about
44
mA
should
satisfy
the
SNS
current
goal.
about 44 mA should satisfy the SNS current goal.
A schematic
schematic view
view of
of ion
ion source
source and
and LEBT
LEBT isis
A
shown in
in Figure
Figure 1,
1, below.
below. By
By now,
now, four
four plasma
plasma gengenshown
erators (including
(including one
one "startup
“startup source")
source”) and
andone
oneLEBT
LEBT
erators
have been
been built
built and
andtested,
tested, and
andthe
thespecific
specificdesign
designfeafeahave
tures and
and performance
performance data
data of
of this
this production
production system
system
tures
are discussed
discussed in
inthe
thefollowing
followingsections.
sections.
are
Based upon
upon the
the main
main design
design features
features of
of the
the SSC
SSC
Based
ion source
source [1],
[1], an
an R&D
R&D version
version of
of the
the SNS
SNSion
ion source
source
ion
was built
built first
first to
to demonstrate
demonstrate the
the viability
viability of
of the
the chochowas
approach, utilizing
utilizing an rf
rf driven
driven discharge
discharge inside
inside aa
sen approach,
multicusp plasma
plasma generator
generator with
with magnetic
magnetic filter,
filter, cecemulticusp
sium enhancement,
enhancement, and
and electron
electron suppression
suppression at
at low
low
sium
energy [2].
[2].
energy
For the
the LEBT,
LEBT, a purely
purely electrostatic
electrostatic focusing
focusing syssysFor
tem [3] was
was chosen,
chosen, thereby
thereby avoiding
avoiding time-dependent
time-dependent
tem
space charge
charge compensation
compensation usually
usually encountered
encountered with
with
space
magnetic LEBTs.
LEBTs. The
The production
production version
version of
of the
the ion
ion
magnetic
source and
and LEBT
LEBT [4]
[4] aims
aims at
at generating
generating and
and transportransporsource
ting aa beam
beam with
with 50-mA
50-mA current,
current, thought
thought to
to be
be suffisuffiting
cient to
to satisfy
satisfy the
thelatest
latest SNS
SNS design
design goal
goal of
of 38
38 mA
mAto
to
cient
FIGURE1.
1. SNS
SNSIon
Ion Source
Source and
andLEBT
LEBTschematic.
schematic.
FIGURE
ION SOURCE
SOURCE
ION
The production-version
production-version ion
ion sources
sources aim
aim atat generatgeneratThe
ing H"
H- beams
beams of
of up
up to
to 50-mA
50-mA current.
current. The
The goal
goal for
forthe
the
ing
normalized,transverse
transverserms
rmsemittance
emittanceisisdetermined
determined
normalized,
* This work is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy
under
Contract No. DE-AC03-76SF-00098.
* This work is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy
under
No. DE-AC03-76SF-00098.
** TheContract
SNS project
is being carried out as a collaboration of six US Laboratories: Argonne National Laboratory (ANL), Brook-
haven National Laboratory (BNL), Thomas Jefferson National Accelerator Facility (TJNAF), Los Alamos National Laboratory
** The SNS project is being carried out as a collaboration of six US Laboratories: Argonne National Laboratory (ANL), Brook(LANL), E. O. Lawrence Berkeley National Laboratory (LBNL), and Oak Ridge National Laboratory (ORNL). SNS is manhaven National Laboratory (BNL), Thomas Jefferson National Accelerator Facility (TJNAF), Los Alamos National Laboratory
aged by UT-Battelle, LLC, under contract DE-AC05-OOOR22725 for the U.S. Department of Energy.
(LANL), E. O. Lawrence Berkeley National Laboratory (LBNL), and Oak Ridge National Laboratory (ORNL). SNS is managed by UT-Battelle, LLC, under contract DE-AC05-00OR22725 for the
U.S. Department of Energy.
th
CP642, High Intensity and High Brightness Hadron Beams: 20 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
276
Recently
collar has
has been
been developed
developed that
that
Recently aa new
new collar
seamlessly
with the
the outlet
outlet aperture
aperture [7].
[7]. This
This
seamlessly merges
merges with
design
major advantages:
advantages: 1),
1), itit provides
provides
design brings
brings three
three major
for
keeping
the
surfaces
around
the
outlet
aperture
for keeping the surfaces around the outlet aperture atat
the
as the
the collar;
collar; 2),
2), with
with the
the help
help of
of
the same
same temperature
temperature as
an
isolating
centering
ring,
it
allows
precise
aligning
an isolating centering ring, it allows precise aligning ofof
the
the outlet
outlet aperture
aperture and
and atat the
the
the collar
collar to
to the
the axis
axis of
of the
same
the entire
entire unit
unit to
to an
an optimal
optimal potenpotensame time
time biasing
biasing the
tial
the source
source body
body [8];
[8]; and
and 3),
3), itit alaltial with
with respect
respect to
to the
lows
modifying
the
contour
of
the
outlet
aperture
as
lows modifying the contour of the outlet aperture as
discussed
below,
without
having
to
build
another
main
discussed below, without having to build another main
outlet
collar/outlet aperture
aperturehas
has
outlet flange.
flange. This integrated collar/outlet
been
been tested
tested in
in the
the
been fabricated
fabricated but so far not yet been
SNS
shown in
in Figure
Figure2.2.
SNS ion
ion source. Its design is shown
bybythetherequirement
requirementofof0.2
0.2nπmm-mrad
mm-mradfor
forthe
thebeam
beamexexiting
itingthe
theLEBT.
LEBT.
Plasma
PlasmaGeneration
Generation
The
Theplasma
plasmaisisproduced
producedby
byaahydrogen
hydrogen discharge
discharge
inside
the
multi-cusp
vessel,
sustained
inside the multi-cusp vessel, sustainedby
bytens
tensofof kW
kW
ofof2-MHz
2-MHzrfrfpower.
power.The
Thepower
powerisistransmitted
transmitted through
through
ananimpedance-matching
impedance-matchingnetwork
network toto an
an antenna
antenna that
that
consists
consistsofofa a2-1/2
2-1/2winding
windingcopper
coppercoil
coil covered
covered by
by aa
multi-layer
multi-layerporcelain
porcelaincoating.
coating.The
Theefficiency
efficiencyof
of beam
beam
generation
generationisisabout
about1.0
1.0mA
mAper
perkW
kWofofrfrfpower.
power.
AnAnantenna
antennawith
with0.25-mm
0.25-mmcoating
coating [6]
[6] underwent
underwent
ananendurance
endurancetest
testatatfull
fullduty
dutyfactor,
factor,and
andthe
thetest
test was
was
intentionally
intentionallystopped
stoppedafter
after 107
107 hours
hours while
while the
the ion
ion
source
sourcedelivered
delivered2020mA
mAofofbeam
beamcurrent.
current.An
Anupgraded
upgraded
antenna
antennawith
with0.8-mm
0.8-mmthick
thick10-layer
10-layercoating
coatingproduces
produces
sameplasma
plasmadensity
densityfor
fora agiven
givenrfrfpower
powerlevel
level as
as
thethesame
thinversion
versionand
andshould
shouldlast
lastsignificantly
significantlylonger.
longer.
thethe
thin
Coolingline
line
Cooling
reliablyignite
ignitethe
theplasma
plasmaatatthe
thebeginning
beginning ofof
ToToreliably
every1-ms
1-mspulse,
pulse,a acontinuous,
continuous,low-power
low-power discharge
discharge
every
sustainedbybyananadditional
additional 13.56-MHz
13.56-MHz rfrf system.
system.
is issustained
Nothaving
havingtotoprovide
provideconditions
conditionssuitable
suitablefor
for ignition
ignition
Not
allowstuning
tuningthe
themain
maindischarge
dischargeparameters
parameters towards
towards
allows
optimum
production
of
H
ions.
optimum production of H" ions.
Slot for
for cesium
cesium
Slot
container
container
Outlet
Outletaperture
aperture
-
H Creation
Creation
H"
FIGURE 2.
2. SNS
SNS Ion
Ion Source
FIGURE
Source and
and LEBT
LEBT schematic.
schematic. InteIntegrated cesium
cesium collar
collar with
grated
with slots
slots for
for cesium
cesium containers
containers (left)
(left)
and outlet
outlet aperture
aperture (right).
and
(right). Heated
Heated or
or room-temperature
room-temperatureair
airisis
conducted around
around the
the collar
collar to
conducted
to stabilize
stabilize the
the temperature.
temperature.
Negativehydrogen
hydrogenions
ionsare
arepreferentially
preferentially created
created
Negative
in
the
space
confined
by
the
cesium
collar,
the
magin the space confined by the cesium collar, the magneticfilter
filterfield,
field,and
andthe
theoutlet
outletelectrode,
electrode, see
see Fig.
Fig. 1.
1.
netic
The
filter
field
keeps
energetic
electrons
that
would
The filter field - keeps energetic electrons that would
destroythe
theH"Hions
ionsaway
awayfrom
fromthe
thecollar
collarregion.
region. VolVoldestroy
ume
production
alone
is
sufficient
only
to
generate
ume production alone is sufficient only to generate
about 15 mA of beam current; cesium enhancement is
about
15 mA of beam current; cesium enhancement is
needed to reach the 50-mA level. For that purpose, the
needed to reach the 50-mA level. For that purpose, the
collar is fitted with eight cesium-chromate containers
collar is fitted with eight cesium-chromate containers
and is thermally isolated from the source body. The
and is thermally isolated from the source body. The
presence of a minute amount of cesium on the inner
presence
of a minute amount of cesium on the inner
collar surface not only multiplies the abundance of
collar
surface
only
multiplies
thebyabundance
of
negative ions not
in the
discharge
plasma
about a factor
negative
ions
in
the
discharge
plasma
by
about
a
factor
of three, but it also reduces the abundance of electrons
ofinthree,
but it also
reduces
theorder
abundance
of electrons
the extracted
beam
by one
of magnitude.
in the extracted beam by one order of magnitude.
LOW-ENERGY BEAM
LOW-ENERGY
BEAM TRANSPORT
TRANSPORT
Electron Dumping
Electron
Dumping
To best utilize the cesium, a freshly cleaned plasma
To best utilize
the cesium,
freshly
cleaned
plasma
generator
is operated
at full aduty
factor
for about
15
generator
is
operated
at
full
duty
factor
for
about
min., heating the cesium collar to more than 500°C 15
by
min.,
heating
thethrough
cesium the
collar
to more
thanutilizing
500°C by
forcing
hot air
collar
wall and
the
forcing
hot as
airathrough
collarAfter
wall this
and initial
utilizing
the
rf power
source the
of heat.
condirf tioning,
power asthe
a source
After down
this initial
collarof isheat.
cooled
by condiroomtioning,
the air
collar
is cooled
roomtemperature
and kept
at about down
280°C by
for optimal
temperature
air and The
keptcesium
at about
280°C
for optimal
beam production.
layer
can then
last for
beam
production.
The cesium
layer
can then last
several
days. Additional
cesium
reconditioning
canfor
be
several
days.
Additional
cesium
reconditioning
can
be
performed in-situ as needed.
When a negative-ion beam is being extracted from
When a negative-ion beam is being extracted from
a plasma, a substantial amount of electrons is extracted
aasplasma,
a substantial amount of electrons is extracted
well. This leads to the problems of increasing the
as
well.
This
leads to especially
the problems
the
space-charge density,
nearoftheincreasing
outlet aperspace-charge
density,
especially
near
the
outlet
aperture, and of the power load to the structure where the
ture,
and beam
of theispower
load With
to thethe
structure
where
the
electron
deposited.
SNS ion
source,
electron
beamare
is deposited.
the SNS ion
source,
the electrons
deposited With
on a dedicated
‘dumping
the
electrons
are deposited
on of
a dedicated
'dumping
electrode’
at moderate
energies
about 5 keV,
aided
electrode'
at
moderate
energies
of
about
5
keV,
aided
by a set of permanent magnets inside the outlet elecby
a
set
of
permanent
magnets
inside
the
outlet
electrode. Usually some fraction of the electrons misses
trode.
Usually
some
fraction
of
the
electrons
misses
the dumping electrode entirely, and these electrons are
the
dumpingtoelectrode
entirely,
and these
are
accelerated
the full beam
energy
of 65 electrons
keV and hit
accelerated
to the full
beam or
energy
of 65 structure.
keV and hit
either the extractor
electrode
its support
A
either
the extractor
its support
water-cooled
shieldelectrode
has nowor
been
installedstructure.
to absorbA
water-cooled
has Itnow
been installed
absorb
the associatedshield
heat load.
is expected
that thetonew,
inthe
associated
heat
load.
It
is
expected
that
the
new,
integrated collar/outlet electrode described above will
tegrated
electrode
described
above will
provide acollar/outlet
more satisfactory
solution
to this problem.
performed in-situ as needed.
provide a more satisfactory solution to this problem.
277
Because of the steering action of the dumping
magnetic field, not only on the electrons, but also on
the ions, the entire ion source is tilted by an adjustable
angle of ~3° with respect to the LEBT axis.
A round-the-clock endurance test of the ion source
and LEBT was conducted over more than a week, continuously producing beam of about 25 mA current at
3% duty factor, with few interruptions. It proved that
the beam-generating system is ready to support commissioning of the SNS accelerators and even SNS operations for the first few years.
Beam Simulations
For the earlier design work, the positive-ion code
IGUN [9] had been used, approximating the electron
space charge by increasing the assumed ion current in
the volume near the meniscus. The negative-ion version of the code PBGUNS [10] appears to better simulate the beam formation process [11] and was used to
design the new outlet aperture contour.
In fact, the commissioning of the SNS RFQ at low
duty factor [5] was aided by very stable performance
of Ion Source and LEBT, and up to 33 mA were
transmitted, out of about 36 mA that had been injected.
The functionality of the LEBT pre-chopper system
has been tested as well, and rise and fall times of 25 ns
were measured, better than the nominal requirement
by a factor of 2. The beam signals were not clear
enough to allow a precise determination of the prechopper attenuation factor, but a value around IxlO"3
appears quite plausible from extrapolations of results
obtained at less than nominal chopping voltages.
Principal LEBT Functions
Apart from forming the beam, the main purpose of
the LEBT is to transport it to the RFQ and match the
injection requirements. To efficiently pump the gas
load produced by the plasma generator the electrode
support structures were given highly transparent
shapes.
ACKNOWLEDGMENTS
The authors would like to acknowledge the support
by a large number of SNS staff at LBNL and ORNL
who supported this work. Thanks are due in particular
to R. Gough, R. Yourd, R. DiGennaro, A. Ratti, S.
Lewis, T. Schenkel, D. Cheng, K. N. Leung, J. Greer,
J. W. Staples, D. Syversrud, W. Abraham, T. Kuneli,
N. Ybarrolaza, C. Lionberger, P. Cull, M. Hoff, J.
Pruyn, R. MacGill, M. Monroy, M. Regis, J. Dougherty, D. Garfield, and K. Barat.
The focusing action of the two-lens electrostatic
system, captured in a tuning matrix, works as predicted by simulations, but generation of less than
nominal beam current results in a narrower beam size
inside the first lens and effectively reduces its focusing
power. To widen beams of less than 35 mA during
RFQ commissioning, the extraction gap was increased
by 4 mm as compared to the nominal size of 20 mm.
For pre-chopping and static steering, pulsed voltage
signals of ±2.5 kV and, independently, dc potentials
are applied to the four quadrants of the center electrode of the second lens. Not all of the chopped beam
is intercepted by the ring target on the LEBT-exit/
RFQ-entrance electrode; the remaining particles are
deposited inside the RFQ cavities
REFERENCES
1. Saadatmand, K., Arbique, G., Hebert, J., Valicenti, R.,
and Leung, K. N., Rev. Sci. Instr. 67 (3), p. 1318 (1996).
2. Leitner, M. A. et al., Proc. PAC '99, Paper WEA 13,
New York (1999).
3. Staples, J. W., Hoff, M. D., and Chan, C. R, Proc. Linac
'96, Geneva (1996).
4. Reijonen, R. J., Thomae, R., and Keller, R., Proc. Linac
2000, Paper MOD19, Monterey (2000).
5. Keller, R. et al., Proc. EPAC '02, Paper THPLE012,
Paris (2002).
6. Welton, R. R, Stockli, M. P., Kang, Y., Janney, M., Keller, R., Schenkel, T., Thomae, R., and Shukla, S., Rev.
Sci. lustrum. 73 (2), p. 1008 - 1012 (2002).
7. Welton, R., Stockli, M., Keller, R., and Thomae, R.,
Proc. EPAC '02, Paper THPLE019, Paris (2002).
8. Peters, J., Rev. Sci. lustrum. 73 (2), p. 900 - 902 (2002).
9. Becker, R., EPAC'98, Paper THP44G, Stockholm(1998).
10. Boers, J. E., PBGUNS Manual, available through Thunderbird Simulations, Garland, TX, 75042.
11. Welton, R. R, Stockli, M. P., Boers, J. E., Rauniyar, R.,
Keller, R., Staples, J. W., and Thomae, R. W., Rev. Sci.
lustrum. 73 (2), p. 1013 -1016 (2002).
BEAM RESULTS
The nominal beam-current goal of 50 mA pulse average measured at full 6% duty factor downstream of
the LEBT was reached about a year ago, still with the
nominal gap width installed. The peak current at the
beginning of every pulse even reached 68 mA. With
the extraction gap increased by 4 mm for RFQ commissioning, up to 36 mA were measured, and the pulse
shape was much more uniform.
The emittances show pronounced distortions at the
10% intensity level, but after subtracting back-ground
signals from the raw data, the normalized rms sizes are
very close to or even better than the nominal values of
0.2 n mm mrad.
278