SNS Beam-in-Gap Cleaning and Collimation
Sarah Cousineau*,Nuria Catalan-Lasheras^ Jeff Holmes**, Daniele Davino* and
Hans Ludewig*
"Indiana University, Bloomington, Indiana
^CERN, Geneva, Switzerland
** Oak Ridge National Laboratory, Oak Ridge, Tennessee
^Brookhaven National Laboratory, Upton, New York
Abstract. The Spallation Neutron Source (SNS) under construction at Oak Ridge National Laboratory will
operate with very high beam power and very strict beam loss requirements. A two stage collimation system
will be used in the accumulator ring to localize beam loss and keep residual radiation to a level that will allow
for hands-on maintenance of the machine. The collimators will also be used in conjunction with a set of fast
kickers to remove the beam from the gap before the rise of the extraction kickers. We have installed a full-scale
collimation package into the ORBIT particle-in-cell tracking code, and have used it optimize both the collimation
system and the beam-in-gap cleaning process. Results of the studies are presented here.
INTRODUCTION
The Spallation Neutron Source currently under construction at Oak Ridge National Laboratory will deliver 1.4 x
1014 to a liquid mercury target at a frequency of 60Hz to
create the most intense source of neutrons in the world.
Due to the very high beam power, unprecedented control of beam loss will be required in order to minimize
radiation activation of the machine and allow for handson-maintenance in most areas. In the accumulator ring
specifically, uncontrolled beam losses are not to exceed
1 x 10~4 of the total beam intensity, or 1 W/m around the
ring. Loss levels of this order can not be achieved without
the use of collimators that capture high emittance particles before they intercept the machine aperture. For this
purpose, the SNS ring will employ a sophisticated twostage collimation system composed of four adjustable
primary scrapers and four fixed-aperture secondary absorbers. The system is designed to absorb either 1 x 10~3
of the beam intensity during every accumulation cycle, or
two full beam pulses in the event of a failure scenario.
In addition to scraping the beam halo, the collimation
system, together with a set of fast stripline kickers, will
provide a means of cleaning the beam in the gap before
the rise of the extraction kickers. The beam gap is expected to reach the 1 x 10~4 intensity level during accumulation due to effects such as nuclear scattering and
energy loss at the foil, RF noise, and collimation inefficiency. Loss of these particles during the rise of the extraction kicker fields translates into over 100W of power
deposition on the extraction region, far in excess of the
IW/m safety level. The proposed solution is to use the
stripline kickers to excite the beam in the gap on the betatron resonance and deposit it into the collimation system
before the rise of the kickers.
To optimize both the beam in gap (BIG) cleaning process and the collimation system in general, we have installed a complete collimation package into the particlein-cell tracking code, ORBIT [2]. The structure of the
collimation routine is modeled after a standalone collimation code K2 [3], with routines adapted specifically
for the SNS energy range. Physical processes included
are multiple coulomb scattering, ionization energy loss,
nuclear inelastic absorption and nuclear elastic scattering, rutherford scattering, and a detailed treatment of the
collimator edge. The ORBIT framework provides a realistic accelerator environment and a complete suite of diagnostics that, combined with the collimation package,
allow for detailed studies of how the beam interacts with
the collimation system. We have used this program to
optimize several parameters of the collimation system,
to determine uncontrolled loss distributions around the
ring, and to test the full beam in gap cleaning process.
We dedicate the first half of this paper to optimization
work associated solely with the collimation system, and
the balance to the beam in gap cleaning simulations.
COLLIMATION SYSTEM
OPTIMIZATION
Much of the design work for the collimators was completed before installation of the ORBIT collimation
package, and detailed descriptions of the system have
been published [3]. The goal of our work was not to redesign the system but to fine-tune such parameters as primary scraper thickness and material, aperture shape, and
collimator location. A few results are discussed here.
In a two-stage collimation system, particles first intercept the primary collimators (scrapers) where they are
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
170
1
1
Efficiency
/ Max.
Efficiency
Efficiency
/ Max.
Efficiency
imparted
a momentum
imparted
a momentumkick.
kick.For
Forbetatron
betatroncollimation,
collimation,the
the
imparted amust
momentum
kick. For
betatron
collimation,
the
momentum
bebetranslated
into
displacement
before
momentum
must
translated
into
displacement
before
momentum
must
be
translated
into
displacement
before
absorption
bybythe
collimators
can
occur.
absorption
thesecondary
secondary
collimators
can
occur.An
An
absorption
by theexists
secondary
collimators
can occur.
An
analytic
expression
for
the
optimum
phase
advance
analytic
expression
exists
for
the
optimum
phase
advance
analytic
expression
exists
for
the
optimum
phase
advance
between
the primary
collimator that
induces the
kick and
between
between the
the primary
primary collimator
collimator that
that induces
induces the
the kick
kick and
and
thethe
secondary
collimator
that
performs
the
absorption:
secondary
collimator
that
performs
the
absorption:
the secondary collimator that performs the absorption:
p
cos(∆
µ ) == p
(ε /ε )
(1)
cos(Aju)
cos(∆µ ) = (1ε1 /2ε2 )
(1)
µ
is
the
phase
advance,
and
ε
and
ε
are
the
Where
∆
Where
A/i
is
the
phase
advance,
and
e
and
z^
are
the
1
Where ∆µ is the phase advance, and 1ε1 and 2ε2 are the
emittance
apertures
emittance
apertures
the
primary
and
secondary
colemittance
aperturesofof
ofthe
theprimary
primaryand
andsecondary
secondarycolcollimators,
respectively.
This
phase
advance
provides
the
limators,
respectively.
This
phase
advance
provides
limators, respectively. This phase advance provides the
the
cleanest
absorption
the
scraped
beam
by
the
secondary
cleanest
absorption
ofof
the
scraped
cleanest
absorption
of
the
scrapedbeam
beamby
bythe
thesecondary
secondary
collimators.
However,
the
equation
neglects
the
exiscollimators.
collimators.However,
However,the
theequation
equationneglects
neglectsthe
the exisexistence
of
other
limiting
machine
apertures.
The
multipletence
of
other
limiting
machine
apertures.
The
multipletence of other limiting machine apertures. The multiplecoulomb-scattered
beam
gaussian
nature
and
poscoulomb-scattered
beam
coulomb-scattered
beamisis
isgaussian
gaussianinin
innature
natureand
andpospossesses
a
long
tail
that
risks
interaction
with
limiting
sesses
sessesa along
longtailtailthat
thatrisks
risksinteraction
interactionwith
withlimiting
limiting
ring
apertures
before
optimum
phase
advance
reached.
ring
apertures
before
optimum
ring
apertures
before
optimumphase
phaseadvance
advanceisis
isreached.
reached.
the
case
of
the
phase-optimized
SNS
system,
the
InIn
the
case
of
the
phase-optimized
SNS
system,
In the case of the phase-optimized SNS system, the
the
scraped
beam
must
negotiate
two
quadrupole
doublets
scraped
beam
must
negotiate
two
quadrupole
doublets
scraped beam must negotiate two quadrupole doublets
and
over
twelve
meters
vacuum
pipe
passage
and
over
twelve
and
over
twelvemeters
metersofof
ofvacuum
vacuumpipe
pipeinin
initsits
itspassage
passage
from
the
primary
scraper
to
the
last
of
the
secondary
from
the
primary
scraper
to
the
last
of
the
secondary
from the primary scraper to the last of the secondary
absorbers.
Collimation
simulations
which
include
both
absorbers.
Collimation
absorbers.
Collimationsimulations
simulationswhich
whichinclude
includeboth
both
beam
pipe
and
quadrupole
apertures
show
that
about
7beam
pipe
and
quadrupole
beam
pipe
and
quadrupoleapertures
aperturesshow
showthat
thatabout
about7-7scraped
beam
lost
the
vicinity
the
sec8%
of
the
scraped
beam
lostinin
inthe
thevicinity
vicinityofof
ofthe
thesecsec8%8%
ofof
thethe
scraped
beam
isisislost
ond
quadrupole
doublet
instead
the
intended
collimaond
quadrupole
doublet
insteadofof
ofthe
theintended
intendedcollimacollimaond
quadrupole
doublet
instead
tor.
This
motivates
a
change
in
location
of
the
last
colliThis
motivates
a changeininlocation
locationofofthe
thelast
lastcollicollitor.tor.
This
motivates
a change
mator
from
after
the
quadrupole
doublet
to
just
before
the
mator
from
after
quadrupole
doublettotojust
justbefore
beforethe
the
mator
from
after
thethe
quadrupole
doublet
doublet.
The
new
arrangement
represents
a
compromise
doublet.
The
new
arrangementrepresents
representsa acompromise
compromise
doublet.
The
new
arrangement
between
optimum
phase
advance
and
limiting
geometric
between
optimum
phase
advanceand
andlimiting
limitinggeometric
geometric
between
optimum
phase
advance
aperture.
Simulations
with
the
new
layout
result
alaperture.
Simulations
with
the
new
layout
result
inalalaperture. Simulations with the new layout result inin
most
no
losses
on
the
doublet,
and
an
overall
efficiency
most
no
losses
on
the
doublet,
and
an
overall
efficiency
most no losses on the doublet, and an overall efficiency
increase
of about
7%.
The
separation
the
last
collimaincrease
about
7%.
Theseparation
separationofof
ofthe
thelast
lastcollimacollimaincrease
of of
about
7%.
The
tor
from
its
optimum
phase
advance
does
allow
aa portion
tor
from
its
optimum
phase
advance
does
allow
portion
tor from its optimum phase advance does allow a portion
of
the
scraped
beam
to
escape
the
collimation
system,
of
the
scraped
beam
to
escape
the
collimation
system,
of the scraped beam to escape the collimation system,
but itit isis below
the limiting
ring
aperture
therefore
combelow
limitingring
ringaperture
aperturetherefore
thereforecomcombutbut
it is below
thethe
limiting
pletes
the
turn
without
interaction
and
is
collimated
on
pletes
the
turn
without
interaction
and
is
collimated
onaaa
pletes the turn without interaction and is collimated on
subsequent
pass
through
the
system.
subsequent
pass
through
system.
subsequent
pass
through
thethesystem.
The
role
of
the
primary
collimators,
the
"scrapThe
role
of
the
primary
collimators,oror
orthe
the“scrap“scrapThe role
of thetheprimary
collimators,
ers",
ers”, is
is to
to kick
kick the halo
halo particles
particles into
into large
large emittances
emittances
ers”,
is to kick the halochanging
particles intobeam
large emittances
without
without significantly
significantly changing the
the beam energy.
energy. The
The
without
significantly
changing
the
beam
energy. The
SNS
SNS primary
primary collimator
collimator is
is comprised
comprised of
of four
four tantalum
tantalum
SNS
primary collimator
is comprised at
of four tantalum
scrapers
scrapers mounted
mounted on
on copper
copper supports
supports at angles
angles of
of 0,
0, 45,
45,
scrapers
mounted
on
copper
supports
at
angles
ofits0, high
45,
90,
and
-45
degrees.
Tantalum
is
chosen
for
90, and -45 degrees. Tantalum
its
90,melting
and -45
degrees.
Tantalum is chosen for its high
melting point
point and
and good
good durability. The thickness of the
melting
point
and
good
The thickness
of the
scraper
impact
of particles
scraper determines
determines the
thedurability.
impact parameters
parameters
scraper
determines
the
impact
parameters
of
particles
on
on the
the secondaries,
secondaries, which
which in
in turn
turn influences
influences the absorpontion
the efficiency
secondaries,
which
in turn influences
absorpFigure the
the
tion
efficiency of
of these
these collimators.
collimators.
1 shows
tion
efficiency
of
these
collimators.
Figure
1
shows
dependence
on
dependence of
of the
the individual
individual collimator efficiencies
efficienciesthe
dependence
the individual
on
the
where
we invokeefficiencies
the definition
the scraper
scraperofthickness,
thickness,
wherecollimator
definition
s o e where
#absorbed
theefficiency
we invoke
=
##impacted
», an<
^ norma
lize to
the
maximum
e fscraper
f iciencythickness,
≡#absorbed
and
normalize
tothe
the definition
maximum
e fefficiency
fefficiency
iciency ≡
,
and
normalize
to
the
maximum
(about
90%).
#impacted
(about
90%).
efficiency
(about
90%).
ItIt isis clear
that
the
clear
that
the thicker
thicker scrapers provide better abIt
is
clear
that
the However,
thicker
scrapers
provide
betterleads
absorption
efficiency.
too
of a scraper
sorption efficiency.
However,
too thick
sorption
efficiency.
However,
too
thick
of
a
scraper
leads
to
scraped
beam
intercepting
the
quadrupole
and
beam
to scraped beam intercepting the
to scraped beam intercepting the quadrupole and beam
(1)
1
0.99
O.99
0.99
0.98
O.98
0.98
0.97
O.97
0.97
0.96
O.96
0.96
0.95
O.95
0.95
0.94
0.94
0.94
0.93
0.93
0.93
Relative Collimator Efficiency vs. Scraper Thickness
Relative Collimator Efficiency vs. Scraper Thickness
Relative Collimator Efficiency vs. Scraper Thickness
1
1
2
3
4
2
3
4
2
3
4
Scraper
Scraper Thickness [mm]
Scraper Thickness [mm]
5
5
FIGURE
1.1. Individual
efficiency
versus scraper
scraper
FIGURE
Individual collimator
collimator efficiency
efficiency versus
versus
scraper
FIGURE 1.
Individual
collimator
thickness.
thickness.
thickness.
% of%Beam
withwith
Larger
Emittance
of Beam
Larger
Emittance
pipe
apertures before
before reaching
collimators.
Based
on
pipe
the collimators.
collimators.Based
Basedon
on
pipeapertures
apertures
before
reaching the
our
studies,
the
optimum
thickness
for
the
SNS
our
studies,
the
optimum
scraper
thickness
for
the
SNS
our studies, the optimum scraper thickness for the SNS
systemisis
is ≈
w 4.0mm.
4.0mm.
system
system
≈
4.0mm.
Figure
2
demonstrates the
effect of
of the
the fully
fully
optiFigure
2
demonstrates
effect
fully optioptiFigure 2 demonstrates
the effect
mized
collimation
system.
aperture
was
set
mized
collimation
system.
The
scrapers
aperture
was
set
mized collimation system. The scrapers aperture was set
200nmm
mrad,and
andabout
about15%
15%of
ofthe
theinitial
initialgaussian
gaussian
toto
15%
of
the
initial
gaussian
to200
200ππmm
mm··•mrad,
mrad,
and
about
beam
distribution exceeded
exceeded that
that value.
value. The
The
initial
emitbeam
value.
The initial
initial emitemitbeam distribution
distribution
exceeded
tance
profile
of
the
beam
is
plotted
along
with
emittance
tance
along with
with emittance
emittance
tanceprofile
profile of
of the
the beam is plotted along
profiles
after 10
10 turns
turns and
and after
after 100
100 turns.
turns.
After
10
turns
profiles
turns. After
After10
10turns
turns
profilesafter
after
10
turns
around
the
machine,
the
beam
halo
is
almost
completely
around
is almost
almost completely
completely
aroundthe
the machine,
machine, the beam halo is
absorbed. Since
Since betatron
betatron phasing
phasing ensures
ensures
that
at
least
absorbed.
ensures that
that at
at least
least
absorbed.
Since
betatron
few
turns
will
be
needed
for
all
of
the
halo
to
intercept
few turns
turns will
will be
be needed
needed for all of the
few
the halo
halo to
to intercept
intercept
the
scrapers, aaa 10
10 turn
turn cleaning
cleaning success
success implies
implies
almost
the scrapers,
scrapers,
10
turn
the
success
implies almost
almost
single-turn
collimation
once
particle
intercepts
the
syssingle-turn
collimation
once
particle
intercepts
the
single-turn collimation
particle intercepts the syssystem.
Typical
loss
distributions
around
the
ring
show
88tem.
Typical
loss
distributions
around
the
ring
show
tem. Typical loss distributions around the ring show888890%
of
particles
being
absorbed
directly
in
the
collima90%of
of particles
particles being
being absorbed directly
90%
directly in
in the
the collimacollimators,
3-4%impacting
impacting the
the radiation-hard
radiation-hard quadrupoles,
quadrupoles,
and
tors,3-4%
3-4%
impacting
radiation-hard
tors,
quadrupoles,and
and
about
7%
impacting
the
vacuum
pipe.
Overall,
98%
of
about 7%
7% impacting
impacting the
the vacuum
vacuum pipe.
about
pipe. Overall,
Overall, 98%
98%of
of
all
losses
occur
within
the
collimation
straight
section.
In
alllosses
lossesoccur
occur within
within the
the collimation
collimation straight
all
straightsection.
section.In
In
our
simulations, both
both the
the beam
beam pipe and
and the quadrupoles
quadrupoles
oursimulations,
simulations,
our
both the
beam pipe
pipe and the
the quadrupoles
are treated
as black
black absorbers,
absorbers, whereas
whereas in reality
reality some
are
treated as
as
are
treated
black absorbers,
whereas in
in reality some
some
fraction
of particles
particles landing
landing there
there will
will scatter
scatter into
into downfraction
of
fraction of particles landing there will scatter intodowndownstream collimators.
collimators. Also,
Also, itit should
should be noted
noted that the
the loss
stream
stream
collimators. Also,
it should be
be noted that
that theloss
loss
distribution
depends heavily
heavily on
on the
the vacuum
vacuum pipe
pipe radius
radius
distribution
depends
distribution
depends
heavily
on
the
vacuum
pipe
radius
which
is still
still under
under design;
design; aa conservative
conservative estimate
estimate of
of
which isis
which
still under
design;
a conservative
estimate of
12.0cm
radius
was
used
for
the
simulation.
12.0cmradius
radius was
was used
used for
for the
12.0cm
the simulation.
simulation.
Beam Distribution
Distribution Before
Before and
and After
After Collimation
Collimation
Beam
Beam Distribution Before and After Collimation
Initial
10 Turns
Initial
100
10 Turns
Turns
100 Turns
100
100
90
90
80
80
70
70
60
60
50
50
40
40
30
30
20
20
10
100
0
0
0
5O
50
50
1OO
100
100
15O 200
2OO 250
25O 300
3OO 350
35O 400
4OO 450
45O 500
5OO
150
Emittance
(pi mm-mrad)
mm-mrad)
Emittance
(pi
150
200 250
300 350 400 450 500
Emittance (pi mm-mrad)
FIGURE 2. Beam emittance
emittance profiles before
before collimation
collimation
(solid red line),
after 10
10emittance
turns (dashed
(dashed
blue line),
line),
andcollimation
after 100
100
FIGURE
2. Beam
profiles
before
blue
and
after
turns (dashed
(solid
red line),black
afterline).
10 turns (dashed blue line), and after 100
turns (dashed black line).
171
BEAM
IN
GAP
CLEANING
BEAM
BEAMIN
INGAP
GAPCLEANING
CLEANING
Fraction
FractionCollimated
Collimated
1
1
1
0.8
0.8
0.8
0.6
0.6
0.6
0.4
0.4
0.4
0.2
O.2
0.2
0
0O0
0
dp/p=.007 (Good Beam)
dp/p-.OO7(Good
(Good Beam)
Beam
dp/p=.007
dp/p=010
dp/p=O1O(RF
(RFAcceptance)
Acceptance
dp/p=010
(RF
Acceptance)
dp/p=.020
dp/p=.O2O(Momentum
(MomentumAperture)
Aperture
dp/p=.020
(Momentum
Aperture)
10
10
20
20
30
40
50
3O
4O
5O
30
40
50
Turn
TurnNumber
Number
Turn
Number
60
60
70
70
80
80
FIGURE
3.3. Fraction
Fraction
of
particles
FIGURE3.
Fraction of
of particles
particles lost
lost versus
versus turn
turn for
for three
FIGURE
lost
versus
turn
for
three
different
momentum
spreads:
different momentum
momentumspreads:
spreads: the
thesolid
solid red
red line
line is
for ∆p/p
Ap/p =
=
different
the
solid
red
line
isis for
for
∆p/p
=
0.007,
the
dashed
blue
line
isisfor
0.007,the
thedashed
dashedblue
blueline
lineis
for∆p/p
&p/p =
= 0.010,
0.010, and
andthe
the dashed
dashed
0.007,
for
∆p/p
=
0.010,
and
black
line
isisfor
for
∆p/p
==0.020.
0.020.
blackline
lineis
for∆p/p
Ap/p =
0.020.
black
Impact
ImpactononScrapers
Scrapers
Impact on Scrapers
80
80
60
60
40
40
20
20
0
0
-20
-20
-40
-40
-60
-60
-80
-80-50-50 -40-40 -30-30 -20- 2 0-10
- 1 0 0 0 1010 2020 3030 4040 50 60
-50 -40 -30 -20 -10 X0 [mm]
10 20 30 40 50 60
X[mm]
X [mm]
Y [mm]
Y [mm]
limators
altogether
and
impact
limatorsaltogether
altogetherand
andimpact
impactsecondary
secondary collimators
collimators inlimators
secondary
collimators
instead.
In
our
simulations,
about
75%
of
the
stead.In
Inour
oursimulations,
simulations,about
about 75%
75% of
of the
the gap
stead.
gap particles
particles
initially
impact
the
primary
initially impact
impact the
the primary
primary scraper,
scraper, while
while the
the remaininitially
scraper,
while
the
remaining
25%
overshoot
into
the
secondaries;
no
particles
ing25%
25%overshoot
overshootinto
intothe
thesecondaries;
secondaries; no
no particles
particles iniiniing
initially
impacted
any
of
the
tiallyimpacted
impactedany
any of
of the
the other
other limiting
limiting apertures.
apertures. ParPartially
other
limiting
apertures.
Particles
that
pass
directly
to
ticlesthat
thatpass
passdirectly
directly to
to the
the secondary
secondary collimators
collimators are
are
ticles
the
secondary
collimators
are
less
likely
to
be
absorbed
than
those
that
pass
through
less
likely
to
be
absorbed
than
those
that
pass
through
less likely to be absorbed than those that pass through
the
primary
collimator
first.
theprimary
primarycollimator
collimator first.
first. Figure
Figure 444 shows
shows typical
typical priprithe
Figure
shows
typical
primary
impact
parameters
on
both
the
scrapers
and
on
one
mary
impact
parameters
on
both
the
scrapers
and
on
one
mary impact parameters on both the scrapers and on one
of
the
secondaries.
The
primary
impact
parameters
of
the
secondaries.
The
primary
impact
parameters
on
of the secondaries. The primary impact parameters on
on
the
secondaries
are
large,
thesecondaries
secondariesare
arelarge,
large,but
but not
not as
as large
large as
as they
they would
would
the
but
not
as
large
as
they
would
have
been
had
the
particles
havebeen
beenhad
hadthe
theparticles
particlesbeen
beenscattered
scattered by
by the
the primary
primary
have
been
scattered
by
the
primary
scrapers.
This
leads
to
an
overall
decrease
collimation
scrapers.This
Thisleads
leadsto
toan
anoverall
overalldecrease
decreasecollimation
collimation efefscrapers.
efficiency
for
beam
in
gap
particles
ficiency for
forbeam
beam in
in gap
gap particles
particles compared
compared to
to non-gap
non-gap
ficiency
compared
to
non-gap
particles.
particles.
particles.
Y [mm]
Y [mm]
Unlike
the
halo
beam,
gap
particles
do
not
necessarily
Unlike
Unlikethe
thehalo
halobeam,
beam,gap
gapparticles
particlesdo
donot
notnecessarily
necessarily
have
large
transverse
emittances
and
therefore
will
not
have
large
transverse
emittances
and
therefore
have large transverse emittances and thereforewill
willnot
not
reach
the
collimators
without
some
type
ofofintervention.
intervention.
reach
reachthe
thecollimators
collimatorswithout
withoutsome
sometype
typeof
intervention.
Five
vertical
stripline
kickers
with
cumulative
kicking
Five
Fivevertical
verticalstripline
striplinekickers
kickerswith
withaaacumulative
cumulativekicking
kicking
strength
ofof111mrad
mrad
will
be
used
totodrive
drive
the
gap
beam
strength
strengthof
mradwill
willbe
beused
usedto
drivethe
thegap
gapbeam
beam
into
the
collimation
system.
The
kickers
are
capable
of
into
intothe
thecollimation
collimationsystem.
system.The
Thekickers
kickersare
arecapable
capableof
of
reversing
polarity
on
a
turn-by-turn
basis
according
toto
reversingpolarity
polarityonona aturn-by-turn
turn-by-turnbasis
basisaccording
accordingto
reversing
programmable
polarity
sequence,
and
therefore
they
programmablepolarity
polaritysequence,
sequence,and
andtherefore
thereforethey
they
aaaprogrammable
can
be
made
fire
in
resonance
with
the
vertical
betatron
canbebemade
madefire
fireininresonance
resonancewith
withthe
thevertical
verticalbetatron
betatron
can
tune
ofofthe
the
ring.
To
determine
the
exact
sequence
ofof
tuneof
thering.
ring.To
Todetermine
determinethe
theexact
exactsequence
sequenceof
tune
kicks
for
any
given
tune,
an
initial
kick
of
arbitrary
kicks
for
any
given
tune,
an
initial
kick
of
arbitrary
kicks for any given tune, an initial kick of arbitrary
polarity
imparted
and
the
beam
centroid
isisfollowed
followed
polarityisisisimparted
impartedand
andthe
thebeam
beamcentroid
centroidis
followed
polarity
around
the
phase
space
for
subsequent
turns.
On
each
around
the
phase
space
for
subsequent
turns.
Oneach
each
around the phase space for subsequent turns. On
turn
kicks
are
assigned
with
the
same
polarity
asas the
the
turnkicks
kicksare
areassigned
assignedwith
withthe
thesame
samepolarity
polarity as
the
turn
centroid
momentum.
Though
the
effect
ofofaaakick
kick
on
each
centroidmomentum.
momentum.Though
Thoughthe
theeffect
effectof
kickon
oneach
each
centroid
individual
particle
sensitive
totothe
the
particle’s
phase,
individualparticle
particleisisissensitive
sensitiveto
theparticle’s
particle's phase,
phase,
individual
using
the
centroid
in
this
manner
guarantees
that
the
usingthe
thecentroid
centroidininthis
thismanner
mannerguarantees
guaranteesthat
that the
the
using
net
effect
is
always
to
increase
the
magnitude
of
the
neteffect
effectisisalways
alwaystotoincrease
increasethe
themagnitude
magnitudeofof the
the
net
momentum
ininthe
the
distribution.
momentumin
thedistribution.
distribution.
momentum
For
very
small
tune
spreads,
isisonly
only
necessary
for
Forvery
verysmall
smalltune
tunespreads,
spreads,itititis
onlynecessary
necessaryfor
foraaa
For
sequence
of
kicks
to
follow
the
main
tune
of
the
lattice.
sequence
of
kicks
to
follow
the
main
tune
of
the
lattice.
sequence of kicks to follow the main tune of the lattice.
However,
the
SNS
expects
tune
spreads
on
the
order
However,the
theSNS
SNSexpects
expectstune
tunespreads
spreadson
onthe
the order
order
However,
of
0.15
−
0.2.
Kicking
on
resonance
with
the
bare
tune
of
0.15
—
0.2.
Kicking
on
resonance
with
the
bare
tune
of 0.15 − 0.2. Kicking on resonance with the bare tune
alone
will
serve
to
excite
only
a
small
fraction
of
the
alone
will
serve
to
excite
only
a
small
fraction
of
the
alone will serve to excite only a small fraction of the
beam
in
the
gap.
We
account
for
the
range
of
tunes
beam
in
the
gap.
We
account
for
the
range
of
tunes
beam in the gap. We account for the range of tunes
by
defining
entire
kicking
sequences
for
several
tunes
bydefining
definingentire
entirekicking
kickingsequences
sequencesfor
forseveral
several tunes
tunes
by
within
the
spread
and
apply
them
sequentially
to
excite
within
the
spread
and
apply
them
sequentially
to
excite
within the spread and apply them sequentially to excite
the
entire
beam.
theentire
entirebeam.
beam.
the
The
simulations
included
initial
gaussian
beam
disThesimulations
simulationsincluded
includedinitial
initialgaussian
gaussianbeam
beamdisdisThe
tributions
of
finite
momentum
spread.
The
five
kickers
tributions
of
finite
momentum
spread.
The
five
kickers
tributions of finite momentum spread. The five kickers
were
modeled
with
single,
mrad
kick,
with
kick
poweremodeled
modeledwith
withaaasingle,
single,111mrad
mradkick,
kick,with
withkick
kickpopowere
larity
sequences
defined
by
the
aforementioned
method.
larity
sequences
defined
by
the
aforementioned
method.
larity sequences defined by the aforementioned method.
Second
order
transport
matrices
were
used
toto induce
induce
Secondorder
ordertransport
transportmatrices
matriceswere
wereused
used to
induce
Second
chromatic
tune
spread
in
the
beam.
The
full
baseline
colchromatic
tune
spread
in
the
beam.
The
full
baseline
colchromatic tune spread in the beam. The full baseline collimation
system
was
used,
with
the
variable-aperture
prilimation
system
was
used,
with
the
variable-aperture
prilimation system was used, with the variable-aperture primary
scrapers
set
to
200
π
mm
·
mrad.
For
SNS,
the
anmary
scrapers
set
to
2QQnmm
mrad.
For
SNS,
the
anmary scrapers set to 200π mm · mrad. For SNS, the anticipated
fractional
momentum
spread
for
the
nominal
ticipated
fractional
momentum
spread
for
the
nominal
ticipated fractional momentum spread for the nominal
(non-gap)
beam
∆p/p
==0.007,
the
RF
acceptance
isis
(non-gap)beam
beamisisis∆p/p
A/?//?=
0.007,the
theRF
RFacceptance
acceptanceis
(non-gap)
0.007,
∆p/p
==0.010,
and
the
momentum
aperture
ofofthe
ring
isis
A/?//?
0.010,
and
the
momentum
aperture
the
ring
∆p/p = 0.010, and the momentum aperture of the ring is
∆p/p
Ap/p ==0.020.
0.020.Results
Resultsofofsimulations
simulationswith
withthe
thethree
threedifdif∆p/p = 0.020. Results of simulations with the three different
ferentinitial
initialbeam
beammomentum
momentumspreads
spreadsare
areshown
shownininFigFigferent initial beam momentum spreads are shown in Figure
ure3,3,where
wherethe
thefraction
fractionofofparticles
particleslost
lostisisplotted
plottedover
over
ure 3, where the fraction of particles lost is plotted over
several
severalturns
turnsofofthe
thebeam.
beam.For
Foreach
eachofofthe
thethree
threecases,
cases,the
the
several turns of the beam. For each of the three cases, the
entirebeam
beamisisabsorbed
absorbedininunder
undersixty
sixtyturns
turnsaround
aroundthe
the
entire
entire beam is absorbed in under sixty turns around the
ring.
ring.
ring.
Theefficiency
efficiency ofofthe
thebeam
beaminin gap
gapcleaning
cleaning process
process
The
The efficiency of the beam in gap cleaning process
fundamentally limited
limitedby
bythe
the collimation
collimation efficiency.
efficiency.
isisfundamentally
is fundamentally limited by the collimation efficiency.
Gapparticles
particlesthat
thatarrive
arriveatatthe
the primary
primary collimator
collimator bebeGap
Gap particles that arrive at the primary collimator beforeany
anyother
otheraperture
aperturewill
willbe
be collimated
collimated inin the
the same
same
fore
fore any other aperture will be collimated in the same
mannerasasthe
thenon-gap
non-gapbeam.
beam.However,
However,the
the drift
drift speed
speed
manner
manner
as
the in
non-gap
However,
the
speed
thebeam
beam
thegap
gapbeam.
much
greaterthan
thandrift
the anticianticiofofthe
in the
isismuch
greater
the
ofpated
the beam
in the
is much
greater than
the anticinon-gap
halogap
drift,
andtherefore
therefore
possible
for
pated non-gap
halo
drift,
and
ititisispossible
for
pated
non-gap
halo
drift,
and
therefore
it
is
possible
for
particlesofofparticular
particularphases
phasestotobypass
bypassthe
theprimary
primarycolcolparticles
particles of particular phases to bypass the primary col-
Fraction of
of Beam Collimated
vs. Turn
Fraction
Fraction ofBeam
BeamCollimated
Collimatedvs.
vs. Turn
Turn
Impact
ImpactononS2S2
Impact on S2
80
80
60
60
40
40
20
20
0
0
-20
-20
-40
-40
-60
-60
-80
-80-50 I -40
-40 -30
-30 -20
-20 -10
-10 0 0 1010 2020 3030 4040 5050 60
-50 -40 -30 -20 -10 X0 [mm]
10 20 30 40 50 60
X[mm]
X [mm]
FIGURE
FIGURE 4.4. The
The figure
figure on
on the
the left
left shows
shows primary
primary impact
impact
FIGURE 4. The figure on the left shows primary impact
parameters
parametersof
ofthe
thebeam
beamininthe
thegap
gapon
on the
the four
four primary
primary scrapers,
scrapers,
parameters of the beam in the gap on the four primary scrapers,
and
andthe
thefigure
fi gureon
onthe
theright
rightshows
showsthe
theprimary
primaryimpact
impactparameters
parameters
and the figure on the right shows the primary impact parameters
one
oneofofthe
thesecondary
secondaryabsorbers.
absorbers.
one of the secondary absorbers.
Thetwo
twoparameters
parametersthat
thatcontrol
controlthe
the distribution
distribution of
of imimThe
The two parameters that control the distribution of impacts among
among the
the collimators
collimators are
are the
the primary
primary collimator
collimator
pacts
pacts among the collimators are the primary collimator
aperture and
and the
the kicker
kicker strength.
strength. The
The primary
primary aperture
aperture
aperture
aperture and the kicker strength. The primary aperture
fixed by
bythe
thenon-gap
non-gapbeam
beam distribution
distribution and
and can
can not
not be
be
isisfixed
is fixed by the non-gap beam distribution and can not be
variedtotoaccomodate
accomodatethe
thebeam
beamin
inthe
thegap.
gap. The
The other
other varivarivaried
varied to accomodate the beam in the gap. The other variableisiskicker
kickerstrength,
strength,which
whichdetermines
determinesthe
the drift
drift velocvelocable
able is kicker strength, which determines the drift velocityof
ofthe
thegap
gapbeam.
beam.A
A decrease
decrease in
in kicker
kicker strength
strength would
would
ity
ity of the gap beam. A decrease in kicker strength would
correspondto
toaaslower,
slower,more
morecontrolled
controlled halo
halo drift;
drift; howhowcorrespond
correspond
to a slower,
more
controlled halo
drift;
however,
this
could
also
lead
to
considerably
slower
beam
in
ever, this could also lead to considerably slower beam in
ever,
this could also lead to considerably slower beam in
gapcleaning.
cleaning.
gap
gap cleaning.
172
CONCLUSIONS
A new collimation simulation package embedded in
a particle-in-cell environment has allowed us to perform detailed studies of the SNS collimation system and
beam-in-gap cleaning process. Results of the work have
led to optimization of collimator location and scraper
thickness. Additionally, we have shown that the collimation system can be used with a set of fast kickers to clean
the beam in the gap before the rise of the extraction kickers.
This work is supported by SNS through UT-Battelle,
LLC, under contract DE-AC05-OOOR22725 for the U.S.
Department of Energy. SNS is a partnership of six
national laboratories: Argonne, Brookhaven, Jefferson,
Lawrence Berkeley, Los Alamos, and Oak Ridge.
REFERENCES
1. See K2 code description in N. Catalan-Lasheras,
"Transverse and Longitudinal Beam Colliamtion in a
High-Energy Proton Collider" PhD Thesis, University of
Zaragoza, Spain (1999).
2. J. Galambos et al., ORBIT User's Manual, SNS/ORNL/AP
Technical Note Oil, 1999.
3. N. Catalan-Lasheras et. al., "Optimization of the
collimation system for the Spallation Neutron Source
accumulator ring", Phys. Rev. ST Accel. Beams 4, 010101
(2001)).
173