Wide Dynamic-Range
Beam-Profile Instrumentation for
Dynamic-Range Beam-Profile
a Beam-Halo
Beam-Halo Measurement: Description and Operation
Operation*•
∗
J. Douglas
O'Hara*,
Douglas Gilpatrick*,
Gilpatrick∗, D. Barr*,
Barr∗, L. A. Day*,
Day∗, D. M. Kerstiens*,
Kerstiens∗, J. F. O’Hara
, M.
∗
∗
#
!
Stettler , R. Valdiviez , M. Gruchalla , J. H. Kamperschroer
Kamperschroer'
&
&
J£
f
∗
* Los Alamos National Laboratory, MS H808, LANL, Los Alamos,
Alamos, NM, 87545
87545
#
Honeywell FM&T/NM,
Albuquerque, NM, 87185
Honeywell
FM&TINM, PO Box 5250, Albuquerque,
87185
!
General Atomics, Los Alamos, NM, 87544
Abstract. Within the halo experiment conducted at the Low Energy Demonstration Accelerator (LEDA)
(LEDA) at
at Los
Los Alamos
Alamos
National Laboratory, specific beam instruments that acquire horizontally and vertically projected particle-density
distributions out to approximately 105:1 dynamic range are located throughout the 52-magnet halo lattice. We measure
the core of the distributions
distributions using traditional
traditional wire scanners, and the tails of the distribution using water-cooled graphite
the
scraping devices. The wire scanner and halo scrapers are mounted on the same moving frame whose location is
controlled with stepper motors. A sequence within the Experimental Physics and Industrial Control System (EPICS)
controlled
(EPICS)
software communicates with a National Instruments LabVIEW
software
Lab VIEW virtual instrument
instrument to control the motion
motion and
and location
location of
of
electrons from the wire scanner 0.033-mm carbon wire and protons impinging
the scanner/scraper assembly. Secondary electrons
on the scraper are both detected with a lossy-integrator electronic circuit. Algorithms implemented within EPICS and
and in
in
System’s Interactive Data Language subroutines analyze and plot the acquired distributions. This paper
Research System's
describes this beam profile instrument and describes our experience with its operation.
describes
INTRODUCTION
At LEDA a 100-mA, 6.7-MeV beam is injected
into a 52-quadrupole magnet lattice (see Fig. 1).
Within this 11-m FODO lattice, there are nine wire
scanner/halo scraper (WS/HS) stations, five pairs of
steering magnets and beam position monitors, five loss
steering
monitors, and three pulsed-beam current monitors [1].
monitors,
instrument’s purpose is to measure the
The WS/HS instrument's
beam’s transverse projected distribution.
These
beam's
measured distributions must have sufficient
sufficient detail to
understand beam halo resulting from upstream lattice
mismatches [2,3]. The first
first WS/HS station, located
mismatches
after
the
fourth
quadrupole
magnet, verifies the beam's
beam’s
after
fourth
characteristics after
after the RFQ exit. A cluster
transverse characteristics
four WS/HS located after
of four
after magnets #20, #22, #24,
and #26 provides phase space information after the
debunched. After
After magnets #45,
#45, #47,
#47, #49,
#49,
beam has debunched.
final four WS/HS stations. These
and #51 reside the final
four WS/HS acquire projected beam distributions
four
under both matched and mismatched conditions.
These conditions
conditions are generated by adjusting
adjusting the firstfirstfour quadrupole magnetic fields so that the RFQ
four
output beam is matched or mismatched in a known
fashion to the rest of the lattice.
lattice. Because the halo
fashion
periods to fully
fully develop, this final
takes many lattice periods
final
positioned to be most sensitive
cluster of WS/HS are positioned
to halo generation.
FIGURE 1. The 11-m,
11-m, 52-magnet FODO lattice
lattice
includes nine WS/HS stations that measure the beam’s
beam's
transverse projected distributions.
As the RFQ output beam is mismatched to the
lattice, the WS/HS actually observe a variety of
distortions to a properly matched Gaussian-like
distribution [2,3].
These distortions appear
[2,3].
appear as
as
distribution tails or backgrounds. It is the size, shape,
and extent of these tails that predict specific
specific types
types of
of
halo. However, not every lattice WS/HS observes the
halo generated in phase space
space because the
the resultant
distribution tails may be hidden from the projection’s
projection's
view. Therefore, multiple WS/HS are used to observe
the various distribution tails.
" Work supported by the US Department of Energy
•
Work supported by the US Department of Energy
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
108
The target position, as defined by the WS/HS
The target
position,
by the screen
WS/HSvia
operator,
is relayed
from as
thedefined
EPICS control
operator,
is
relayed
from
the
EPICS
control
screen
via
a database process variable to a National Instruments
aLabVIEW
database process
variable
to
a
National
Instruments
Virtual Instrument (VI). The VI also
LabVIEW
Virtual
Instrument
also
calibrates the
relative
position (VI).
of the The
linearVIencoders
calibrates
the
relative
position
of
the
linear
encoders
based on the measured position of the limit switches,
based on the measured position of the limit switches,
and provides some error feedback information. The
and provides some error feedback information. The
total error between the target wire position and the
total error between the target wire position and the
actual wire position attained is within a total 4% range
actual wire position attained is within a total 4% range
of
a typical 1-mm rms-width beam.
of a typical 1-mm rms-width beam.
WS/HS DESCRIPTION
WS/HS DESCRIPTION
Each station consists of a horizontal and vertical
Eachassembly
station consists
horizontal
anda vertical
actuator
(see Fig.of2)a that
can move
33-|imactuator
assembly
(see
Fig.
2)
that
can
move
a 33-µmcarbon monofilament and two graphite/copper
scraper
carbon monofilament
two graphite/copper
scraper
sub-assemblies.
The and
carbon
wire and scrapers
are
sub-assemblies. The carbon wire and scrapers are
connected
to the same movable frame. Attached to
connected to the same movable frame. Attached to
this movable frame is a linear encoder that provides
this movable frame is a linear encoder that provides
the wire and scraper edges' relative position to within
the wire and scraper edges’ relative position to within
aatypical
rms error of 5 |im, and an additional linear
typical rms error of 5 µm, and an additional linear
potentiometer
approximate
potentiometer provides
provides an
an absolute
absolute approximate
position
for
LEDA's
run
permit
systems.
A stepper
stepper
position for LEDA’s run permit systems. A
motor
coupled
to
a
ball
lead
screw
is
used
to
drive
the
motor coupled to a ball lead screw is used to drive the
moveable
frame.
A
motor
brake
along
with
moveable frame.
A motor brake along with
microswitches
microswitcheslimit
limitthe
theframe's
frame's movement.
movement.
Asthe
thewire
wireisismoved
movedthrough
throughthethebeam,
beam,it itsenses
senses
As
the
projected
beam
core
distribution.
A
small
portion
the projected beam core distribution. A small portion
of the
thebeam’s
beam'senergy
energyisisimparted
impartedtotothethewire
wirecausing
causing
of
secondary
electron
emission
to
occur.
The
secondary
secondary electron emission to occur. The secondary
electrons leaving
leavingthe
thewire
wireare
arereplaced
replacedbybynegative
negative
electrons
chargeflowing
flowingfrom
fromthe
theelectronics.
electronics.This
Thiscurrent
current
flow
charge
flow
for both
bothaxes
axesisisconnected
connectedthrough
througha bias
a biasbattery
battery
for
to to
anan
electroniclossy
lossyintegrator
integratorcircuit
circuitand
andfollowed
followedbyby
electronic
anan
amplification
stage.
amplification stage.
The integrator
integratorcapacitance
capacitanceand
andamplifier
amplifiergain
gainareare
The
set toto allow
allow a a very
very wide
wide range
rangeofofvalues
valuesof of
set
accumulatedcharge.
charge. Data
Dataare
areacquired
acquiredbybydigitizing
digitizing
accumulated
the
at at
theaccumulated
accumulatedcharge
chargethrough
throughthe
thelossy
lossyintegrator
integrator
two
two different
different times
times within
withinthe
thebeam
beampulse.
pulse. This
This
charge
charge difference,
difference, acquired
acquiredbybysubtracting
subtractingthethetwo
two
values
values ofof charge,
charge,provides
providesa alow
lownoise
noisemethod
methodof of
relative
relative beam
beam charge
charge acquisition.
acquisition. The
Thewire
wireandand
scraper
accumulated
charge
signals
are
digitized
using
scraper accumulated charge signals are digitized
using
12and
14-bit
digitizers,
respectively.
The
analog
12- and 14-bit digitizers, respectively. The analog
noise
noisefloor
floorhas
hasbeen
beenmeasured
measuredtotobebe0.03
0.03pC,
pC,a noise
a noise
level
noise
levelslightly
slightlylower
lowerthan
thanthe
thescraper
scraperdigital
digitalLSB
LSB
noise
level
within
levelofof0.15
0.15pC
pCusing
usingthe
thehighest
highestgain
gainsettings
settings
within
the
thedetection
detectionelectronics.
electronics.
FIGURE
contains aa
FIGURE 2.2. The
The WS/HS
WS/HS assembly contains
movable
carbon wire
wire
movable frame
frame on
on which
which a 0.033-mm carbon
residesbetween
betweentwo
twowater-cooled
water-cooled graphite scrapers.
resides
scrapers.
Thecarbon
carbon wire,
wire, which
which senses the beam's
beam’s core,
The
core, isis
cooledby
bythermal
thermal radiation.
radiation. If
If the beam macropulse
cooled
macropulse
toolong,
long,the
thewire
wiretemperature
temperature continues above
isistoo
above 1800
1800
K
resulting
in
the
onset
of
thermionic
K resulting in the onset of thermionic emission.
emission.
Thermionicemission
emission causes
causes an
an inaccurate
inaccurate appearance
Thermionic
appearance
thedistribution
distribution by
by exaggerating
exaggerating the
the core's
totothe
core's current
current
density. To
To eliminate
eliminate these
these effects
effects for
density.
for the
the halo
halo
experiment, the
the maximum
maximum pulse
pulse length
length and
experiment,
and repetition
repetition
rate isis limited
limited to
to approximately
approximately 30
30 µs
rate
|is and
and 11 Hz,
Hz,
respectively.
respectively.
The
The front-end
front-end electronic
electroniccircuitry,
circuitry,mounted
mountedonona a
daughter
printed
circuit
board,
is
connected
daughter printed circuit board, is connectedto toa a
motherboard that has all of the necessary interface
motherboard that has all of the necessary interface
electronics to communicate with EPICS via a
electronics to communicate with EPICS via a
controller module within the same electronics crate. A
controller module within the same electronics crate. A
software state machine sequence was written within
software state machine sequence was written within
EPICS to control and operate WS/HS instrumentation.
EPICS
control and
operate
instrumentation.
The statetomachine
instructs
theWS/HS
VI to move
the wire
The
state
machine
instructs
the
VI
to
the wire
and scraper to a specific location, acquire move
synchronous
and scraper data
to a specific
location,
distribution
from either
the acquire
wire orsynchronous
scraper,
distribution
dataroutine
from to
either
the wire
scraper,
trigger
the IDL
normalize
the oracquired
trigger
the
IDL
routine
to
normalize
the
acquired
charge with a nearby toroidal current measurement,
chargethewith
a nearby
toroidal
current
measurement,
graph
normalized
data,
and write
the distribution
to
graph
the
normalized
data,
and
write
the
distribution
a file. The sequence also instructs IDL to calculate the to
a file.through
The sequence
instructs IDL
first
fourthalso
moments,
fit to
a calculate
Gaussianthe
first through
moments,
fit calculate
a Gaussian
distribution
to thefourth
wire scanner
data, and
the
distribution
to the
data, disappears
and calculate
point
at which
the wire
beamscanner
distribution
intothe
point
at whichnoise.
the beam distribution disappears into
the
background
Thehalo
haloscrapers
scrapers are
are composed
composed of
of aa 1.5-mm
The
1.5-mm thick
thick
graphite plate brazed to a water-cooled 1.5-mm thick
graphite plate brazed to a water-cooled 1.5-mm thick
copper plate. Since 6.7-MeV protons average range in
copper plate. Since 6.7-MeV protons average range in
carbon is approximate 0.3 mm, the beam is completely
carbon is approximate 0.3 mm, the beam is completely
stopped within the graphite plate.
Cooling via
stopped
within the graphite plate. Cooling via
conduction lowers the average temperature of the
conduction
lowers theand
average
scraper sub-assembly
allowstemperature
the scraper of
to the
be
scraper
sub-assembly
and
allows
the
scraper
to be
cooled more rapidly than the wire. The lower average
cooled
more rapidly
than
the wire.
The
average
temperature
and faster
cooling
allows
thelower
scraper
to be
temperature
andfar
faster
allows the
to be
driven in as
as cooling
2 rms widths
fromscraper
the beam
driven
in
as
far
as
2
rms
widths
from
the
beam
distribution peak without the peak temperature
distribution
peak1800
without
the peak temperature
increasing above
K.
increasing above 1800 K.
The movement and positioning of each wire and
The movement
and positioning
of control
each wire
and
scraper
pair is controlled
by a motion
system
scraper
pair
is
controlled
by
a
motion
control
system
that contains a stepper motor, stepper motor controller,
that
contains
a stepper
stepper motor
a linear
encoder,
andmotor,
an electronic
driver controller,
amplifier.
aThe
linear
encoder, digital
and anPID
electronic
driver the
amplifier.
controller’s
loop controls
speed
The
loop controls
the speed
and controller's
accuracy at digital
which PID
the assembly
is moved
and
and
accuracy at which the assembly is moved and
placed.
the background noise.
To plot the complete beam distribution for each
complete
beam
distribution
each
axis, To
the plot
wirethe
scanner
and two
scraper
data setsfor
must
axis,
the wire
scanner and
scraper
data analysis
sets must
be
joined.
To accomplish
thistwo
joining,
several
placed.
be joined. To accomplish this joining, several analysis
109
wire detection signal. Furthermore, it appears that the
tasks are performed on the wire and scraper data
tasks are scraper
performed
wire and
scraper data
including,,
dataonarethe
spatially
differentiated
and
including,,
scraper
data
are
spatially
differentiated
and
averaged, wire and scraper data are acquired with
averaged, spatial
wire and
scraperanddata
are acquiredscraper
with
sufficient
overlap,
differentiated
sufficient
spatial
overlap,
and
differentiated
scraper
data are normalized to the wire beam core data.
wire
signal. Furthermore,
it appears
that the
wiredetection
collects positive
ions with < -25
V bias potentials
wire
collects
positive
ions
with
<
-25
V
bias
potentials
well after the beam pulse. This ion collection
well
after the
beam
pulse. ofThis
ion collection
additionally
limits
the amount
negative
bias that is
additionally
limits
the
amount
of
negative
biasemission
that is
applied to the wire for proper secondary
applied
to
the
wire
for
proper
secondary
emission
operation.
operation.
data are normalized to the wire beam core data.
The scraper data need only be normalized in the
The scraper data need only be normalized in the
relative charge axis since the distances between each
relative charge axis since the distances between each
wire and scraper edge are known to within 0.25-mm.
wire and scraper edge are known to within 0.25-mm.
InIn addition,
the first four moments and the point at
addition, the first four moments and the point at
which
the
beam
into the
the noise
noise
which the beam distribution
distribution disappears
disappears into
are
also
calculated
for
the
combined
distribution
data.
are also calculated for the combined distribution data.
The scraper detection goal is to inhibit secondary
The scraper detection goal is to inhibit secondary
emission and detect only 6.7-MeV protons. With
emission and detect only 6.7-MeV protons. With
approximately +25 V bias applied to the scraper, the
approximately +25 V bias applied to the scraper, the
secondaryemission
emissionisisalmost
almostentirely
entirelyinhibited
inhibitedand
and
secondary
thethe
net
current
reduces
to
the
nominal
proton
current.
net current reduces to the nominal proton current.
ACQUIRED
ACQUIRED DISTRIBUTIONS
DISTRIBUTIONS
SUMMARY
SUMMARY
Fig.
#26 with
with some
some
Fig. 33 shows
shows data
data from
from WS/HS
WS/HS #26
slight
the field
field
slight mismatch
mismatch generated
generated by
by increasing
increasing the
above
quadrupole
abovenominal
nominal by
by 5%
5% in
in the
the first
first matching
matching quadrupole
magnet.
These
profiles show
show
magnet.
These typical
typical WS/HS
WS/HS profiles
55:1 and
distributions
with
a
dynamic
range
of
~
10
distributions with a dynamic range of ~ 10 :1 and
provide
> 5X
5X to
to 7X
7X times
times
provide distribution
distribution information
information to
to >
typical
calculated rms
rms
typical rms
rms widths
widths of
of the
the beam. The calculated
widths
widths are
are 1.10
1.10 and
and 1.13 mm
mm for the horizontal and
vertical
verticaldistributions,
distributions, respectively.
respectively.
wire scanner
scanner and
and halo
halo scraper
scraper have
havebeen
been
AA wire
integrated into
intoaabeam
beamprofile
profileinstrument
instrumentcapable
capableofof
integrated
5
105:1
:1 dynamic
dynamicrange.
range.This
ThisWS/HS
WS/HScombination
combinationwas
was
10
used
extensively
to
acquire
wide
dynamic
rage
data
used extensively to acquire wide dynamic rage data in in
order toto understand
understand beam
beam halo
halo generation.
generation. The
The
order
scanner
and
scraper
V-I
curves
show
that
the
wire
and
scanner and scraper V-I curves show that the wire and
scraper are
are optimally
optimally biased
biasedatat–12
-12 VVand
and+25
+25V,V,
scraper
respectively.
respectively.
Relative Charge
10 0
10 -1
10 -2
10 -3
10 -4
10 -5
10 -6
-15
REFERENCES
REFERENCES
Horizontal
Veritcal
-10
-10
--5
5
5 00
5
Position (mm)
10
10
D.Gilpatrick,
Gilpatrick,etetal.,
al.,"Experience
"Experiencewith
withthetheLow
LowEnergy
Energy
1.1. J.J.D.
Demonstration Accelerator
Accelerator(LEDA)
(LEDA)Halo
HaloExperiment
Experiment
Demonstration
Beam
Beam Instrumentation,”
Instrumentation," Proceedings
Proceedings ofof thethe 2001
2001
Particle
Particle Accelerator
Accelerator Conference,
Conference, June
June18-22,
18-22,2001,
2001,
pp.2311-2313.
pp.2311-2313.
2.2. T.
T. Wangler,
Wangler,etetal.,
al.,“Linac
"LinacBased
BasedProton
ProtonDrivers,”
Drivers,"this
this
workshop.
workshop.
15
15
3.3. P.P. L.L.Colestock,
Colestock,etetal.,
al.,“Measurement
"Measurementofofa aBeam
BeamHalo
Halo
Generation
this
GenerationininananIntense
IntenseProton
ProtonBeam,”
Beam,"
thisworkshop.
workshop.
FIGURE 3.
3. WS/HS
WS/HS distributions,
distributions, such
FIGURE
such as
as5 #26
#26
shownhere,
here,have
have aa typical
typical dynamic
dynamic range
range of
shown
of >> 10
105:1.
:1.
4.4. J.J.D.
D.Gilpatrick,
Gilpatrick,etetal.,
al.,”Biasing
"BiasingWire
WireScanners
Scannersand
andHalo
Halo
Scrapers
Scrapers for
forMeasuring
Measuring6.7-MeV
6.7-MeVProton-Beam
Proton-BeamHalo,”
Halo,"
Proceedings
Proceedings ofof the
the 2002
2002 Beam
Beam Instrumentation
Instrumentation
Workshop
Workshop held
heldatatBrookhaven
BrookhavenNational
NationalLaboratory,
Laboratory,onon
May
May6-9,
6-9,2002.
2002.
WIRE AND
AND SCRAPER
SCRAPER PHYSICS
WIRE
PHYSICS
The WS
WS wire
wire is
is biased
biased negative
negative to
The
to optimize
optimize
secondary
emission
(S.E.)
yield,
where
the
secondary emission (S.E.) yield, where the yield
yield isis
defined as
as the
the ratio
ratio of
of the
the emitted
emitted secondary
secondary electron
defined
electron
current and
and the
the proton
proton beam
beam current
current intercepted
current
intercepted by
by the
the
wire.
All
of
the
wires
in
the
halo
lattice
WS
wire. All of the wires in the halo lattice WS are
are
configured with a 33-µm, carbon monofilament. The
configured with a 33-|im, carbon monofilament. The
HEBT WS is configured with a 100-µm SiC wire.
HEBT WS is configured with a 100-|im SiC wire.
The choice of bias potential was determined by
The choice of bias potential was determined by
measuring the wire and scraper currents as a function
measuring the wire and scraper currents as a function
of bias potential. The resulting data showed that the
ofwire
biasis potential.
The resulting
dataVshowed
the
optimally biased
at -6 to -12
and the that
scraper
wire
is
optimally
biased
at
-6
to
-12
V
and
the
scraper
is optimally biased at +20 to +30 V [4].
is optimally biased at +20 to +30 V [4].
As the wire bias is positively increased from 0 V to
theV,
wire
is positively
from 0 V is
to
> As
+100
thebias
wire
secondary increased
electron emission
>inhibited
+100 V,
the
wire
secondary
electron
emission
and the net wire current reduces to very nearis
inhibited
and
the net wire
current
reduces
very near
zero. As expected,
a large
positive
bias to
reduces
the
zero. As expected, a large positive bias reduces the
110