lr
The Mechanical Design and Preliminary Testing
Results of Beam Position Monitors for the
LANSCE Isotope Production Facility and
Switchyard Kicker Projects
J. F. O'Hara, J. D. Gilpatrick, J. E. Ledford, R. B. Shurter, R. J. Roybal,
B. E. Bentley
Los Alamos National Laboratory
Los Alamos, NM 87545
Abstract. The Los Alamos Neutron Science Center (LANSCE-1) Beam Diagnostic Team is
providing Beam Position Monitors (BPMs) to the LANSCE Facility for use in two on-going
projects: The Isotope Production Facility (IPF) and The Switchyard Kicker Upgrade (SYK).
The BPM designs for both projects are very similar. The BPMs are classic, four, micro-stripline
units having one end terminated in a 50-ohm load. This paper will discuss the position
measurement requirements, mechanical design, fabrication, and alignment issues encountered
for both sets of BPMs, as well as report the results obtained from the initial taught wire testing of
the IPF BPMs.
INTRODUCTION
The Los Alamos Neutron Science Center (LANSCE) is currently in the process of
implementing two different projects to upgrade the 800 MeV accelerator facility's
capabilities. The two projects are the construction of the Isotope Production Facility
and the Switchyard Kicker Project upgrade. Both of these projects require Beam
Position Monitors (BPMs), which will be provided by the LANSCE-1 Beam
Diagnostic Instrumentation Team (BDIT). The IPF projects BPMs have been
fabricated and tested, four of the eight required units have been installed during the
recent LANSCE maintenance outage. The Switchyard Kicker BPMs are in the
process of being fabricated. The Switchyard Kicker mechanical design is based on the
IPF design with some intended improvements.
Isotope Production Facility
Radioisotopes can be introduced into the body where their absorption by different
organs can be detected and used for diagnosis and treatment of diseases [1]. LANSCE
has been producing radioisotopes for the nation's health program for over 20 years,
and it is essential that Los Alamos National Laboratory along with other isotope
1
This work supported by the US Department of Energy.
CP648, Beam Instrumentation Workshop 2002: Tenth Workshop, edited by G. A. Smith and T. Russo
© 2002 American Institute of Physics 0-7354-0103-9/02/$19.00
305
production facilities at Brookhaven National Laboratory, TRIUMPF (Vancouver,
Canada), Institute of Nuclear Research (Troitsk, Russia), National Accelerator Centre
(Faurve, South Africa), and Paul Scherrer Institute (Villigen, Switzerland) continue to
deliver a dedicated year-round supply of key medical isotopes. In order to help meet
this demand, a new facility is being built at LANSCE that consists of a new beam line
leading to a new target irradiation area (located below ground), and a new target
equipment handling area (above ground). The new beam line will contain eight
BPMs, seven four-inch ID and 1 six-inch ID, whose main purpose is to ensure proper
beam/target interaction.
Switchyard Kicker Project
Typical LANSCE operation [2], [3], consists of two simultaneously accelerated
beams using alternating cycles of the linac RF. Two 0.75 MeV Cockroft-Walton
injectors are used; one supplies protons and the other supplies H" ions. Beams are
accelerated up to 800 MeV. The beam switchyard is used to direct the two beams
from the linac to the different experimental areas. One problem with the existing
switchyard configuration is that it is not currently possible to deliver simultaneous
beam to experimental areas served by lines D and X. It requires a few hours in order
to reconfigure beam delivery from one line to the other and during this time delivery
of beam to all users is interrupted.
The Switchyard Kicker project involves removing existing unused magnets and
replacing them with a kicker system consisting of two C-magnet benders, two pulsed
kicker magnets, and assorted beam diagnostics including three, four inch ID, BPMs.
The purpose of the BPMs is to monitor and track the kicker system operation by
acquiring the H" beam's position as the beam is switched between lines D and X. The
Switchyard Kicker BPMs are placed such that they will only see accelerated H"
particles.
BEAM POSITION MONITORS
Position Measurement Requirements
For both facilities, the overall position measurement requirements are similar with
some differences. As Table 1 shows, the beamlines transport H+ (IPF) and H" (SYK)
beams, their dynamic range, and timing constraints are also somewhat different. The
Switchyard Kicker BPM has various chopped beam structures that required the beam
position measurement to detect and monitor the chopped beam's position with as few
as 10 to 20 beam bunches per chopped beam pulse. Whereas the IPF will be both
tuned and operated with no chopped beam pattern. However, due to the tuning and
operating procedures both have similar range of detected beam position and dynamic
range requirements. Finally, one additional but no less important requirement was
added. Both sets of instruments must have an online method of both unambiguously
verifying the instrumentation's operational health and maintaining the electronics
306
calibration. For this reason, the calibration and operational verification method, first
applied in the LEDA beam position measurements, was adopted. This method
allowed the operators to initiate a calibration of the electronics processors from the
control room and to verify that the cable and BPM electrode are working in a known
and unambiguous manner. This additional cable and electronics verification step is
performed by injecting a signal into the downstream termination and acquiring the
know attenuation through the electrodes and full cable plant. Further detail of the
processing electronics and calibration and verification processes will be discussed in a
later paper.
Table 1. Overall position measurement requirements.
Measurement or Beam Parameter
IPF
Accelerated Beam Species
Beam Repetition Rate (Hz)
Data Acquisition rate (Hz)
Macropulse length (ms)
Chopped Beam Rate (MHz)
Chopped Beam Duty Factor (%)
Macropulse Beam Current (mA)
Bunching Frequency (MHz)
Position Measurement Range (% pipe radius)
Position Measurement Dynamic Range (dB)
Bandwidth (kHz)
Position Precision (% pipe radius)
Beam Pipe Radius (mm)
H+
30 (possibly 1 to 120)
ItolO
0.05 to 1
No chopped beam
No chopped beam
16.5 to 0.1
201.25
50
63
15
0.3
50.4 and 76.2
Switchyard
Kicker
H1 to 100
ItolO
0.15 to 1.225
2.8 and 5.6
22 to 56
13tol
201 .25
50
-70
-2500
0.25
50.4
IPF BPM Mechanical Design
The BPMs consist of four micro-stripline units having one end terminated in a 50ohm load, two LANSCE style modified flanges, and the center-body housing. Figure
1 is a photograph of the fabricated IPF BPMs. Two different size BPMs were built for
IPF, a four-inch ID and a six-inch ID. There will be a quantity of 7, four inch ID
BPMs and 1, six inch ID BPM used in the IPF line. The six-inch ID BPM is required
where the beam is being expanded as it approaches the target.
The BPM is fabricated in two separate sub-assemblies, which are joined at the final
assembly step. The first sub-assembly is the housing. This consists of the BPM
center-body and flanges, all fabricated from 316-stainless steel. The BPMs are located
near quadrupole magnets and therefore are desired to be non-magnetic so as to not
interfere with the quadrupoles magnetic fields. 316-stainless steel was chosen because
it possess good non-magnetic stability during cold work [4]. The flanges are initially
fabricated in a rough machined condition. One flange is slightly different from the
other. The BPM has four flats cut into the downstream flange's outer-diameter; each
flat also receives a 0.250" hole for an alignment target. The flats and holes are later
used in the characterization and alignment of the BPM. Some material is left on the
sealing face so that later it can be removed once the flanges have been welded onto the
307
center-body.
center-body. This
Thisfinal
final machining
machiningstep
stepallows
allowsthe
the sub-assembly
sub-assembly to
to be
be brought
brought within
within
dimensional
dimensionaltolerance
toleranceafter
afterany
anydistortion
distortiondue
duetotothe
the welding
welding operation.
operation. The
The shape
shape in
in
the
the center-body
center-body was
was cut
cut out
out using
using aa wire
wire EDM
EDM (electrical
(electrical discharge
discharge machining)
machining)
technique.
technique. The
The flanges
flanges themselves
themselves are
are aa modified
modified version
version of
of the
the standard
standard LANSCE
LANSCE
flange.
This
style
of
flange
utilizes
an
aluminum
wire
sandwiched
flange. This style of flange utilizes an aluminum wire sandwiched between
between mating
mating
flanges
flangesasasthe
thevacuum
vacuumsealing
sealingmechanism.
mechanism.
FIGURE
FIGURE1.1. Fabricated
Fabricated4”
4"ID
IDand
and6”
6"ID
IDIPF
IFFBPMs.
BPMs.
The
Thecenter-body
center-bodyhousing
housingand
andstripline
striplineelectrode
electrodegeometry
geometry were
were optimized
optimized to
to give
give
aamatched
50-ohm
impedance
[5].
The
electrodes
are
recessed
1
mm
from
the
bore
matched 50-ohm impedance [5]. The electrodes are recessed 1 mm from the bore of
of
the
theflange
flange for
fortwo
two reasons.
reasons. The
The first
first reason
reason is
is to
to protect
protect the
the electrode
electrode from
from stray
stray beam
beam
impingement.
impingement.The
Thesecond
secondreason
reasonisis to
to minimize
minimizethe
the signal
signal coupling
coupling between
between adjacent
adjacent
electrodes.
electrodes.
The
The electrode
electrode sub-assembly
sub-assembly units
units consist
consist of
of aa 316
316 stainless
stainless steel
steel electrode
electrode 0.030
0.030
inches
inchesthick
thickconnected
connectedtototwo
twoKAMAN
KAMAN[6]
[6]UHV
UHVmicrowave
microwavefeed
feed throughs.
throughs. The
The feed
feed
throughs
throughsare
areattached
attachedtotoaatransition
transitionpiece
piecethat
thatisisininturn
turn welded
welded into
into the
the BPM
BPM centercenterbody.
body. Figure
Figure 22 shows
shows the
the electrode
electrode sub-assembly.
sub-assembly. The
The process
process for
for assembling
assembling the
the
electrode
electrodeisis toto first
first weld
weld the
the transition
transition piece
piece to
to each
each of
of the
the individual
individual feed
feed throughs.
throughs.
The
The KAMAN
KAMAN feed
feed throughs
throughs have
have aa borosilicate
borosilicate strengthened
strengthened glass
glass seal
seal that
that has
has aa
temperature
temperature limitation
limitation ofof 300
300 °C.
°C. Care
Care must
must be
be taken
taken during
during welding
welding to
to avoid
avoid
overheating
overheatingthe
thefeed
feedthrough,
through,damaging
damagingthe
theseal,
seal,and
andcausing
causingaavacuum
vacuumleak.
leak. The
Thenext
next
step
stepisistotoattach
attachthe
thefeed
feed through
through center-pins
center-pins to
to the
the electrode.
electrode. The
The center-pin
center-pin material
material
isisaamolybdenum
molybdenumalloy
alloy(TZM)
(TZM) that
that needs
needs to
to be
be secured
secured to
to the
the 316-stainless
316-stainless electrode.
electrode.
AA torch
torch braze
braze technique
technique has
has been
been found
found to
to be
be extremely
extremely effective
effective in
in making
making this
this aa
secure
securejoint.
joint. The
Thealloy
alloyused
usedisisAWS
AWS(American
(AmericanWelding
WeldingSociety)
Society) Bag-13a
Bag-13a alloy
alloy with
with
AWS
AWSFB3C
FB3Cflux,
flux,again
againcare
caremust
mustbe
betaken
takentotoavoid
avoidoverheating
overheatingthe
thefeed
feedthrough.
through.
308
Kaman
KamanFeed
Feedthrough
through
Transition
TransitionPiece
Piece
Electrode
Electrode
TZM
TZMMolybdenum
Molybdenumpin
pin
Figure
Electrode
sub-assembly.
Figure
Figure2.2.
2. Electrode
Electrodesub-assembly.
sub-assembly.
The
final
step in
in assembling
assembling the
the BPM
BPM isis
is to
to weld
weld the
the
electrode
assembly
into
the
The
The final
final step
step
in
assembling
the
BPM
to
weld
the electrode
electrode assembly
assembly into
into the
the
center-body.
The
electrode
assembly
is
installed
from
the
inside
of
the
center-body.
center-body.
The
electrode
assembly
is
installed
from
the
inside
of
the
center-body.
center-body. The electrode assembly is installed from the inside of the center-body.
The
feed
throughs
are passed
passed
through
the
holes
in
the
center-body
until
the
tops
of
the
The
The feed
feed throughs
throughs are
are
passed through
throughthe
theholes
holesin
inthe
thecenter-body
center-bodyuntil
untilthe
thetops
topsof
ofthe
the
transition
pieces
are flush
flush with
with the
the flats
flats in
in the
the center-body.
center-body. Figure
Figure 333 depicts
depicts the
the
transition
transition pieces
pieces are
are
flush
with
the
flats
in
the
center-body.
Figure
depicts
the
installation
ofthe
thesub-assembly
sub-assemblyinto
intothe
theBPM
BPMcenter-body.
center-body.
installation
installation of
of
the
sub-assembly
into
the
BPM
center-body.
Final
FinalWeld
WeldJoint
Joint
Weld
WeldJoint
Joint
BPM
BPMCenter-body
Center-body
Silver
SilverBraze
BrazeJoint
Joint
Figure
Figure
Electrode
sub-assemblyinstallation.
installation.
Figure3.3.
3. Electrode
Electrodesub-assembly
sub-assembly
installation.
IPF
IFF
BPM Characterization
IPF BPM
Characterization
The
The fabricated
fabricated BPMs
BPMs are
are then
then characterized
characterized using
using the
the BDIT’s
BDIT's BPM
BPM mapping
mapping
BDIT’s
system
system [7],
[7], [8].
[8]. The
The mapping
mapping system
system consists
consists of
of aa 0.004-inch
0.004-inch diameter,
diameter,stainless
stainlesssteel
steel
wire,
wire, stretched
stretched tightly
tightly between
between two
two horizontal
horizontal plates
plates and
and running
running through
through the
the bore
bore of
of
the
onto
the
wire,
which
induces
RF
the BPM.
BPM. A
A 201.25
201.25 MHz
MHz RF
RF signal
signal isis injected
injected
onto
the
wire,
which
induces
RF
injected
currents
currents on
on the
the BPM’s
BPM's
electrodes. Figure
Figure 44 shows
shows the
the BDIT
BDIT mapping
mapping system
system
BPM’s electrodes.
schematically.
schematically. AA stepper
stepper motor
motor system
system isis used
used to
to move
move the
the upper
upper and
and lower
lower plates,
plates,
while
while the
the BPM
BPM remains
remainsstationary,
stationary,such
suchthat
thatthe
thewire
wireisispositioned
positionedatatdiscrete
discretelocations
locations
throughout
throughout the
the bore
bore of
of the
the BPM.
BPM. The
Theinduced
inducedRF
RFsignal
signalon
oneach
eachelectrode
electrodeisisread
readout
out
and
and
recorded
on aaaBoonton
Boonton
[9],4300
4300
power
meter.
Thepower
power
ratio
between
opposing
and recorded
recorded on
on
Boonton[9],
[9],
4300power
powermeter.
meter. The
The
powerratio
ratiobetween
betweenopposing
opposing
309
electrodes is then calculated for each wire location. This data set of power ratios is
fitted to a third order, two dimensional equation using the Levenberg-Marquardt [10]
least-squares fit method using IDL programming environment [11].
The reference features (fiducial flats and alignment target holes) on the BPM flange
described previously are used to locate the BPM in the mapping system. The offsets
determined from the mapping process are based on these reference surfaces. These
reference surfaces are also used during BPM installation. The BPMS are installed in
the beam line and their locations are measured using a theodolyte system. The
theodolyte system uses alignment targets, which are placed in the holes provided in
the BPM flange. This way alignment data and mapping data are based on the same
reference surfaces. The offsets determined from the mapping and alignment processes
can then be incorporated into the measurement software and accounted for during a
measurement.
Parker Daedal linear Z drive
Compumotor 6000
series indexer/driver
Figure 4. EDIT BPM mapping system schematic.
IFF BPM Mapping Results
Each of the eight (seven required and one spare) four inch ID BPMs has been
mapped using the above-mentioned method. Each of the eight BPMs had at least three
mapped data sets. Each data set is analyzed and the fit coefficients are generated.
Table 1 shows the offsets and sensitivities of each of the eight four inch BPMS. The
values listed in table 1 are the mean value of the coefficients based on the number of
data sets taken for each BPM. The calculated theoretical sensitivity for these 50-mm
310
radius BPMs was 0.654 dB/mm which is approximately 4% higher than the actual
measured sensitivities shown in Table 2.
Table 2. 4" IFF BPM Offset and Sensitivity Data
X offset (dB)
Y offset (dB)
IPFBPM
BPM#1
BPM #2
BPM #3
BPM #4
BPM #5
BPM #6
BPM #7
BPM #8
-1.9661E-01
-6.9556E-02
-1.1288E-01
2.0089E-02
-1.2761E-01
-1.8416E-01
1.3820E-02
-1.1351E-01
-4.9739E-02
-1.3268E-01
-2.4562E-01
-1.9652E-01
-8.1582E-02
-4.2807E-01
-1.2591E-01
-1.5327E-01
X Sensitivity
(dB/mm)
Y Sensitivity
(dB/mm)
6.2674E-01
6.2608E-01
6.2602E-01
6.2580E-01
6.2588E-01
6.2573E-01
6.2594E-01
6.2547E-01
6.2674E-01
6.2595E-01
6.2615E-01
6.2548E-01
6.2603E-01
6.2587E-01
6.2591E-01
6.2623E-01
The maximum standard deviation of the offsets values for each individual BPM
was 0.020 dB (0.032 mm). This value was generated from the numerous data sets
related to each individual BPM and gives insight into the mapping system's precision.
Other important information can be gained by looking at the standard deviation of the
offset values for all of the different BPMs. From this value one can get an indication
of the BPM manufacturing tolerances. The standard deviation of the offsets of the
entire BPM set was 0.106 mm (0.004 inches).
Switchyard Kicker BPM Mechanical Design
The Switchyard Kicker BPM design is very similar to the IPF BPM. The same
modified LANSCE style of flanges, the same recessed electrode design, and the same
KAMAN feed throughs are used. There were a few areas where some improvements
were attempted. The Switchyard Kicker BPM design calls for the brazing of the
flanges to the center-body. This is done to eliminate any potential virtual vacuum
leaks caused by trapped volumes due to the outside weld required in the IPF BPM.
The manufacturing process for the Switchyard Kicker BPMs has been modified from
the IPF plan by including more rough machining steps during the housing subassembly fabrication stages. These extra steps allow for the precise location of key
features such as the reference flats and alignment target holes to be accomplished after
the parts have gone through the braze process, as opposed to the IPF design where
potential distortion due to welding could adversely impact the location of these
features.
Another improvement is a modification to the electrode sub-assembly. A transition
piece was added between the two feed throughs. This added piece keeps the feed
throughs supported during the time prior to installation in the center-body. The
interface between the transition piece and the center-body becomes the final sealing
weld. Plans call for this weld joint to be done with at laser to minimize the amount of
heat used to make the joint and also minimize any electrode placement distortion.
Figure 5a shows the Switchyard Kicker BPM with the upstream flange removed and
figure 5b shows the modified electrode sub-assembly.
311
Transition Piece
FIGURE 5A
5A &
& 5B.
5B. Switchyard
Switchyard Kicker
Kicker BPM
BPM with
with upstream
upstream flange
flange removed and modified
modified electrode
FIGURE
sub-assembly.
sub-assembly.
Summary
Summary
Two similar
similar style
style BPMs
BPMs have
have been
been designed
designed for two different
Two
different on-going upgrade
upgrade
projects atat LANSCE.
LANSCE. The
The BPMs
BPMs are classic, four, micro-stripline units having one
projects
end terminated
terminated in
in aa 50-ohm
50-ohm load.
load. The
The four-inch
four-inch ID IPF
IFF BPMs have been
end
been
characterized and
and the
the results
results were
were presented.
presented. The results show that the mapping
characterized
mapping
system has
has aa precision
precision of
of 0.020
0.020 dB
dB (0.032
(0.032 mm),
mm), the BPMs have a sensitivity of 0.626
system
dB/mm (standard
(standard deviation
deviation of
of 3.55e-4
3.55e-4 dB/mm),
dB/mm), which
which compared well with the
dB/mm
theoretical value
value of
of 0.64
0.64 dB/mm.
dB/mm. Based
Based on
on the
the offset data the error associated with
theoretical
themanufacturing
manufacturing process
process can
can be
be described
described to
to be within 0.004 inches. Four of the
the
the IPF
IPF
BPMs
have
been
installed
in
the
beam
line.
The
Switchyard
Kicker
BPM
design
BPMs have been installed in the beam line.
design is
is
similar to
to the
the IPF
IPF design
design with some modification that should improve these BPMs
similar
overall performance.
performance. The
The Switchyard
Switchyard Kicker
Kicker Upgrade
Upgrade schedule
schedule calls
calls for
overall
for the
the BPMs
BPMs to
to
stst of 2002.
be
delivered
and
tested
by
October
1
be delivered and tested by October 1 of 2002.
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