FNAL Instrumentation: Lessons Learned
James L Crisp
Fermilab1
PO Box 500
Batavia, IL 60540
Abstract. Experience gained during the recent commissioning of the Main Injector accelerator
and the Recycler storage ring at Fermilab will be discussed. Some of the more interesting
problems involve; ground differences, cabling for bpm and multiwire systems, electromagnetic
noise, and magnetic shielding
LOCAL CURRENTS
"Ground"
According to Mr. Webster, definitions of "ground" include:
• The position or portion of an electric circuit at zero potential with respect to
the earth.
• A conducting connection to such a position or to the earth.
• A large conducting body, as the earth, used as a return for electric currents and
as an arbitrary zero of potential.
However, ohm's law requires "ground" to have different voltage potentials that
depend on how currents flow from one part to another.
Main Injector Bending Magnet System
The Fermilab Main Injector uses conventional magnets to contain the beam as it
accelerates from 8GeV to ISOGeV. Twelve 1KV power supplies in series with 344
bending magnets are ramped to 9375 amps in about ¥2 second. To build the desired
waveform, each supply ramps up separately in about 30 milliseconds. Bending
magnets have a series impedance of O.SmQ and 2mH with a coil to ground
capacitance of 30nf.
1
Operated by Universities Research Association, Inc.
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
103
TABLE 1. Main Injector bending magnet system.
344 Bending magnets
0.8 mQ and 2 mH each
capacitance to ground
30 nf each magnet
12 power supplies
1KV each supply
Maximum bend current
9375 Amps
1/2 second ramp overall
30 msec each supply
As the supplies ramp up, the changing voltage to ground induces "local currents"
through the magnet capacitance to ground. These currents flow along the path of least
resistance such as beam pipe, signal cables, and cable tray. Local currents make the
bend buss current different for each supply and magnet. Generally, the bend magnet
current at only one point is monitored and controlled. At Fermilab, orbit distortions
lead to the realization that magnet current was not the same around the ring and
depended on ramp waveform, power supply turn on order, and geometry of the bend
magnet system. Local currents also produce significant potential differences in the
ground system that result in problems with some instrumentation systems.
lmo<g
FIGURE 1. Power supply with two magnets in series. Local currents are driven by the changing
voltage to ground. Even though all of the magnets and supplies are in series, they don't necessarily all
have the same current.
Vacuum Chamber Impedance
If the vacuum chamber of the Main Injector were cut in one place, the impedance
across the cut could be estimated as shown below.
Rm =£*-„
p ='
cPt
'
CM = machine circumference = 3320 meters
CP = pipe circumference = (0.0254) * 13.9"
t = wall thickness = (0.0254) * 0.0595"
p = resistivity of stainless = 9e -1 Q - m
The inductance of the 2 mile diameter ring is estimated below:
104
(1)
(2)
a = radius ~ 528 meters
N =1 = number of turns
Magnet laminations surround the beam pipe and increase this inductance. With a
typical "H" magnet lamination geometry, the reluctance for the path surrounding the
pipe will be about 4 times that for the normal bending field. The beam pipe has only
one turn compared to about 10 for the normal magnet coils. The increase in
inductance attributed to the laminations of all 344 magnets is estimated below.
i- —
=1.7 mH
The corner frequency defined by the ring resistance and inductance is R/27iL=l IHz
(5.6Q, 2.7mH). The ring is resistive below this frequency and inductive above. The
inductance from the magnet laminations will tend to steer beam image currents
through the beam pipe and local currents through other pathways such as
instrumentation cables.
The stainless steel beam pipe wall thickness (.0595") is one skin depth at 100 KHz.
At frequencies below this, the magnetic fields from the beam current will leak through
the wall and begin to produce measurable potential differences that follow the beam
around the ring. For lell particles distributed within 1/7 of the circumference
(1.6usec) the voltage to ground would be:
Estimate of Local Current
For one power supply turning on, the voltage to ground on the magnet buss has a
simple distribution. Symmetry suggests the average voltage to ground is about +1A of
the supply voltage on l/i of the ring and -1A on the other half. This voltage will induce
currents through 1A the capacitance and 1A of the resistance. The resistance is assumed
to be only that of the stainless beam pipe. The Fermilab Main Injector also has a
ground conductor around the circumference. The capacitive coupling to ground
through the magnets will make the ground voltage proportional to dV/dt as estimated
below.
105
Vqnd
FIGURE 2.
2. Voltage
Voltage to
to ground
ground of
of the
the magnet
With
FIGURE
magnet buss
buss with
with one
one power
power supply.
supply.
With aa single
single power
power supply
supply
l
turning on,
on, symmetry
symmetry suggests
suggests the
/4 of
turning
the average
average voltage
voltage to
to ground
groundisis about
about++¼
ofthe
thesupply
supplyvoltage
voltageon
onhalf
half
of the
the ring
ring and
and –1/4
-1/4 on
on the
the other
other half.
of
half.
1 V ps C tot Rtot
= 30 mVolts
4 ∆t 4 4
VpsPS=IKV
V
= 1 KV
V ground =
Att = 30 m
msec
∆
sec
=344x30
M =
C tot
344 × 30 nf
(5)
(5)
Rtotut=s.6n
R
= 5.6 Ω
Measurement
Voltages
Measurement of
of Local
Local Current
Current Induced
Induced Voltages
The measured
measured voltage
voltage between
between the
the tunnel
tunnel ground
ground and
and the
service building
The
the service
building ground
ground is
is
shown below.
below. The
The measurement
measurement was
was made
shown
made by
by connecting
connecting both
both the
the center
center conductor
conductor
and the
the shield
shield of
of aa coaxial
coaxial cable
cable to
to the
shield of
and
the beam.
beam. The
The shield
of the
the service
service building
building end
end
was
connected
to
ground
through
the
chassis
of
the
oscilloscope.
The
impedance
was connected to ground through the chassis of the oscilloscope. The impedance of
of
the test
test cable
cable shield
shield is
is large
large compared
the
compared to
to all
all of
of the
the other
other ground
ground connections
connections and
and does
does
not disturb
disturb the
the measurement.
measurement.
not
106
FIGURE 3.
3. Measurement
Measurement of
FIGURE
of the
the voltage
voltage between
between the
the tunnel
tunnel and
and service
service building
building grounds
groundsduring
duringthe
the
magnet ramp.
ramp. The
The voltage
magnet
voltage measured
measured on
on the
the tunnel
tunnel ground
ground induced
inducedby
bytwo
twoconsecutive
consecutiveramps
rampsisisshown
shown
on the
the left
left (50mV,
(50mV, .5sec
.5sec per
per division).
on
division). One
One magnet
magnet ramp
ramp is
is shown
shown on
on the
the right
right (3000Amps,
(3000Amps, 1.25
1.25sec
sec
per division).
division). The
The voltage
voltage is
is proportional
per
proportional to
to dV/dt.
dV/dt.
Effects of
of Voltage
Effects
Voltage Differences
Differences Between
BetweenGrounds
Grounds
The voltage
voltage between
between tunnel
The
tunnel ground
ground and
and building
building ground
ground can
can induce
induceerrors
errorsinto
intothe
the
signal path
path of
of instrumentation
instrumentation cables.
signal
cables. The
The voltage
voltage will
will appear
appear across
across the
the shield
shield
impedance. For
For equal
impedance.
equal source
source and
and receiving
receiving impedance,
impedance, 1/2
1/2 of
of the
the noise
noisevoltage
voltagewill
will
appear
across
the
load
resistor
as
shown
below.
appear across the load resistor as shown below.
R1
Tunnel
ground" JL
R1
6
R2
R2
R1 + R2
Building
"ground"
R2
FIGURE 4. Schematic of a coaxial cable between the tunnel ground (or beam pipe) and the building
FIGURE
ground. 4. Schematic of a coaxial cable between the tunnel ground (or beam pipe) and the building
ground.
107
The noise voltage across the load resistor can be reduced by inserting a transformer
as shown below. Typically, these are made by wrapping as many turns as possible of
coaxial cable on a torroidal tape wound core.
For wL»Rshield
FIGURE 5. Schematic demonstrating the benefit of torroidal cores. For wL»Rshield the noise well
be removed from the load impedance.
For an inductance L of the core, and total cable shield resistance Rs, the amount of
noise reduction is given below.
* nut
*Vc
A
forf»f0 =
(6)
* out _ J o
V noise ~ J f
The core has no effect below the corner frequency fo=Rs/27iL. Above this
frequency, the noise is reduced by fo/f. Such a core was not effective above 50 KHz
because of turn to turn capacitance and associated resonance's in the windings
themselves. Common mode filters using such an approach can be effective but only
for a limited frequency range. The core must be inserted into the signal cable between
the tunnel and building grounds. Well intentioned, but less informed, people have
inserted a transformer inside the relay rack with the cable shield grounded were it
entered the rack. If the shield of the cable is grounded before the torroid, it does no
good.
108
TABLE 2. Typical common mode filter using a torroidal core____________
Arnold T8027 tape wound Supermalloy core
3" OD, 2" ID, 1" Ht
104 uH/N2
500 ft RG58 cable
4 Q/lOOOft
2Q
30 turns on core
94 mH
fo
R/27iL
3.4 Hz
60 Hz noise reduction
60/3.4
17.6 (or25db)
Maximum frequency________________________________50 KHz
Fast Kicker Magnets
Fast kicker magnets are another source of voltage differences between grounds. At
Fermilab, fast kicker magnets have rise times of 20 to 100 nsec and pulse widths of
1.6 to 20 usec. Several RG220 cables are used in parallel with up to 600 amps in each
cable to carry the pulse to the magnet in the tunnel. The impedance of the RG220
cable shield is 0.05Q for a modest cable length of 200ft. The 600 amp return current
will make 30 volts on it's way back to the power supply! The shield of the RG220 is
tied to the building ground upstairs and to the magnet case in the tunnel. If the magnet
case is connected to the tunnel ground or beam pipe, this 30 volt signal will drive
currents through any instrumentation cables connected between the tunnel and the
building.
If the magnet case is left floating, beam image currents will produce
electromagnetic waves as they cross from one side of the magnet to the other. The
kicker magnet becomes an antenna broadcasting the beam frequency with substantial
power. This rf interference can induce substantial signals into nearby signal cables.
This is a difficult problem because the fast rising kicker currents have frequency
components as high as the beam. The decision to ground kicker magnet cases has not
been resolved at Fermilab.
Multiwires
Multiwire Signals and Ground
Multiwires are secondary emission profile monitors made from wire grids. The
small charges induced on the wires by the passing beam are integrated and the relative
amplitudes on the wires in the grid are measured to produce a transverse beam profile.
A 50 um wire centered on the beam collects 1 charge for each 2000 protons. This
depends on many things but is typical for the beam density at Fermilab. The 40
picocoulombs collected from Sell protons will produce a 0.4 volt signal in a lOOpf
integrator. The signal to noise ratio is often several hundred and results in good
profiles.
109
FIGURE
FIGURE6.6. Multiwire
Multiwire(or
(orsecondary
secondaryemission)
emission) profile
profile monitor.
monitor. Horizontal
Horizontal wires
wires are
are placed
placed on
on one
one
sidewith
withvertical
verticalwires
wireson
onthe
theother.
other. Each
Eachplane
planehas
has48
48 wires.
wires. This
This one
one has
has 2mm
2mm spacing.
spacing.
side
The collected
collected charge
charge isis stored
stored in
in the
the cable
cable capacitance
capacitance and
and isis later
later bleed
bleed into
into the
the
The
integrating capacitor
capacitor through
through aa resistor.
resistor. IfIf the
the cable
cable shield
shield were
were connected
connected to
to ground
ground
integrating
both atat the
the tunnel
tunnel and
and the
the service
service building
building the
the difference
difference between
between grounds
grounds would
would
both
swampthe
the signal.
signal. Just
Just 11mV
mV difference
difference between
between grounds
grounds would
would integrate
integrate to
to 55 volts
volts
swamp
withthe
the10KΩ
10KQ100pf
lOOpfintegrator
integratorused.
used.
with
Thesolution
solution toto this
this problem
problem lies
lies in
in exploiting
exploiting the
the essentially
essentially infinite
infinite beam
beam source
source
The
impedance. The
Thebeam
beamwill
willinduce
inducethe
the same
same secondary
secondary emission
emission for
for any
any wire
wire voltage
impedance.
withinreason.
reason. Figure
Figure77indicates
indicatesthat
thatwith
withthe
theshield
shield grounded
grounded in
in the
the tunnel,
tunnel, AC
AC noise
noise
within
can
be
induced
in
the
load
impedance
above
the
corner
frequency
given
by
the
cable
can be induced in the load impedance above the corner frequency given by the cable
capacitance and
and the
the load
load impedance.
impedance. For
For aa 500
500 foot
foot 20pf/ft
20pf/ft cable
cable into
into 10KΩ
10KQ the
the
capacitance
corner frequency
frequency would
would be
be 1.6KHz.
1.6KHz. IfIf the
the shield
shield isis not
not connected
connected to
to ground
ground in the
corner
tunnelthen
then noise
noisevoltage
voltage becomes
becomes Rload/Rbeam,
Rload/Rbeam, essentially
essentially zero.
zero. The
The signal
signal return
return
tunnel
currents
will
flow
through
what
ever
ground
connections
it
can
find.
Because
the
currents will flow through what ever ground connections it can find. Because the
beam source
source impedance
impedance isis so
so large
large the
the return
return current
current path
path can
can have
have substantial
substantial
beam
impedancewithout
withoutaffecting
affecting the
theprofile
profilemeasurement.
measurement.
impedance
Unfortunately, cables
cables are
are run
run through
through cable
cable trays
trays with
with many
many other
other conductors
conductors that
that
Unfortunately,
canhave
havesubstantial
substantial“signals”
"signals"on
ontheir
theirshields.
shields. Capacitive
Capacitivecoupling
coupling between
between the
the shield
shield
can
ofthe
themultiwire
multiwirecable
cableand
andit’s
it'senvironment
environmentcan
canstill
stillinduce
inducenoise.
noise.
of
110
r——-p(
(±^ —— ,
Rbeam
T1 r
^~
Rlodd J
Rqnd ^^
y
Vnoise
> Rlood
7
Vnolse
((a>l.6KHz)
ω > 1.6 KHz )
forcoC
«RRkload
cable<<
for ω C cable
RRload
Vout
load
*Vnoise
noise
=
switch
closed
switch closed
R gnd + Rload
Vout
Rload
=
R gnd + Rload + Rbeam
*Vnoise
noise
(7)
(7)
switch
open
switch open
FIGURE
FIGURE7.
7. Multiwire
Multiwire noise
noise analysis.
analysis.
Multiwire
Multiwire Cable
Originally
instrument
Originally Fermilab
Fermilab used
used custom
custom bundles of RG174 coaxial cable to instrument
multi
wires. The
wires measured only the horizontal
horizontal or
or vertical
vertical profiles
profiles
multiwires.
The original
original multi
multiwires
using
and vertical
vertical profiles
profiles with
with
using 24
24 wires.
wires. The
The latest
latest design measures both horizontal and
48
bundles precluded
precluded
48 wires
wires in
in each
each plane.
plane. The
The cost
cost and burden of terminating these bundles
their
their use
use and
and itit was
was decided
decided to
to switch to a shielded ribbon cable. Figure 8 indicates
how
shield. The
The “flat
"flat to
to
how the
the ribbon
ribbon is
is folded
folded into a round
round shape and shows the braided shield.
round"
terminate using
using mass
mass
round” cable
cable is
is significantly
significantly cheaper and is much easier to terminate
connectors.
reduction in
in wiring
wiring errors.
errors.
connectors. The
The geometry
geometry constraints leads to a significant reduction
However,
However, we
we still
still manage
manage to
to get the polarity wrong on a regular basis.
Capacitive coupling
1 .U
; signal ;onductjor
1-
-v
0.5 -
x
-0.5 -
I
coupled cond jctor
-1 1
-
2
0
1
2
1
4
6
8
time [usec]
FIGURE8.
8. Effect
Effect of
of capacitive
capacitive coupling
coupling between
between signal
FIGURE
signal conductors
conductors in
in Multiwire
Multiwire cable.
cable.
Ill
The multiwire scanner simultaneously
simultaneously integrates
integrates all
all 96
96 channels.
channels. One
One feature
feature of
of
the
channel exceeds
exceeds aa limit,
limit, integration
integration
the firmware
firmware running the scanner is that if any one channel
is
capacitive coupling
coupling between
between conductors
conductorsinin
is stopped for all channels. Unfortunately, capacitive
the
cause ghost
ghost profiles
profiles or
or satellites.
satellites. Figure
Figure88suggests
suggestswhat
what
the "flat
“flat to round"
round” cable can cause
aa capacitively induced signal might
might look
look like
like on
on aa single
single conductor.
conductor. IfIfthe
theintegration
integration
window
coupled signal
signal integrates
integrates to
to zero,
zero, then
then aanet
net
window is closed before the capacitively coupled
signal
will appear
appear as
as shown
shown in
in Figure
Figure 9.9. The
The
signal will be measured and a ghost profile will
normal
long to
to make
make capacitive
capacitive coupling
couplinginsignificant.
insignificant.
normal integrate window is sufficiently long
The
round" cable
cable outweigh
outweigh this
this undesirable
undesirablefeature.
feature.
The advantages of "flat
“flat to round”
FIGURE 9.
9. If
If the
the integration
integration window
window is
FIGURE
is closed
closed before
before the
the capacitivly
capacitivly coupled
coupled signal
signal averages
averages out,
out,
satellite profiles
profiles are
are left
satellite
left in
in the
the profile.
profile.
BPM’s
BPM's
Resonant
Resonant BPM’s
BPM' s
The Fermilab
Fermilab Recycler
Recycler uses
The
uses aa split
split tube
tube BPM.
BPM. An
An 11
11 inch
inch section
section of
of elliptical
elliptical
beam tube
tube is
is sliced
beam
sliced at
at an
an angle
angle to
to form
form the
the two
two BPM
BPM electrodes.
electrodes. The
The electrodes
electrodes are
are
held inside
inside of
of aa concentric
concentric vacuum
chamber.
The
system
isis configured
to
measure
the
held
vacuum
chamber.
The
system
configured
to
measure
the
rd
harmonic of
33rd harmonic
of the
the bunch
bunch spacing
spacing at
at 7.5MHz.
7.5MHz. To
To improve
improvethe
thesignal
signaltotonoise
noiseratio,
ratio,aa
resonator is
is formed
formed with
resonator
with the
the plate
plate to
to ground
ground capacitance
capacitance and
and an
an inductor.
inductor. AA
preamplifier
is
located
near
the
BPM
and
connected
to
it
through
a
two
preamplifier is located near the BPM and connected to it through a two foot
foot cable.
cable.
The
plate
impedance
at
resonance
is
1.5
KΩ
and
is
determined
by
the
preamp
The plate impedance at resonance is 1.5 KQ and is determined by the preampinput
input
impedance as
as shown
impedance
shown in
in Figures
Figures 10
10 and
and 11.
11.
112
I be.'dm / J ,
FIGURE 10. Beam Position Monitors used in the Fermilab Recycler are made to resonate at 7.5MHz
by connecting the capacitive plate in parallel to an inductor through a short cable.
Cp=1Q5pf
Cc=6pf
L=4uH
R=1.5K
fo= 7.5MHz
0=8
Plate B
Plate A
FIGURE 11. The beam position monitor plates are coupled through the small capacitance between
them. Coupling will be significant if the plate to ground impedance is comparable to the coupling
impedance.
Plate to plate capacitance causes problems. An impulse or swept sine wave applied
to one plate is coupled to the other plate, changing measured position. At 7.5 MHz,
the coupling impedance (1/jwC = 3.5KQ) is comparable to the 3KQ preamplifier input
impedance (1.5KQ each side). The coupling impedance can be reduced by shunting it
with an inductor between the two electrodes, Equation 8. Coupling will be reduced by
a factor of Q for the resonant circuit. Figure 12 shows the coupling with and without
the inductor for an impulse and a swept sine wave applied to one electrode.
Q=
k Energy Stored
Energy Lost,
'Cycle
= Q\ a><C
113
-cv2
2 R f
(8)
1 -I
1.0
1.5
time [usec]
FIGURE 12. Measurements of coupling
coupling before
before and
and after
after adding
adding the
the de-coupling
de-couplinginductor
inductorinintime
timeand
and
frequency. On the left,
frequency.
left, the
the signal
signal on
on both
both the
the driven
driven and
and coupled
coupled electrodes
electrodesare
areshown
shownfor
forboth
boththe
the
coupled and de-coupled
de-coupled condition.
condition. On
On the
the right,
right, only
only the
the coupled
coupled plate
plate isis shown
shownfor
forboth
boththe
thecoupled
coupled
and de-coupled condition.
condition. Coupling
Coupling was
wasreduced
reducedby
by27db
27dbatat7.5MHz.
7.5MHz.
BPM
BPMCables
Cables
The Main Injector
Injector at Fermilab
Fermilab has
has 208
208 bpm's
bpm’s that
that use
use 262000
262000feet
feetof
ofRG8/U
RG8/Ucable
cable
(49.6 miles)! The longest
longest cable
cable length
length isis 1400
1400 feet and the shortest is 120 feet. The
cable was pulled as pairs to obtain
obtain similar
similar length
length and
and facilitate
facilitate phase
phasematching.
matching. The
The
spread in propagation velocity
velocity required
required pulling
pulling an
an extra
extra 2%
2%or
or30
30feet
feettotoeach
eachbpm.
bpm. AA
bpm's were outside of
number of bpm’s
of the
the specification
specification and
and required
required adding
addingcable
cabletotoone
one
side to obtain matched delay
delay through
through the
the pair.
pair. Figure
Figure 13
13 indicates
indicates the
the percent
percent
difference between
between cables
difference
cables in
in aa pair
pair as
as they
they were
were pulled
pulled in.
in. The
Thecable
cabledid
didnot
notmeet
meetthe
the
manufacturers
specification
±2%
variation
in
velocity.
manufacturers specification ±2% variation in velocity.
Figure 13
13 also
Figure
also shows
shows the
the measured
measured characteristic
characteristicimpedance
impedancefor
forone
onepair
pairofofcables.
cables.
Only
a
half
dozen
were
measured
with
similar
results.
Again,
the
cables
Only a half dozen were measured with similar results. Again, the cablesdid
didnot
notmeet
meet
the manufacturer’s
manufacturer's specification
specification of
the
of ±2Q.
±2Ω.
114
100 -,
40
60
80
100
120
frequency [MHz]
FIGURE
FIGURE 13.
13. Distribution
Distribution of
of the
the percent
percent difference
difference in
in cable
cable length
length for
for the
the 208
208 pairs
pairs of
of cables
cables used
used for
for
the
the Fermilab
Fermilab Main
Main Injector
Injector BPM's.
BPM's. Measured
Measured characteristic
characteristic impedance
impedance of
of one
one pair
pair of
of RG8/U
RG8/U BPM
BPM
cables.
cables.
After
After installing
installing the
the bpm’s
bpm's in
in the
the Booster
Booster to
to Main
Main Injector
Injector transfer
transfer line
line we
we were
were
plagued
plagued with
with complaints
complaints of
of poor
poor accuracy
accuracy from
from one
one particular
particular bpm.
bpm. Eventually
Eventually what
what
was
was found
found is
is that
that one
one cable
cable of
of the
the cable
cable pair
pair had
had aa resonance
resonance very
very near
near the
the frequency
frequency
used
the measured
measured characteristic
characteristic impedance
impedance as
as aa
used by
by the
the bpm
bpm system.
system. Figure
Figure 14
14 shows
shows the
function
of
frequency.
The
marker
is
placed
at
the
beam
frequency
component
used
function of frequency. The marker is placed at the beam frequency component used
by
replacing the
the pair
pair of
of
by the
the bpm
bpm system,
system, 52.813MHz.
52.813MHz. The
The problem
problem was
was corrected
corrected by
by replacing
cables.
cables. The
The offending
offending cable
cable was
was closely
closely inspected
inspected but
but no
no obvious
obvious problem
problem could
could be
be
found.
found. The
The cable
cable was
was cut
cut into
into sections
sections with
with all
all sections
sections displaying
displaying the
the same
same
resonance.
resonance. ItIt is
is speculated
speculated that
that perhaps
perhaps the
the cable
cable reel
reel sat
sat on
on one
one side
side and
and placed
placed
periodic
periodic deformations
deformations at
at just
just the
the right
right spacing
spacing to
to make
make aa resonance
resonance at
at exactly
exactly the
the
l
worst
at
worst possible
possible frequency.
frequency. A
A diameter
diameter of
of 2.3
2.3 feet
feet corresponds
corresponds to
to ½
/2 wavelength
wavelength at
52.813
from other
other
52.813 MHz.
MHz. This
This is
is consistent
consistent with
with the
the diameter
diameter of
of the
the reels
reels used.
used. Cables
Cables from
nearby
nearby bpm’s
bpm's had
had similar
similar resonance’s
resonance's at
at higher
higher and
and lower
lower frequencies
frequencies as
as if
if they
they were
were
to
to come
come from
from different
different depths
depths of
of the
the reel.
reel. A
A problem
problem in
in the
the manufacturing
manufacturing process
process is
is
also
also possible.
possible.
115
impedance of aa problem BPM cable. The beam bunch frequency
frequency being
FIGURE 14. Characteristic impedance
measured (at the marker) was nearly at the peak, making position measurement
measurement very
very sensitive.
sensitive.
Magnetic Interference
DC
Transformer
DC Current Transformer
A Bergoz Parametric Current Transformer was purchased and installed in the
Fermilab Recycler to measure average beam charge. The device worked satisfactory
satisfactory
until it was moved. The magnetic field from
from the now nearby Main Injector
Injector bend buss
induced a substantial error in the measured beam charge, Figure 15. According to the
100 uAmp/gauss
uAmp/gauss radial sensitivity
sensitivity to magnetic fields.
manual, the transformer has a 100
Typical magnetic fields from the buss are a few gauss. The
The error
error in
in Figure
Figure 15 is
is about
about
lelO
and
corresponds
to
1
mAmp
or
10
gauss.
A
magnetic
shield
was
constructed
1e10
1
with 1/16
1/16 inch thick high permeability mu-metal. The shield was 9 inches in
in diameter
diameter
and 18
18 inches long.
long. The shield reduced the magnetic field by 22db, Figure 16.
116
Sun
H-JAH^-er!
ie:4S :p*%
engineering units
FIGURE
FIGURE
15.
Beam
charge
in
Main
Injector
is
transferred
into
the
Recycler
at
about
1.2seconds.
seconds. The
The
FIGURE 15.
15. Beam
Beamcharge
chargein
inMain
MainInjector
Injectoris
istransferred
transferredinto
intothe
theRecycler
Recyclerat
atabout
about1.2
Main
Main
Injector
ramps
between
and
seconds.
The
measured
Recycler
beam
charge isisaffected
affected
bythe
the
Main Injector
Injector ramps
ramps between
between 555and
and888seconds.
seconds. The
Themeasured
measuredRecycler
Recyclerbeam
beamcharge
affectedby
current
current
in
the
Main
Injector
bend
buss.
currentin
inthe
theMain
MainInjector
Injectorbend
bendbuss.
buss.
FIGURE 16. A high permeability mu-metal shield was placed around the Bergoz Parametric Current
FIGURE
FIGURE 16.
16. A
A high
high permeability
permeability mu-metal
mu-metal shield
shield was
was placed
placed around
around the
the Bergoz
Bergoz Parametric
Parametric Current
Current
transformer. The Main Injector bend buss is shown in the background attached to the enclosure wall.
transformer.
transformer. The
The Main
Main Injector
Injector bend
bend buss
buss is
is shown
shown in
in the
the background
background attached
attached to
to the
the enclosure
enclosure wall.
wall.
The plot on the left indicated the measured magnetic field as a function of frequency without the shield,
Theplot
plot on
on the
the left
left indicated
indicated the
the measured
measured magnetic
magnetic field
field as
as aa function
function of
of frequency
frequency without
without the
the shield,
shield,
The
at the edge of the shield, and at 2.5, 5, and 9” from the edge. The shield is 9” in diameter.
theedge
edgeof
ofthe
theshield,
shield,and
andatat2.5,
2.5, 5,
atatthe
5, and
and 9”
9" from
from the
the edge.
edge. The
The shield
shield is
is 9”
9" in
in diameter.
diameter.
117