Laser Beam-Profile Monitor Development at
BNL for SNS1
R. Connolly, P. Cameron, J. Cupolo, D. Gassner, M. Grau,
M. Kesselman, S. Peng and R. Sikora
Brookhaven National Lab
Upton, NY, USA
Abstract. A beam profile monitor for H" beams based on laser photoneutralization is being
developed at Brookhaven National Laboratory (BNL) for use on the Spallation Neutron Source
(SNS) [1]. An H" ion has a first ionization potential of 0.75eV and can be neutralized by light
from a Nd:YAG laser (^=1064nm). To measure beam profiles, a narrow laser beam is passed
through the ion beam neutralizing a portion of the H" beam struck by the laser. The laser
trajectory is stepped across the ion beam. At each laser position, the reduction of the beam
current caused by the laser is measured. A proof-of-principle experiment was done earlier at
750keV. This paper reports on measurements made on 200MeV beam at BNL and with a
compact scanner prototype at Lawrence Berkeley National Lab on beam from the SNS RFQ.
INTRODUCTION
Photoneutralization of H- beams [2,3,4] has been used for measuring beam
parameters and for beam manipulation. The first ionization potential of an H- ion is
0.75 eV which is the energy of a 1.67 jim photon. As shown in fig. 1, any photon with
?l<1.6 jim can neutralize an H- ion. In these applications light from a laser is used to
mark a portion of the beam. Downstream from the laser interaction point the beam
has three components: H- ions, neutral atoms and unbound electrons. A magnetic
field is used to separate one or two of these from the rest of the beam and
measurements are made on the remaining beam.
Laser marking of the beam has been done in three ways. The first is to use very
short light pulses to neutralize a small phase slice of the entire cross section of the
beam. This technique was developed at Los Alamos National Lab to measure
longitudinal emittance [5]. Light from a Q-switched NdiYAG laser was passed
through a pulse slicer and frequency doubler to produce 23ps-long pulses. These short
light pulses passed through the H- beam. The charged beam was deflected into a
beam stop and a time-of-flight measurement was made on the neutralized beam
component to measure momentum spread. A clever modification on this idea using a
mode-locked laser and spectrometer was proposed but never built [6].
A second marking technique is to neutralize the entire cross section of the beam with a
laser pulse several rf periods long. At Los Alamos this was done to measure the
transverse emittance of beams at the exit of an rf cavity [7]. The beam power was too
1
SNS is managed by 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.
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
150
great to intercept the full beam with a slit but allowing the beam to drift would
introduce space-charge emittance growth. A laser neutralized the full cross section of
the beam at the exit of the cavity and then a magnet removed the charged beam. A slit
and parallel-channel collector was placed after the clearing magnet. Since the
measured beam was neutralized at the cavity exit the actual phase space there could be
determined by transforming the measured phase space through a simple drift with no
space-charge corrections.
At Fermilab a laser has been used to place a notch in the beam when it sweeps
over a Lamberston magnet to reduce activation of the magnet [8]. In this case the
neutralized beam hits a beam stop and the charged beam is bent 90° down the
transport line. A 5ns, 99% notch was produced.
The third marking technique is to focus the laser light into a narrow ribbon and
neutralize a small transverse slice of the beam. The transverse profile can be
measured by translating the laser 'wire' across the beam and, at each position,
measuring the size of the effect it makes on the beam. A measurement which would
collect the removed electrons was proposed by D.R. Swenson et al [9]. This paper
reports on efforts to develop a laser profile monitor (LPM) at Brookhaven National
Lab which measures the beam current notch created by the laser pulse [10].
Profiles of the SNS H" beam will be measured in the medium energy transport line
(MEET) between the radio frequency quadrupole (rfq) and the linac entrance, along
the linac, and in the linac-ring transport line. Stepped carbon-wire scanners are the
primary profile diagnostic. However beam heating will limit wire scanners to tuning
and matching applications with either the beam pulses shortened or the current
reduced. Also there are concerns about placing wires near the superconducting
cavities where wire failure can cause cavity damage. These concerns have motivated
the effort to develop a laser profile monitor (LPM) which is noninvasive.
5
VI
i 4
'o
3
O
0
200
400
600
800
1000
1200
1400
Wavelength (nm)
FIGURE 1. Calculated cross section for H" photoneutralization as a function of photon wavelength.
Data are from a table in ref. [2].
PHOTONEUTRALIZATION
Figure 1 shows the photoneutralization cross section as a function of photon
wavelength in the center-of-mass frame. If the laser beam crosses the H- beam at a lab
151
angle
in the
the moving
moving frame
frame is
is Lorentz
Lorentz shifted
shifted by
by the
the amount
amount
angle of
of θL the
the photon
photon energy
energy in
[11,12],
[11,12],
E
=γEL[1-βcos(θL)]
ECM
CM=yEL[l-pcos(eL)]
(1)
(1)
For
will be
be 90°
90° so
so at
at the
the full
full energy
energy of
of IGeV
1GeV the
the centercenterFor the
the SNS
SNS laser
laser installation
installation θ0LL will
of-mass
photon
energy
will
be
double
the
lab
energy.
For
these
measurements
and
of-mass photon energy will be double the lab energy. For these measurements and
probably
will be
be aa Q-switched
Q-switched NdiYAG
Nd:YAG laser
laser
probably for
for the
the final
final SNS
SNS installation
installation the
the laser
laser will
operating
the full
full energy
energy of
of IGeV
1GeV the
the
operating at
at its
its fundamental
fundamental of
of λ=1064
?t=1064 nm
nm so
so at
at the
neutralization
cross
section
will
be
about
70%
of
the
low
energy
cross
section.
neutralization cross section will be about 70% of the low energy cross section.
The
neutralized passing
passing through
through the
the laser
laser beam
beam
The fraction
fraction of
of beam
beam ions
ions which
which get
get neutralized
is,
is,
-σ(E)Ft
= 1 - ea-o(E)Ft
f•f
(2)
(2)
neut =
Jneut
Here
cross section,
section, FF is
is the
the photon
photon flux,
flux, and
and tt isis the
the time
time
Here σ(E)
a(E) is
is the
the energy-dependent
energy-dependent cross
the
ion
is
in
the
light
beam.
The
photon
flux
in
the
moving
reference
is
also
the ion is in the light beam. The photon flux in the moving reference is also
transformed
the
same
as
the
photon
energy
[13],
transformed the same as the photon energy [13],
FCM=γFL[1-βcos(θL)]
(3)
(3)
The
laser initially
initially drops
drops with
with increasing
increasing beam
beam
The neutralization
neutralization fraction
fraction from
from aa given
given laser
energy
to 1GeV.
In this
this range
range the
the decreased
decreased
energy then
then becomes
becomes almost
almost flat
flat from
from 400MeV
400MeV to
IGeV. In
reaction
Lorentz boosted
boosted photon
photon energy
energy isis approximately
approximately offset
offset
reaction cross
cross section
section from
from the
the Lorentz
by
by the
the Lorentz
Lorentz boost
boost in
in photon
photon flux.
flux.
For
SNS MEET
MEBT experiment
experiment produces
produces aa 20ns-long
20ns-long pulse
pulse
For example,
example, the
the laser
laser on
on the
the SNS
with
focused to
to aa rectangular
rectangular spot
spot 1mm
1mm wide
wide by
by 3mm
3mm
with an
an output
output energy
energy of
of 50mJ.
50mJ. It
It is
is focused
along
variation of
of neutralization
neutralization fraction
fraction with
with beam
beam
along the
the beam.
beam. The
The approximate
approximate variation
energy
in fig.
fig. 2.
2. This
This calculation
calculation does
does not
not include
include the
the
energy this
this laser
laser will
will produce
produce is
is shown
shown in
Lorentz
power lasers
lasers are
are required
required for
for
Lorentz shift
shift of
of neutralization
neutralization cross
cross section.
section. Higher
Higher power
higher
level in
in the
the detector.
detector.
higher beam
beam energies
energies to
to achieve
achieve the
the same
same signal
signal level
1.0
Neutralization fraction
0.8
0.6
0.4
0.2
0.0
20
40
60
(MeV)
Beam energy (MeV)
80
80
100
100
FIGURE 2. Calculated
Calculated neutralization
neutralization fraction vs. beam energy
FIGURE
energy for
for aa 20ns-long,
20ns-long, 50mJ
50mJ laser
laserpulse
pulse
focused to
to aa spot
spot size
size of 1mm
1mm xx 3mm.
3mm.
focused
152
THE 750 KeV EXPERIMENT
THE 750 KeV EXPERIMENT
Our first profile measurement was made on the BNL linac between the rfq and the
Our first profile measurement was made on the BNL linac between the rfq and the
first drift tube linac tank, fig. 3. A light pulse from a Q-switched Nd:YAG passed
first drift tube linac tank,
fig. 3. A light pulse from a Q-switched NdiYAG passed
through the 750keV H- beam from the linac rfq neutralizing most of the beam the light
through the 750keV H" beam from the linac rfq neutralizing most of the beam the light
passed through. A downstream current transformer measured a dip in the beam
passed through. A downstream current transformer measured a dip in the beam
current
of the
the beam
beam hit
hit with
with the
the light,
light,fig.
fig.4.4.
currentwhich
which was
was proportional
proportional to
to the
the fraction
fraction of
The
laser
beam
was
stepped
across
the
ion
beam
and
the
profile
constructed
The laser beam was stepped across the ion beam and the profile constructed byby
plotting
position.
plottingthe
thedepth
depthof
ofthe
thecurrent
current notch
notch vs.
vs. laser
laser beam
beam position.
Pearson transformer
Pearson transformer
s
N
H- beam
N
s
scope
Signal
Sigi
Trigger
Q switched
Nd:YAG
10 ns pulse
FIGURE
first of
of two
two 10
10 Gm
Gm dipole
dipolemagnets
magnetsremoves
removes
FIGURE3.3. Laser
Laserscanner
scannerexperiment
experiment on
on BNL
BNL linac.
linac. The
The first
the
the beam.
beam.
thefree
freeelectrons
electronsfrom
fromthe
thebeam
beam and
and the
the second
second straightens
straightens the
FIGURE4.4. Scope
Scopetrace
traceof
ofthe
thecurrent
current transformer
transformer signal
signal showing
FIGURE
showing notch
notchcreated
createdby
bythe
thelaser
laserpulse.
pulse.
Thearrangement
arrangementof
ofthe
thelaser
laser and
and optics
optics on
on the
the linac
The
linac beamline
beamline isis shown
shownininfig.
fig.5.5. AA
CFR200
laser
from
Big
Sky
Laser
[14]
was
mounted
on
a
shelf
at
the
top
left.
CFR200 laser from Big Sky Laser [14] was mounted on a shelf at the top left. Three
Three
45°mirrors
mirrorswere
weremounted
mountedinside
inside the
the vacuum
vacuum on
on linear
linear motion
45°
motion feedthroughs.
feedthroughs. The
Thetoptopleft mirror was used to switch between vertical and horizontal scans and the other two
left mirror was used to switch between vertical and horizontal scans and the other two
did the scanning. The top-right mirror scanned horizontally and the bottom-left mirror
did the scanning. The top-right mirror scanned horizontally and the bottom-left mirror
scanned vertically. Both scanning mirrors are shown with arms to hold lenses. In this
scanned vertically. Both scanning mirrors are shown with arms to hold lenses. In this
experiment the lenses were not installed.
experiment the lenses were not installed.
The CFR200 puts out 200 ml pulses that are about 20 ns long. Without lenses the
Thediameter
CFR200isputs
out0.6
200
pulses
that areflux
about
20 ns
long.
lenses
26
2
beam
about
cmmJ
giving
a photon
of 1.9
x 10
/cmWithout
s. About
97% the
of
beam diameter is about 0.6 cm giving a photon flux of 1.9 x 1026/cm2s. About 97% of
153
the ions passing through the center of the laser beam were neutralized. The laser is
triggered 400|is before the measurement is to made. The laser then returns a timing
pulse synchronous with the Qswitch firing which was used to trigger a scope.
When an ion is neutralized the free electron continues to move along with the
beam. These electrons have to be removed from the beam to measure a current drop.
In an accelerator installation this is accomplished by either rf cavities or quadrupoles
but in the experiment the current transformer had to be placed in the same vacuum
chamber as the laser optics. For this reason we placed two weak permanent-magnet
dipoles on either side of the transformer. The pole tips are 2.5cm square and 5cm
apart and the field is about 400 G. The first magnet deflects the electrons from the
beam and the second one straightens out the beam.
The data were taken by moving the mirrors manually and measuring the notch
depth on an oscilloscope set to average 15 shots. We measured a maximum notch
depth of about 40% on the horizontal scan. If the laser beam power was uniformly
distributed over the spot the maximum notch depth should have been closer to 60%.
Based on this we conclude the laser power was not uniform over the spot.
FIGURE 5. Laser scanning assembly installed on linac beamline. View is looking up beamline.
Measured Profiles
Figure 6 shows the measured horizontal and vertical profiles. In each plot the
measured points are indicated by markers and the curve is a gaussian fit to the data.
>
s
1.
40
45
50
55
MIRROR POSITION (mm)
MIRROR POSITION (mm)
FIGURE 6. Measured horizontal (left) and vertical beam profiles.
154
The
fitted curves
curves are
are aσxx=3.32±0.05
=3.32±0.05 mm
mm and
and aσyy=7.3±0.6
=7.3±0.6
The rms
rms widths
widths of
of the
the two
two fitted
mm.
These
values
agree
with
expectations
from
previous
measurements
at this
this
mm. These values agree with expectations from previous measurements at
location,
however
for
this
experiment
there
was
no
profile
measurement
by
another
location, however for this experiment there was no profile measurement by another
method.
carbon wire
wire installed
installed in
in the
the beam
beam box
box but
but we
we were
were operating
operating
method. We
We had
had aa carbon
parasitically
during
a
production
run
and
we
were
never
able
to
get
the
short
beam
parasitically during a production run and we were never able to get the short beam
pulses
necessary
to
prevent
damage
to
the
wire.
pulses necessary to prevent damage to the wire.
THE
MEBT EXPERIMENT,
EXPERIMENT, 2.5MeV
2.5MeV
THE MEET
Based
promise of
linac experiment
experiment we
we designed
designed aa laser
laser platform
platform
Based on
on the
the promise
of the
the BNL
BNL linac
which
wire-scanner chambers
chambers on
on the
the SNS
SNS MEET
MEBT at
at
which would
would attach
attach to
to one
one of
of the
the wire-scanner
Lawrence
Berkeley National
National Lab.
Lab. Mounted
Mounted on
on the
the platform,
platform, fig.
fig. 7,
7, are
are aa 50mJ/pulse
50mJ/pulse
Lawrence Berkeley
laser
holder, and
three linear
linear actuators
actuators which
which move
move 45°
45° mirrors
mirrors in
in aa more
more
laser head,
head, aa lens
lens holder,
and three
compact
identical arrangement
to the
the linac
linac experiment
experiment of
of fig.
fig. 5.
5. There
There
compact but
but otherwise
otherwise identical
arrangement to
are
are four
four profile
profile measurement
measurement stations.
stations. Each
Each has
has aa wire
wire scanner
scanner and
and four
four windows
windows for
for
laser-beam
access.
We
installed
the
platform
on
the
most
upstream
chamber
after the
the
laser-beam access. We installed the platform on the most upstream chamber after
MEBT
under vacuum
vacuum and
MEET was
was under
and ready
ready for
for first
first beam.
beam.
A
300mm-focal-length
cylindrical
lens
mounted directly
front of
of the
the laser
laser
A 300mm-focal-length cylindrical lens is
is mounted
directly in
in front
head
and
the
two
optical
path
lengths
from
the
lens
to
the
beam
center
are
the
same.
head and the two optical path lengths from the lens to the beam center are the same.
The
produces aa 1mm
by 3mm
3mm long
long light
light ribbon
ribbon across
across the
the beam
beam producing
producing aa
The lens
lens produces
1mm wide
wide by
measurement
window
of
rms
width
0.3mm.
Since
the
measured
rms
width
of the
the
measurement window of rms width 0.3mm. Since the measured rms width of
horizontal
profile
is
1.48mm
the
width
of
the
laser
beam
caused
about
2%
broadening
horizontal profile is 1.48mm the width of the laser beam caused about 2% broadening
of
of the
the profile.
profile.
FIGURE
platform mounted
on the
the SNS
SNS MEET
MEBT wire
wire scanner
scanner chamber.
chamber.
FIGURE 7.
7. Laser
Laser scanning
scanning platform
mounted on
The
detected with
with the
the existing
existing beam
beam transformer
transformer at
at the
the end
end of
of the
the
The signal
signal was
was detected
MEBT.
half was
was fed
fed into
into aa LeCroy
LeCroy LT374L
LT374L scope
scope
MEET. The
The BCM
BCM signal
signal was
was split
split and
and our
our half
155
with
the scope
scope was
was used
used to
to average
averagefor
for
with ethernet
ethernet connection
connection [15].
[15]. A
A math
math channel
channel on
on the
several
rf pickup.
pickup. The
The rf
rf pickup
pickup could
could have
have been
been greatly
greatly
several pulses
pulses to
to reduce
reduce noise
noise and
and rf
reduced
low-pass filter
filter as
as was
was done
done on
on the
the 750keV
750keV
reduced with
with the
the use
use of
of aa 50MHz
50MHz low-pass
experiment
In the
the profiles
profiles shown
shown in
in fig.
fig. 8,
8,the
thesignals
signalsfrom
from25
25
experiment but
but none
none was
was available.
available. In
beam
40dB signal/background
signal/background in
in the
thebeam
beamcenter.
center.
beampulses
pulses were
were averaged
averaged giving
giving about
about 40dB
The
The program
program switched
switched the
the laser,
laser,
The experiment
experiment was
was controlled
controlled in
in Labview.
Labview. The
moved
the
mirrors,
initialized
the
scope
for
each
new
position,
and
read
the
data.
moved the mirrors, initialized the scope for each new position, and read the data.
Cursors
on
the
scope
were
set
manually
around
the
pulse.
For
each
set
of
averaged
Cursors on the scope were set manually around the pulse. For each set of averaged
data
between the
the cursors,
cursors, summed
summed an
anequal
equalnumber
number
datathe
the program
program summed
summed the
the channels
channels between
of
two 'integrals'
'integrals' to
to give
giveone
onedata
datapoint
point
of channels
channels before
before the
the pulse,
pulse, and
and subtracted
subtracted these
these two
in
inthe
the profile.
profile.
25x10
12x10
-3
-3
Laser notch depth
Laser notch depth
10
20
15
10
8
6
4
5
2
0
5
10
Laser mirror
mirror position
position (mm)
(mm)
Laser
15
5
10
Laser
Lasermirror
mirrorposition
position
15
FIGURE8.
8. Beam
Beam profiles
profiles measured
measured on
on the
FIGURE
the SNS
SNS MEET
MEBT with
with the
thelaser
laserprofile
profilemonitor.
monitor. The
Thehorizontal
horizontal
profile
(left)
has
a
measured
width
of
a=1.60±0.04mm
and
the
vertical
profile
has
a=4.16±0.16mm.
profile (left) has a measured width of σ=1.60±0.04mm and the vertical profile has σ=4.16±0.16mm.
MEASUREMENTS AT
MEASUREMENTS
AT 200
200 MeV
MeV
After the
the measurements
measurements on
on the
the BNL
BNL linac
After
linac at
at 750keV
750keV the
the entire
entire apparatus
apparatusshown
shown
schematically
in
fig.
5
was
moved
to
the
high
energy
end
of
the
linac
schematically in fig. 5 was moved to the high energy end of the linac toto measure
measure
200MeV beam.
beam. ItIt was
was installed
installed in
in the
the linac-AGS
linac-AGS transfer
200MeV
transfer line
line which
which isisno
nolonger
longerused
used
for
beam
transport,
fig.
9.
The
200mJ
laser
hear
is
mounted
on
the
covered
shelf
for beam transport, fig. 9. The 200mJ laser hear is mounted on the covered shelfatatthe
the
topright.
right. The
The beam
beam line
line chamber
chamber also
also had
had the
top
the carbon
carbon wire
wire installed.
installed.
In this installation there were two cylindrical lenses which moved with each
In this installation there were two cylindrical lenses which moved with each
mirror. A 300mm-focal-length lens produced a waist perpendicular to the ion beam
mirror. A 300mm-focal-length lens produced a waist perpendicular to the ion beam
and a 50mm lens spread the light beam longitudinally to reduce power density on the
and a 50mm lens spread the light beam longitudinally to reduce power density on the
laser beam stop. The light beam which crossed the ion beam was 1mm wide by 20mm
laser beam stop. The light beam which crossed the ion beam was 1mm wide by 20mm
long. The calculated neutralization of beam passing through the laser light is 72%.
long. The calculated neutralization of beam passing through the laser light is 72%.
For this experiment the goal was to use stripline beam position monitors (BPMs)
For this experiment the goal was to use stripline beam position monitors (BPMs)
to measure the laser notch. A single-plane RHIC BPM was installed before and after
to
the laser
A single-plane
BPM
was
before and
after
themeasure
beam box.
In thenotch.
superconducting
linacRHIC
of SNS
there
areinstalled
BPMs between
rf tanks
the
beam
box.
In
the
superconducting
linac
of
SNS
there
are
BPMs
between
rf
tanks
with a current transformer at the exit of the linac. Using the striplines as detectors
with
at and
the downstream
exit of the linac.
Using
thebystriplines
detectors
givesauscurrent
accesstransformer
to a upstream
detector
spaced
a single rfasstructure.
gives us access to a upstream and downstream detector spaced by a single rf structure.
156
Using the
the transformer for
signal
transmission
through
the
Using
signal detection
detection will
willrequire
requiregood
goodtransmission
transmissionthrough
throughthe
the
Using the transformer
transformer for
for signal
detection
will
require
good
full linac
linac before
before profiles
can
be
made
atatany
point.
full
profiles
can
be
made
any
point.
full linac before profiles can be made at any point.
FIGURE 9. LPM measurement station at 200 MeV in the BNL linac.
FIGURE9.
9. LPM
LPM measurement
measurement station
station at
FIGURE
at 200
200 MeV
MeV in
in the
theBNL
BNL linac.
linac.
To
measure
the
phase
and attenuation
of
stripline
signals
and
Tomeasure
measure the
the notch
notch we
we adjust
adjust the
the
phase
attenuation
ofoftwo
two
stripline
signals
and
To
notch
we
adjust
theproduce
phase and
and
attenuation
two
stripline
signals
and
combine
them
with
a
hybrid
to
a
nulled
signal
in
the
absence
of
laser
combine them
them with
with aa hybrid
hybrid to
produce
aa nulled
signal
inin the
absence
ofoflaser
combine
to
produce
nulled
signalwhich
the
absence
laser
neutralization.
The
laser
pulse
causes
a
signal
imbalance
appears
either
as
a
neutralization. The
The laser
laser pulse
pulse causes
which
appears
either
neutralization.
causes adepending
a signal
signal imbalance
imbalance
which
appears
eitherasasa a
wide
or
narrow
spot
in
the
rf
envelope
on
how
the
signals
are
combined.
wide or
or narrow
narrow spot
spot in
in the
the rf
wide
rf envelope
envelope depending
depending on
onhow
howthe
thesignals
signalsare
arecombined.
combined.
Different
no dc
dc
Different from
from the
the transformer
transformer measurements,
measurements, now
now the
the signals
signals are
are bipolar
bipolar with
no
Different
from
the
transformer
measurements,
nowdata
the are
signals
arethrough
bipolarwith
with
no dc
component.
To
integrate
the
laser
notch,
the
scope
passed
a
Labview
component. To integrate the laser notch, the scope data are passed through a Labview
component.
To integrate
the laserofnotch,
the scope
data are passed through a Labview
VI
VI which
which takes
takes the
the absolute
absolute value
value of each
each data
data point.
point.
VI which
takes thefrom
absolute
value
of experiments,
each data point.
Also
different
the
first
two
we
Also different from the first two experiments, we have
have had
had extremely
extremely noisy
noisy beam
beam
Also different
fromwith
the care
first we
twohave
experiments,
wematch
have the
hadbeam
extremely
atat 200MeV.
However
been
able
to
pattern
frombeam
the
200MeV. However with care we have been able to match the beam patternnoisy
from
the
at
200MeV.
However
with
care
we
have
been
able
to
match
the
beam
pattern
from
the
upstream
and
downstream
BPMs
and
produce
a
null
signal
with
over
20dB
common
upstream and downstream BPMs and produce a null signal with over 20dB common
upstream
and
downstream
BPMs
and
produce
a
null
signal
with
over
20dB
common
mode
rejection.
Figure
10
shows
a
scope
trace
of
the
laser
notch
in
the
nulled
signal.
mode rejection. Figure 10 shows a scope trace of the laser notch in the nulled signal.
mode rejection. Figure 10 shows a scope trace of the laser notch in the nulled signal.
FIGURE
FIGURE10.
10. Laser
Lasernotch
notchin
inthe
thebeam
beam rfrf envelope
envelope picked
picked up
up on
on the
the BPM
BPM striplines.
striplines. The
The top
top trace
trace is
is
the
from
aaphotodiode
near the
head.
thesignal
signal10.
fromLaser
photodiode
the laser
laser
head. picked up on the BPM striplines. The top trace is
FIGURE
notch in near
the beam
rf envelope
the signal from a photodiode near the laser head.
157
Experimental
Experimental Difficulties
Difficulties
The
where we
we had
had beamline
beamline
The first
first two
two experiments
experiments were
were conducted
conducted at
at low
low energy,
energy, where
access
with
the
beam
on,
and
inside
clean
vacuum
systems.
At
200
MeV
we
have had
had
access with the beam on, and inside clean vacuum systems. At 200 MeV we have
toto operate
remotely
from
outside
the
tunnel.
Also
we
have
been
in
a
section
of
beam
operate remotely from outside the tunnel. Also we have been in a section of beam
line
linewhich
whichnormally
normally isisnot
not used
used to
to transport
transport beam
beam so
so we
we have
have had
had limited
limited beam
beam time.
time.
There
was
a
period
of
two
months
between
installation
of
the
experiment
and
There was a period of two months between installation of the experiment and
available
beam
during
which
time
the
laser
controller
was
in
the
tunnel.
Less
than
available beam during which time the laser controller was in the tunnel. Less than aa
month
controller failed
failed from
from radiation
radiation and
and
monthafter
after we
we started
started getting
getting beam
beam time
time the
the laser
laser controller
had
old and
is slightly
slightly contaminated
contaminated with
with
hadto
to be
be replaced.
replaced. The
The beamline
beamline is
is thirty
thirty years
years old
and is
pump
for beam,
beam, they
they
pump oils.
oils. All
All the
the optics
optics are
are in
in the
the vacuum
vacuum and,
and, during
during the
the wait
wait for
became
the laser
laser light
light burned
burned the
the
becamecontaminated
contaminated with
with oil.
oil. When
When we
we started
started taking
taking data
data the
oil
During the
the early
early
oil on
on the
the optics
optics forming
forming milky
milky patches
patches which
which scattered
scattered the
the light.
light. During
measurements
time. We
We discovered
discovered the
the
measurements the
the signal
signal continued
continued to
to get
get worse
worse over
over time.
damaged
damagedoptics
opticswhen
when replacing
replacing the
the damaged
damaged laser.
laser.
Profile
Profile Measurements
Measurements
For
the measurements
measurements at
at
For several
several reasons
reasons including
including those
those mentioned
mentioned above
above the
200MeV
first profile
profile data
data at
at 200
200
200MeV have
have not
not been
been as
as clean
clean as
as the
the low-energy
low-energy ones.
ones. The
The first
MeV,
was taken
taken by
by
MeV, fig.
fig.11,
11, made
made with
with 100µA
lOOjiA of
of beam
beam from
from the
the polarized
polarized source,
source, was
hand
handby
bymeasuring
measuring the
the notch
notch depth
depth with
with the
the scope
scope cursors.
cursors.
Notch depth (mV)
0.6
0.5
0.4
0.3
0.2
-10
-10
-5
-5
00
55
10
10
Laser position (mm)
Laser
15
15
FIGURE11.
11. Profile
Profileof
of100µA
lOOuApolarized
polarizedbeam.
beam. The
The width
width of
of the
the fitted gaussian is 8.1±2.2 mm.
FIGURE
After the
the polarized
polarized run
run we
we had several weeks of
After
of 400|is-long,
400µs-long, 10mA
10mA beam
beam pulses
pulses
before the
the laser
laser failed
failed from
from radiation.
radiation. During
During the
the early
early portion
before
portion of
of this
this time
time we
we were
were
able to
to get
get clean
clean laser
laser signals
signals but
but as
as time
time progressed
progressed the
able
the signal
signal progressively
progressively got
got
worse. In
Inhindsight
hindsight we
we realize
realize this
this was
was from
from the
the oil
oil contamination
contamination problem.
worse.
problem.
Figure 12
12 shows
shows aa LPM-measured
LPM-measured profile
profile and
and aa wire-scanner
wire-scanner profile
Figure
profile taken
taken the
the day
day
before
the
LPM
profile.
We
were
unable
to
get
a
LPM
profile
and
wire
profile
before the LPM profile. We were unable to get a LPM profile and wire profile at
at the
the
sametime
timebecause
because the
the wire
wire scanner
scanner failed
failed after
after one
one use.
use. The
The linac
same
linac had
had been
been returned
returned
the set
setpoints
points of
of the
the previous
previous day
day although
although between
between the
totothe
the two
two times
times we
we had
had beam
beam the
the
158
linac
linac
had
been
switched
back
to
normal
production operation.
operation. The
The measured
measured widths
widths
linac had
had been
been switched
switched back
back to
to normal
normal production
production
operation.
The
measured
widths
are:
σ
=5.3±1.3mm
and
σ
=3.51±0.34mm.
are:
<JLpM=5.3±1.3mm
and
a
ire=3.51±0.34mm.
LPM
wire
are: σ =5.3±1.3mm and σw =3.51±0.34mm.
LPM
wire
3.0
3.0
wiresignal
signal(mV)
(mV)
CCwire
60
60
Lasernotch
notch(mV)
(mV)
Laser
2.5
2.5
1
I"
50
50
2.0
2.0
g>
'
o 1.5
C1.5
1.5
40
40
CD
'i
o
1.0
lo
1.0
30
30
0.5
0.5
-5
-5
0
5
0
5
Laser
Laser position
position
(mm)
Laser
position (mm)
(mm)
10
10
0
0
5
10
5
10
Wire
(mm)
Wire position
position
(mm)
Wire
position
(mm)
15
15
FIGURE
FIGURE
12.
Beam profile
profile taken
takenwith
withaaawire
wirescanner
scanner(right)
(right)and
andaaabeam
beamprofile
profiletaken
takenthe
thenext
nextday
daywith
with
FIGURE 12.
12. Beam
Beam
profile
taken
with
wire
scanner
(right)
and
beam
profile
taken
the
next
day
with
the
the
laser
scanner
(left).
Profiles
were taken
taken on
on different
different days
days but
but the
the linac
linac had
had been
been restored
restored to
tothe
the set
set
the laser
laser scanner
scanner (left).
(left). Profiles
Profiles were
were
taken
on
different
days
but
the
linac
had
been
restored
to
the
set
points
points
of
the
previous
day.
points of
of the
the previous
previous day.
day.
Our
Our last
last beam
beam run
run before
before this
this paper
paper produced
produced aa fully-automatic
horizontal profile
profile
fully-automatic horizontal
horizontal
profile
scan.
The
Labview
software
was
the
same
as
that
used
at
LBNL
scan. The Labview software was the same
same as
as that
that used
used atat LBNL
LBNL with
with the
the beam
beam
with
the
beam
current
current transformer.
transformer. Since
the stripline
bipolar with
with no
no dc
dc component
component we
we
Since the
stripline data
data are
are bipolar
bipolar
with
no
dc
component
we
added
added an
an absolute
absolute value
to rectify
rectify the
summed the
the data
data between
between
value VI
VI to
the scope
scope data
data then
then summed
summed
the
data
between
cursors
cursors to
to produce
produce aa notch
integral. Figure
13 shows
shows the
the rectified
scope trace
trace of
of the
the
notch integral.
integral.
Figure 13
13
shows
the
rectified scope
scope
trace
of
the
laser
notch.
Figure
14
shows
the
laser
profile
and
a
profile
taken
with
the
carbon
laser notch. Figure 14 shows the laser profile and a profile taken with
with the
the carbon
carbon wire
wire
wire
at
at the
the same
same time.
time. With
With the
the width
a=2.82±0.76mm
the laser
laser the
width of
of the
the fitted
fitted gaussian
gaussian is
is σ=2.82±0.76mm
σ=2.82±0.76mm
and
the
wire
measured
σ=3.61±0.22mm.
and the wire measured σ=3.61±0.22mm.
a=3.61±0.22mm.
FIGURE
FIGURE 13.
13. At
At
each
laser
position
the
scope
averaged
forty
traces.
The
scope
was
read
into
Labview
FIGURE
13.
At each
each laser
laser position
position the
the scope
scope averaged
averaged forty
forty traces.
traces. The
Thescope
scopewas
wasread
readinto
intoLabview
Labview
and
and passed
passed through
through an
an absolute
absolute value
value VI
VI to
to rectify
rectify the
the
bipolar signal.
signal.
Cursors
on
the
scope
marked
the
and
passed
through
an
absolute
value
VI
to
rectify
the bipolar
bipolar
signal. Cursors
Cursors on
on the
the scope
scopemarked
markedthe
the
notch
notch channels
channels and
and all
all of
of
the
data
between
the
cursors
were
added
to
produce
an
integral.
notch
channels
and
all
of the
the data
data between
between the
the cursors
cursors were
were added
added to
to produce
produce an
an integral.
integral.
159
16x10
-3
70
14
60
2
Wire signal (mV)
Notch integral (mV)
12
E,
10
8
6
50
40
30
4
2
20
32
34
36
38
40
42
44
46
84
Laser
Laser position
position (mm)
(mm)
86
88
90
92
94
96
98
100
Carbonwire
wireposition
position(mm)
(mm)
Carbon
102
104
106
FIGURE
the left
left gives
gives aa width
width of
of a=2.82±0.76mm.
σ=2.82±0.76mm.The
Thewire
wireprofile
profileon
on
FIGURE 14.
14. The
The fit
fit to
to the
the laser
laser date
date on
on the
the
the right
right measured
measured σ=3.61±0.22mm.
a=3.61±0.22mm.
DISCUSSION
Transverse profiles of H- beams can be measured
Transverse
measured by
by scanning
scanning aa laser
laserbeam
beamacross
across
the ion beam and detecting the notch in the beam current
the
current downstream.
downstream. This
This technique
technique
is attractive
attractive because no components are in the vacuum,
is
vacuum, profile
profile measurements
measurements can
can be
be
made without
without disrupting machine operation, and measurements can
made
can be
be made
made on
on high
high
power beams. As we demonstrated on the
the SNS
SNS MEET,
MEBT, profile
profile measurement
measurement
capability can be added to an operating accelerator if a suitable window
capability
window exists
exists and
and aa
downstream current transducer is available.
downstream
Q-switched Nd:YAG
NdiYAG lasers are perfect
Q-switched
perfect for these
these measurements
measurements on
on beams
beams with
with
energies up to about 1GeV.
IGeV. These lasers are readily
energies
readily available
available with
with aa wide
wide range
range of
of
output energies.
energies. Lasers with pulse energies of close to
output
to aa Joule
Joule are
are available
available with
with
compact laser
laser heads attached to power units by
compact
by umbilicals
umbilicals which
which make
make them
them suitable
suitable
for mounting on compact platforms on beamlines.
for
Our experiments have placed the laser controller
Our
controller and
and cooling
cooling unit
unit next
next to
to the
the
beamline for
for convenience. Two laser controllers have
beamline
have failed
failed from
from radiation.
radiation. Any
Any
installation of
of aa LPM
LPM in
in aa radiation
installation
radiation area
area has
has to
to have
have the
the controller
controller in
in aa nonradiation
nonradiation
area.
We
do
not
know
the
radiation
doses
the
laser
heads
can
take.
The
area. We do not know the radiation doses the laser heads can take. The plan
plan for
forSNS
SNS
is
to
place
the
entire
laser
outside
of
the
tunnel
and
transport
the
light
by
mirrors
is to place the entire laser outside of the tunnel and transport the light by mirrors or
or
fiber optics.
optics.
fiber
The two
two LPM
LPM experiments
experiments which
which used
used beam
The
beam current
current transformers
transformers for
for beambeamcurrent
detection
produced
extremely
clean
signals
with
very
little
set
up
time.
current detection produced extremely clean signals with very little set up time. The
The
measurement at
at 750keV
750keV gave
gave aa signal/noise
measurement
signal/noise ratio
ratio at
at beam
beam center
center of
of 25dB
25dB and
and the
the
MEET experiment
experiment gave
gave 40dB.
MEBT
40dB. Measurements
Measurements on
on 200MeV
200MeV beam
beam with
with BPM
BPM striplines
striplines
have been
been made
made but
but we
we need
need to
to make
make improvements
improvements in
in the
the data
data processing.
have
processing.
160
ACKNOWLEDGEMENTS
The authors thank Brian Briscoe and Vinnie LoDestro at the BNL linac. Also we
thank Alex Ratti, Larry Doolittle and John Staples at LBNL for letting us try the
MEBT platform during commissioning. SNS personnel who have become involved
include Saheed Asadi, Warren Grice, Sasha Alexander and Tom Shea.
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161