**LASER BEAM-PROFILE MONITOR DEVELOPMENT AT BNL FOR SNS**

R. Connolly, P. Cameron, J. Cupolo, C. Dawson, C. Degen, A. Della Penna,

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 using laser photoneutralization is
being developed at Brookhaven National Laboratory \[1\] for use on the
Spallation Neutron Source (SNS) \[2\]. An H^-^ ion has a first
ionization potential of 0.75eV and can be neutralized by light from a
Nd:YAG laser (λ=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, and the perturbation of the beam current caused by
the laser is measured. The laser trajectory is stepped across the ion
beam generating a transverse profile. Proof-of-principle experiments
were done at 750keV and 200MeV. Also a compact scanner prototype was
used at Lawrence Berkeley National Laboratory (LBNL) \[3\] during
commissioning of the SNS RFQ.

**1. INTRODUCTION**

The H^-^ ion, a bound state of one proton and two electrons, has no
excited bound states \[4\]. When the ion gains 0.75eV one of the
electrons becomes a free particle. Negative hydrogen ions can thus be
neutralized with beams of light with λ≤1.67μm. The detached electron is
boosted into the continuum so the cross section variation with photon
energy is a broad curve with a gentle maximum at 1.3eV (λ=930nm), fig. 1
\[5\].

Photoneutralization can be used to select a small portion of an H^-^
beam for measurement. For instance at Los Alamos National Lab
longitudinal emittance has been measured by neutralizing a short phase
slice of a bunch with a pulsed laser, removing the rest of the beam with
a magnet, and measuring the momentum spread of the selected beam \[6\].
Also at Los Alamos the transverse emittance of an intense beam was
measured by passing the light from a Q-switched laser through the full
cross section of the beam. A magnet removed the charged beam, and a slit
and parallel-channel collector was placed after the clearing magnet.
Beam loading on the slit was avoided and there were no space-charge
effects since the measurement was made on neutral beam \[7\].

This paper reports on the development of a profile measurement system in
which a laser beam serves the same role as the wire in a stepped-wire
profile scanner. Focused light from a Nd:YAG laser passes through the
ion beam and neutralizes some of the ion beam it passes through.
Downstream either the dip in the beam current is measured or the pulse
of detached electrons is deflected into a Faraday cup and measured. The
laser beam is stepped across the ion beam and a profile is built up
\[8\].

![](media/image1.pct){width="3.8006944444444444in"
height="2.3465277777777778in"}

Figure 1: Calculated cross section for H^-^ photoneutralization as a
function of photon wavelength. Data are from a table in ref. \[5\].

This technique is being considered for SNS as an alternative to placing
carbon wire scanners into the superconducting linac. 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.

**2. PHOTONEUTRALIZATION**

Figure 1 shows the photoneutralization cross section as a function of
photon wavelength in the center-of-mass frame. All the experiments
reported here use λ=1064nm light. For low ion-beam velocities the
neutralization cross section is about 3.7 x 10^-17^cm^2^. As the ion
velocity increases, the photon energy in the ion rest frame increases.
If the laser beam crosses the H^-^ beam at a lab angle of θ~L~ the
photon energy in the moving frame is Lorentz shifted by the amount
\[9\],

E~CM~=γE~L~\[1-βcos(θ~L~)\] (1)

For the SNS laser installation θ~L~ will be 90° so, at the full energy
of 1GeV, the center-of-mass photon energy will be double the lab energy.
For these measurements and probably for the final SNS installation the
laser will be a Q-switched Nd:YAG laser operating at its fundamental of
λ=1064 nm so at the full energy of 1GeV the neutralization cross section
will be about 70% of the low energy cross section.

The fraction of beam ions which get neutralized passing through the
laser beam is,

*f~neut~* = 1 - e^-σ(E)Ft^ (2)

Here σ(E) is the energy-dependent cross section, F is the photon flux,
and t is the time the ion is in the laser light. For ion beam energies
up to about 200MeV the cross section changes very little and the time
the ions take to cross the laser path decreases so the neutralization
fraction drops with increasing beam energy.

The photon flux in the moving reference is also transformed the same as
the photon energy \[10\],

F~CM~=γF~L~\[1-βcos(θ~L~)\] (3)

The relativistic increase in photon flux compensates for the decreased
time the ions spend in the laser path and for the drop in cross section
as the Lorentz-shifted photon wavelength drops below 700nm resulting in
an almost flat neutralization fraction from 400MeV to 1GeV.

For example, the laser on the SNS MEBT experiment produces a 20ns-long
pulse with an output energy of 50mJ. It is focused to a rectangular spot
1mm wide by 3mm along the beam. The approximate variation of
neutralization fraction with beam energy this laser will produce is
shown in fig. 2.

![](media/image2.pct){width="3.245138888888889in"
height="2.2868055555555555in"}

Figure 2: Calculated neutralization fraction vs. beam energy for a
20ns-long, 50mJ laser pulse focused to a spot size of 1mm x 3mm.

**3. 750 KeV EXPERIMENT**

Our first profile measurement was made on the BNL linac between the rfq
and the first drift tube linac tank. A light pulse from a 200mJ/pulse
Q-switched Nd:YAG passed through the 750keV H^-^ beam from the linac rfq
neutralizing \>95% of the beam the light passed through, fig. 3. The
beam passed through a weak magnet to remove the detached electrons, and
a downstream current transformer measured a dip in the beam current
which was proportional to the fraction of the beam hit with the light,
fig. 4. The laser beam was stepped across the ion beam and the profile
constructed by plotting the depth of the current notch vs. laser beam
position, fig. 5.

![](media/image3.pct){width="3.25in" height="2.1875in"}

Figure 3: Laser scanner experiment on BNL linac. The first of two 10 Gm
dipole magnets removes the free electrons from the beam and the second
straightens the beam.

![](media/image4.pct){width="1.8444444444444446in"
height="1.476388888888889in"}

Figure 4: Scope trace of the current transformer signal showing notch
created by the laser pulse.

![](media/image5.pct){width="2.6125in" height="1.8138888888888889in"}

Figure 5: Measured horizontal profile with Gaussian fit to the data.

**4. SNS MEBT MEASUREMENTS**

We built a laser platform that attached to a wire-scanner chamber on the
SNS medium energy beam transport line (MEBT) at LBNL. Mounted on the
platform, fig. 6, are a 50mJ/pulse laser head, a lens holder, and three
linear actuators which move 45° mirrors. The top-left mirror is a
switch. For vertical scans it is inserted into the laser beam directing
the laser light to the bottom-left mirror which moves vertically. For
horizontal scans it is removed and the top-right mirror is moved
horizontallly. We installed the platform on the MEBT after it was under
vacuum and ready for first beam.

A 300mm-focal-length cylindrical lens is mounted directly in front of
the laser head and the two optical path lengths from the lens to the
beam center are the same. The lens produces a 1mm-wide by 3mm-long light
ribbon across the beam producing a measurement window of rms width
0.3mm. Since the measured rms width of the horizontal profile is 1.60mm
the width of the laser beam caused 2% broadening of the profile.

![](media/image6.jpeg){width="3.045138888888889in"
height="2.8569444444444443in"}

Figure 6: Laser scanning platform mounted on the SNS MEBT wire scanner
chamber.

The signal was detected with the existing beam transformer at the end of
the MEBT. The BCM signal was fed into a LeCroy LT374L scope with
ethernet connection \[11\]. A math channel on the scope was used to
average for several pulses to reduce noise and rf pickup. For the
profile shown in fig. 7, the signals from 25 beam pulses were averaged
giving about 40dB signal/background in the beam center. This was the
first beam profile measured during the SNS MEBT commisioning.

The experiment was controlled in Labview. The program switched the
laser, 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 data the program summed the channels
between the cursors, summed an equal number of channels before the
pulse, and subtracted these two \'integrals\' to give one data point in
the profile.

![](media/image7.pct){width="2.3409722222222222in"
height="1.8347222222222221in"}

Figure 7: Horizontal beam profile measured on the SNS MEBT with the
laser profile monitor. The Gaussian fit to the data has a measured width
of σ=1.60±0.04mm

**5. MEASUREMENTS AT 200 MEV**

One concern driving this development is the risk of placing carbon-wire
scanners near superconducting cavities where the formation or release of
particulate matter (from actuator mechanisms, hot wires, breaking wires,
etc.) may contaminate cavity surfaces. Superconducting cavity surfaces
must be clean to semiconductor-grade standards. Contamination can
introduce field-emission sites leading to degraded performance from
field-emission heating and reduced operating gradients. Once a cavity is
degraded remedial action, which may include rebuilding the cryomodule,
is required in order to restore the cavities to their specified level of
performance \[12\]. The SNS linac has normal-conducting rf cavities to
accelerate the beam to 185MeV and superconducting cavities to take the
beam to 1 GeV. For this reason we needed to make a test in the energy
range of the superconducting linac (SCL)

The apparatus that was used to measure the 750keV profiles was installed
in the linac-AGS transfer line, which is no longer used for beam
transport, to measure 200-MeV beam. Also a single-plane RHIC stripline
beam position transducer was installed before and after the beam box.
The goal of this experiment was to measure SNS-energy ion beams using
stripline beam position monitors (BPMs) to measure the laser notch.

In the SCL there are BPMs between rf tanks but only one current
transformer at the exit of the linac. Using the striplines as detectors
gives us access to an upstream and downstream detector spaced by a
single rf structure. Using the transformer for signal detection will
require good beam transmission through the full linac before profiles
can be made at any point.

The arrangement of the laser and optics on the linac beamline is shown
in fig. 8. A CFR200 laser from Big Sky Laser \[13\] is mounted on a
shelf at the top left. Three 45° mirrors are mounted inside the vacuum
on rail-mounted trollies driven by linear motion feedthroughs. The
top-left mirror was used to switch between vertical and horizontal scans
and the other two were used for scanning. The top-right mirror scanned
horizontally and the bottom-left mirror scanned vertically. Arms to hold
lenses are attached to the scanning mirror trollies. Figure 9 is a
photograph of the tunnel installation.

![](media/image8.pct){width="3.248611111111111in" height="2.73125in"}

Figure 8: Laser scanning assembly installed on linac beamline. View is
looking up beamline.

![](media/image9.jpeg){width="3.2354166666666666in"
height="2.426388888888889in"}

Figure 9: Laser profile monitor at 200 MeV in the BNL linac.

Experiments at 200MeV were conducted over a period of several months.
During this time several techniques were tried to produce clean
measurements of the laser-induced current notch. The detection method
that has worked best so far is shown in fig. 10.

Signals from one upstream and one downstream stripline are passed
through filters at the linac bunch frequency of 200MHz to remove higher
harmonics and adjusted to be in phase. A 180° hybrid subtracts the two
signals to produce a nulled signal in the absence of laser
neutralization. The laser pulse causes a signal imbalance which appears
either as a wide or narrow spot in the rf envelope depending on how the
signals are combined.

The 200-MHz carrier frequency is removed from the difference signal
using a homodyne receiver \[14\]. The difference signal is mixed with a
reference signal from the linac rf system and the mixer output is passed
through a 50-MHz low-pass filter. Figure 11 shows the signal generated
by the laser pulse striking a photodiode and the output of the mixer.
The beam-current notch was generated by averaging 50 pulses.

![](media/image10.pct){width="3.2465277777777777in"
height="1.2027777777777777in"}

Figure 10: Circuit used to detect current notch in beam.

![](media/image11.pct){width="3.2444444444444445in"
height="2.433333333333333in"}

Figure 11: Oscilloscope trace of laser pulse (top) and beam-current
notch generated by circuit in fig. 10.

The laser platform was located 40m from the exit of the linac. In this
drift distance the energy jitter of 2.2MeV in the linac beam causes a
time-of-flight jitter of ±1ns which is ±70° at 200MHz. For this reason
there was a lot of rf noise in this measurement which will be not be
present in measurements made along the SNS linac.

Figure 12 shows beam profiles measured with the laser (dotted line) and
with a carbon wire scanner located in the laser beam chamber. The
Gaussian fitted curve to the laser data has an rms width of
σ~laser~=6.67±0.6mm. The spatial flux distribution of the laser spot was
not measured but a uniform round laser spot of 6mm diameter would have
an rms width of 1.7mm. When the profile broadening caused by the size of
the laser beam is removed the measured beam width is
σ~laser~=6.45±0.6mm. The width of the fit to the carbon wire data is
σ~wire~=5.7±0.3mm.

![](media/image12.pct){width="3.24375in" height="2.35625in"}

Figure 12. Beam profiles measured with the laser scanner (blue, dotted
curve with round markers) and with the carbon wire scanner.

**6. DISCUSSION**

Transverse profiles of H- beams can be measured by scanning a laser beam
across the ion beam and detecting the notch in the beam current
downstream. This technique is attractive because no components are in
the vacuum, profile measurements can be made without disrupting machine
operation, and measurements can be made on high power beams. As we
demonstrated on the SNS MEBT, profile measurement capability can be
added to an operating accelerator if a suitable window exists and a
downstream current transducer is available.

Q-switched Nd:YAG lasers are perfect for these measurements on beams
with energies up to about 1GeV. These lasers are readily available with
a wide range of output energies. Lasers with pulse energies of close to
a Joule are available with compact laser heads attached to power units
by umbilicals which make them suitable for mounting on compact platforms
on beamlines.

Our experiments have placed the laser controller and cooling unit next
to the beamline for convenience. Two laser controllers have failed from
radiation. Any installation of a LPM in a radiation area has to have the
controller in a nonradiation area. We do not know the radiation doses
the laser heads can take. The plan for SNS is to place the entire laser
outside of the tunnel and transport the light by mirrors or fiber
optics.

The two LPM experiments which used beam current transformers for
beam-current detection produced extremely clean signals with very little
set up time. The measurement at 750keV gave a signal/noise ratio at beam
center of 25dB and the MEBT experiment gave 40dB. Measurements on 200MeV
beam using BPM striplines for the current transducers have been made but
they suffered from large rf backgrounds. These backgrounds are from
phase jitter caused by the 1% energy jitter of the linac beam together
with the 240ns flight time from linac to detector. The SNS installations
in the linac will have far more phase stable beam.

**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.

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