**Laser Profile Measurements of an H^-^ Beam**

R. Connolly, P. Cameron, J. Cupolo, M. Grau, M. Kesselman,

C-J. Liaw, and R. Sikora

Brookhaven National Lab, Upton, NY, USA

*Abstract*

A non-intercepting beam profile monitor for H^-^ beams is being
developed at Brookhaven National Lab. An H^-^ ion has a first ionization
potential of 0.75eV. Electrons can be removed from an H^-^ beam by
passing light from a near-infrared laser through it. Experiments have
been performed on the BNL linac to measure the transverse profile of a
750keV beam by using a Nd:YAG laser to photoneutralize narrow slices of
the beam. The laser beam is scanned across the ion beam neutralizing the
portion of the beam struck by the laser. The electrons are removed from
the ion beam and the beam current notch is measured.

.

**1. INTRODUCTION**

The Spallation Neutron Source (SNS) under construction at Oak Ridge
National Lab consists of a 1GeV H^-^ linear accelerator (linac), a
storage ring, target, and connecting beam-transport lines \[1\]. The
linac delivers a 1.04 ms macropulse which is chopped into 10^3^ pulses
of 10^11^ protons each. As this pulse enters the storage ring from the
linac each chopped pulse lines up longitudinally with all proceeding
pulses forming a single 550-ns bunch and a 250-ns gap. After stacking
beam in the storage ring for 1000 turns the proton beam is dumped onto a
metal target producing a 550 ns pulse of neutrons.

Profiles of the H^-^ beam will be measured in the medium energy
transport line (MEBT) 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.

We are developing a laser beam profile monitor (LPM) which is
non-invasive and suitable for continuous profile monitoring. The
technique selects a transverse slice of an H^-^ beam by
photoneutralization by a laser beam \[2,3\]. An H^-^ ion has a first
ionization potential of 0.75eV and can be neutralized by interacting
with a photon with wavelength\<1500 nm, fig.1. The 1064-nm light from a
Nd:YAG laser is very near the peak of the cross section.

![](media/image1.wmf){width="3.9770833333333333in"
height="2.4520833333333334in"}

Figure 1: Calculated cross section for H^-^ photoneutralization as a
function of photon wavelength. Data are from a table in ref. \[4\].

**2. LINAC EXPERIMENT**

Figure 2 shows the experiment on the BNL linac. A light pulse from a
Q-switched Nd:YAG passes through the 750 keV H^-^ beam from the linac
rfq neutralizing most of the beam the light passes through. A downstream
current transformer measures a dip in the beam current which is
proportional to the fraction of the beam hit with the light, fig. 3. The
laser beam is stepped across the ion beam and the profile is constructed
by plotting the depth of the current notch vs. laser beam position.

![](media/image2.wmf){width="3.2493055555555554in" height="2.04375in"}

Figure 2: 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/image3.wmf){width="2.696527777777778in"
height="2.1395833333333334in"}

Figure 3: Scope trace of the current transformer signal showing notch
created by the laser pulse.

The arrangement of the laser and optics on the linac beamline is shown
in fig. 4. A CFR200 laser from Big Sky Laser \[5\] is mounted on a shelf
at the top left. Three 45° mirrors are mounted inside the vacuum on
linear motion feedthroughs. The top-left mirror is used to switch
between vertical and horizontal scans and the other two do the scanning.
The top-right mirror scans horizontally and the bottom-left mirror scans
vertically. Both scanning mirrors are shown with arms to hold lenses. In
this experiment the lenses were not installed.

![](media/image4.wmf){width="3.248611111111111in"
height="2.172222222222222in"}

Figure 4: Laser scanning assembly installed on linac beamline. View is
looking up beamline.

The CFR200 puts out 200 mJ pulses that are about 10 ns long. Without
lenses the beam diameter is about 0.6 cm giving a photon flux of 3.5 x
10^26^/cm^2^s. The neutralization fraction *f~neut~* is calculated from,

*f~neut~* = 1 - e^-(E)Ft^ (1)

where (E)=3.7 x 10^-17^cm^2^ is the cross section from fig. 1, F is the
photon flux and t is the flight time of the ion beam through the laser
beam. The flight time of 750keV ions through the 0.6cm diameter beam is
420ps. Using these numbers we calculate that over 99% of the ions
passing through the center of the laser beam were neutralized.

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.

**3. MEASURED PROFILES**

Figure 5 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.

![](media/image5.wmf){width="2.4944444444444445in"
height="1.7277777777777779in"}

![](media/image6.wmf){width="2.7430555555555554in"
height="1.7951388888888888in"}

Figure 5: Measured horizontal (top) and vertical beam profiles.

The rms widths of the two fitted curves are ~x~=3.32±0.05 mm and
~y~=7.3±0.6 mm. These values agree with expectations from previous
measurements at this location, however for this experiment there was no
profile measurement by another method.

These 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 is not uniform over the spot.

**4. DISCUSSION**

Beam profiles of an H- beam can be measured by laser photodetachment
followed by current measurement. To automate the data collection we are
going to use a gated boxcar integrator triggered by the Q-switch timing
output of the laser. The output of the integrator will be digitized and
averaged over several laser bursts to accommodate shot-to-shot
variations in the laser power output and the ion beam current.

A compact laser scanning station, fig. 6, is being built for measuring
profiles in the SNS medium energy transport line between the rfq and the
linac entrance. This will use a 50mJ/pulse laser and all of the optical
components will be mounted on a 35x20 cm plate. The plate will attach to
tapped holes on the viewports of the wire-scanner chamber.

![](media/image7.wmf){width="3.248611111111111in"
height="2.5256944444444445in"}

Figure 6: Laser profile scanner plate that will be tested on the SNS
beamline between the rfq and linac entrance. It is shown mounted to the
wire-scanner chamber.

**Acknowledgements**

We thank Vincent Lo Destro and Brian Briscoe for their help in setting
up the experiment and for providing support at the linac. Robert Shafer
at LANL has done calculations and picked out the laser. This work was
performed under the auspices of the United States Department of Energy.

**REFERENCES**

\[1\] http://www.sns.gov/

\[2\] R.C. Connolly, K.F. Johnson, D.P. Sandoval and V. Yuan, \"A
transverse phase-space measurement technique for high-brightness H^-^
beams\", Nucl. Instr. and Meth. A132 (1992) 415-419.

\[3\] V.W. Yuan, R.C. Connolly, R.C. Garcia, K.F. Johnson, K.
Saadatmand, O.R. Sander, D.P. Sandoval and M.A. Shinas, \"Measurement of
longitudinal phase space in an accelerated H^-^ beam using a
laser-induced neutralized method\", Nucl. Instr. and Meth. A329 (1993)
381-392.

\[4\] J.T. Broad and W.P. Reinhardt, Phys. Rev. A14 (6) (1976) 2159.

\[5\] Big Sky Laser Technologies, Inc., Bozeman, MT 59715.