Laser Beam Profile Monitor Development at Bnl for Sns. Page: 2 of 5
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The fraction of beam ions which get neutralized
passing through the laser beam is, '(2)
fnew = 1 - e
Here o(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],FcM-=yFLE1- cos(9O)]
(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
5OmJ. 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. 2.0 z00 400 000 000
Beam energy (MeV)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.
Current transformer
5 H- beam
scope
Signal Q switched
Nd:YAGJ0 ns pulse
Trigger
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.
Figure 4: Scope trace of the current transformer signal
showing notch created by the laser pulse.20
1000 1200
iooo zoo2Figure 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 which0 65 70
MIRROR POSfTION (mm]75 B0
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 theC
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Connolly, R.; Cameron, P.; Cupolo, J.; Dawson, C.; Degen, C.; Della Penna, A. et al. Laser Beam Profile Monitor Development at Bnl for Sns., article, August 19, 2002; Upton, New York. (https://digital.library.unt.edu/ark:/67531/metadc739113/m1/2/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.