Considerations on ODR beam-size monitoring for gamma = 1000 beams Page: 1 of 5
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CONSIDERATIONS ON ODR BEAM-SIZE
MONITORING FOR GAMMA = 1000 BEAMS*
A.H. Lumpkin, Fermilab, Batavia, IL, U.S.A. 60510
C.-Y. Yao, Argonne National Laboratory, Argonne, IL U.S.A. 60439
E.Chiadroni, M. Castellano, LNF-INFN, Frascati, Italy, A. Cianchi, Univ. of Rome Tor Vergata
We discuss the feasibility of monitoring the beam size
of y--1000 beams with 3000 times more charge in a video
frame time and with a more sensitive 12- to 16-bit camera
than were used in the previous electron beam studies at 7
GeV at the Advanced Photon Source. Such a beam would
be generated at Fermilab in a new facility in the coming
years. Numerical integrations of our base model show
beam size sensitivity for + 20% level changes at 200- and
400-pm base beam sizes. We also evaluated impact
parameters of 5 ay and 12 ay for both 800-nm and 10-pm
observation wavelengths. The latter examples are related
to a proposal to apply the technique to an ~ 0.98 TeV
proton beam, and this study shows there are trades on
photon intensity and beam size sensitivity to be
considered at such gammas. In addition, we report on first
results at y--1800 on a superconducting if linac.
Characterization of the high-power electron beams of
the superconducting rf (SCRF) accelerator to be installed
in the New Muon Laboratory (NML) building at Fermilab
will be an important aspect of the project . Beam size,
position, divergence, emittance, and bunch length
measurements are all of interest. Due to the projected high
beam power with 3000 micropulses of up to 3 nC each in
a macropulse at 5 Hz at eventually up to 1800 MeV, the
need for nonintercepting (NI) diagnostics is obvious.
Although position is readily addressed with standard if
beam position monitors (BPMs), the transverse size, and
hence emittance, are less easily monitored
noninterceptively in a linear transport system. Besides an
expensive laser-wire system, one of the few viable
solutions appears to be the use of optical diffraction
radiation (ODR) [2-8] which is emitted when a charged-
particle beam passes near a metal-vacuum interface.
Appreciable radiation is emitted when the distance of the
beam to the screen edge (impact parameter) b ~ y/27,
where y is the Lorentz factor and , is the observation
wavelength. Previous near-field imaging experiments at
the Advanced Photon Source (APS) with 7-GeV beams
used an impact parameter of 1.25 mm from a single edge
of a plane as compared to the scaling factor of ~1.4 mm
(with an assumed operating wavelength of 0.628 gm) .
The near-field images were obtained with a single, 3-nC
*Work supported by U.S. Department of Energy, Office of Science,
Office of High Energy Physics, under Contract No. DE-AC02-
micropulse using a standard CCD camera. Since for the
NML case, with its much lower gamma, the fields are
reduced exponentially as e2 t. We either have to use the
longer wavelengths in the NIR or FIR or have more
charge integrated in the image and a more sensitive
camera. The NML design-goal beam intensity gives a
factor of 3000, and the intensified or low-noise camera
should give another factor of 1000. These two factors
combined should allow visible to IR near-field imaging of
a beam that is up to 10 tol5 times lower in gamma than
the APS case, if similar impact parameters can be used.
We considered focus-at-the-object or near-field
imaging optics and established that the perpendicular
polarization component of ODR has the beam-size
sensitivity that would be needed for a transport line with
400- to 1000- m rms sizes in the x-plane. These
parameters are compatible with the proposed test-area
location in the lattice after the SCRF linac  as shown in
Fig. 1. In addition, we evaluated the possible extension of
the technique to a very high intensity hadron beam with
y-1000 as would be found in the Fermilab Tevatron
[2,10]. In the latter application, we also consider a larger
impact parameter constraint and the possible
compensation of the consequently reduced signal by
going to much longer wavelengths. We show there are
trades to be considered in this paradigm.
The basic strategy is to convert the particle-beam
information into optical radiation and to take advantage of
the power of imaging technology to provide two-
dimensional displays of intensity information. These
images can be processed for beam size information.
Possible radiation sources are optical transition radiation
(OTR), ODR, and optical synchrotron radiation (OSR).
For completeness, the near-field ODR model as described
in Ref. 6 is provided here.
As stated before, ODR is produced when an electron
beam passes near a region where different dielectric
materials are present. This is generally a vacuum-to-metal
interface, and the theory [3-6] is usually for the far-field
diffraction pattern produced by a beam passing through
apertures or slits in conducting planes. In the present case,
we effectively integrate over angle and frequency since
our optical system is focused on the ODR source itself,
i.e. the near-field image on the screen. Therefore we
proposed a simplified model of the
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Lumpkin, A.H.; /Fermilab; Yao, C.-Y.; /Argonne; Chiadroni, E.; Castellano, M. et al. Considerations on ODR beam-size monitoring for gamma = 1000 beams, article, April 1, 2008; Batavia, Illinois. (digital.library.unt.edu/ark:/67531/metadc900483/m1/1/: accessed October 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.