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\documentclass{JINST}


\title{A radiation-hard beam position monitor for high energy photon beams}

\author{R.T. Jones$^a$\thanks{Corresponding author.},
A.M. Carroll$^a$, 
C. Pelletier$^a$,
and I. Senderovich$^a$\\
\llap{$^a$}University of Connecticut,\\
  2152 Hillside Road, Storrs, CT, USA\\
  E-mail: \email{richard.t.jones@uconn.edu}}


\abstract{A novel photon beam position monitor has been designed and
constructed for use in the coherent bremsstrahlung beam collimator in
Hall D at Jefferson Lab.  The beam position monitor will be placed directly
in front of the tungsten collimator, to count the relative flux of
photons on the face of the collimator as a function of radius and
azimuthal angle.  Real-time information on the centroid of the photon
beam spot at the collimator will be fed back to steering magnets on the
electron beam to maintain the required alignment of $\pm 200$~$\mu$m
between the photon beam centroid and the collimator axis.  A prototype of
this detector was built and tested in the tagged photon beam in Hall B
in Spring 2011.  Results from this test show a beam centroid position
resolution of 46~$\mu$m rms when the beam centroid is within 5~mm of
the collimator axis, and no worse than 120~$\mu$m rms for misalignments
between 5~mm and 15~mm.  These results exceed the requirements for Hall D
with a comfortable margin, and were obtained with photon beam intensities
considerably lower than those planned for use in Hall D.  Detector signals
were found to be close to the middle of the designed dynamic range of
$10^6$.  The bandwidth limit was determined to be 1~kHz, sufficient to
monitor the dominant frequency components of beam motion that are expected
for the 12~GeV beam.
}


\keywords{beam position monitor; active collimator; coherent bremsstrahlung; photon beam profile}

\begin{document}

\section{Collimation requirements}

The GlueX experiment~\cite{GlueXref1} is designed to search for exotic mesons
in the mass range 1.5 -- 2.5~GeV/c$^2$ produced by a 9~GeV polarized photon
beam interacting in a liquid hydrogen target.  The polarized photon beam will
be generated from a primary electron beam from the CEBAF accelerator at
Jefferson Lab in Newport News, Virginia following the upgrade of the
accelerator to 12~GeV.  The secondary photon beam is produced by passing the
electron beam through a thin bremsstrahlung radiator, then passing the two
beams through a dipole ``tagger'' magnet which bends the electrons into a
beam dump, and at the same time analyzes the energy of those electrons which
lost a significant fraction of their energy to a photon in the radiator.
A timing coincidence between an event in the photoproduction target and a
counter on the tagger focal plane tags the energy of the beam photon that
produced the event.

Normally a bremsstrahlung beam is unpolarized because it is an incoherent
superposition of photons generated by scattering from each of the atoms in
the radiator independently.  However, if the radiator is crystaline and it
is oriented so that one of the crystal planes is slightly inclined with
respect to the incident electron direction, a process known as coherent
bremsstrahlung takes place in which the momentum transfer from the electron
is absorbed by all of the atoms in the crystal instead of by a single atom.
Coherent bremsstrahlung photons are polarized in the plane containing the
incident electron momentum and the normal to the scattering plane in the
radiator crystal.

The photon beam for the GlueX experiment will be generated from a diamond
crystal radiator that is oriented such 12~GeV electrons passing through it
produce coherent bremsstrahlung peak in the photon intensity spectrum at
9~GeV, as shown in figure~\ref{fig:collimspec}.  The upper curve shows
the general characteristics of a bremsstrahlung spectrum, dropping with
photon energy like $1/E$, except that discrete peaks with sharp edges
appear at particular locations in the spectrum.  This spectrum was computed
for the particular radiator orientation that places the largest peak edge
at 9~GeV.  

The lower curve in the same figure shows the effect of collimation on the
beam rate spectrum.  Collimation can only remove flux from the beam, so the
rates for all energies are lower after the collimator than before, but the
collimation cuts much more deeply into the incoherent component (the baseline
under the coherent peaks) than it does into the coherent peaks.  In fact,
the height of the peak at 9~GeV above the underlying continuum is essentially
the same for the upper and lower curves, even though the continuum has been
reduced by roughly a factor 4.  Further reducing the collimator aperture 
would begin to reduce the coherent and incoherent yields together; in fact
the post-collimator rate spectrum shown in figure~\ref{fig:collimspec} is
very close to optimal for Hall D.

\begin{figure}
\begin{center}
\includegraphics[width=.75\textwidth]{collimspec.pdf}
\caption{Photon beam rate spectrum under nominal running conditions of
polarized beam for the GlueX experiment.  The upper (black) curve shows
the rate spectrum at the entrance to the collimator, and the lower curve
(red) curve shows the spectrum after the collimator.  The flat baseline
(green) under the lower curve indicates the remaining incoherent (unpolarized)
component of the beam after collimation, showing that it is a small fraction
of the coherent bremsstrahlung component in region of the primary coherent
peak.
\label{fig:collimspec}
}
\end{center}
\end{figure}

From figure~\ref{fig:collimspec} one deduces that 84\% of all photons that
are produced in the radiator are absorbed on the collimator, and only 16\%
of them reach the target.  The Hall D collimator has a circular aperture
of diameter 3.4~mm and is located a distance 76~m from the radiator.  This
corresponds to a collimation half-angle of 0.53 in units of the characteristic
bremsstrahlung angle $m_e/E$, where $E=12GeV$ is the energy of the primary
electron beam and $m_e$ is the mass of the electron.  Under these conditions,
the collimated beam contains $10^8$~$\gamma$/s of tagged photons in the
energy range 8.4 -- 9.0~GeV, with an average linear polarization within that
window of 36\%.

\begin{figure}
\begin{center}
\includegraphics[width=.75\textwidth]{misspolar.pdf}
\caption{Photon beam polarization after collimation as a function of the
offset of the beam centroid from the collimator axis.  The polarization is
the average value within the tagged window 8.4 -- 9.0~GeV containing the
primary coherent peak.
\label{fig:misspolar}
}
\end{center}
\end{figure}

Alignment of the photon beam on the collimator is accomplished by steering
the electron beam using steering magnets located upstream of the radiator.
These magnets are more than 80~m from the collimator, and must be capable
of steering the electron beam such that its spot projected downstream onto
the face of the collimator is centered on the aperture to within a fraction
of its radius.  A quantitative measure of the sensitivity to misalignment
at the collimator is shown in figure~\ref{fig:misspolar}, which shows the
average beam polarization in the primary coherent peak as a function of the
displacement of the photon beam centroid from the collimator axis. The design
goal for the systematic error on the average beam polarization is 1\%,
which corresponds to a maximum photon beam misalignment at the collimator
of $\pm 200$~$\mu$m.

The electron beam line design contains $x$ and $y$ RF beam position monitors
in the straight region just upstream of the radiator, but their position
resolution and absolute stability are not sufficient to insure alignment
at the collimator to the required degree of accuracy.
The purpose of the active collimator is not just to monitor the position
of the photon beam at the collimator, but to provide a pair of signals
representing the $x,y$ of the beam centroid in real time, so that they
can be fed back to electron beam steering magnets and form an orbit lock
on the electron beam at its approach to the radiator.  Such a feedback
loop must have sufficient bandwidth to constrain the principal frequency
components of the beam motion.  Besides slow drifts in the sub-Hz region,
beam motion is expected to be dominated by the line frequency of 60~Hz
and its low-lying harmonics.  Actual measurements with the 6~GeV beam in
Hall B confirm this, as reported later in this article.  The design goal
for the active collimator is to obtain an absolute accuracy of 200~$\mu$m
in $x$ and $y$ for all frequencies up to a bandwidth of 600~Hz.

\begin{table}
\begin{center}
\caption{Summary of design requirements for the Hall D active collimator.
\label{table:requirements}
}
\begin{tabular}{}{lcl}
\hline\hline
quantity & value & unit \\
\hline
centroid position rms & 200 & $\mu$m \\
maximum beam power & 3.6 & W \\
minimum beam power & 1.2 & mW \\
bandwidth (3db power) & 500 & Hz \\
\hline\hline
\end{tabular}
\end{center}
\end{table}

All of the design requirements for the active collimator are collected
together in table~\ref{table:requirements}.




\section{Detector and readout}

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\section{Simulation}

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asoiuh siuhihosi soiu hskjd.

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soiu hskjd. Asoiuh siuhihosi soiu hskjd asoiuh siuhihosi soiu hskjd.

\section{Beam test results}

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\section{Conclusions}

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\acknowledgments

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hskjd. Asoiuh siuhihosi soiu hskjd asoiuh siuhihosi soiu hskjd. Asoiuh
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Asoiuh siuhihosi soiu hskjd asoiuh siuhihosi soiu hskjd.


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\end{document}
