\chapter{The Photon Beamline}
\label{chapter_photon_beamline}

\begin{center}
Sections left out here...
\end{center}

\section{The Active Collimator}

The primary photon beam collimator on the Hall D photon beam line 
has a circular aperture of diameter 3.4~mm, and is located 76~m
downstream of the radiator.  Effective collimation requires that the
photon beam spot be centered on the collimator axis to a small fraction
of its radius.  The specification of the tolerance on this alignment
during beam operation is a circle of radius 200 microns.  The reason
for this demanding requirement is illustrated in panel two of
Fig.~\ref{fig:active collimator simulation}, which shows how the
collimated flux of tagged photons varies as the beam spot centroid
deviates from the center of the collimator.  Parity violation experiments
at CEBAF have shown that it is possible to obtain electron beam position
stability at the level of 200 microns without employing active stabilization.
However, the lever arm of 80~m from the last magnetic elements on the electron
beam and the entrance to the photon collimator means that electron beam
displacements less than 100 microns back at the source might potentially
result in shifts of the photon beam spot by more than 200 microns at the
collimator position.  For this reason, the GlueX experiment has 
incorporated photon beam centroid monitoring and active stabilization
into the source design.  The device that performs this function
is the active collimator.

\begin{figure}
\begin{center}
\epsfig{file=collimasym.eps,width=2.5in}
\epsfig{file=collimsyst.eps,width=2.5in}
\caption{
\label{fig:active collimator simulation}
Results from Monte Carlo studies of the active collimator in the
Hall D photon beam line.  Panel one shows the current asymmetry
(difference over sum) between two opposing inner sectors, as the
beam position is shifted along the line between their centers.
Panel two shows the systematic shift in the rate of
tagged photons within the polarized peak that reach the GlueX target,
as a function of the shift of the photon spot from the collimator
axis.
}
\end{center}
\end{figure}

The basic design for the Hall D active collimator was originally developed
for electron beam lines at SLAC \cite{active_collimator_idea_article}.
This device is very radiation hard, highly linear, and highly stable
in its gain.  It consists of a large tungsten plate divided azimuthally
into four quadrants and radially into two rings.  Attached to each
sector is a large array of stout tungsten wires that rise out of the
block along the direction of the beam.  Beam photons interacting in
the plates create showers that radiate into the pin arrays.  The charge
asymmetry of electrons over positrons in the showers due to high-energy
delta rays called {\em knock-ons} generates a small net current in the
plates.  Currents in each of the plates measure the photon beam
intensity integrated over the region of the pin arrays.  Current
differences between the azimuthal sectors in a ring are the signature
of beam displacement from the central axis.

The first panel in Fig.~\ref{fig:active collimator simulation} 
shows the current asymmetry between opposing inner sectors of the active
collimator as a function of the offset between the beam centroid and
the active collimator axis, derived from Monte Carlo simulation.  The
results show that the device achieves maximum sensitivity when the
beam is nearly centered, and that 5\% resolution in the current
asymmetry is sufficient to measure the beam centroid offset with an
error of 200~microns r.m.s.  The second panel in 
Fig.~\ref{fig:active collimator simulation} shows the collimated fraction
of the photon beam in the coherent peak as a function of the offset
of the beam centroid from the collimator axis.  This variable reflects
the sensitivity of the collimated spectrum to beam spot offsets at
the collimator, and shows that 200~microns is about the right tolerance
on the alignment if the collimated spectrum is to be stable at the 1\%
level.

\begin{figure}
\begin{center}
\epsfig{file=colbeamtest.eps,width=3.5in}
\caption{
\label{fig:active collimator beam test}
Results from a test of the prototype active collimator in the
Hall B coherent photon beam during 2007.  The two curves are
the currents in inner opposing sectors, as the beam is moved
across the collimator face along an axis passing through the
centers of the two sectors.  Only the two inner sectors were
connected to amplifiers during this test.
}
\end{center}
\end{figure}

Fig.~\ref{fig:active collimator beam test} shows the measured response
of the active collimator in a beam test conducted in the Hall B coherent
bremsstrahlung beam line using a prototype of the final device.  The two
curves in the plot show the current in two opposing inner sectors as
the collimator is moved across the beam.  Even though the intensity
of the Hall B beam was only a few percent of the full intensity of
the photon beam in Hall D, it is clear from the smoothness and symmetry
of the curves in the plot that the errors on the current measurement
are less than 5\%.

The current-sensitive amplifiers used to read out the active collimator
have sufficient bandwidth to allow a beam centroid measurement many
times per second, which allows a feedback signal to be derived from
it that can be used to control micro-steering magnets on the electron
beam to suppress drifts in beam position, and also beam motion at
60~Hz and its first few harmonics.  A feedback system is being designed,
in concert with the accelerator group, to enable a fast-feedback beam
stabilization system.  It is expected that such a system would only be
usable at beam currents above 50-100~$\mu$A.

