\chapter{Introduction}\label{chapter_Introduction}

One of the unique opportunities presented by a \cebaf{} upgrade to energies
of 12~GeV and beyond is the possibility of generating high-intensity
continuous photon beams for high-energy photoproduction experiments. In this
regime, photon beams represent an interesting extension to the meson 
spectroscopy program that has been actively pursued using beams of
pseudoscalar mesons at hadron accelerator laboratories: with high
energy photons one has essentially a beam of {\em{vector}} mesons.
It is difficult, in fact, to conceive of any other way to obtain such 
a vector beam.

A key component of the 12~GeV Jefferson Lab upgrade is the \GX\
experiment, devoted to the search for mesons with gluonic excitations.
The experiment requires a photon beam with energies in the range 8-9~GeV
and linear polarization.  The preferred method for producing such a beam
is coherent bremsstrahlung in an oriented crystal.  Electrons of 12~GeV
from the accelerator pass through a thin diamond radiator, generating
an intense beam of high-energy photons with a continuous energy spectrum
that is dominated by a single peak.
A significant fraction of the total power
in the beam is concentrated inside this peak, which has a width of less than
5\%~f.w.h.m.  At a fixed electron beam energy $E_{0}$, the peak energy 
of the photon beam can be varied anywhere up to 90\%~$E_0$ simply by
rotating the crystal.  The photon spectrum inside the intensity peak has
a large degree of linear polarization.
The precise energy of an individual photon
inside the peak is determined (``tagged'') from the momentum of the recoil
electron measured in a dedicated tagging spectrometer.

This report begins with a survey of the techniques for producing
high-energy photons that were considered in the development of this design,
and the reasons for the choice of coherent bremsstrahlung.  The coherent
bremsstrahlung source is then described in greater detail, followed by
a discussion of the requirements that the design places on the electron
beam properties at the source.  The tagging spectrometer design is described
next, followed by the photon beam collimation and shielding design.  The
report concludes with a description of the pair spectrometer and
methods for monitoring the properties of the beam.
