\chapter{Introduction}\label{chapter_Introduction}

One of the unique opportunities presented by a \cebaf{} upgrade to energies
of $12$ \eunit\
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.

The requirements for photon beam energy and polarization were described
in Chapter 3.  This chapter describes a design for a real photon source
that meets these requirements.  Starting with a beam of monochromatic
electrons, it provides an intense beam of high-energy photons with an 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
$10$\% 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 a 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.  The design is
formed around the expected parameters for the \cebaf{} beam following the
energy upgrade to $12$ \eunit, although nothing prevents its operation at 
lower energies before the time that $12$\eunit{} beams are available.

This chapter 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 that feeds the source.  The tagging spectrometer design is described
next, and the chapter concludes with a discussion of the considerations
that govern the choice of beam intensity for particular measurements.

