In this chapter we examine various aspects of experimentation that directly interacts with the accelerator design. Of particular concern in this area is the background to the physics experiments that are caused by the beams passing through the detector.
The characteristics of background events at JLC will be very different from those at
typical 
colliders, except the SLC. The features of the background strongly
depend on numerous operational parameters of the accelerator, such as the beam aspect ratio
(typically 
), a high beam intensity
(
particles/bunch) and possible tails in the particle distribution that
deviates from a Gaussian distribution. The population of low energy 
pairs that are
created during collisions are directly related to the beam aspect ratio, while the tail is
mostly responsible for synchrotron radiation and muon background. The optimization
of the machine operational parameters must be considered by taking this ``interaction"
between the experimentation and the machine into account. The highest-priority goal here is,
of course, to maximize the luminosity while minimizing the background. With such
a motivation in mind, the effects of pairs have been estimated by detailed Monte Carlo
simulations with the proposed JLC-1 detector[1], in addition to simulations of masking
of synchrotron radiation and the attenuation of the muon flux that are produced by interactions
of the beams with upstream collimator materials.
Another important issue is the need for measurements of the distribution of the
center-of-mass energy within each beam collision, hereafter called the ``luminosity
spectrum.'' At TeV linear colliders, particles in the colliding beams loose a significant
amount of energy before ``collisions" take place. This is due to the emission of synchrotron
radiation in a strong electromagnetic field produced by the opposite beam, known as
the ``beamstrahlung" phenomenon. Therefore, the effective luminosity at 
is always smaller than the nominal value that does not take beamstrahlung into
account. The luminosity spectrum as a function of the center-of-mass energy depends on the
magnitude of beamstrahlung. Knowledge on such issues is very important for
conducting precision measurements, especially for studies of toponium physics[2] and
detailed investigations of SUSY physics[3], which are research opportunities unique
to linear colliders. A method based on the measurement of acollinearity angles in Bhabha
scattering events is examined as a possible technique to measure the luminosity spectrum.
Detailed engineering design studies for the interaction region are not ready at this moment, and, thus, they will not be given in this chapter. No consideration of the support system of the final focus quadrupole magnets and the heavy masking system are given. The designs of the extraction beam lines and measurements of electron polarizations are not discussed, either. These issues are left for subsequent design studies in the near future.