  The Jefferson Lab Q_weak experiment determined the weak charge of the proton
by measuring the parity-violating elastic scattering asymmetry of
longitudinally polarized electrons from an unpolarized liquid hydrogen target
at small momentum transfer. A custom apparatus was designed for this experiment
to meet the technical challenges presented by the smallest and most precise
${\vec{e}}$p asymmetry ever measured. Technical milestones were achieved at
Jefferson Lab in target power, beam current, beam helicity reversal rate,
polarimetry, detected rates, and control of helicity-correlated beam
properties. The experiment employed 180 microA of 89% longitudinally polarized
electrons whose helicity was reversed 960 times per second. The electrons were
accelerated to 1.16 GeV and directed to a beamline with extensive
instrumentation to measure helicity-correlated beam properties that can induce
false asymmetries. Moller and Compton polarimetry were used to measure the
electron beam polarization to better than 1%. The electron beam was incident on
a 34.4 cm liquid hydrogen target. After passing through a triple collimator
system, scattered electrons between 5.8 degrees and 11.6 degrees were bent in
the toroidal magnetic field of a resistive copper-coil magnet. The electrons
inside this acceptance were focused onto eight fused silica Cerenkov detectors
arrayed symmetrically around the beam axis. A total scattered electron rate of
about 7 GHz was incident on the detector array. The detectors were read out in
integrating mode by custom-built low-noise pre-amplifiers and 18-bit sampling
ADC modules. The momentum transfer Q^2 = 0.025 GeV^2 was determined using
dedicated low-current (~100 pA) measurements with a set of drift chambers
before (and a set of drift chambers and trigger scintillation counters after)
the toroidal magnet.
