The beamline is composed of several inter-dependent subsystems for continuous control and diagnostics of the electron flow from
the accelerator, through the fixed-target, and to the Faraday cup at the beam dump. Dipole and quadrupole
magnets act on electrons like optical lenses would on light, deflecting and focusing the beam.
Antenna cavity Beam Position Monitors operate at continuous currents down to 1 nA with accuracies of
20 μm at 100 Hz. A coincidence double arm Møller polarimeter, with a permadur target
(49% Fe, 49% Co, 2% V) inside Helmholtz coils, periodically analyzes the elastic polarized electron-electron scattering angular asymmetry
to provide the beam polarization with a few percent accuracy. When performing a Møller measurement, or when establishing the beam
after a machine change, an insertable tungsten calorimeter blocks the beam in the upstream tunnel.
To ensure the quality of the beam, wire scanners with 20 μm thick tungsten measure its profile by
passing through at two stereo angles. The profile is reconstructed either by photomultiplier counters downstream of the wires, or by
measuring directly the changes in wire mechanical vibrations due to beam heating. Scintillating fibers
surround the target to monitor the beam offset continuously. An insertable YAG screen also allows for
checking of beam position and profile downstream at the dump. Finally, the beam current is monitored by the charge deposited in a
Faraday cup. All these measurements are relayed to the operators in the counting room during data taking,
and also provide inputs for orbit lock feedback and fast shutdown of the accelerator if necessary.
Last modified: April 30, 2013
Daniel S. Carman