Accelerator Operations Orientation
Setting Up Beam to the Halls
Most work done in the halls is performed with the hall in "Restricted Access". Before relocking the hall for beam delivery, the hall must be thoroughly searched (called a "sweep") to ensure that all personnel have exited. Announcements will be made 15 minutes and 5 minutes prior to the sweep to allow personnel ample time to leave the hall. The sweep is performed by two people: one a trained sweeper who actually performs the sweep, and a second "untrained" person to act as a guard so that people cannot reenter the area. It is very helpful to the MCC if the hall provides a person to act as the guard. Freeing up the second operator will help you get beam back faster.
During a sweep, the operator searchs the hall in a pre-determined pattern while arming the run/safe boxes. If any of the detector huts (in Halls A and C) have been opened, they will be searched and closed at this time. The run/safe boxes in the detector huts disarm themselves if the door to the hut has been opened. The huts will, therefore, need to be searched, and the boxes re-armed after each entry.
Establishing Beam to Hall A and/or Hall C
When the operators initially set up beam to Halls A or C, it is necessary for the orbit to be checked with "PULSED BEAM" (also referred to as "TUNE BEAM.") This beam has a duty factor of 1.5%. This means that 8 uA of pulsed beam is equivalent to:
Low-current pulsed beam will generally not affect hall equipment, except for possibly a polarized target (if that is the case, the potential for damage should be communicated to the Experiment Liaison and Program Deputy well in advance of initial running). The MCC Control Room will always notify the hall when pulsed beam is about to be run for initial steer up or to check the orbit to the hall.
Establishing Beam to Hall B
When beam is initially being established to Hall B, the orbit at the beginning of the Hall B line is set up using pulsed beam to the Tagger Dump. After tuning beam in this region, the hall performs a harp swipe with low current CW to check the profile of the beam. Once the profile is deemed acceptable, the hall switches to straight-ahead running to the Faraday Cup (that is if the experiment calls for electron as opposed to photon running), and the Tagger Magnet is degaussed and turned off. Degaussing the magnet takes about 15 minutes. Before sending initial beam to the Hall B Faraday Cup, the target solenoid and the raster must be OFF, and the cryotarget ladder must be removed or set to an empty cell. The beamline from the Tagger Magnet to the Faraday Cup is steered with small amounts of CW beam. The hall will then be asked to turn on the target solenoid and torus and insert the target. If the drift chambers remain on without tripping and the downstream PMT counts are acceptable, beam has been successfully established to the Faraday Cup. Minor steering to the desired positions on the nA BPMs (2C22A, 2C24A, and 2H01) can usually be accomplished during the physics run.
The nA BPMs (nano-Amp Beam Position Monitors) are beam position diagnostics designed specifically to work with the low beam currents used in the Hall B line. Keep in mind that these monitors do not accurately measure the beam position at currents of less than 1 nA. When the beam current drops below the 1 nA threshold, even though the beam has not physically changed position, it will appear to have on the BPM readbacks.