BNMR: Dual Channel Mode
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Introduction
When the beam is switched alternately between the two experiments bnmr and bnqr, the experiments are running in dual channel mode. When no switching takes place, and the beam is sent to either bnmr or bnqr experiment, the experiment is running in single channel mode. Switching the beam alternately between the two experiments is done by the Controls Group using EPICS switches. During the beam period for bnmr / bnqr, the experimenters telephone the controls group and ask them to change between dual channel and single channel modes as required. Experimenters must also ensure that the DAQ parameter "Enable dual channel mode" is set to the appropriate value, otherwise the DAQ will not run (assuming EPICS switch checking is enabled).
The EPICS switches are called ...
Helicity Switch box
The PPG requests the helicity is requires for the next cycle by a signal from the PPG Output "Pol" connected to the Helicity Switch box. In Single Channel mode, the helicity is immediately set to this value (UP/DOWN). In Dual Channel mode, the helicity state is not changed until that channel is active (Beam Ready).
Timing
In dual channel mode, the PPG is started by an external signal (Beam Ready). Usually the PPG cycle is divided into 3 regions: prebeam, beam on , and post-beam. Once the PPG cycle is started, the PPG Output Beam On controls when and how long the beam is on.
The experimenter must be careful to set the timing of the BNMR and BNQR cycles such that the Beam On period has finished before the beam is switched to the other channel. The post-beam period of one channel generally overlaps part of the cycle of the other channel. The BNMR Delay and BNQR Delay times must be set to give the Helicity time to change. They are equivalent to "Helicity Sleep time" in Single Channel mode.
