BNMR: Helicity
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Introduction
The bnmr and bnqr experiments control the helicity via the PPG "POL DRV" output. The helicity is changed using a plate that is moved into and out of the beam. The two helicity states are called "UP" and "DOWN". An experimental parameter flip helicity is provided for each Experimental Mode. When this is set to true, the helicity will be flipped. Some modes flip the helicity after each PPG cycle, others at the end of the scan. See BNMR Mode Parameters for details.
Helicity Switch Box
Since both experiments bnmr and bnqr control the same hardware (the helicity plate) signals from both experiments are connected to a helicity switch box (Rolf's box). These signals are from the PPG "POL DRV" output. The electronics is edge-driven, so it only changes when it sees an edge. To ensure a particular helicity state at the begin-of-run in single-channel mode (e.g. DOWN), the PPG must drive the helicity DOWN-UP-DOWN.
The experiment requests the helicity state for its next cycle by setting the PPG "POL DRV" output. In single channel mode, the helicity state is changed immediately. In dual channel mode, the helicity is not changed until that experiment receives the beam (see BNMR: Dual Channel Mode#Helicity Switching. A circuit diagram of the helicity switch box in single and dual channel modes is shown in Figure 1.
Helicity Read Back
Helicity read back signals have been provided in recent years so that the state of the helicity can be read with certainty, particularly in dual channel mode. The helicity state is read using a VMEIO32
In case, the frontend software sets the helicity it requires for its next cycle via the PPG "POL DRV" output