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 the #EPICS switch. Currently, only Type 2 (TD) experiments can be run in dual channel mode. When running in dual channel mode, both the DAQ systems bnmr and bnqr may be running and taking data, therefore dual channel mode makes better use of the beam time. In single channel mode, only one DAQ system (bnmr or bnqr) is active (see #Single channel mode).
EPICS switch
The EPICS parameter that controls switching is called "BNMR:BNQRSW:ENBSWITCHING". Switching must be enabled for dual channel mode, and disabled for single channel mode. During the beam period for bnmr / bnqr, the experimenters telephone the controls group and ask them set this switch as required to run in dual channel or single channel mode. Figure 1 shows the switching set for Dual channel mode (scheduled switching enabled, manual switching disabled). This page can be found on the EPICS page display by clicking on the button "beam line switch" on the page "ILE2A3/BNMR OPTICS".
Experimenters must also ensure that the DAQ ODB parameter Enable dual channel mode is set to the appropriate value. The ODB parameter enable epics switch checks (on the custom status page) should also be set true (except for testing). This causes rf_config to compare the EPICS switch position with the value of Enable dual channel mode. If the switch is not set correctly, rf_config prevents the run from starting.
Helicity
The PPG requests the helicity it 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 (see #Timing).
Timing
Figure 2 shows a timing diagram for dual channel mode.
In dual channel mode, the PPG is started by an external signal (PPG Start/Beam Ready) that occurs at the end of the BNMR or BNQR Delay (Figure 2). 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. This timing is controlled by the EPICS parameters BNMR Period (s), BNQR Period (s), BNMR Delay (s), BNQR Delay (s). These are available on the EPICS Kicker Control/BNMR-BNQR Switching page (Figure 1). The BNMR and BNQR Delays must be set to give the Helicity time to change. They are equivalent to the ODB parameter Helicity Sleep time in Single Channel mode. This parameter is ignored in dual channel mode.
Single Channel Mode
In single channel mode, the beam is switched to one of the channels (bnmr or bnqr). Only that DAQ system is active. The other DAQ system must not be run (except by "experts" for #Testing under certain conditions). This is because there is NO CHECK against both DAQ systems running simultaneously in single channel mode, and both trying to flip the helicity. This would ruin the data for the active channel with the beam.
Testing
Both DAQ systems may be run in single channel mode simultaneously for testing under certain conditions. Outside the beam period, it is sufficient to disable helicity flipping on one of the DAQ systems, so that for example BNMR flips the helicity, and BNQR does not. In that case, the check on helicity flipping enable all hel checks may be disabled on BNQR.
Alternatively, and especially during the beam period, the helicity signal from the PPG should be disconnected so that the second DAQ cannot interfere with the data taking run. In this case, helicity flipping can be enabled on the disconnected system, but the check on helicity flipping enable all hel checks must be disabled.