Introduction

The calorimeter and scintillators modules are all read out by three different types of PMTs. The PMT signal gain is highly dependent on the supplied high voltage. The HV therefore needs to be adjusted on the calorimeter modules when the beam conditions changes (beam type and electron energy) in order to maintain a signal which is fully measurable in the digitizer which has a 2 V readout range and around 1mV noise level. The scintillator signals should be less dependent on the beam and are not foreseen to be varied during the test beam period.

Calorimeter HV settings

It is foreseen to mainly operate with three voltage settings, tuned to low (about 3x10³), medium (about 3x10⁴) and high (about 3x10⁵) gain. The gains vs HV is not exactly the same in each PMT which will therefore be tuned individually during the initial setup. Dedicated scans of the response vs HV will also be done during the test beam period. Note that the calorimeter HV should not exceed 1600 V.

	Low gain		Medium gain		High gain
Channel	Nominal Voltage	Expected Current	Nominal Voltage	Expected Current	Nominal Voltage	Expected Current	Comments
Calo 0	-700 V	-160 muA	-1220 V	-277 muA	-1552 V	-353 muA	HV's adjusted to equal MIP signal (without filter) from Run 3408 on-wards
Calo 1	-700 V	-160 muA	-1120 V	-255 muA	-1425 V	-324 muA	Medium baseline was 100V lower before Run 3567
Calo 2	-700 V	-160 muA	-1124 V	-225 muA	-1431 V	-325 muA
Calo 3	-700 V	-160 muA	-1100 V	-225 muA	-1400 V	-319 muA
Calo 4	-700 V	-160 muA	-1104 V	-225 muA	-1406 V	-320 muA
Calo 5	-700 V	-160 muA	-1228 V	-225 muA	-1562 V	-356 muA

Scintillator HV settings

These are not expected to be changed during test beam period beyond the initial setup:

Channel	Nominal voltage	Expected Current	Max Voltage	Comment
Preshower 0	-1100 V	-372 muA	2000 V	Was -1500 V until switching to electrons in Run 3500
Preshower 1	-1100 V	-372 muA	2000 V	Was -1500 V until switching to electrons in Run 3500
Trigger 0	-950 V	-342 muA	1000 V
Trigger 1	-950 V	-342 muA	1000 V

Monitoring and changing the PMT HV settings

The status of the PMTs can most easily be viewed in grafana (updates once a minute). One can also bring up the DCS control panel by logging into faser-dcs-001:

ssh shifter@faser-dcs-001
/data/runCaloHVFSM.sh

Note that Calo channels 4-5 are connected to "VETO1", while the Trigger scintillator channels are connected to "VETO2". The "TIMING" channels are not used.

The panel allows one to switch ON and OFF each channel and to set the voltage working point (Set V0). To change the voltage, enter a new value in the box and press enter for it to immediately take effect, if the channel is ON. Do not change any of the other settings.

As changing all six channels repeatedly by hand is error prone, a script can be used to set all of the calorimeter voltages at the same time:

ssh shifter@faser-dcs-001
/home/shifter/setCaloVoltages.py <gain> <offset>

where gain is either "=low=", "=medium=" or "=high=", and offset is offset that will be added to the nominal voltage. Only positive offsets, i.e. reducing gain, are allowed. As this bypasses the DCS panels, the panel will not show the correct voltage setting under V0 in channel settings, but the voltage will be shown in "Actual".

For HV scans, a script is provided that takes care of setting HV and starting/stopping runs. Once the table is the right position, run the following:

ssh shifter@faser-daq-002
cd /home/shifter/software/faser-daq/scripts/Sequencer
./sequencer.py HVScanHighGainElectrons.json

One should then first see the HV change (or not for the first step) and after 30-50 seconds a run should start. It stop automatically when 100,000 events are recorded and moves to next step. Follow the progress in the RunList. In case a transition fails or HV setting fails, the sequencer should stop. In this case one has to stop the run by hand and restart the sequencer. One can restart from the current step by doing:

./sequencer.py -s <stepNumber> HVScanHighGainElectrons.json

-- BrianPetersen - 2021-07-24