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A great and detailed explanation can be found here: LCLS 1 Event Generator for Dummies
In summary, the main LCLS-I timing is based on a 360 Hz frequency, which is divided in 6 time slots (TS), each running at 60 Hz, but with a different phase. The 120 Hz beam is a combination of slots TS1 and TS4, while the other ones are "empty".
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As shown below, event codes are generated for each time slots, from 60 Hz down to 0.5 Hz. Special event codes combining multiple time slots are also generated. For example, event codes 10 and 40 combine TS1 and TS4, giving a marker for the nominal 120 Hz beam.
When multiplexing with SXR as the secondary branch, the SXR line gets their shots on TS1. For a 10 Hz secondary beam to SXR (event code 13), this means that HXR gets the full 60 Hz of TS4, but only 50Hz on TS1 (110 Hz total). When using the sequencer, one needs to be mindful of that, as detailed below.
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To do so, the sync marker should be put on 60 Hz and the DAQ / detector trigger event code generated after an EVEN number of ΔBeam.
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