Brief discussion on the LCLS timing

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".

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.

To be verified: when HXR is secondary, HXR is put on TS4.

Event sequencer

Sync marker

The sync marker defines on which phase the sequence can be started. For HXR, the sync marker follow the event codes 4X.
A sync marker of 120 Hz thus means that the sequence will start on the next 120 Hz tick (TS1 or TS4). With a sync marker of 60 Hz or below, the sequence is guaranteed to start on TS4. This is important to ensure a deterministic start of the sequence when multiplexing.

When running a sequence in a loop (N times or repeat forever), the sync marker determines the start for each loop. This means that it will limit the frequency at which the sequence will run. For example, if a sequences that could run at 30 Hz (4 delta beams total, see below) is run with a 10 Hz sync marker, it will only start on a 10 Hz frequency and thus run at 10 Hz.

Lower rate operation

When using the sequencer to operate at lower rep rate (60 Hz and below) or on a custom sequence of beam, one wants to make sure that the DAQ is not synchronized with the 10 Hz shot sent to SXR. That means DAQ must be triggered only on TS4.

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.

Working example

In the above figure, the sequencer is used to run at 20 Hz with the pulse picker. The total number of ΔBeam is thus 6 (120 Hz/20 Hz=6) and can't be changed without changing the repetition rate of the loop. In addition, one must allow for 2 ΔBeam after triggering the pulse picker to get the beam through.

The corresponding sequence is executed as follow:

Here, event code 197 triggers the pulse picker and event code 198 the DAQ readout. Note how if the sequence were to start on TS1, the DAQ read out would also end up on TS1. In this case (20 Hz), this means that every second shot falls on the dropped shots that are sent to SXR.