ELOG OPERATION-2024

10 May 2024, 00:04

our pp laser experienced timing jumps by 18 ns today. this happened for the first time at ~7:30 in the morning without us noticing and a couple of more times during the morning. we realized it at 15:30 due to the fact that the signal on our pulse arrival monitor (X-ray/laser cross correlator downstream of the experiment) disappeared. we spent some time to investigate it, until we also see this jump in two different photodiodes different triggers (oscilloscope and digitizer) so it was clear that it is indeed the laser timing jumping. we also see the laser spectrum and intensity change for the trains that are offset in timing.

we called laser OCD at ~16:30. they have been trying to fix the problem by readjusting the instability zone multiple times and by adjusting the pockels cell, but this did not change the timing jumps. at 21:30 they concluded that there was nothing more they could do tonight and a meeting should happen in the morning to discuss how one can proceed.

effectively, this means that we lost the entire late and night shifts today for the pump-probe beamtime and have some data of questionable quality from the early shift as well.

Laser status:

After the laser issues listed in https://in.xfel.eu/elog/OPERATION-2024/99 the late shift was finally able to find a timing setting for the amplifier trigger that was beyond stable settings under usual conditions, but which allowed to continue taking data runs for the rest of the beamtime reliably. The users were already able to reveal indications of a pump-probe signal in their data.

The issue itself remains but didn't show up in form of unstabilities anymore. After the beamtime LAS took some effort to investigate and narrow down to either a faulty trigger going into the front-end amplifier or on the front-end amplifier itself. To the best of our knowledge this circumstance will need to involve EEE support or even a service from the manufacturer and cannot be solved solely by LAS. LAS ensured that the highest priority will be to deliver stable laser pulses for next week's beamtime as well.

FEL status:

we found the FEL poynting to be very stable troughout the last weeks beamtime (despite some short down-times), which allowed to reliably verify the spatial overlap.

Today in the afternoon, SASE1 was tuned (a description of the activities in att 4). We now realize that this has in some form changed the SQS pulse parameters.

att 1 shows ion spectra recorded under nominally the same photon parameters before (run 177) and after (run 247)

it's also visible in the history of the SASE viewer that the behavior of the SASE3 pulse energy is different before and after 15:45, which is right when SASE1 was tuned. it clearly started fluctuating more. however, we don't think that the pulse energy fluctuations themselves are the reason for the large change in our data, we rather suspect a change in the pulse duration and/or the source point to cause this. however, we have no means to characterize this now after the fact.

it is particularly unfortunate that the tuning happened exactly BEFORE we started recording a set of reference spectra on the SASE3 spectrometer, which were supposed to serve as a calibration for the spectral corelation analysis to determine the group duration, as well as for a training data set to use the PES with machine learning as a virtual spectrometer for the large data set taken in the shifts before.

We asked the machine for two color mode in the morning. Tuning gave good results at first (1.6mJ second color, 100uJ first color), but as soon as any of the colors was switched off and on again the pulse energy dropped (~600uJ) and BKR had to retune to even get a fraction of the pulse energy back back. The same happened when changing the delay