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Check all diodes at the beginning of the shift (ideally this should also be done a few days before the experiment to give you time to fix things that don't work), by plugging them into Lecroy and visual check (with bias-T).

Zoomed-in view:

If timing is desired to be done at atmospheric pressure in SC1, then the MPS interlock must be bypassed to allow the beam to come to CXI with the detector gate valve close. this requires bypassing Link Node 40, Card 3 Channel 15 to open for the duration of the timing measurement at atmosphere. This bypass can only be enabled by ACR and you must call them to request this at x2151.

If needed, align the fs laser to the diode (usually not needed). Move laser delay time (LAS:R52B:EVR:31:TRIG0:TDES) to bring laser within 100 picoseconds of the X-rays using the scope. Alternatively, change the  Target Time in the fs laser window. Align the fs laser trace so that the leading edges of the traces line up. You can center the diode signal on the Lecroy by changing the Delay. Try to match the amplitude of the X-ray and fs laser signals. Type the time below the ns Offset in the fs laser window in the ns Offset box, which will reset the Target Time to 0.

See the for more details about how the time tool works.

Fine timing is done by using the photoelectrons generated by the interaction of the X-rays with a material to effectively turn a non-metal into a metal (sea of electrons produced from photoelectrons) that changes the index of refraction and, more importantly, the reflectivity and transmission of the material.  If an optically transparent material, such as Si3N4 or a clear YAG, is irradiated with an optical laser, the vast majority of the photons will be transmitted through the material.  However, if the X-rays arrive before the optical laser, the non-metal to metal transition (effectively) caused by the photoelectrons will increase the reflectivity of the material and decrease the transmission of the optical laser through the material.  This effect can be measured either through a decrease in the transmission of the optical laser or through the increased reflection of the optical laser.  In the case of fs timing at CXI we use a measurement of the decrease in transmission of the optical laser (SC1 measurement) or white light produced by the laser (time tool, done using a portion of the optical laser diverted from the sample chamber).In SC1, the change in transmission is measured with a simple diode but the diode cannot be put directly in the beam path after the target since both the x-rays and optical laser beams would hit that diode. Therefore, the 45 degree mirror which sends the beam to another diode on the chamber door is used to separate the optical laser from the x-ray beam. The mirror reflects the laser and the x-rays are absorbed. It is OK to dump the full x-ray beam on this mirror. Even if it damages slowly with time, the mirror is cheap.

Set up Aquiris

Perform manual scan

Find approximate t0

You have multiple options here. You can set up a “Takahiro” plot. Set up an AMI plot of integrated diode intensity vs. DG2 Wave8 or FEE gas detector intensity. You can also watch the fast timing diode signal on the Acquiris and watch for the signal to decrease when X-rays are turned on. 

If using the Takahiro plot, when the X-rays come after the fs laser, these values are uncorrelated. When the X-rays come before the fs laser, there is a negative correlation for these values. Using a binary search, vary the fs laser target time in the Time Tool Plugin window until you find the point where the AMI plot shows a correlated line of points with slope half of the maximum.

Perform am automatic scan with icxi

Load icxi by typing "icxi" in any terminal on the cxi machines (cxi-daq, cxi-monitor, etc.)

Load the time scan by typing "%timescans" into the ipython terminal

For time scan, type "ts.scan_times(start_delay, end_delay, step_size, nevents_per_timestep=360, randomize=True)" in the icxi terminal

The time scan, if done properly, will produce a plot that looks similar to the one below

What is identified as t0 is a never ending matter of debate. However, the majority of the time, the width of the falloff in transmission fairly closely matches the distribution of the jitter in arrival times. Since the measurements made here average many shots, we are sensitive to this jitter. The photoelectron release is assumed to be essentially instantaneous with the width driven by the time difference in propagation of the x-rays and the laser through the material and the jitter in the laser arrival time. Typically, the jitter dominates with a thin target like a 20 um YAG. If the measurement is jitter dominated, then the inflection point is the right choice for time zero. If it is determined that jitter is not the dominant factor in the width of the falloff in transmission, then one can determine a more suitable reference point earlier in time. Since the jitter is typically >200fs and approaching 400 fs, as was the case for the example here, then a 400 fs width of the drop as shown above makes the inflection point likely the right choice.  In the example from LS87 above, a t0 value of -400 fs would be a reasonable number (with -300 fs perhaps being a bit better)  with a +/- 100 error bar. One thing to be careful is in the past, we have seen "periodic jitter" where the timing would drift back and forth every 20-30 seconds with an amplitude of more than 1 ps sometimes. Under those circumstances, the phase of that "priodic jitter" when the scan is close to time zero could easily move the apparent t0 by a few 100fs. The measurements at every data point of the scan should ideally be longer than any periodic jitter period, if this situation is present on a given day.

The t0 value would be updated in the laser timing screen

In this example, the ns Target Time should be set to -400 fs to determine the value of the final timing number (LAS:FS5:VIT:FS_TGT_TIME, in the above image it is 4392.074100).

The ns Offset should be changed to be the value of LAS:FS5:VIT:FS_TGT_TIME.  Once this change is made the ns Target Time should reset automatically to 0.000000.

Once timing has been established in SC1, timing must be found at the time tool.  If the time tool delay stage position is roughly known, then timing at the time tool is relatively easy and you can skip to the time tool fine timing section.