Step-by-step guide - Femtosecond (fs) timing in sample chamber 1 (SC1) at CXI.
Setup
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).
Install a fast diode and mirror (roughly at 45 degrees with respect to the beam) on the SC1 post_sample_x stage (CXI:SC1:MMS:02.RBV) downstream on the interaction region. There is a danger of driving this stage into the Jungfrau or other components in SC1, so it might be disabled. It is also good practice to disable the motor when not in use.
Roughly align the fast diode and mirror using the reference laser at the time tool.
Install a regular diode connected to the Acqiris on the sample chamber door and align the reflected reference laser beam on the mirror onto that diode.
Repeat with the fs laser and maximize the Acqiris signal by moving the diode on the chamber door. An ND filter may be necessary to reduce the signal, as shown in the side-on view below
Also install a fs laser diagnostic diode (OPC-TP) and connect it to the Acqiris. This is useful to confirm that the fs laser is firing during the experiment.
Insert a nozzle rod with both a clear YAG and a frosted YAG.
If doing the timing with the shroud on, the bracket needs to be facing in the -Z direction when inserted into the sample chamber or it will collide with the exit cone.
Determine the position of the X-ray spot at the IP using the clear YAG and the beam with 1x10-4 Transmission (although the beam can be seen with less intensity). Mark the position of the beam with crosshairs (cyan and magenta in the example image below).
Rough timing
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.
Move the fast diode into position (at this point should only need to move the SC1 sample_x stage) and move the IP YAGs out of the beam.
Measure the laser signal on the diode using the LeCroy scope to make sure there is overlap
Shutter the fs laser and unshutter the X-rays. Move the diode around in x and y to maximize the X-ray signal (if the steps are very large then the laser may need to be re-pointed)
Click Store on the Lecroy to save a trace of the X-rays on the scope
Shutter X-rays off and unshutter fs laser.
Temporally overlap the X-rays and the fs laser.
Fine timing
Background Information
Align the laser with crosshairs on the inline cameras that represent the location of the X-ray beam.
Enlarge the X-ray spot size using DG2 lenses.
Find t0 at the IP with a fine timing scan.
For the timing scan you will need close to 100% transmission in the sample chamber (likely > 25%). This is required because the desired transmission change is driven by high intensity x-ray effects.
For the timing scan we (as of 22-Oct-2018) are using a timing scan in icxi (a hutch python instance)
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
The laser signal should be on the acqiris using the diode installed on the chamber door. In the case of LS87 we were using Channel 1 to look at the diode outside SC1 and we should strive for a peak height of ~1V to ensure that the cross correlation signal is strong.
For time scan, type "ts.scan_times(start_delay, end_delay, step_size, nevents_per_timestep=360, randomize=True)" in the icxi terminal
Before running the scan, a plot needs to be set up that plots the integrated laser intensity on the Acqiris vs the laser ns Target Time (LAS:FS5:VIT:FS_TGT_TIME_DIAL)
Set cursors both earlier and later than the laser peak on the Acqiris and in "Expr" under Expression manually add "a integral b" using the integral sign
Plot A integral b as a function of LAS:FS5:VIT:FS_TGT_TIME_DIAL and choose the hi and low values consistent with the scan. The above example shows the typical final low and hi values chosen when we are very close to t0
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.
Rough timing at the time tool
- The most -Y position of the time tool
Fine timing at the time tool
- If fine timing was successful in SC1 and immediately doing fine timing at the time tool, take the different in t0 measured for the current experiment and compare it to the previous experiment that used the time tool
Concept: Light travels roughly 300 um in 1 picosecond. So for every 1 ps difference in t0 between the current fs timing experiment and the previous one, the white light path needs to be changed by 300 um total. However, the time tool delay stage has a double bounce mirror setup, meaning that changing the time tool delay stage value by 100 um increases the white light path by 200 um. This is important for mapping the t0 found in SC1 to the time tool - Take the difference in t0 compared to previous measurement (Δt0 in ps) and do the following calculation (Δt0 * 300 um/ps /2)
- Change the time tool delay stage by the calculated value (it is not guaranteed that adding the number actually increases the path length so try one direction and if that doesn't work, try the other. If both don't work then you did something wrong).
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