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| Cloak |
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The exact procedure for doing a fine timing scan tends to change often depending on the status of various scripts. Currently (as of Feb. 2022), the icxi script does not seem to work. A manual scan will always work if you cannot get an automated scan to work. 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
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For time scan, type "ts.scan_times(start_delay, end_delay, step_size, nevents_per_timestep=360, randomize=True)" in the icxi terminal
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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. |
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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) |
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. 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 |
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: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. Click Reset Plots in the AMI window. Change the Target Time in the laser timing window by +-1 ps in 100 fs steps. Allow the data to be averaged for a few seconds at each time point. You should get a plot with a clear edge, like this: 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 shown above, a t0 value of -200 fs would be a reasonable number with a +/- 100 fs 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 "periodic 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. Update t0 in the laser timing window: In this example, the ns Target Time should be set to -200 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. 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 |
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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 |
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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
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