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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. |
Fine timing
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| Once the laser is roughly timed, it is time to move onto the finer timing measurement using a cross-correlation measurement |
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See the |
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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. |
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Procedure
- Align the laser with crosshairs on the inline cameras that represent the location of the X-ray beam
- In the image below, the bright spot is the laser spot at the interaction point (IP) visualized using a frosted YAG. The frosted YAG is on the upper half of the image. There is a small gap followed by the bottom 1/4 of the image having a clear YAG in this particular instance but it is likely that the mounting of the 2 YAGs will vary with changing experiments and if Ray Sierra broke them or not...
- The laser needs to be repointed using Pico X (CXI:LAS:PIC:03) and Pico Y (CXI:LAS:PIC:02). These PVs can be changed and which of the motors does pitch and yaw can change so these are not set in stone.Trial and error is required to verify the direction of motions.
- When the laser is roughly centered on the X-ray spot (the laser should be on the order of 100-150 um but this can vary between experiments with larger spots being possible), the X-ray beam needs to be made similar in size at the interaction point to the laser to maximize the signal, to make sure that the part of the target the laser sees is fully modified by the x-ray beam. In order to do this at CXI, insert Be lenses at the DG2 location to focus the beam upstream of SC1 and produce a larger, divergent beam at the IP.
- The DG2 lens screen is found in the DG2 section of CXI home. it is password protected with the usual CXI password
- The DG2 lens screen is found in the DG2 section of CXI home. it is password protected with the usual CXI password
- The Be lenses will need to be aligned in both x and y using the DG2 PIM and then the DG2 slits will need to be closed to stop any intensity outside the Be lenses aperture
- Beam on DG2 without lenses
- Beam on DG2 with lenses but without slits aligned
- Beam on DG2 with lenses and with DG2 slits aligned
- The DG2 slits need to be closed such that there is no beam intensity outside the lens aperture. If intensity remains outside of the lens aperture it will be focused to the 1.3 um spot size in the IP and damage the YAG at full intensity
- Beam on DG2 without lenses
- Open all gate valves and remove objects upstream of SC1 before bringing the beam to the IP.
- Since the beam is being refocused upstream of SC1 to make the larger beam, we could inadvertently place the focused beam on upstream diamond windows or other objects and damage them
- Look at the beam on the clear YAG in SC1 (or the frosted YAG is fine for this as it allows for a check of the spatial overlap of the x-rays and optical laser)
- The beam should be similar in size to the laser beam or the signal used for timing in SC1 will be low
- (zoomed in view)
- Optional: Use the large beam at SC1 to align the DSB slits, SC1 jaws, and SC1 ap0 and ap1 motors to be roughly centered. it is easy and the SC1 +Y jaw and DSB +Y slits are particularly useful for cleaning up scatter from the nozzle that may make the data quality worse
- The beam should be similar in size to the laser beam or the signal used for timing in SC1 will be low
- 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 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 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.
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