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Step-by-step guide

Femtosecond (fs) timing in sample chamber 1 (SC1) at CXI.

 

SC1 setup for fs laser experiments

  1. Install a fast diode and mirror (roughly at 45 degrees with respect to the beam) on the SC1 sample_x stage (CXI:SC1:MMS:02.RBV)
    1. The fast diode has a small area and is therefore difficult to align. Ideally, a Y stage (PP-30 piezo) is used to allow remote motion of the diode to align to the laser and x-rays.
    2. Due to the limited travel range of the long X-stage in SC1, the diode and mirror must be installed as far as reasonable towards the -X side, overhanging away from the carriage
  2. Roughly align the fast diode and mirror using the reference laser at the time tool (-4 mm in position but this will soon be changed to 0 mm.   If the "out" position is 92 mm, then -4 mm, if 96 mm, then 0 mm).
  3. Install a regular diode connected to the Acqiris on the sample chambe door and aligned the reflected laser beam on the mirror onto that diode.
    1. Repeat with the fs laser and maximize the acqiris signal by moving the diode on the chamber door.
  4. Insert a nozzle rod with both a clear YAG and a frosted YAG. 
    1. At CXI, the clear YAGs are typically thinner, around 20 um, whereas the frosted YAGs are thicker, around 50 um
  5. Determine the position of the X-ray spot at the IP using the clear YAG and the beam with 1x10-4 Transmission
    1. (zoomed in view)

Rough timing

  1. Move the fast diode into position (at this point should only need to move the SC1 sample_x stage)
  2. Measure the laser signal on the diode using the LeCroy scope to make sure there is overlap
  3. Turn off the laser and turn on 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 repointed)
  4. Save a trace of the X-rays on the scope
  5. Turn X-rays off and turn on laser
  6. Move laser delay time (LAS:R52B:EVR:31:TRIG0:TDES) to bring laser within 100 picoseconds of the X-rays using the scope
  7. Once the laser is timed roughly, it is time to move onto the finer timing measurement using a cross-correlation measurement

Fine timing

General concept

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 phtooelectrons) that changes the index of refraction and, more importantly, the reflectively 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).

Procedure

  1. Align the laser with crosshairs on the inline cameras that represent the location of the X-ray beam
    1. 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.
    1. 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 the motors does pitch and yaw can change so these are not set in stone.
  2. When the laser is roughly centered on the X-ray spot (the laser should be on the order of 100-150 um), the X-ray beam needs to be made similar in size at the interaction point to the laser.  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.
    1. The DG2 lens screen is found in the DG2 section of CXI home
  3. 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
    1. Beam on DG2 without lenses
    2. Beam on DG2 with lenses but without slits aligned
    3. Beam on DG2 with lenses and with DG2 slits aligned
    4. 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
  4. Open all gate valves and remove objects upstream of SC1 before bringing the beam to the IP. 
    1. 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
  5. Look at the beam on the clear YAG in SC1
    1. The beam should be similar in size to the laser beam or the signal used for timing in SC1 will be low
    2. (zoomed in view)

    3. 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
  6. For the timing scan you will need 100% transmission in the sample chamber
  7. For the timing scan we (as of 22-Oct-2018) are using a timing scan in iCXI( a hutch python instance)
  8. Load icxi by typing "icxi" in any terminal on the cxi machines (cxi-daq, cxi-monitor, etc.)
  9. Load the time scan by typing "%timescans" into the ipython terminal
  10. The laser signal should be on the acqiris.  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
  11. For time scan, type "ts.scan_times(start_delay, end_delay, step_size, nevents_per_timestep=360, randomize=True)" in the icxi terminal
  12. 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)
    1. 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
    2. 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
  13. The time scan, if done properly, will produce a plot that looks similar to the one below

    1. The current convention used at CXI is that the first dip in the integrated intensity is used as t0 (not the inflection point).  In the example from LS87 above, a t0 value of -400 fs would be a reasonable number.
  14. The t0 value would be updated in the laser timing screen
    1. 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).
    2. 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 to 0.000000.
  15. 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

  1. The most -Y position of the time tool

 

Fine timing at the time tool

  1. 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
  2. Take the difference in t0 compared to previous measurement (Δt0 in ps) and do the following calculation  (Δt0 * 300 um/ps /2)
  3. 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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