MHATT-CAT Micromonochromator Issues


The micromonochromator ("micromono") in 7ID-B was designed by researchers from Oak Ridge National Laboratory and Howard University.  Its purpose was to deliver either white beam or monochromatic beam to the same spot on a sample (with focusing done by a KB mirror pair).  The micromono also has a "pass-through" option to allow either white beam or monochromatic beam (from the High-Heat-Load Monochromator, or HHLM) to pass by the optics in the micromono.

A long-standing problem with the micromono is that it sometimes blocks the pass-thru beams.  It has been known for a while that the lower edge of the pass-thru white beam can be blocked unless the micromono crystals are rotated to their lowest angle.  Details are here, on the MHATT-CAT restricted-access website.  Here is a photo (taken thru a viewport on the inboard side of the micromono) highlighting the bracket which can block the beam:


However, we have also observed that the upper edge of the HHLM monochromatic beam (which is at a height about 35 mm above than the white beam) is sometimes blocked.  We know the edge which blocks the beam is downstream of the HHLM, and  have long suspected that it is inside the micromono.  The following is an image of the top of the beam being cut by the mysterious edge:


MHATT-CAT has received help from the APS Survey and Alignment group to check whether any beamline components are out of alignment.  Fortunately, nothing in 7ID-A is significantly out of alignment, and the micromono vacuum chamber is also within 0.003" of its correct position (although slightly tilted).  Therefore, on January 29, 2004, MHATT-CAT staff opened the micromono to try to determine if any internal parts were blocking the beam.

Inside the MHATT-CAT micromono

 
In the above image, the beam proceeds from right to left, past the water-cooled first slits, the double-crystal monochromator, and then the second slits.

It turns out that the likely source of the beam edge is the housing around one or both sets of slits.  The copper structures are apparently Compton shields.  The APS survey group found that the bottom of the top Cu plate is 36.5 mm above the white-beam height.  With the nominal height of the HHL mono beam being 35 mm above the white-beam height, the 1.5 mm of clearance is very small.

Using a leveling laser at roughly the height of the HHLM beam, the following photo shows that the copper housing above the second slit is very close to the HHLM-mono beam height (look for the small red dot intersecting the front of the top copper piece):


Shimming the Compton shields

On January 30, 2004, MHATT-CAT staff installed Cu block spacers to shim up the top plates of the Compton shields.  Advice from Jon Tischler of UNI-CAT is gratefully acknowledged for this step.  The blocks were made of spare OFHC (at least, that was how the material was labeled) by the APS machine shop on short notice.  The following pictures show the first Compton shield before and after being shimmed, as well as both Compton shields with shims.
no shim  with shims  

Since the micromono was closed and pumped down, the pressure has been decreasing slowly; after two weeks, the pressure is in the high 10-9 Torr, a factor of 3 higher than the previous base pressure.

Various digital photos of the micromono can be found here



Don Walko, MHATT-CAT, XOR, APS, ANL
Last updated February 12, 2004

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