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.
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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