Stanford Synchrotron Radiation Lightsource
Engineering Note
Authors
Thomas Rabedeau
Serial
M693
Revision
Rev. 0
Page
1 of 8
Date
9/17/2014
Title
BL10-1 M0 Mirror System Fix SOW
Revisions:
Rev 0. Archive BL10-1 M0 mirror fix SOW as Engineering Note M693.
BL10-1 M0 Mirror System Fix SOW rev 4
Rev 0 - 7/24/2013 by TR
Rev 1 – 8/6/2013 by TR, Added more details regarding required acceptance (BC 6 and acceptance section),
power (BC 8 and power section), possible mask geometry, and flux (flux section).
Rev 2 – 9/4/2013 by TR, Added more details of power profile on mask to facilitate mask FEA. See mask
geometry section. Added as Figure 3 a sketch of the mask concept in section and renumbered the flux plot
from Figure 3 to Figure 4.
Rev 3 – 9/6/2013 by TR, Corrected power profile equation for beam at 3.0deg and normal incidence. Rev. 2
provided the normal incidence equation errantly labeled as 3.0deg incidence.
Rev 4 – 9/18/2013 by TR, Added information about vertical beam mis-steers and beam power footprint on
the fan allocation mask in the Mask Geometry section.
Overview & Background:
The BL10-1 M0 mirror system was first designed and installed in the late 1980’s to deflect about 2mrad of
off axis radiation sourced by the BL10 wiggler to the BL10-1 soft x-ray side station (see BL10 1/10th scale
drawing AD-441-146-00-c6 and ray trace GP-451-029-20-c6). The horizontally deflecting, flat mirror is
centered at z=8700mm and does not include a fan allocation mask upstream. Instead the mirror is inserted
into the beam from the SSRL side until the tip of the mirror just avoids clipping the central 1.5mrad of beam
delivered to the BL10-2 end station. The original mirror was externally cooled via contact to a water cooled
heat exchanger.
A replacement CVD SiC mirror featuring enhanced cooling but the original vacuum system and mover was
installed in 2004 as part of the SPEAR3 500mA BL upgrade (see SA-451-029-00). Owing to lack of
available space without wholesale vacuum system modifications, the new mirror system fan acceptance
geometry replicated the old system geometry including the absence of a fan allocation mask. The Pt coated
mirror, which cuts off at approximately 1500eV, absorbs >90% of the incident radiation power. As such, this
mirror operates with the highest absorbed power density of any mirror at SSRL.
During the 2013 run this mirror system manifest several troubling issues: (a) higher base vacuum and
evidence of Compton heating of the mirror vacuum tank, (b) degraded focus, and (c) degraded beam
intensity. The mirror system was vented 7/10/2013 and inspected. The mirror was observed to be cracked as
depicted below. The crack is believed to have resulted from thermally induced stress at the interface between
the SiC mirror body and a thick surface veneer of SiC.
Stanford Synchrotron Radiation Lightsource
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M693
Engineering Note
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Thomas Rabedeau
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Date
9/17/2014
Title
BL10-1 M0 Mirror System Fix SOW
Figure 1: The BL10-1 mirror
surface as seen via an
inspection mirror. The crack
evident is thought to extend
through a thick surface veneer
of SiC to the interface with SiC
mirror body which had a
90degree rotated growth axis
relative to the surface veneer.
The mirror fix involves several elements:
(1) Employ the spare BL13 M0 SiC mirror to replace the cracked BL10-1 M0 mirror.
(2) Provide a fan allocation mask to create a shadow between the BL10-2 centerline acceptance and the
BL10-1 off axis acceptance and limit the BL10-1 mirror acceptance to 1.0mrad.
(3) Design and fabricate a new vacuum system and mirror mover such that the existing mirror can continue
to operate (poorly) until the new system is installed.
Deliverables:
(1) Design package, travelers, etc in time to fabricate, process, assemble, and install mirror system and
associated mask during the December 2013 shutdown.
(2) Archive above drawing package (model, drafts, and images).
(3) Fabrication problem resolution, assembly, installation, and alignment oversight.
Boundary Conditions:
(1) The revised system shall employ the 40mm x 20mm x 360mm SiC replacement mirror per SSRL
engineering note M547 (attached).
(2) The BL5-2 M1 mirror system (see SA-451-409-00) provides a reference design as a departure point for
the mirror system design.
Stanford Synchrotron Radiation Lightsource
Engineering Note
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Thomas Rabedeau
Serial
M693
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Date
9/17/2014
Title
BL10-1 M0 Mirror System Fix SOW
(3) The mirror cooling shall use the updated heat exchanger system consisting of a water cooled copper rail –
Si pad (PF-451-409-26) – mirror body circuit with the thermal contact between elements provided by a
0.05mm GaIn layer. The BL5-2 M1 mirror system heat exchanger (SA-451-409-01) provides a reference
design.
(4) The mirror system shall include a Compton mask if feasible to minimize the diffuse secondary power
absorbed by the vacuum chamber walls.
(5) The mirror system and associated mask must fit into the space between the BL10 injection stopper system
(item 2, AD-441-146-00-c6) and the downstream mask (item 5, AD-441-146-00-c6). The reflected beam axis
must remain the same as the existing system. Provided the mirror and mask are independently positioned, the
vacuum system can be connected directly to the injection stopper tank without intervening bellows. A
bellows shall be included between the mirror vacuum system and the downstream mask.
(6) The fan allocation mask shall provide a shadow between the BL10-2 acceptance (ie., +/-0.75mrad about
ID centerline) and the BL10-1 acceptance. The required BL10-1 acceptance is 1.0mrad horizontal by
0.95mrad vertical centered at (2.0mrad, 0.0mrad). The fan allocation mask can limit the out board acceptance
edge of the BL10-1 mirror acceptance (preferred) or a water cooled chin guard mask can be fixed to the
mirror to protect the leading edge of the mirror. If the fan allocation mask limits the out board acceptance
edge, then a TC instrumented, but uncooled, chin guard shall be provided.
(7) The glidcop Al-15 fan allocation mask shall operate with a maximum Von Mises stress of 30ksi and
maximum wet wall temperature of 130degC. This will require a grazing incidence mask. SA-451-070-03
provides a reference design for a compact fan allocation mask though this particular example defines
multiple acceptances and employs crenelated mask surfaces which may not be appropriate for the BL10
application. It should be noted that the maximum Von Mises stress allowed can rise to 40ksi or maximum
tensile stress to 30ksi if the mask element is not monolithic with water cooling channels. In other words, one
can tolerate higher stress if any stress cracks will terminate at a braze interface before propagating to a water
channel. As standard practice, air guards are required around in vacuum joints of cooling passages.
(8) The radiation source is the BL10 wiggler consisting of 32 effective poles with 1.27T peak field and
128.5mm period. Power profiles are discussed below.
(9) Tim Montagne is lead engineer (design authority). TR is SSRL technical contact. Design meetings will be
conducted on Mondays at 1400 at a location to be designated by Tim.
(10) SSRL vacuum shop assembly with lead tech TBD. Machining by most cost effective means (ie., SLAC
or SSRL for modifications, SLAC, SSRL, or vendor for new parts). Julie Greer will support effort as
expeditor.
Acceptance: (See bl10_power_2013_09_04.xlsx for more information)
Horizontal – The old horizontal acceptance was completely determined by the mirror positioning and mirror
length as discussed in Overview and Background. This resulted in the downstream tip of the mirror being
inserted into the highest absorbed power density without the benefit of one direction of longitudinal heat
flow. The revised horizontal acceptance shall be limited to 1.00 horizontal mrad centered at 2.0mrad off the
ID axis (2.0mrad towards SSRL not SPEAR). The revised acceptance shall be established by the addition of
a mask upstream of the mirror. (Note the inboard mask edge can be moved from the 0.75mrad edge of the
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BL10-1 M0 Mirror System Fix SOW
BL10-2 acceptance to 0.895mrad to permit a SSRL standard mask thermal design with 5.08mm channel
repeat spacing. See SSRL Engineering Notes M484 and M488.)
Vertical – The old vertical acceptance was established by the exit aperture of the front end fixed mask. This
8.0mm aperture at z=6100mm passed a vertical acceptance of 1.31mrad to the mirror. Downstream of the
mirror, however, the mask located at z=9575mm provides an 8.0mm vertical aperture resulting in 0.836mrad
vertical acceptance being passed along to the monochromator. At 250eV the resulting vertical acceptance is
81.6% of the full beam flux while at 1200eV it is 99.5%. The revised vertical acceptance of the mirror as set
by the new mask shall be 0.95mrad so as to slightly overfill the downstream aperture yet not illuminate the
mirror with more power than necessary. (Slightly over filling the mirror improves intensity stability and
ensures that the accepted beam passed to the monochromator does not include beam reflected by that portion
of the mirror with a strong thermal gradient hence greater slope error.) The entrance acceptance of the
vertical masking must be configured for the mis-steers depicted in the ray trace GP-451-029-20-c6 with front
end fixed mask to new mask relative alignment tolerance (ie., about 13.0mm mis-steer plus a couple mm of
alignment tolerance).
Power: (See bl10_power_2013_09_04.xlsx for more information)
The masking defined above significantly reduces the mirror incident and absorbed power with more modest
changes in the peak absorbed power density. Table 1 below compares the integrated power values for the old
mirror configuration, the inboard horizontal mrad (ie., higher absorbed power portion) of the old
configuration, and the revised configuration at 500mA SPEAR3 current. The total absorbed power under the
revised configuration is reduced to 46% of the old configuration while the total absorbed power per
horizontal mrad is reduced to 68% of the power in the inboard horizontal mrad under the old configuration.
Table 1: Integrated power at the mirror for various masking configurations at 500mA.
configuration
old
old inside mr
revised
incident (W)
reflected (W)
absorbed
(W)
3008.8
188.7
1986.9
92.5
1388.0
100.4
2820.1
1894.4
1287.6
The vertical power density profile on the ID centerline and at the lateral limits of the mask to be inserted
upstream of the mirror are illustrated in Figure 2. Details of the power profile in both horizontal and vertical
directions are provided in the spreadsheet referenced above.
Stanford Synchrotron Radiation Lightsource
Engineering Note
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Thomas Rabedeau
Serial
M693
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Date
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Title
BL10-1 M0 Mirror System Fix SOW
Figure 2: Vertical power density profiles along the ID centerline (x=0mrad), at the mask inboard edge
(x=0.75mrad), at the mask edge defining the inboard side of the BL10-1 acceptance (x=1.5mrad), and at the
mask edge defining the outboard edge of the BL10-1 acceptance (x=2.5mrad). The power density decreases
with x observation angle owing to the ID magnetic field roll off with observation angle.
Possible Mask Geometry:
As noted above the BL4 comb mask per SA-451-070-03 provides something of a reference design. The
SSRL standard 0.125” ball end mill mask FEA (EN M488) suggests a maximum mask incident angle of
approximately 1.5deg. The SSRL standard 0.125” ball end mill crenelated mask FEA (EN M484) suggests a
maximum mask incident angle of approximately 3.0deg. It may be advantageous to combine the two
concepts with the “folded mask” concept of BL4. Specifically make a ball end mill multi-channel mask
operating at 1.5 degrees and occluding from at least 7.5mm off median plane to 4.0mm off median plane,
then add crenlated mask teeth operating at 3.0 degrees occluding at least 7.5mm off median plane down to
the median plane. This requires approximately 150mm mask length. The “folded mask” concept of BL4
implies one such mask for above the median plane and one below the median plane. See Figure 3 for concept
section sketch.
The tooth of the mask closest to the wiggler centerline is illuminated with the highest power density as
represented by the x=0.75mrad green dashed curve in Figure 2. At normal incidence at z=8.4m, the
x=0.75mrad vertical power envelope shown in Figure 2 translates to an approximate Gaussian power
envelope with peak amplitude of 120.8W/mm^2 and rms height of 0.775mm. One can conservatively model
this power envelope as a boxcar with constant power 120.8W/mm^2 and full vertical height of 1.94mm.
Ignoring the weak x dependence of the power envelope, this integrates to 234.5W/horz_mm. If one assumes
the mask intercepts the beam at 3.00deg grazing incidence, then the boxcar approximation to the absorbed
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BL10-1 M0 Mirror System Fix SOW
power density will have constant power of 6.32W/mm^2 and a footprint full “height” along the mask of
37.07mm. This simple boxcar heat load should permit conservative modeling of the real power load. Though
the extra complication is generally not warranted, alternatively one could apply a step wise approximation to
the Gaussian heat load which is …
P(y) (W/mm^2) = 120.8*exp[-y*y / (2*0.775*0.775)] for normal incidence with y in mm
P(y) (W/mm^2) = 6.32*exp[-y*y / (2*14.81*14.81)]
for 3.0deg incidence with y in mm
Figure 4: Mask concept section sketch at downstream end of mask tooth. Note that relative to earlier sketches
the glidcop to copper bond line has been moved 2mm farther from beam to allow for bond line wander in the
explosive bond. Dimensions in mm.
The beam vertical mis-steer is controlled by the SPEAR3 orbit interlock. The orbit interlock ensures the
beam steering satisfies the following condition after all tolerances are accounted (ref. SSRL EN M344r2):
|y| / 1.505mm + |y’| / 0.59mrad < 1
Assuming maximum angle mis-steer (i.e, y’=+/-0.59mrad) results in the center of the mis-steered beam at
y=+/-4.96mm at the position of the fan allocation mask. This implies the beam centroid can strike the 4.0mm
offset mask surface depicted in figure 4 above.
To simplify power footprint calculations for arbitrary beam incident angle α on the mask, we can use the
generalized version of the boxcar power footprint discussed above. To wit,
power density in W/mm^2 (@z=8.4m) = 120.8*sin(α)
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footprint full length in mm (@z=8.4m) = 1.94/sin(α)
This power footprint can be centered anywhere inside the vertical window -4.96mm ≤ y ≤ 4.96mm.
Flux: (See bl10_power_2013_09_04.xlsx for more information)
The ideal integrated flux with the revised acceptance is ~55% of the ideal flux under the old configuration as
depicted in Figure 4. However, it is likely that even before the mirror was damaged the extreme thermal load
the mirror system resulted in substantially less than ideal reflected beam properties. Thus we anticipate that
the revised mirror configuration will deliver more than 55% relative to the 500mA pre-damaged mirror
system flux.
Figure 4: The ideal integrated flux at 500mA for the old and revised beam line acceptance configurations.
Stanford Synchrotron Radiation Lightsource
Engineering Note
Authors
Thomas Rabedeau
Title
BL10-1 M0 Mirror System Fix SOW
Referenced spreadsheet bl10_power_2013_09_04.xlsx
bl10_power_2013_0
9_04.xlsx
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Date
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