SLAC Linac Preparations for FACET Page: 2 of 3
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experiments. These blocks were cut to fit tightly together
to span the height and width of the tunnel with numerous
small penetrations to accommodate existing pipes and
cable trays and the electron and positron transport lines
that were part of the PEP-II injection system. The wall is
approximately two feet thick in the beam direction and is
supplemented with steel plates. The blocks are secured to
the tunnel floor and walls to withstand seismic shocks.
The design of the wall was complicated by the need to
provide an emergency egress path through the tunnel in
both directions. To provide this feature, the wall was
constructed with a pedestrian maze that allows a person to
walk through a passageway along the south side of the
tunnel. Steel doors at both ends of the passageway are
interlocked to trip off the beam hazards if either door is
opened during operations. Figure 2 is a cutaway view of
the wall structure showing the passageway. Figure 3 is a
photograph of the finished shielding wall, taken from the
FACET side looking downstream.
Figure 2: Mechanical design of Sector 20 shielding wall.
Figure 3: Completed shielding wall as seen from the
tunnel in Sector 20.
The 2-ft diameter alignment light pipe is visible in both
the figure and photograph. This pipe extends for the full
length of the linac and provides an optical path for a laser-
based alignment system. This alignment feature was
judged to be too valuable to abandon, and so the wall was
constructed to fit closely around the pipe. To provide
radiation shielding over the light pipe opening, a movable
stopper was constructed in a rectangular vacuum
enclosure integrated into the light pipe. The stopper
enclosure box is visible in the lower left corner of figure
Persons working in the FACET area upstream of the
wall are well shielded from any radiation that might back-
scatter from the LCLS injection area in Sector 21.
However, the converse is not true; no one will be allowed
in the tunnel downstream of the wall when a beam is
present in the FACET area. Enforcing the new access
requirements requires significant modifications to the
personnel protection system. These modifications are
being done as part of a general upgrade of the linac PPS
SECTOR 19 STAIRWAY
As originally built, each sector is accessible by way of
a vertical ladder installed in a "manway" shaft, which is a
cylindrical opening from the Klystron Gallery to the
tunnel approximately 35 feet below. Over the years,
these manway shafts have provided the primary means for
entering the linac tunnel. However, the height of the
ladders makes them inherently hazardous, especially for
persons carrying tools or other equipment. In addition to
the manway shafts, larger equipment access shafts already
exist at five-sector intervals and have been used for
moving 10-foot accelerator structures and other large
objects in and out of the tunnel. Sometime after the
original construction, a stairway was installed in the shaft
in Sector 4, and more recently a stairway was installed in
Sector 24 to facilitate easy access to the LCLS injection
and bunch compressor systems. A similar stairway will
be installed in the equipment access shaft in Sector 19.
This stairway will provide convenient access to the
FACET focal point area where experimenters will set up
E r taNew stC nwjyrin
Secldr 19 shaft
Underground accelerator housing
ssesFACET fndusig sytm
t ~ Positron vaul
Figure 4: Cutaway view of the linc tunnel in Sector 20
showing the locations of the new stairway, the FACET
systems, and the shielding wall.
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Erickson, R.; Bentson, L.; Kharakh, D.; Owens, A.; Schuh, P.; Seeman, J. et al. SLAC Linac Preparations for FACET, article, February 7, 2011; United States. (https://digital.library.unt.edu/ark:/67531/metadc834666/m1/2/: accessed March 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.