Optical and Mechanical Design of C-Mod Motional Stark Effect Diagnostic Page: 2 of 3
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maintaining a component parallel to the beam to Doppler
shift the Stark components away from unshifted H coming
from the plasma edge. The MSE optics relay collected light
out of the vessel through a 10 cm diameter quartz window
on the same port but 9" above the neutral beam that it is
imaging. The final factor determining the geometry is the
need to avoid impinging on the plasma. Any stainless steel
part in the optical path was darkened using a vacuum
compatible hydrogen furnace heat treatment to suppress
stray light. The MSE vacuum housing is bolted to the
inside flange of the quartz viewport. A split ring bracket
(see Figure 1) is employed to support the MSE and protect
the diagnostic from a plasma disruption. A heat shield is
used to protect the thin first lens. See Figures 1 and 2.
Figure 1. View of MSE from inside C-Mod showing input
lens, vertical housing for second lens set, split ring support,
and thin wall housing containing last two mirrors.
Figure 2. Close-up view from inside C-Mod of MSE
showing input lens, heat shield and lower portion of split
ring support bracket.
A stainless steel shutter (not shown) has been made but not
yet installed. The shutter fits between the heat shield and
the housing for the first lens. It is designed with an aperture
for unobstructed viewing of the neutral beam and on the
opposite side an aperture with a Polorcor polarizer. The
shutter is designed to rotate 1800. A small braided monel
wire wraps around the shutter and connects to a pair of
linear feedthroughs on the outside of the vessel which drive
the motion. The shutter rides on a small inconel ring
located just below the heat shield (see Figure 2).
IV. OPTICS AND HOUSING OUTSIDE C-MOD
The exterior optics are not yet installed on C-Mod. The
exterior optics relay the light from an intermediate image
plane just outside the quartz viewport to a more accessible
location for the fiber optic array about 60 cm away. There
is a set of crossed photoelectric modulators (PEMs) used in
the polarization measurement located just before the last
lens (see Figure 3). This final lens focuses the light to a
concave image plane with a 19.5 mm radius of curvature.
The image plane is populated with closely spaced bundles
of 1 mm diameter fiber optics. The MSE fibers are bundled
in sets of seven fibers and placed in stainless steel ferules
with a 3.66 mm inside diameter. The BES fibers are
bundled in sets of four fibers and placed in stainless steel
ferules with a 3 mm inside diameter. A delrin and
aluminum composite fixture containing a two dimensional
set of holes locate the fibers in the x-y plane. Delrin is used
to provide friction to hold the ferrules in place, while the
aluminum is needed to provide structural rigidity to the
holes spaced as close as 0.25 mm apart. The location of the
fibers along the z-axis is set by pushing the ferrules through
the delrin and aluminum composite fixture until they
contact a temporary convex fixture made from carefully
positioned pins which matches the image plane.
Figure 3. View of MSE external optics enclosure showing
(from right) the phenolic window adapter and lens barrel,
PEMs, and fiber optic array holder with cutout for
temporary alignment fixture.
Outside the vacuum boundary there are high magnetic
fields and restricted access. The exterior optics enclosure is
made of phenolic elements in a lens barrel design. Spacers
made of G-10 fix the location of the lenses inside the
phenolic tube along the z-axiz. The lenses were mounted in
G-10 barrels using a three point contact similar to that used
for the interior lenses described above. The inside surface
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Simon, D.I.; Marmar, E.; Bretz, N.L.; Bravenec, R. & Parsells, R.F. Optical and Mechanical Design of C-Mod Motional Stark Effect Diagnostic, report, November 1, 1999; Princeton, New Jersey. (digital.library.unt.edu/ark:/67531/metadc620759/m1/2/: accessed December 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.