Kavli Affiliate: Tomotake Matsumura
| First 5 Authors: Takayuki Toda, Yuki Sakurai, Hirokazu Ishino, Tomotake Matsumura, Kunimoto Komatsu
| Summary:
The polarization modulator unit for the low-frequency telescope in LiteBIRD
employs an achromatic half-wave plate (AHWP). It consists of five layers of
a-cut sapphire plate, which are stacked based on a Pancharatnam recipe. In this
way, the retardance of the AHWP is a half-wave over a bandwidth of 34-161 GHz.
The diameter of a single sapphire plate is about 500 mm and the thickness is 5
mm. When a large diameter AHWP is used for a space mission, it is important for
the AHWP to survive launch vibration. A preliminary study indicates that the
five-layer stacked HWP has a risk of breakage at the launch unless the five
layers are glued together and mechanically treated as one disk. We report our
investigation using a sodium silicate solution that can bond the sapphire
plates. This technique has been previously investigated as a candidate of
cryogenic bonding for a mirror material, including sapphire, of the
gravitational wave experiments: LIGO, VIRGO, and KAGRA. We experimentally
studied the mechanical strength of the bonded interface for two different
surface conditions: polished and unpolished. We demonstrated that the tensile
and shear strength > 20 MPa for samples with a polished surface. This satisfied
the requirement of 5.5 MPa derived from the mechanical simulation assuming a
launch load of 30G. We identified that samples glued on a polished surface
exhibit higher strength than unpolished ones by a factor of 2 for tensile and
18 for shear strength. We measured the millimeter-wave transmittance between 90
and 140 GHz using sapphire plates with a diameter of 50 mm before and after
bonding. We did not find any optical effects caused by the bonded interface
within 2% error in transmittance, which originates from the measurement system.
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