Need: Vacuum-sealing window, lossless in our 100/150 GHz bands.

ACBAR (4" aperture) used 3-cm thick Zotefoam PPA30:

• "Closed cell" => low He gas permeability, low water absorption
• "Cross-linked" => strong
• "PP foam (pure N₂ expanded)" => low loss @ mm wavelength
• Small cell size (0.3 mm) => low scattering @ mm wavelength
• Low density (30 mg/cm³) => low surface scattering

Transmission measured @ ~150 GHz

Stress analysis of 1/8"-thick Al window holder

Preventing snow accumulation

The vacuum window with a 30-cm aperture is made of 10-cm thick Zotefoam PPA30 (closed-cell nitrogen expanded polypropylene, http://zotefoams.com), based on the experience of ACBAR (Runyan et al. 2003). While being opaque to infrared radiation, this 4-layer heat-laminated slab of Zotefoam was measured to have ~1% loss at 150 GHz. Scattering at mm wavelength is very low thanks to the small cell size (0.3 mm) and low density (30 mg/cm3). The window is estimated to add an optical loading of 2~3 K_RJ on the detectors. Because the foam window is radiatively cooled to below ambient temperature from both sides, frost can form from residual moisture in the air. To prevent this, we have installed a taut 18-μm polypropylene film as a cover, with the airspace between the cover and the Zotefoam window slightly pressurized and continuously purged by warm dry nitrogen gas.