Cryogenic millimeter-wave receivers require a low-loss vacuum window through which the incoming beam can pass on its way to the cold feed horn. Thick dielectric plates have been used as windows, but their loss can contribute significantly to the receiver noise temperature, and their useful bandwidth is limited even when matching grooves or dielectric matching layers are used. A simpler approach, which has been in use for many years at Berkeley [1] and Caltech [2], is to use a thin sheet of mylar (polyethylene terephthalate) as a vacuum window. Atmospheric pressure causes such a window to balloon alarmingly, but the great strength and flexibility of mylar are sufficient in many applications so long as no sharp object pricks the window (in which event the results could be hazardous). Radiation cooling of the mylar film by the cold innards of the dewar can cause condensation of water on the outside of the window, which can contribute significant electrical loss. To prevent this, the mylar can be dried by air from a small fan. This report describes a window in which a thin plastic film vacuum barrier is supported by a thick slab of low density dielectric foam. The dielectric foam bears the full atmospheric pressure, and also acts as an infrared filter to reduce radiative cooling of the plastic film and radiative warming of the cold contents of the dewar. This scheme was inspired by the window for circular waveguide described of Bradley and Norrod [3, 4], which uses a plug of polystyrene foam inside the waveguide and a mylar film glued on the atmospheric side of the window to prevent moisture absorption by the foam over long periods of time.
View a PDF version of ALMA Memo #090.Last modified: 2001 Aug 06
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