Achromatic Half Wave Plate
This project focuses on the construction of a sapphire AHWP and measurements of its properties at a wavelength of 2.7 mm (110 GHz), as well as analysis of the design of a three-plate sapphire AHWP for a wavelength of 2 mm. There is currently interest in an AHWP that is suitable for the mm-wave band because of the increase in experimental efforts to measure the polarization of the cosmic microwave background radiation. Several experiments will use HWPs and increasing the bandwidth where the efficiency is high will increase the signal-to-noise ratio of the experiment.
Superconducting Magnetic Bearing
A design for a cosmic microwave background (CMB) polarimeter in which a cryogenically cooled half-wave plate rotates by means of a high-temperature superconducting (HTS) bearing has been presented. A prototype bearing, consisting of a commercially available ring-shaped permanent magnet and an array of YBCO bulk HTS material, has been constructed. We measured its coefficient of friction and vibrational property as a function of several parameters, including temperature between 15 and 83 K, rotation frequency between 0.3 and 3.5 Hz, levitation distance between 6 and 10 mm and ambient pressure of 10^- 7 Torr. We concluded that the low rotational drag of the HTS bearing would allow rotations for long periods with minimal input power and negligible wear and tear, thus making this technology suitable for a future satellite mission. We have given experimental evidence for the connection between magnetic field inhomogeneity of a permanent magnet and torque on the rotor in a high temperature superconducting bearing. The results imply that arcminute accuracy angular encoding of the rotor can be achieved with a single measurement of angle in each period, which is neccesary for a polarimeter to detect B-mode variations in the polarization of the CMB (such as in EBEX).