We discuss the C<sub>l</sub>over cryostats, which are dry cryostats containing three stages of cooling; a pulse tube cooler,
a sorption fridge and a continuous miniature dilution refrigerator. We describe the thermal architecture of the
system and present thermal data for the various stages including its performance when tilted.
Finlines are planar structures which allow broadband and low loss transition from waveguide to planar circuits.
Their planar structure and large substrate makes them ideal for integration with other planar circuits and
components, allowing the development of an on chip polarimeter. We have developed a method of extending the
employment of finlines to thick substrates with high dielectric constants by drilling or etching small holes into
the substrate, lowering the effective dielectric constant. We present the results of scale model measurements at
15GHz and cryogenic measurements at 90GHz which illustrate the excellent performance of finline transitions
with porous substrates and the suitability of this technique for extending the bandwidth of finline transitions.
CℓOVER is a multi-frequency experiment optimised to measure
the Cosmic Microwave Background (CMB) polarization, in
particular the B-mode component. CℓOVER comprises two
instruments observing respectively at 97 GHz and 150/225 GHz.
The focal plane of both instruments consists of an array of
corrugated feed-horns coupled to TES detectors cooled at 100
mK. The primary science goal of CℓOVER is to be sensitive to
gravitational waves down to r ~ 0.03 (at 3σ)in two years of operations.
We have fabricated TES bolometers with finline transitions for the CℓOVER project. We have measured the
optical response of CℓOVER's first prototype 97-GHz detectors and find that they have a detection efficiency
close to 100%. We have also investigated the effects of misalignment of the finline in the waveguide and of
thinning the substrate. The prototype detectors have dark NEPs as low as 1.5 x 10<sup>-17</sup>W/√Hz and satisfy
the requirement of photon-noise limited operation on CℓOVER. We describe the optical tests of CℓOVER's
prototype 97-GHz detectors and discuss their implications for the design of the science-grade detectors.
Several technologies are now being considered for modulating the polarization in various B-mode instruments, including rotating quasioptical half-wave plates in front of the focal plane array, rotating waveguide half-wave plates and Faraday rotators. It is not at all clear that any of these techniques is feasible without heavy penalty in cost or performance. A potentially much more efficient method is to use a pseudo-correlation polarimeter in conjunction with a planar circuit phase switch.
We investigate three different devices for use as mm-wave switches, SIS tunnel junctions, capacitively coupled superconducting nanostrips and RF MEMS. The SIS tunnel junction switches operate by switching between two different bias voltages, while the nanostrip switch operates by changing the impedance of a resonant circuit by driving the nanostrip from the superconducting to normal state. In each case the RF signal sees two substantially different complex impedance states, hence could be switched from one transmission line branch to another. In MEMS this is achieved by mechanical movement of one plate of a parallel plate capacitor system. Although RF MEMS have been reported at high microwave and low mm-wave frequencies, in this work we have investigated cryogenic MEMS for operation at high mm-wave frequencies (225 GHz) using superconducting transmission lines.
We present and compare designs and simulations of the performance of phase switches based on all three switching technologies, as well as preliminary experimental results for each of the switches. Finally we also present designs of phase shift circuits that translates the on/off switching into phase modulation.