Detecting the B-mode Polarisation of the CMB with Clover
ArXiv 0805.3690 (2008)
Abstract:
We describe the objectives, design and predicted performance of Clover, which is a ground-based experiment to measure the faint ``B-mode'' polarisation pattern in the cosmic microwave background (CMB). To achieve this goal, clover will make polarimetric observations of approximately 1000 deg^2 of the sky in spectral bands centred on 97, 150 and 225 GHz. The observations will be made with a two-mirror compact range antenna fed by profiled corrugated horns. The telescope beam sizes for each band are 7.5, 5.5 and 5.5 arcmin, respectively. The polarisation of the sky will be measured with a rotating half-wave plate and stationary analyser, which will be an orthomode transducer. The sky coverage combined with the angular resolution will allow us to measure the angular power spectra between 20 < l < 1000. Each frequency band will employ 192 single polarisation, photon noise limited TES bolometers cooled to 100 mK. The background-limited sensitivity of these detector arrays will allow us to constrain the tensor-to-scalar ratio to 0.026 at 3sigma, assuming any polarised foreground signals can be subtracted with minimal degradation to the 150 GHz sensitivity. Systematic errors will be mitigated by modulating the polarisation of the sky signals with the rotating half-wave plate, fast azimuth scans and periodic telescope rotations about its boresight. The three spectral bands will be divided into two separate but nearly identical instruments - one for 97 GHz and another for 150 and 225 GHz. The two instruments will be sited on identical three-axis mounts in the Atacama Desert in Chile near Pampa la Bola. Observations are expected to begin in late 2009.The ClOVER Experiment
MILLIMETER AND SUBMILLIMETER DETECTORS AND INSTRUMENTATION FOR ASTONOMY IV 7020 (2008) ARTN 70201E
Tests of finline-coupled TES bolometers for Cℓ OVER
IRMMW-THz2007 - Conference Digest of the Joint 32nd International Conference on Infrared and Millimetre Waves, and 15th International Conference on Terahertz Electronics (2007) 180-181
Abstract:
CLOVER aims to detect the signature of gravitational waves from inflation by measuring the B-mode polarization of the cosmic microwave background. CLOVER consists of two polarimeters. One operates at 97 GHz, using finline-coupled Transition Edge Sensors (TES). The other has a combined 150/220-GHz focal plane populated by radial-probe coupled TES detectors. The 97-GHz instrument will have 100 feedhorns and 200 detectors while the combined 150 and 220-GHz instrument will have a total of 200 horns. To achieve the target NEP of 1.5 x 10 -17 W Hz-1 the 97-GHz detectors will have a transition temperature of 190 mK and will operate with a base temperature of ∼ 100 mK. CLOVER's detectors are fabricated on 225-micron silicon substrates. In the 97-GHz instrument a finline transition feeds a microstrip which is terminated by a matched resistor on the silicon nitride island that carries the TES. Each detector is fabricated as a single chip to ensure a 100% operational focal plane. The detectors are mounted in linear modules made of copper which form split-block waveguides. Each detector module contains a time-division SQUID multiplexer to read out the detectors. The multiplexed signals are further amplified by SQUID series arrays. The first prototype detectors for CLOVER have a transition temperature of 350 mK and were fabricated to validate the detector design and the polarimeter technology. We have characterised these detectors in a dedicated test facility. The CLOVER testbed contains cryogenics similar to those in the final instrument: a pulse-tube cooler, He-7 sorption fridge, and a mini dilution fridge so that the detectors are tested in a realistic environment. The test bed has a cryogenic black-body source with band-defining filters for optical testing. As well as the multi-channel electronics that will be used on the final instrument the test bed has an analogue SQUID readout which allows us to characterise the readout fully. We discuss the results of the detector tests and the design changes needed to achieve the required sensitivity.The design of potter horns for THz applications using a genetic algorithm
International Journal of Infrared and Millimeter Waves 28:12 (2007) 1103-1114
Abstract:
We describe the design and performance of Potter horns at millimetre and submillimetre wavelength employing a novel software package that we have developed, using Genetic Algorithm. The horn is easy to fabricate and exhibits excellent beam circularity and low cross polarization over a 15% bandwidth which is sufficient for many applications. Excitation of the required higher order modes is done by either a step or a flare discontinuity at the horn throat. In each case we provide design curves that give the optimum parameters of the horn geometry as a function of frequency and beamwidth. The range of values provided covers the parameters required for the design of horns for telescope feeds and various other instruments. The design curves show clearly that the flare-step performance is superior to the traditional groove-step Potter horn. The simulations for designing these horns were carried out at millimetre and submillimetre wavelengths but the results can be scaled to lower or higher frequencies. A key component in the design method is the optimization software that searches for the correct magnitude and location of the flare discontinuities. We have developed a software package based on the combination of modal matching, a genetic algorithm (GA) and downhill simplex optimization. The genetic code is first used to locate the proximity of the global minimum. The set of parameters obtained are then used as a starting point for the simplex method, which refines the parameters to the required accuracy. © 2007 Springer Science+Business Media, LLC.An integrated superconducting phase switch for cosmology instruments
Physica C: Superconductivity and its Applications 466:1-2 (2007) 115-123