30 research outputs found
Sorption-cooled miniature dilution refrigerators for astrophysical applications
The next generation of balloon-borne and ground-based mm/sub-mm astronomy experiments will require operating temperatures near or below 0.1 K. When these experiments are operated remotely on platforms or at sites with limited infrastructure and maintenance sup port, a compact and reliable dilution refrigerator becomes essential. We have investigated two different dilution refrigerators in order to evaluate which system is most suitable for these applications. We have carried out a feasibility study of the simplest of the two technologies, a single-shot dilution refrigerator. A thermal model for predicting its performance has been developed, and a first prototype which achieved temperatures of about 70 mK was built. We discuss advantages and disadvantages of a single-shot system and show how minor changes to the current design can make it useful for many astronomy applications. The second dilution refrigerator is based on the principle of condensation pumping. We have built and integrated such a refrigerator with a pulse-tube cooler in order to create a completely cryogen-free system. Temperatures below 50 mK have been achieved, and temperatures below 100 mK have been maintained for more than 10 hours with several micro-Watt of cooling power. Using two 3 He sorption coolers and gas-gap heat switches we have also demonstrated how this cooler can be operated in a continuous mode. The entire system is fully automatic in operation and can be controlled and monitored remotely through a standard http protocol. We show how existing thermal models can be used to predict the cooling power and lowest achievable temperatures of the refrigerator. Experimental results are analysed and used to estimate the condensation efficiency, the performance of the heat exchangers and the 3 He circulation rate
Clover - A B-mode polarization experiment
Clover is a new instrument being built to detect the B-mode polarization of
the CMB. It consists of three telescopes operating at 97, 150, and 220 GHz and
will be sited in Chile at the Llano de Chajnantor. Each telescope assembly is
scaled to give a constant beam size of 8 arcmin and feeds an array of between
320 and 512 finline-coupled TES bolometers. Here we describe the design,
current status and scientific prospects of the instrument.Comment: 11 pages, 6 figures. Proceedings of the Fundamental Physics With CMB
workshop, UC Irvine, March 23-25, 2006, to be published in New Astronomy
Review
A continuous dry 300 mK cooler for THz sensing applications
We describe and demonstrate the automated operation of a novel cryostat design that is capable of maintaining an unloaded base temperature of less than 300 mK continuously, without the need to recycle the gases within the final cold head, as is the case for conventional single shot sorption pumped 3He cooling systems. This closed dry system uses only 5 l of 3He gas, making this an economical alternative to traditional systems where a long hold time is required. During testing, a temperature of 365 mK was maintained with a constant 20 μW load, simulating the cooling requirement of a far infrared camera
QUBIC: The QU Bolometric Interferometer for Cosmology
One of the major challenges of modern cosmology is the detection of B-mode
polarization anisotropies in the CMB. These originate from tensor fluctuations
of the metric produced during the inflationary phase. Their detection would
therefore constitute a major step towards understanding the primordial
Universe. The expected level of these anisotropies is however so small that it
requires a new generation of instruments with high sensitivity and extremely
good control of systematic effects. We propose the QUBIC instrument based on
the novel concept of bolometric interferometry, bringing together the
sensitivity advantages of bolometric detectors with the systematics effects
advantages of interferometry. Methods: The instrument will directly observe the
sky through an array of entry horns whose signals will be combined together
using an optical combiner. The whole set-up is located inside a cryostat.
Polarization modulation will be achieved using a rotating half-wave plate and
interference fringes will be imaged on two focal planes (separated by a
polarizing grid) tiled with bolometers. We show that QUBIC can be considered as
a synthetic imager, exactly similar to a usual imager but with a synthesized
beam formed by the array of entry horns. Scanning the sky provides an
additional modulation of the signal and improve the sky coverage shape. The
usual techniques of map-making and power spectrum estimation can then be
applied. We show that the sensitivity of such an instrument is comparable with
that of an imager with the same number of horns. We anticipate a low level of
beam-related systematics thanks to the fact that the synthesized beam is
determined by the location of the primary horns. Other systematics should be
under good control thanks to an autocalibration technique, specific to our
concept, that will permit the accurate determination of most of the systematics
parameters.Comment: 12 pages, 10 figures, submitted to Astronomy and Astrophysic
Detecting the B-mode Polarisation of the CMB with Clover
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.Comment: 5 pages, 3 figures. To appear in the proceedings of the XXXXIIIrd
Rencontres de Moriond "Cosmology". Figure 1 update