Ultra-low temperature polarization-sensitive receiver for millimeter-wavelength Atacama Cosmology Telescope
Cool science below 100 mK at 5140 meters elevation!
Janis Research’s ultra low temperature (ULT) group is proud to announce that it has developed, in collaboration with multi-agency scientific collaboration team called ACTPol, a new pulse-tube cooled cryogenic platform for the Atacama Cosmology Telescope (ACT). The ACT is a six-meter Gregorian telescope located at an altitude of 5,200 meters (17,000 ft) on Cerro Toco, in Northern Chile, and is dedicated to studies of the structure and evolution of the early universe through direct observation of the Cosmic Microwave Background (CMB) radiation at different polarizations and with arcminute resolution. Its new focal plane contains 3000 polarization-sensitive transition edge senor (TES) bolometers, which must be cooled to below 100 mK to work. The new platform, together with new optical tubes and detectors developed by ACTPol collaboration, constitutes the telescope “light receiver” (camera). When fully deployed, its three light-sensing detector packages, as well as parts of the optical system, will be cooled by an integrated cryogen-free 3He – 4He dilution refrigerator developed by Janis Research. During its first season of operation in 2013, the first 150 GHz detector package array (PA1) was successfully tested on site and first light was received with detectors below 100 mK, which made this experiment the only CMB experiment to operate at such a low temperature.
Figure 1 ACT telescope is located inside of the micro-wave shield seen here at 5140 meters above the sea-level in site on Cerro Toco, in Northern Chile.
The Receiver temperature-controlled cabin is attached to the telescope primary-secondary mirror superstructure, and is moving together with the telescope during the scans (Figure 2).
Figure 2 ACT telescope superstructure.
Figure 3. Receiver positioning scheme on the telescope optical axis inside of the cabin. Neutral cabin floor position is tilted down at 5% to horizon, bringing both PT-tubes to vertical positions.
Figure 4 Receiver cross-section, showing preliminary design with two of three optic tubes.
The dilution refrigerator (DR), installed inside of the receiver, is remotely operated via Ethernet link and can move with acceleration of 1 G both vertically in the scan range of +/- 35 degree from neutral position, as well as horizontally. The air cooling system for electronics and pumps allows continuous operation at low atmospheric pressure of 500 mbar.
The receiver schematic cross-section is shown on Figure 4. It is designed as a vacuum chamber (blue) with 50 K (purple) and 4 K (turquoise) shields on G-10 isolating supports (yellow), cooled by a dedicated Cryomech pulse-tube (PT) cryo-cooler PT-415 seen in the left lower corner (grey).
The PT-407 cooled removable DR insert is shown in the left upper corner as a circle, and in detail on Figure 9 and Figure 10. The PT as well as dilution core are installed tilted to the optical central axis to allow DR operation in full scan range.
The refrigerator set-up for factory tests at Janis Research is shown on Figure 6, while the summary of test results is plotted on Figure 5. The test results both as is and integrated with the receiver have shown that it is fully compliant with technical specification set up by the ACTPol collaboration. Employing the highly-customized remotely controlled liquid-cryogen-free dilution refrigerator, without thermal cycling of commonly used single-shot adiabatic demagnetization refrigerator (ADR) cooling systems, significantly improved enabled day-time scanning strategies. Base temperature well below the target of 100 mK and cooling power of 120 µW@100mK was achieved with air cooled HighPace 300 Turbo and Edwards XDS35 scroll pumps.
Figure 5 Cooling power and base temperature at indicated 3He flow rates
Figure 6 JDry-100-ACTPol in its test enclosure, attached to the automated Gas Handling System GHS4, during its tests at Janis Research.
Figure 7 GHS4 exposed: Janis Automated Control Box (JACoB) sitting atop of Janis Mixture Enclosure (JAMiE100)
Figure 8 Setting up remote connection to GHS4 via Ethernet link.
Figure 9 JDry-100-ACTPol Insert solid model and assembly process.
Figure 10 Insert with mechanical heat-switch and house-keeping wiring installed.
In Spring 2013, the first 150 GHz kilo-TES polarimeters array package (PA1) was deployed for operation in the ACTPol receiver on the ACT site. In early 2014, a second 150 GHz array package (PA2), and a multichroic 90/150 GHz array package (PA3) will be deployed for a complete focal plane across three optics tubes.
Figure 11 PA1, top view. Note feed-horn monolithic array (courtesy of ACTPol collaboration).
Figure 12 PA1, bottom view. Note 100 mK gold-plated thermal links to polarimeters arrays (courtesy of ACTPol collaboration).
Fully assembled Receiver before and during integration with the telescope is shown on, Figure 13, Figure 14 and Figure 15.
Figure 13 The Receiver during final tests at University of Pennsylvania test facility before shipping to Chile, fully assembled and with electronics attached. Three optical inputs are seen, one with Teflon window exposed.
As of June 2013, the ACTPol receiver has been successfully shipped to and integrated on the ACT site in Chile. First light and commencement of season one operations (PA1) was in July 2013.
Figure 14 On site before installation.
Figure 15 Inside ACT moving cabin.
Figure 16 Receiver on the telescope optical path.
Operations with full focal-plane deployment are projected for Spring 2014.
Figures courtesy of ACTPol collaboration.
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