Studying the Cosmic Microwave Background at ACT using Cryogenic Cooling Detectors

A new pulse-tube cooled cryogenic platform has been developed by the ultra low temperature (ULT) group of Janis Research in partnership with ACTPol, a multi-agency scientific collaboration team, for a six-meter Gregorian telescope called the Atacama Cosmology Telescope (ACT).

Pulse Tube Cooled Dilution Refrigerator for the ACT

The ACT, situated at an altitude of roughly 5,200m on Cerro Toco, in Northern Chile (Figure 1), is built to study the structure and evolution of the early universe by directly observing the Cosmic Microwave Background (CMB) radiation at arc-minute resolution and at different polarizations.

The new focal plane of the telescope consists of 3000 polarization-sensitive transition edge sensor (TES) bolometers. It is necessary to cool the bolometers to below 100mK for their proper function.

ACT telescope is located inside of the micro-wave shield seen here at 5140m above the sea-level in site on Cerro Toco, in Northern Chile.

Figure 1. ACT telescope is located inside of the micro-wave shield seen here at 5140m above the sea-level in site on Cerro Toco, in Northern Chile.

The ACT’s “light receiver” (camera) contains the new pulse-tube cooled cryogenic platform as well as the new optical tubes and detectors designed by the ACTPol collaboration. When fully installed, the new platform’s three light-sensing detector packages as well as the optical system components will be subjected to cooling by an integrated cryogen-free 3He – 4He dilution refrigerator devised by Janis Research.

The testing of first 150 GHz detector package array (PA1) was successfully completed on site during the first season of operation of the new pulse-tube cooled cryogenic platform in 2013, allowing the detectors to receive the first light below 100mK.

This was the only CMB experiment conducted at such a low temperature. The receiver temperature- controlled cabin is connected to the telescope primary-secondary mirror superstructure. As a result, it moves along with the ACT during the scans (Figure 2).

ACT telescope superstructure.

Figure 2. ACT telescope superstructure.

Figure 3 depicts receiver positioning scheme on the telescope optical axis within the cabin. Both PT-tubes is brought to vertical positions by tilting down neutral cabin floor position at 5% to horizon.

The dilution refrigerator (DR) is deployed within the receiver and is remotely operated by means of Ethernet link (Figure 4). It is capable of moving at 1G acceleration both horizontally and vertically, in the scan range of +/- 35° from neutral position.

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.

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 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.

Setting up remote connection to GHS4 via Ethernet link.

Figure 4. Setting up remote connection to GHS4 via Ethernet link.

Continuous operation at very low atmospheric pressure of 500mbar is ensured by the air cooling system for pumps and electronics. Figure 5 illustrates the schematic cross-section of the receiver.

The receiver is a vacuum chamber (blue) consisting 4K (turquoise) and 50K (purple) shields on G-10 isolating supports (yellow). A Cryomech pulse-tube (PT) cryo-cooler PT-415 (grey) is used to cool the receiver. The PT-407 cooled removable DR insert is seen in the left upper corner as a circle.

Receiver cross-section, showing preliminary design with two of three optic tubes.

Figure 5. Receiver cross-section, showing preliminary design with two of three optic tubes.

The PT-407 cooled removable DR insert is depicted in details in Figures 6 and 7.

JDry-100-ACTPol Insert solid model and assembly process.

JDry-100-ACTPol Insert solid model and assembly process.

Figure 6. JDry-100-ACTPol Insert solid model and assembly process.

Insert with mechanical heat-switch and house-keeping wiring installed.

Figure 7. Insert with mechanical heat-switch and house-keeping wiring installed.

The installation of the PT and dilution core is in tilted position to the optical central axis, enabling DR operation in full scan range.

Advantages of Liquid-Cryogen-Free Dilution Refrigerator

Figure 8 illustrates the refrigerator setup for tests at Janis Research. The tests results as is, and integrated with the receiver are summarized in Figure 9, showing the complete compliance of the receiver with technical specifications outlined by the ACTPol collaboration.

JDry-100-ACTPol in its test enclosure, attached to the automated Gas Handling System GHS4, during its tests at Janis Research.

Figure 8. JDry-100-ACTPol in its test enclosure, attached to the automated Gas Handling System GHS4, during its tests at Janis Research.

ooling power and base temperature at indicated 3He flow rates.

Figure 9. Cooling power and base temperature at indicated 3He flow rates.

The use of highly customized, remotely controlled liquid-cryogen-free dilution refrigerator, with no thermal cycling associated with commonly used single-shot adiabatic demagnetization refrigerator (ADR) cooling systems, enabled improved day-time scanning strategies.

The use of Edwards XDS35 scroll pumps and air cooled HighPace 300 Turbo helped achieving a cooling power of 120µ[email protected] and a base temperature lower than the target of 100mK.

The first 150GHz kilo-TES polarimeters array package (PA1) was installed into the ACTPol receiver on the ACT site in Spring 2013. A second 150GHz array package (PA2) and a multichroic 90/150GHz[2] array package (PA3) will be installed to get a full focal plane across three optics tubes in the beginning of 2014. Top and bottom view of PA1 are presented in Figures 10 and 11.

PA1, top view. Note feed-horn monolithic array (courtesy of ACTPol collaboration).

Figure 10. PA1, top view. Note feed-horn monolithic array (courtesy of ACTPol collaboration).

PA1, bottom view. Note 100 mK gold-plated thermal links to polarimeters arrays (courtesy of ACTPol collaboration).

Figure 11. PA1, bottom view. Note 100 mK gold-plated thermal links to polarimeters arrays (courtesy of ACTPol collaboration).

Figures 12, 13, and 14 illustrate fully assembled receiver before and during integration with the ACT.

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.

Figure 12. 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.

On site before installation.

Figure 13. On site before installation.

Inside ACT moving cabin.

Figure 14. Inside ACT moving cabin.

Conclusion

The shipping and integration of the ACTPol receiver on the ACT site in Chile were successfully performed as of June 2013. July 2013 marked the first light and start of season one operations (PA1). Operations with the deployment of complete focal plane are expected in Spring 2014.

This information has been sourced, reviewed and adapted from materials provided by Janis Research.

For more information on this source, please visit Janis Research.

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