Matthaeus Leitner

Several CMB-S4 technical subsystems made important advancements during the last few months. The project is entering an exciting development phase.

The Module Assembly and Testing team at Fermilab was able to start the first 100 mK test of an integrated detector module including a detector wafer and readout module. Fig. 1 (below) shows the test setup which required a few cool-down iterations before it became fully operational. The arrangement contains a full detector module assembly with interface wafers, horn arrays, and wafer cooling connections. A 100 mK readout enclosure is mounted adjacent to the wafer. Three similar cryostat configurations (at UIUC, SLAC, and FNAL) are now operational and able to test CMB-S4 prototype detector modules.

We can also report notable successes on the detector design side: NIST characterized several LAT-MF prototype single pixel designs plus a CMB-S4 dual-TES bolometer chip. Fig. 2 (below) highlights the NIST single pixel measurement apparatus which utilizes an adiabatic demagnetization refrigerator (ADR). The unique setup allows qualification of passbands, optical efficiency, and polarization responses. Fig. 2 also shows the compact test-board which enables measuring several discrete pixel configurations in a single cool-down. This approach permits rapid development of different design iterations.

A first SEEQC full SAT-MF wafer prototype test was completed at LBNL (see Fig. 3, below), and dark bolometer data were extracted. Based on these results, SEEQC fabricated a second prototype wafer with further tuned annealing properties to raise the critical temperature of the AlMn transition edge sensors.

The recent Summer Collaboration meeting at SLAC focused on the future publication of a second edition of the CMB-S4 Science Book. SLAC organized an excellent gathering, and the opportunity to discuss topics in person strengthened our collaboration. It was great seeing everyone!

 

Originally posted in the CMB-S4 July-August 2023 Newsletter

The CMB-S4 South Pole Large Aperture Telescope (SPLAT) team has recently authored two key scientific publications [ref. 1, ref. 2] related to the telescope and mirror designs and is working on a third publication describing the detailed optics design. In addition, Eric Chauvin Consulting has submitted to the project a SPLAT preliminary engineering design report which includes mechanical design details as well as engineering analysis. In Fig. 1, below, the SPLAT design, originally conceived by S. Padin [ref. 3] is based on a three-mirror anastigmat design with a large field of view. To reduce systemic errors the telescope incorporates co-moving shields, boresight rotation, and monolithic aluminum mirrors.

Fig. 2, below, is an example of the SPLAT structural analysis evaluating deflections and pointing errors. The image shows modal analysis results displaying the mount’s first resonance shape.

The South Pole Large Aperture Telescope incorporates a novel combination of features which will allow CMB measurements over a wide range of angular scales. The telescope’s large (~5 m) primary mirror provides the necessary angular resolution to determine the angular distortions in CMB caused by gravitational lensing from the large scale structure in the universe. These distortions lead to a low level of B-mode polarization signal that must be subtracted from the CMB measurements to detect the primordial B-mode signal. As a novel development, the telescope will provide exceptional control of systematic errors by providing co-moving shields and baffles, boresight rotation capability, and uniquely fabricated monolithic aluminum mirrors. The combination of these new features will enable unprecedented levels of sensitivity for LAT measurements of B-mode polarization on large angular scales. Fig. 3, below, shows the optical layout of the three mirror anastigmat configuration (left) plus lens arrangement of the 85 receiver optics tubes (right) located at the telescope’s final focus.

Recently, a full-scale prototype monolithic mirror was machined to required precision and delivered to the University of Chicago. Deformation tests including a mirror support structure will be performed in the near future. The photos below show the recently-completed SPLAT prototype mirror. The upper image shows the surface of the mirror after machining while the lower image shows the structural support on the backside of the mirror.

Key Dates and Upcoming Activities

The project started preparations for a Director’s Review at LBNL (November 14th to 17th, 2023). A
sequence of external subsystem conceptual design reviews is also planned for late summer to fall.
Thanks to everyone who supports the preparations for these reviews!

• CMB-S4 Summer Collaboration Meeting (at SLAC) – July 31 – Aug 3, 2023
• Subsystem Conceptual Design Reviews – July – September, 2023
• Director’s Review (at LBNL) – November 14 – 17, 2023

 

Originally posted in the CMB-S4 June 2023 Newsletter

Fabrication of CMB-S4 prototype detector wafers is a key development goal for the project. The February newsletter highlighted the successful delivery of a first CMB-S4 prototype wafer produced by SEEQC/LBNL. This month we want to highlight ongoing work in our other wafer fabrication sites: NIST, JPL, UC Berkeley, and ANL.

NIST is currently focused on building several single pixel test structures to validate detector design features. This modular approach enables precise optimization of fabrication conditions for the final wafer design. The completed test structures, see Fig. 1 (below: Microscope photo of a recently completed LAT mid-frequency prototype detector pixel at NIST. The superconducting circuit elements are clearly visible.) for a LAT mid-frequency pixel, are wirebonded to a circuit board that integrates into a cryostat for rapid testing at cryogenic temperatures.

JPL completed the layout and mask design of a SAT mid-frequency single pixel, see Fig. 2, and a corresponding prototype wafer. JPL is making significant progress with validating their proposed fabrication steps by building and measuring properties of subcomponent structures. Parallel work has focused on optimizing the final, full wafer design to adapt to the fabrication capabilities at JPL.Fig. 2, below, is JPL’s recent design for the SAT mid-frequency detector pixel.

UC Berkeley’s development is focused on building low frequency detector wafers, which require large-diameter and free-standing nitride membranes to support the long-wave-length optimized OMT probes. Fig. 3, below, shows a test wafer successfully incorporating these fragile structures which is their most recent achievement. Support of the OMTs (Orthomode Transducers) – paddles coupling the CMB radiation to the detector RF circuits – requires large-diameter, free-standing, low-stress nitride membranes to hold the long-wave-length optimized OMT probes in place. UC Berkeley (with SEEQC support) has recently demonstrated this critical fabrication step.

ANL concentrates effort on the development of the LAT mid-frequency wafers. Recent work has been focused on evaluating material properties using single pixel structures as shown in Fig. 4, below. ANL has validated material properties by carrying out systematic mm-wave loss studies of microstrip lines. The image shows a test setup with MKIDs and the microstrip line extending beyond the bottom of the photo. In addition, ANL is developing an optical test setup for rapid, cryogenic testing of single-pixel structures – similar to what has been developed at NIST.

 

Key Dates and Upcoming Activities

Thanks for an exciting Spring Collaboration Meeting! The meeting focused on CMB-S4’s project baseline development after the recent conclusion of the Analysis of Alternatives. We heard about significant advancements developing the new project baseline (covering scientific, technical, simulation, and project management areas). The next Collaboration Meeting will be held in person at SLAC this summer – more details to follow.

• CMB-S4 Summer Collaboration Meeting – week of July 31 – Aug 4, 2023

 

Originally posted in the CMB-S4 April 2023 Newsletter

The Small Aperture Telescope (SAT) design team at CfA | Harvard & Smithsonian recently completed the assembly of a mini-cryostat for cryogenic testing of state-of-the-art optical materials to optimize the SAT telescope components. Fig. 5 (below) shows the newly fabricated vacuum chamber for the testing of optics components.

In parallel, Berkeley Lab engineers continue to refine the design of the telescope focal plane and cryobus, especially in regards to integration with other sub-systems. Fig. 6, below, shows the SAT focal plane wiring design.

 

Key Dates and Upcoming Activities

The project-focused Spring Collaboration Meeting is right around the corner. Please go to the meeting Indico page for more information. The meeting will be held remotely and registration is
required to receive a zoom link.

• CMB-S4 Spring Collaboration Meeting – April 3 – 6, 2023
• CMB-S4 Summer Collaboration Meeting – week of July 31 – Aug 4, 2023

 

Originally posted in the CMB-S4 March 2023 Newsletter 

Fermilab is currently assembling components required to perform optical tests with the Large Aperture Telescope mid-frequency (LAT-MF) detector wafers.

Fig. 1, below, shows the Fermilab dilution refrigerator being prepared for testing detector modules. 100 mK electronics circuits have been installed recently, and Fermilab scientists are now preparing the choke, waveguide interface plate, and backshort wafers.

These precision etched wafers hold the superconducting detector wafers in place and form a precisely shaped optical cavity for detecting the CMB microwave signal. Fig. 2, below, shows a recently completed precision-etched LAT-MF waveguide interface plate made out of Cu coated silicon. Radiation is coupled to the detector wafers through feedhorn arrays which are precision-machined to specific requirements of the telescope and its frequency band.

Fig. 3, below, shows a recently completed LAT-MF feedhorn array after inspection at Fermilab and the final gold-plating step. In addition, Fermilab’s engineering team has developed a novel feedhorn array configuration for the Small Aperture Telescopes (SAT).

That design incorporates sloped sidewall corners to enable a tighter module packing for the curved SAT focal plane. Fig. 4, below, shows a newly developed compact feedhorn array design for the SAT curved focal plane.

 

Originally posted in the CMB-S4 March 2023 Newsletter