THE BICEP3 INSTRUMENT
3.2 Cryostat receiver CryostatCryostat
Metal mesh edge filter
Metal mesh low-pass edge filters with a cutoff at 4 cm−1are added to reject out-of- band signal [25]. These filters are made out of multiple randomly orientated metal grid layers, each of them is a thin polypropylene substrate coated with copper film, and hot-pressed fused together to form a shape cut-off edge filter.
The largest filter can be made is smaller than the require size for the Bicep3 focal plane [29]. In order to make sure all detectors are covered and protected, multiple filters are cut to small, 3" by 3" pieces, and they are individually placed directly on top of each detector module, and cooled to 280 mK. Combination of the fabrication process and increase machining on the filter’s edges, we found delamination on the filter in 2016, which degrades the efficiency and shows non-uniform in band spectral response. All the metal-mesh filters are redesigned and replaced prior to 2017 season (Figure 3.5); Fourier Transform Spectroscopy shows the new filter gives a uniform in-band respond (Section 4.1) and increase in optical efficiency measurements indicate functioning filter.
3.2 Cryostat receiver
Figure 3.5: Metal-mesh filters mounted on Bicep3 in 2017. The filters are cut to bigger piece, each covering 5 detector modules to minimize the machining required for each filter. Mounting holes are carefully slotted to account for differential thermal contraction. New fabrication process is used for these filters, and each of them are cold tested before installing into Bicep3.
Bicep3 uses a single PT-415 pulsetube cryocooler2, which provides continuous cooling to 35 K at the 1ststage under typical 26 W load and 3.3 K at the 2nd stage under 0.5 W load. A non-continuous, three-stage (He-4/He-3/He-3) helium sorption fridge3 is heat sunk to the nominal 4 K stage and cooled the sub-Kelvin focal plane and supporting structures. The sorption fridge provided continuous sub-Kelvin operation on the focal plane for> 48 and> 80 hours for the 2016 and 2017 season, respectively, with 6 hours of recycling time. These hold time allow us to have a continuous two and three day observation schedule.
The focal plane and ultra-cold (UC) stage (nominal 280 mK) is a planar copper assembly mounted in a vertical stack on two buffer stages (nominal 350 mK and 2 K), each supported and isolated by carbon fiber trusses. The UC stage is comprised
2Cryomech Inc., Syracuse, NY 13211, USA (www.cryomech.com)
3Chase Research Cryogenics Ltd., Sheffield, S10 5DL, UK (www.chasecryogenics.com)
of the 9 mm thick, 46 cm diameter copper focal plane plate that supports the detector modules and a thinner secondary copper plate. The two plates are separated by seven 5 cm tall stainless steel blocks that serve as low-pass thermal filters and dampen thermal fluctuations before reaching the focal plane. Both the focal plane and the secondary UC stage are actively temperature controlled with resistive heaters to 274 mK and 269 mK, respectively. Thermal fluctuations on the focal plane during CMB observation are monitored and controlled by multiple temperature control modules (TCMs). The TCMs include two Germanium NTD thermistors, two heaters, bias and readout circuitry. The JFET readout for the NTDs is located in a readout module mounted to the 4 K baseplate.
Housekeeping
General thermometry uses silicon diode thermometers (Lakeshore4 DT-670) and sub-Kelvin stages are measured with thin-film resistance temperature detectors (Lakeshore Cernox RTDs). Germanium NTD thermistors are mounted directly to the detector tile substrates for more sensitive measurements of the TES thermal bath temperatures, and they are used on both the focal plane and secondary UC stage for the active temperature control input (TCMs). The thermometry, heater, and thermal control signals interface to an external electronics ‘backpack’ mounted directly to the cryostat vacuum jacket that provides biasing, signal pre-amplification, and buffering. A BLASTbus2 ADC system [32] interfaces between the control com- puters and the backpack. Figure 3.6 shows the housekeeping schematic in Bicep3.
RF shielding
Several levels of RF shielding are designed into the 4 K stage and sub-Kelvin structure to prevent RF coupling to the detector signal. Except for the short length of flex ribbon cables that connect the detector modules to the focal plane, all cabling in the cryostat are twisted pair. The ribbon cables are caged by the detector module, copper focal plane module cutout, and the ground plane of the wiring board that accepts the cable. Upon exiting the cryostat, all of the detector signal lines immediately interface with a low-pass filtered connection on the MCE readout electronics box. The 4 K non-optics volume is designed as a Faraday enclosure, with all seams taped, and all cabling passing through additional low-pass filtered connectors. The cage continuously encloses the stack of sub-Kelvin stages by wrapping and sealing a single layer of aluminized mylar between the 4 K stage and
4Lake Shore Cryotronics, Westerville, OH 43082, USA (www.lakeshore.com)
Figure 3.6: Bicep3 housekeeping schematic.
the edge of the focal plane (Figure 3.10). Finally, the niobium enclosure of each detector module, and detector tile ground plane, enclose the SQUID amplifier/MUX chips.
The cryostat tube acts as a waveguide with microwave frequency cutoff. But the larger diameter in Bicep3 is found to be susceptible to 450 MHz interference from the South Pole station radio system. The interference introduced an azimuth- synchronous signal, as well as transient disturbances to the feed-back based detector readout, and potentially contributed to increased 1/f noise within the science band.
Reduced output power and installation of a directional antenna at the main South Pole station emitter in 2017 reduced the radio signal by 35 dB, resulting in the RF signal to be a sub-dominate effect in the science band.
Magnetic shielding
Magnetic shielding is crucial to minimize coupling to the SQUID amplifiers while the telescope scans through the Earth’s field. Cryostat-level shielding is composed of cylindrical, high-permeability Amuneal5 A4K layers, with open ends to avoid
5Amuneal Manufacturing Corp (www.amuneal.com)
Figure 3.7: Magnetic shield in Bicep3. The 50 K A4K is highlighted in red (the 300 K vacuum jacket are not shown), as the location of the second-stage SQUID series array (SSA), and the first-stage SQUIDs inside the detector module. The SSA are packaged in niobium boxes and further shielded by Metglas.
interference with the optics and allow data cabling at the bottom. There is one layer on the inner surface of the vacuum jacket spanning the length of the cryostat, and a second, shorter layer on the 4 K stage surrounding the focal plane (Figure 3.7).
Laboratory Helmholtz coil measurements of these cylindrical shields in Bicep3 shows a shielding factor of∼30 along the optical axis. The niobium detector module housing provides further shielding of the first-stage SQUID amplifier chips on the sub-Kelvin focal plane, as described in Section 3.3. The second-stage SQUID series arrays on the 4 K stage are packaged in niobium boxes and additionally wrapped with ∼10 layers of Metglas 2714A6. Section 4.6 shows the magnetic shielding performance in Bicep3.
6Metglas Inc., (www.metglas.com)
Figure 3.8: Bicep3 focal plane with 20 tiles.