LiteBIRD is the Cosmic Microwave Background (CMB) radiation polarization satellite mission led by ISAS/JAXA. The main scientific goal is to search for primordial gravitational wave signals generated from the inflation epoch of the Universe. LiteBIRD telescopes employ polarization modulation units (PMU) using continuously rotating half-wave plates (HWP). The PMU is a crucial component to reach unprecedented sensitivity by mitigating systematic effects, including 1/f noise. We have developed a 1/10 scale prototype PMU of the LiteBIRD LFT, which has a 5-layer achromatic HWP and a diameter of 50 mm, spanning the observational frequency range of 34-161 GHz. The HWP is mounted on a superconducting magnetic bearing (SMB) as a rotor and levitated by a high-temperature superconductor as a stator. In this study, the entire PMU system is cooled down to 10 K in the cryostat chamber by a 4-K Gifford-McMahon (GM) cooler. We propagate an incident coherent millimeter-wave polarized signal throughout the rotating HWP and detect the modulated signal. We study the modulated optical signal and any rotational synchronous signals from the rotation mechanism. We describe the testbed system and the preliminary data acquired from this setup. This testbed is built to integrate the broadband HWP PMU and evaluate the potential systematic effects in the optical data. This way, we can plan with a full-scale model, which takes a long time for preparation and testing.
The two most common components of several upcoming CMB experiments are large arrays of superconductive TES (Transition-Edge Sensor) detectors and polarization modulator units, e.g. continuously-rotating Half-Wave Plates (HWP). A high detector count is necessary to increase the instrument raw sensitivity, however past experiments have shown that systematic effects are becoming one of the main limiting factors to reach the sensitivity required to detect primordial B-modes. Therefore, polarization modulators have become popular in recent years to mitigate several systematic effects. Polarization modulators based on HWP technologies require a rotating mechanism to spin the plate and modulate the incoming polarized signal. In order to minimize heat dissipation from the rotating mechanism, which is a stringent requirement particularly for a space mission like LiteBIRD, we can employ a superconductive magnetic bearing to levitate the rotor and achieve contactless rotation. A disadvantage of this technique is the associated magnetic fields generated by those systems. In this paper we investigate the effects on a TES detector prototype and find no detectable Tc variations due to an applied constant (DC) magnetic field, and a non-zero TES response to varying (AC) magnetic fields. We quantify a worst-case TES responsivity to the applied AC magnetic field of ∼ 105 pA/G, and give a preliminary interpretation of the pick-up mechanism.
We develop a continuously rotating achromatic half-wave plate (HWP) for LiteBIRD. An achromatic HWP is made of five-layer sapphire plates following a Pancharatnam design. The two surfaces employ broadband anti-reflection (AR) sub-wavelength structures (SWS) fabricated with ultra-short pulsed laser ablation. For designing AHWP with SWS, we fabricated three representative structures using laser ablation. One has a symmetric SWS shape and the other two have different asymmetric shapes in ordinary and extraordinary directions. We modeled five-layer AHWP with SWS based on fabricated shapes and numerically evaluated their transmittance, modulation efficiency, and phase of the modulated signal using the rigorous coupled-wave analysis (RCWA) method. We also added instrumental polarization (IP) as the figure-of-merit, which is a conversion of unpolarized to polarized light. IP creates an undesired modulated signal, which may cause a non-linear response in a bolometric detector. The typical cause of IP is the imperfection of AR SWS. From calculations, we did not find a significant difference in IP among the three cases. However, we found the impact on the modulation efficiency because the retardance depends on the SWS shapes. Furthermore, the retardance depends on frequency. We numerically analyzed the impact of the extra retardance from SWS on the overall AHWP performance. We show one of the three cases has the broadest modulation efficiency by compensating for the frequency dependence of the retardance from the SWS and the AHWP sapphire stacks.
This conference presentation was prepared for the Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy XI conference at SPIE Astronomical Telescopes + Instrumentation, 2022.
LiteBIRD is a future space mission designed to observe the polarization of the cosmic microwave background (CMB) radiation. LiteBIRD employs polarization modulator units (PMUs) at telescope apertures to mitigate 1/f noise and systematic uncertainties. The PMU employed in the Low-Frequency Telescope (LFT) consists of a broadband achromatic half-wave plate (HWP) and a cryogenic rotation mechanism. A superconducting magnetic bearing, which is a rotor levitation type bearing, is used to eliminate physical friction. A contactless AC synchronous motor consisting of SmCo permanent magnets and copper coils is employed as the drive mechanism. One of the technical challenges for the PMU development is to reduce the heat dissipation generated by the rotation mechanism during cryogenic operation. We evaluated the heat dissipation owing to the eddy currents generated from the rotor in the rotation mechanism at room temperature. We performed a rotor spindown measurement using a breadboard model of the PMU. We established that eddy currents generated from the motor coil were dominant in the rotor at room temperature, and its estimated value was 3.91 ± 0.91 mW.
LiteBIRD, the Lite (Light) satellite for the study of B-mode polarization and Inflation from cosmic background Radiation Detection, is a space mission for primordial cosmology and fundamental physics. JAXA selected LiteBIRD in May 2019 as a strategic large-class (L-class) mission, with its expected launch in the late 2020s using JAXA's H3 rocket. LiteBIRD plans to map the cosmic microwave background (CMB) polarization over the full sky with unprecedented precision. Its main scientific objective is to carry out a definitive search for the signal from cosmic inflation, either making a discovery or ruling out well-motivated inflationary models. The measurements of LiteBIRD will also provide us with an insight into the quantum nature of gravity and other new physics beyond the standard models of particle physics and cosmology. To this end, LiteBIRD will perform full-sky surveys for three years at the Sun-Earth Lagrangian point L2 for 15 frequency bands between 34 and 448 GHz with three telescopes, to achieve a total sensitivity of 2.16 μK-arcmin with a typical angular resolution of 0.5° at 100 GHz. We provide an overview of the LiteBIRD project, including scientific objectives, mission requirements, top-level system requirements, operation concept, and expected scientific outcomes.
LiteBIRD has been selected as JAXA’s strategic large mission in the 2020s, to observe the cosmic microwave background (CMB) B-mode polarization over the full sky at large angular scales. The challenges of LiteBIRD are the wide field-of-view (FoV) and broadband capabilities of millimeter-wave polarization measurements, which are derived from the system requirements. The possible paths of stray light increase with a wider FoV and the far sidelobe knowledge of -56 dB is a challenging optical requirement. A crossed-Dragone configuration was chosen for the low frequency telescope (LFT : 34–161 GHz), one of LiteBIRD’s onboard telescopes. It has a wide field-of-view (18° x 9°) with an aperture of 400 mm in diameter, corresponding to an angular resolution of about 30 arcminutes around 100 GHz. The focal ratio f/3.0 and the crossing angle of the optical axes of 90◦ are chosen after an extensive study of the stray light. The primary and secondary reflectors have rectangular shapes with serrations to reduce the diffraction pattern from the edges of the mirrors. The reflectors and structure are made of aluminum to proportionally contract from warm down to the operating temperature at 5 K. A 1/4 scaled model of the LFT has been developed to validate the wide field-of-view design and to demonstrate the reduced far sidelobes. A polarization modulation unit (PMU), realized with a half-wave plate (HWP) is placed in front of the aperture stop, the entrance pupil of this system. A large focal plane with approximately 1000 AlMn TES detectors and frequency multiplexing SQUID amplifiers is cooled to 100 mK. The lens and sinuous antennas have broadband capability. Performance specifications of the LFT and an outline of the proposed verification plan are presented.
Sapphire, alumina, and silicon present the following characteristics that make them suitable as optical elements for millimeter and sub-millimeter applications: low-loss, high thermal conductivity at cryogenic temperatures, and high refractive index ~3. However, the high index also leads to high reflection. We developed a technique to machine sub-wavelength structures (SWS) as a broadband anti-reflection coating on these materials through laser ablation. We describe here the status of our development: transmission measurements of fabricated samples in a diameter of 34.5 mm agree with predictions, and we are now focusing on increasing the fabrication area with high processing rate. This is motivated by the need of ~500 mm diameter optical elements for the next-generation cosmic microwave background polarization experiments. We show our large area machining method on the alumina and sapphire over an area of < 5200 mm2 with the processing rate of < 4:0 mm3=min:, and the transmission measurements are consistent with the predictions.
LiteBIRD is a JAXA-led Strategic Large-Class mission designed to search for the existence of the primordial gravitational waves produced during the inflationary phase of the Universe, through the measurements of their imprint onto the polarization of the cosmic microwave background (CMB). These measurements, requiring unprecedented sensitivity, will be performed over the full sky, at large angular scales, and over 15 frequency bands from 34 GHz to 448 GHz. The LiteBIRD instruments consist of three telescopes, namely the Low-, Medium-and High-Frequency Telescope (respectively LFT, MFT and HFT). We present in this paper an overview of the design of the Medium-Frequency Telescope (89{224 GHz) and the High-Frequency Telescope (166{448 GHz), the so-called MHFT, under European responsibility, which are two cryogenic refractive telescopes cooled down to 5 K. They include a continuous rotating half-wave plate as the first optical element, two high-density polyethylene (HDPE) lenses and more than three thousand transition-edge sensor (TES) detectors cooled to 100 mK. We provide an overview of the concept design and the remaining specific challenges that we have to face in order to achieve the scientific goals of LiteBIRD.
We report the development of an optical encoder and its readout system for a cryogenically-cooled continuously rotating half-wave plate (HWP) polarization modulator unit (PMU) in the LiteBIRD low-frequency telescope. LiteBIRD is a cosmic microwave background polarization satellite mission to probe B-mode polarization, which originates from primordial gravitational waves, observing from the second Lagrange point (L2). LiteBIRD employs a continuously-rotating HWP to mitigate systematic effects. The knowledge of the position angle of the HWP is in a one-to-one relationship to the incident polarization angle. The required reconstruction accuracy is about 1 arcmin and the targeted rotational frequency stability is 1 mHz. A unique development constraint comes from a telemetry bandwidth limitation between the Earth and L2, and thus we implement a digital process to reduce the data volume assuming a future implementation of on-board processing of the encoder data before the downlink. The demonstrations were done experimentally using a breadboard model of the PMU: a readout system using FPGA (Spartan-6) and a rotational mechanism using a superconducting magnetic bearing and AC motor. We acquired the encoder data from the rotational mechanism operating under two conditions: liquid nitrogen at room pressure and below 10 K in a cryostat. We demonstrated the reconstruction of the position angle accuracy < 0.5 arcmin and the corresponding data volume of 0.12 GB/day, which is at least an order of magnitude smaller than the total data volume per day. We further discuss the sources of the position angle uncertainty and its implications to the observations.
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