We report on the breadboard model of a polarization modulator unit (PMU) using a sapphire-based achromatic half-wave plate (HWP) for the low-frequency telescope (LFT) of LiteBIRD, the JAXA-led space mission to probe cosmic inflation by observing the polarization of the cosmic microwave background. The PMU is a key component to reduce 1/f noise and the systematic effects between the two orthogonal polarized detectors. For the HWP, we glued together the surfaces of five 330 mm diameter sapphire plates using hydro catalysis bonding, working as HWP in LFT bands. We also fabricated anti-reflective sub-wavelength structures using ultra-short pulsed laser ablation. For the rotation mechanism, we use a superconducting magnetic bearing (SMB) and contactless synchronous motor to levitate, and rotate the HWP without any contact. Optical measurements show that fabricated HWP archives broadband transmission and polarization efficiency to obtain a sensitivity close to an ideal HWP. We investigate and characterize each component of the rotation mechanism, SMB, encoder to measure the rotation angle of HWP, and the holder mechanism. We improved the design of the rotation mechanism and reduced the total mass from 34.7 kg to 21.7 kg, which is significant reduction for the mass limited satellite mission. The knowledge of characterization for each component can be scaled to the size of the flight model of 480 mm diameter.
LiteBIRD is an JAXA-led strategic L-class satellite mission designed to measure the primordial B modes of the cosmic microwave background radiation (CMB) to test cosmic inflation. The LiteBIRD Low-Frequency telescope employs a polarization modulator unit (PMU). The PMU is placed at the telescope aperture to modulate the incoming CMB polarization signal by using a continuous rotating half-wave plate to reduce the impact of 1/f noise and differential systematic effects. The current PMU design employs three cryogenic holder mechanisms that hold the rotor until the superconducting magnet bearing cools below its critical temperature after launch. They also serve a conductive path to the rotor when they are held. Minimizing the heat dissipation of this holder is one of the key development goals of the PMU due to the limited cooling power on the satellite system. In this paper, We report on the detailed design of the holder and the developed cryogenic stepping motor that actuates the holder. Also, we conducted the preliminary thermal characterization at around 7 K. The preliminary estimated total heat dissipation of the holder is 2.39 ± 0.09 mW when we activated it for 532 s.
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.
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 present a breadboard model development status of the polarization modulator unit (PMU) for a low-frequency telescope (LFT) of the LiteBIRD space mission. LiteBIRD is a next-generation cosmic microwave background polarization satellite to measure the primordial B-mode with the science goal of σr < 0.001. The baseline design of LiteBIRD consists of reflective low-frequency and refractive medium-and-high-frequency telescopes. Each telescope employs the PMU based on a continuous rotating half-wave plate (HWP) at the aperture. The PMU is a critical instrument for the LiteBIRD to achieve the science goal because it significantly suppresses 1/f noise and mitigates systematic uncertainties. The LiteBIRD LFT PMU consists of a broadband achromatic HWP and a cryogenic rotation mechanism. In this presentation, we discuss requirements, design and systematic studies of the PMU, and we report the development status of the broadband HWP and the space-compatible cryogenic rotation mechanism.
LiteBIRD is a candidate for JAXA’s strategic large mission to observe the cosmic microwave background (CMB) polarization over the full sky at large angular scales. It is planned to be launched in the 2020s with an H3 launch vehicle for three years of observations at a Sun-Earth Lagrangian point (L2). The concept design has been studied by researchers from Japan, U.S., Canada and Europe during the ISAS Phase-A1. Large scale measurements of the CMB B-mode polarization are known as the best probe to detect primordial gravitational waves. The goal of LiteBIRD is to measure the tensor-to-scalar ratio (r) with precision of r < 0:001. A 3-year full sky survey will be carried out with a low frequency (34 - 161 GHz) telescope (LFT) and a high frequency (89 - 448 GHz) telescope (HFT), which achieve a sensitivity of 2.5 μK-arcmin with an angular resolution 30 arcminutes around 100 GHz. The concept design of LiteBIRD system, payload module (PLM), cryo-structure, LFT and verification plan is described in this paper.
We present our design and development of a polarization modulator unit (PMU) for LiteBIRD space mission. LiteBIRD is a next generation cosmic microwave background (CMB) polarization satellite to measure the primordial B-mode. The science goal of LiteBIRD is to measure the tensor-to-scalar ratio with the sensitivity of δr < 10-3. The baseline design of LiteBIRD is to employ the PMU based on a continuous rotating half-wave plate (HWP) at a telescope aperture with a diameter of 400 mm. It is an essential for LiteBIRD to achieve the science goal because it significantly reduces detector noise and systematic uncertainties. The LiteBIRD PMU consists of a multi-layered sapphire as a broadband achromatic HWP and a mechanism to continuously rotate it at 88 rpm. The whole system is maintained at below 10K to minimize the thermal emission from the HWP. In this paper, we discuss the current development status of the broadband achromatic HWP and the cryogenic rotation mechanism.
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