H. Sugai 40 , P. A. R. Ade ², Y. Akiba 41,52 , D. Alonso ², K. Arnold 75 , J. Aumont¹¹, J. Austermann 72, C. Baccigalupi²7,19,25, A. J. Banday 11, R. Banerji 7, R. B. Barreiro 59, S. Basak 16,27, J. Beall 72, S. Beckman 74 , M. Bersanelli 30,22 , J. Borrill 70,74 , F. Boulanger 9, M. L. Brown 66, M. Bucher 5, A. Buzzelli 9, E. Calabrese 62, F. J. Casas 59, A. Challinor65,63, V. Chan 4 , Y. Chinone74,40 , J.-F. Cliche1, F. Columbro²6,24, A. Cukierman74, D. Curtis74, P. Danto7, P. de Bernardis²6,24, T. de Haan 70, M. De Petris²6,24, C. Dickinson 66, M. Dobbs¹, T. Dotani³7, L. Duband®, A. Ducout 40 , S. Duff 72 , A. Duivenvoorden 61 , J.-M. Duval , K. Ebisawa 37 , T. Elleflot 75 , H. Enokida³5, H. K. Eriksen⁵7, J. Errard⁵, T. Essinger-Hileman⁷1, F. Finelli²0, R. Flauger 75, C. Franceschet 30,22, U. Fuskeland57, K. Ganga 5, J.-R. Gao 56, R. Génova-Santos 58,60, T. Ghigna 67,40, A. Gomez 7, M. L. Gradziel¹ , J. Grain 9, F. Grupp¹¹ , A. Gruppuso 20, J. E. Gudmundsson 61 , N. W. Halverson 76 , P. Hargrave 62 , T. Hasebe37, M. Hasegawa41,52, M. Hattori53, M. Hazumi41,37,40,52, S. Henrot-Versille¹², D. Herranz⁵9, C. Hill 74,70, G. Hilton7², Y. Hirota³5, E. Hivon 8, R. Hlozek 4, D.-T. Hoang 68,77, J. Hubmayr 72, K. Ichiki 45, T. Iida 40, H. Imada 12, K. Ishimura 54, H. Ishino 48,41, G. C. Jaehnig 76 , M. Jones 67 , T. Kaga 37 , S. Kashima 46 , Y. Kataoka 48 , N. Katayama 40 , T. Kawasaki 42 , R. Keskitalo 70 , A. Kibayashi 48 , T. Kikuchi 32 , K. Kimura 49 , T. Kisner 70 , Y. Kobayashi 38 , N. Kogiso 49 , A. Kogut71, K. Kohri41, E. Komatsu13, K. Komatsu48, K. Konishi36, N. Krachmalnicoff 7,19,25, C. L. Kuo 73,69, N. Kurinsky 73,69, A. Kushino 44 , M. Kuwata-Gonokami 34 , L. Lamagna 26,24 , M. Lattanzi²¹, A. T. Lee 74,70 , E. Linder 70,74 , B. Maffei 9, D. Maino 30,22 , M. Maki 41 , A. Mangilli 11 , E. Martínez-González 59 , S. Masi 26,24 , R. Mathon¹¹, T. Matsumura 40 , A. Mennella 30,22 , M. Migliaccio 29,24 , Y. Minami 41 , K. Mistuda 37 , D. Molinari 28 , L. Montier 11 , G. Morgante²0, B. Mot¹¹, Y. Murata³ ⁷, J. A. Murphy¹ ⁷, M. Nagai 46, R. Nagata 41 , S. Nakamura 55 , T. Namikawa 65 , P. Natoli 28,21 , S. Nerval 4 , T. Nishibori 39 , H. Nishino 50 , Y. Nomura 51 , F. Noviello 62 , C. O'Sullivan¹ ⁷ , H. Ochi ⁵⁵ , H. Ogawa ⁴⁹ , H. Ogawa³ ⁷ , H. Ohsaki ³⁵ , I. Ohta 43 , N. Okada 37 , N. Okada 49 , L. Pagano 28,21 , A. Paiella 26,24 , D.
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Updated design of the CMB polarization experiment satellite LiteBIRD

H. Sugai, et al.
Abstract Recent developments of transition-edge sensors (TESs), based on extensive experience in ground-based experiments, have been making the sensor techniques mature enough for their application on future satellite CMB polarization experiments. LiteBIRD is in the most advanced phase among such future satellites, targeting its launch in Japanese Fiscal Year 2027 (2027FY) with JAXA's H3 rocket. It will accommodate more than 4000 TESs in focal planes of reflective low-frequency and refractive medium-and-high-frequency telescopes in order to detect a signature imprinted on the cosmic microwave background (CMB) by the primordial gravitational waves predicted in cosmic inflation. The total wide frequency coverage between 34GHz and 448GHz enables us to extract such weak spiral polarization patterns through the precise subtraction of our Galaxy's foreground emission by using spectral differences among CMB and foreground signals. Telescopes are cooled down to 5Kelvin for suppressing thermal noise and contain polarization modulators with transmissive half- wave plates at individual apertures for separating sky polarization signals from artificial polarization and for mitigating from instrumental 1/f noise. Passive cooling by using V-grooves supports active cooling with mechanical coolers as well as adiabatic demagnetization refrigerators. Sky observations from the second Sun-Earth Lagrangian point, L2, are planned for three years. An international collaboration between Japan, USA, Canada, and Europe is sharing various roles. In May 2019, the Institute of Space and Astronautical Science (ISAS), JAXA selected LiteBIRD as the strategic large mission No. 2. Keywords satellite • cosmic microwave background • polarization • inflation • primordial gravitational wave
1 Concept of LiteBIRD
It has been suggested since the 1980s [1,2,3] that inflation occurred in the very early, high energy Universe, to resolve remaining issues of the Big Bang theory, such as uniformity, flatness, and monopole problems. Cosmic inflation predicts primordial gravitational wave production, with quantum fluctuations of spacetime as its origin.  The LiteBIRD (Lite (Light) satellite for the studies of B-mode polarization and Inflation from cosmic background Radiation Detection) aims to detect signatures of these primordial gravitational waves in the form of specific curl patterns [4,5] of polarization angle distribution of the Cosmic Microwave Background (CMB). The sizes of expected spiral patterns in the sky are characterized by Hubble lengths at the electron scattering eras of the CMB, since primordial gravitational waves
Updated design of the CMB polarization experiment satellite LiteBIRD
that enter the horizon then most effectively produce tensor anisotropies. It is therefore essential to cover the whole sky to investigate these large Hubble lengths at the recombination era as well as the reionization era. LiteBIRD is focused on this point: targeting [6] both the recombination era with the multipole moment ℓ between 11 and 200 and of the reionization era with ℓ between 2 and 10, optimizing the angular resolution. The other important concepts of this satellite are a warm launch without the requirements of heavy vessels/tanks, and use of multichroic detectors for the effective exploitation of finite focal plane areas. Advantages of measurements from space are being free from atmospheric effects, providing high sensitivity, stability with less systematic uncertainties [e.g.,7], and no restrictions on observing-band selection. Space measurements also give no pickup from the ground. The Sun-Earth L2 point has been selected, since the Sun, the Earth, and the Moon are all located in almost the same direction, which makes it easier to avoid facing them in terms of optical and thermal aspects. Care should be taken, however, on cosmic ray effects [8] because the satellite is more directly exposed to them. Sky observations are planned for 3 years: the presently guaranteed cooling-chain lifetime is 3.5years, in which 0.5 year is assigned to the transitional period to the normal observation phase on course to L2. A scanning strategy with a combination of boresight spin angle of 50° around the satellite axis and its precession-like rotation around the anti-Sun direction of 45° is used (Fig.1), since this combination provides not only a fairly uniform sky coverage but also the minimization of instrumental systematic uncertainties in polarization measurements.

2 Status of LiteBIRD
In May 2019, the Institute of Space and Astronautical Science (ISAS), JAXA confirmed that LiteBIRD completed activities planned during Pre- phase A2 (previously called Phase-A1 and dedicated to the concept study and development of key technologies) and selected LiteBIRD as the strategic large mission No. 2, with modifications to the cooling chain and subsequently the focal plane designs: we have chosen to use adiabatic demagnetization refrigerators (ADRs) in series, removing ideas [6,9] of using a JAXA-provided 1-K Joule-Thomson cooler and of having 2-K telescope apertures. To recover the degraded sensitivity with 5-K apertures instead, we have increased the number of detectors. With the international collaboration between Japan, USA, Canada, and Europe, the satellite is planned to be launched with JAXA’s H3 rocket in 2027FY (Fig. 1). Tables 1 and 2 summarize the updated basic parameters and current baseline design. 3 Current design and technical progresses of LiteBIRD 3.1 Cooling chain
H. Sugai, et al.
With the design modification required, we have selected the following baseline combination for the cooling chain: i) down to 5K a sunshield and passive cooling with V grooves [10], 15-K pulse tube coolers, and a 4-K J-T with 2ST precoolers; ii) from 5K to 1.75K a parallel three-stage ADR for providing continuous cooling at 1.75K; and iii) from 1.75K to 100mK a multi-staged ADR with continuous cooling at 300mK and 100mK. Detailed schemes for this cooling chain are described in [11]. 3.2 Telescopes and polarization modulator units The accurate, precise, and robust foreground cleaning requires 15 frequency bands covering a wide range of 34 to 448GHz. This will be achieved with two kinds of telescopes (Table 2): a reflective one for the lower frequency range (LFT); and refractive ones for medium and high frequency ranges (MHFT [12]). While the LFT uses crossed-Dragone reflective optics [13,14] to minimize effects from multi-reflection among refractive surfaces, the MHFT uses lens optics with the advantage of compactness. For the LFT, the F#3.0 and the crossing angle of 90° have been selected, since this combination suppresses the straylight [9,15] (Fig. 2), and aluminum is used as the mirror material [16]. All the telescopes are cooled down to 5K for suppressing thermal noise. Each telescope has a polarization modulator unit (PMU) [17,12,18,19], which consists of a transmissive half-wave plate (HWP) system rotated with a superconductive magnetic bearing (SMB), as the first optical element, close to the aperture stop to distinguish the sky and instrumental polarization components and to reduce the instrumental I/f noise. The SMB enables contactless operation of the HWP system, which minimizes heat generation, through magnetic levitation by using YBCO bulk and permanent SmCo magnets. The LFT HWP system consists of multi-layer stacked sapphire HWPs with optical axes shifted relative to each other to achieve high polarization efficiency over the wide frequency range [20], while the MHFT employs the metal-mesh HWPs [19]. At the top and bottom surfaces of the LFT HWP system, we adopt anti-reflection with sub-wavelength structure produced by laser machining. A cryogenic testbed is designed to study interactions, such as between space-optimized detectors and the PMU, consisting of a magnetically levitated and rotating HWP system [21].
3.3 Detectors and readout
Transition-edge sensor (TES) bolometers are used to form multichroic pixels, based on maturity from successful ground-based and balloon experiments TES bolometers are coupled with a silicon lenslet and a sinuous [22]. antenna for individual broadband pixels for LFT and MFT, while silicon platelet-based corrugated horn and orthomode transducers are used for HFT [23,24]. The noise equivalent power of TES bolometer is proportional to
Updated design of the CMB polarization experiment satellite LiteBIRD
N(Psat Tb), with the Tc/Tb ratio being optimized. Here Psat , Tc, and Tb are the saturated power, transition temperature, and thermal bath temperature, respectively. The TES parameter optimization for LiteBIRD, towards low saturation-power detectors for the satellite environment, has been carried out, as well as a sensor impedance that readily couples to the frequency domain multiplexer. The electro-thermal time constants are controlled by slowing down with the additional heat capacity. Details of these experimental results are described in [25]. Cosmic-ray mitigation methods have been developed by reducing propagation with palladium structures to absorb phonons and with the removal of bulk silicon to block phonons [26]. Readout will be carried out through digital frequency multiplexing, with 68 bolometers connected to each SQUID array amplifier [27,28]. The updated focal-plane designs and the expected sensitivities are shown in Fig. 2.
4. Foreground cleaning and systematic uncertainty studies
The focal-plane designs have been determined through iterations with sensitivity calculations based on thermal studies [10], as well as with foreground cleaning (by using methods described in [29,30,31,32]) and systematic uncertainty studies. The baseline foreground-cleaning results have been obtained based on an eight-dimensional parameterization of the foregrounds, including synchrotron power-law spectral index plus curvature and effective temperature of dust, plus power-law index of dust emissivity for Q and U Stokes parameters, each of which is allowed to vary across the sky. Systematic uncertainties have also been studied [33], including beam systematics [14], instrumental polarization and HWP harmonics, polarization efficiency, relative and absolute gain, pointing, polarization angle, time correlated noise, cosmic ray glitches, bandpass mismatch [34], transfer function, non-linearity, and non-uniformity in HWP, with realistic ground/inflight calibration methods taken into account [e.g.,15,35]. Through all these studies, it has been shown that the updated focal-plane designs satisfy LiteBIRD's full success of the total uncertainty in the tensor-to-scalar ratio of less than 0.001, with its uncertainty budgets distributed comparably into a statistical part (including foreground residual and lensing B-mode), a systematic part, and a margin. Acknowledgements This work was supported by World Premier International Research Center Initiative (WPI), MEXT, Japan, by JSPS Core-to-Core Program, A. Advanced Research Networks, and by JSPS KAKENHI Grant Numbers JP15H05891, JP17H01115, and JP17H01125. The Italian contribution to the LiteBIRD phase A is supported by the Italian Space Agency (ASI Grant No. 2016-24- H.1-2018) and the National Institute for Nuclear Physics (INFN). The French contribution to the LiteBIRD phase A is supported by the Centre National d'Etudes Spatiale (CNES), by the Centre National de la Recherche Scientifique (CNRS) and by the Commissariat à l'Energie Atomique (CEA). A Concurrent Design Facility study focused on the MHFT and Sub-Kelvin coolers has been led by the European Space Agency (ESA). The Canadian contribution to  LiteBIRD is supported by the Canadian Space Agency. The US contribution is supported by NASA grant no. 80NSSC18K0132.
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