The LiteBIRD satellite is a Japan Aerospace Exploration Agency (JAXA)-led mission with substantial contributions from the USA and Europe. It is a complex measurement system, designed to reach unprecedented levels of precision and accuracy the measurement of a subtle feature of the microwaves coming from the sky: their polarization (link to the box) status.
Sir George Gabriel Stokes invented, around 1860, a method to measure the polarization status of light (or in general the polarization of electromagnetic waves (EMWs)) without measuring every oscillation of the electric field (which is extremely fast, and cannot be followed by most of the available light detectors). In a Stokes polarimeter, the light beam under study interacts with two special optical components (a half-wave plate (HWP) and a polarizer) before reaching a power-sensitive light detector. Measuring the detected power from the light beam for different orientations of the HWP, while keeping the polarizer still, allows to retrieve the linear polarization status of the light under analysis.
The LiteBIRD instrument consists of three Stokes polarimeters for microwave EMWs. Each polarimeter features a rotating HWP, followed by a telescope feeding power detectors with integrated polarizers. It is designed to be extremely sensitive, through the use of arrays of cryogenic bolometric detectors (cooled to 0.1K, i.e. 3000 times colder than room temperature, thus freezing any thermal agitation disturbance) and cryogenically cooled optical components collecting the microwaves from the sky. It is designed to be extremely accurate (through the use of HWP, polarizer and optical components cooled at cryogenic temperatures to minimize their own emission, and avoiding the presence of any optical device in front of the HWP). Moreover, multiple observation frequency bands allow the extraction of the faint polarized Cosmic Microwave Background, which is the target of the measurement, from overwhelming foreground polarized radiation (see foregrounds section, link). Finally, operating in the Sun-Earth L2 point, 1.5 million km away from the Earth, LiteBIRD can observe the dark universe avoiding the bright microwave emissions from the Sun, the Earth and the Moon.
The implementation of such an ambitious design involves the most advanced technology solutions:
- An autonomous cryogenic system must keep the arrays of detectors at 0.1K for the entire duration of the mission (several years), while the satellite bus is kept at 300K, under a solar irradiation power of several kW. This challenging task is achieved by means of passive radiating V-grooves, active mechanical cryocoolers, and adiabatic demagnetization refrigerators. (link to LTD paper on cryogenics).
- Three telescopes are used to feed detector arrays in different frequency bands (Low-Frequency, Medium-frequency, High-frequency telescopes, LFT, MFT, HFT) and are optimized to reject very efficiently radiation outside the main beams of the detectors, minimizing straylight and diffraction by means of custom absorbing shields and apertures, and high efficiency detectors feedhorns. The measurement of primordial CMB polarization does not require high angular resolution, so the aperture of the telescopes is not very large: 40, 30, 20 cm diameter for the LFT, MFT and HFT respectively. (link to LTD paper on telescopes – Baptiste Mot)
- The HWP is a precision, large diameter optical component, placed at the entrance aperture of the telescope, and rotated by means of a superconducting magnetic bearing. This means that the HWP levitates in front of the telescope, and its rotation is nearly frictionless. This involves high temperature superconductors on the stator and strong permanent magnets on the rotor, with high-tech clamp/release systems and motion control units (links to papers
on HWP: G. Pisano, et al., SPIE Proc. 9153, 915317 (2014), DOI: 10.1117/12.2056380; K. Komatsu, et al., SPIE Proc. 10708, 1070847 (2018), DOI: 10.1117/12.2312431
on SMB: Y. Sakurai, et al., SPIE Proc. 10708, 107080E (2018), DOI: 10.1117/12.2312391
on Clamp/Release: F. Columbro, P. de Bernardis, S. Masi, Rev. Sci. Instrum. 89, 125004 (2018), DOI: 10.1063/1.5035332).
- The detector arrays include a very large (order of 4k) number of Transition Edge Sensor (TES) bolometers, arranged in polarization-sensitive multichroic pixels to maximize the survey efficiency of the telescopes (link to paper of ….). These detectors are so sensitive that they would be able to detect the thermal emission of …. on …. To avoid excessive heat load on the cryogenic system, an advanced multiplexing readout electronics is used (link to paper on readout: Tsujimoto, M.; Nishino, H.; Hazumi, M.; Sekimoto, Y.; Dotani, T.; Ishino, H.; Kibayashi, A.; Sakurai, Y.; Matsumura, T.; Dobbs, M.; Cliche, J.-F.; Smecher, G.; Suzuki, A.; Lee, A. T.; Arnold, K.; Montier, L.; Mot, B.; Signorelli, G.; de Bernardis, P. Current design of the electrical architecture for the payload module of LiteBIRD, Proceedings of the SPIE, Volume 10698, id. 1069847 9 pp. (2018)).
- A sky-scan 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. This combination provides not only a fairly uniform sky coverage but also the minimization of instrumental systematic uncertainties in polarization measurements. (link to paper Hajime Sugai LTD)
The main characteristics of the LiteBIRD mission are summarized in table XXX (insert tables 1 & 2 Hajime Sugai LTD paper)
The calibration of such a complex polarimeter is as important as its design and implementation. Extremely detailed plans of ground validation measurements and accurate in-flight calibration are implemented, following the scheme in table YYY
The expected performance of the instrument can be summarized as in table ZZZ (insert performance table).
BOX: Polarization of light
An electromagnetic wave (EMW) transfers energy through space via the oscillations of orthogonal electric and magnetic fields. If the oscillation of the electric field happens along a constant direction, the EMW is fully linearly polarized. This happens for example when the EMW is generated by an electronic oscillator, like the klystron in a microwave owen. If, instead, the oscillation direction changes randomly, the EMW is unpolarized. This is the case of light from thermal sources, like sunlight. In general, an EMW is partially polarized, i.e. the oscillation direction of the electric field is the superposition of a random one plus a constant one. An initially unpolarized EMW can become polarized through the interaction with matter. For example, unpolarized sunlight becomes partially linearly polarized when reflected in a pond, or when scattered by atmospheric atoms (skylight is partially polarized). Devices aimed at measuring the polarization status of EMWs are called polarimeters, and are useful in astronomy to investigate the physical process producing radiation in the source, as well as subsequent interactions of the radiation with intervening matter (interstellar dust, for example, polarizes starlight).