LiteBIRD: mission overview and design tradeoffs

Abstract

We present the mission design of LiteBIRD, a next generation satellite for the study of B-mode polarization and inflation from cosmic microwave background radiation (CMB) detection. The science goal of LiteBIRD is to measure the CMB polarization with the sensitivity of δr = 0.001, and this allows testing the major single-field slow-roll inflation models experimentally. The LiteBIRD instrumental design is purely driven to achieve this goal. At the earlier stage of the mission design, several key instrumental specifications, e.g. observing band, optical system, scan strategy, and orbit, need to be defined in order to process the rest of the detailed design. We have gone through the feasibility studies for these items in order to understand the tradeoffs between the requirements from the science goal and the compatibilities with a satellite bus system. We describe the overview of LiteBIRD and discuss the tradeoffs among the choices of scientific instrumental specifications and strategies. The first round of feasibility studies will be completed by the end of year 2014 to be ready for the mission definition review and the target launch date is in early 2020s.

Development of Helium-3 Compressors and Integration Test of Closed-Cycle Dilution Refrigerator System

“…In principle, a cooling power is determined by a 4 He flow rate as a function of a temperature at a mixing chamber when 3 He flow rate is enough. Table. 1 in 9) shows that a 4 He flow rate of 350 µmol/sec is needed to obtain a cooling power of 1 µW at 47 mK for a 3 He circulation rate of 28.8 µmol/sec at a still pressure of 10 kPa. Therefore, the target flow rate of 3 He has been 33 µmol/sec (100 µg/sec) in this study.…”

“…When a cooling chain using the closed-cycle dilution refrigerator is considered for a space mission, it requires a heat lift of 5∼10 mW at 1.7 ∼ 1.8 K. One primal candidate as a pre-cooler is a 1K-JT, which provides a cooling power of 10 mW at 1.7 K. Therefore, 1.75 K is targeted as a heat sink of a 3 He circulation line and a fountain pump outlet 4 He return line, and the rejected heat into the pre-cooler, mainly generated by a fountain pump must be lower than 10 mW.…”

“…In the closed-cycle dilution refrigerator, the cross section of the capillary is designed to be plugged by these 3 He-rich phase droplets where the capillary forces play the role of gravity. The 4 He circuit does not exist in a standard dilution refrigerator and such a refrigerator cools by moving 3 He from a 3 He-rich phase to a dilute phase. On earth, gravity separates the 3 He-rich phase from the dilute phase in a mixing chamber.…”

“…To achieve this, a space qualified helium-3 circulator is needed as a new important research and development, and the technology of the low pressure compressor for 1K-class Joule Thomson cooler is one of primal candidate. 7) Then, a 4 He / 3 He separator in a still and a fountain pump to circulate a superfluid 4 He at low temperature are also needed.…”

“…The 4 He is extracted from the still through a superleak by means of a fountain pump, which can only work below the superfluid transition temperature. The 3 He is extracted by pumping on the gaseous phase much richer in 3 He (> 90 %) than the liquid phase (∼ 10 %) and then re-condensed into the dilution.…”