The LiteBIRD low-frequency telescope polarization modulation unit
Thuong D. Hoang1, Takashi Hasebe1, Yuki Sakurai2, Tomotake Matsumura1, Ryota Takaku1,3, Nobuhiko Katayama1, Tommaso
Ghigna4, Guillaume Patanchon5, Susanna Azzoni6, Ryota Uematsu2, Teruhito Iida7, Clément Leloup1 for the LiteBIRD collaboration.
1

Kavli IPMU -The University of Tokyo, 2Okayama University, 3Department of Physics-The University of Tokyo, 4QUP-KEK, 5APC-Paris University, 6
University of Oxford, 7space inc.

I. Introduction

thuong.hoang@ipmu.jp

II. LFT Polarization Modulation UnitLiteBIRD is the future Cosmic Microwave Background (CMB) radiation polarization
satellite searching for primordial gravitational waves generated during theinflationary epoch prior to the Big Bang. One of the main scientific goals is toachieve an upper limit on, or measure, the amplitude of the primordial gravity
waves (B mode polarization), which is parameterized by the tensor-to-scalarratio r < 0 . 001 (for 2 ≤ ℓ ≤ 200) . LiteBIRD payload module will deploy a lowfrequency telescope (LFT), a medium-frequency telescope (MFT), and a highfrequency telescope (HFT). All telescopes employ polarization modulator units(PMU) using continuously rotating half-wave plates (HWP). The PMU is a crucialcomponent to mitigate/avoid several systematic effects including 1/f noise, to reach
Angular scale
unprecedented sensitivity.
90
183
1
0.2
0.1
We have developed a second
breadboard model (BBMv2) 10
PMU for the LFT. In thisposter, we present the10design and testbed of theLiteBIRD
LiteBIRD LFT PMU
BBMv2.
10±±±±±frameFramePolarization
Modulation UnitApertureaperture
stopstop (5K)A 5-layer sapphire Achromatic Half-Wave
Plate (AHWP): Rotating birefringent plates to(First optical element)
modulate CMB polarization signal.

front hoodsecondary
Secondaryreflector
reflector 5°(5K)Five-layer AHWP
primary reflector

Primaryreflector (5K)ωhwp

HWP birefringent axis

Focalplane

focal plane

4ωhwp

±3

Polarization
angle

Payload module
MFT HFT LFT

CMB- TT

WMAP
Planck
ACT
SPTpolPOLARBEAR
BICEP/Keck1

OpticalOpticalaxisaxis

PMU

Input light

Output lightdm(t) = I + Q cos 4ωhwpt + U sin 4ωhwpt°1

CMB- BBPowerspectrum`(` + 1)C`/2º [µK2]

CMB- EE

10°3Lensing•LiteBIRD will improvecurrent sensitivity on r atlarge angular scale.

1/f

noPrimordial
r=0.004

10°5

1030

80

200

500

4ωhwp

Frequency•Current upper limit r < 0.032 (Planck+BICEP)
2

ise

1000

Unpolarized signal
Modulated polarized signal
Demodulated polarized signal

2000

Multipole `III. Breadboard model (BBM) of the PMU

Optical encoder

Encodersystem

Encoder disk
(128 slots)

Encoder disk

Drive motor magnets
Drive motor coils
330 mmSpacers
For HWP holder
Gripper

SMBmagnet•We employ a custom AC driver
motor which outputs a 3-phasesignal to accelerate and maintainthe rotor/HWP at a constantrotational frequency.

SmComagnet ring
YBCO
superconductors

Motor drive

Base plate• The second version of the breadboard model (BBMv2) of the PMU
contains a cryogenic rotational mechanism (a YBCO ring, and a rotor),a holder mechanism (grippers), and an optical encoder system.

•The cryogenic rotational mechanism is a superconducting magnetic bearing(SMB) which consists of high-temperature superconductors (HTS) YBCO (46pieces arranged in a ring shape) and a SmCo permanent magnet (32 pieces arranges
in a ring shape).
•At cryogenic temperature, the HTS will levitate the SmCo ring as the rotor.
Therefore, the SMB with no mechanical contact can avoid the heat dissipation fromthe physical friction.

IV. Results: We present some preliminary results of the BBMv2 cryogenic test.
• HWP angle reconstruction is critical fordata analysis. We evaluate the instability
in the HWP angle using the period of theencoder.

Acceleration

Spin stability

2πδρA;i = 2πfhwpΔtA;i −128
Measurement• We accelerated the rotor to a constant frequency. The
minimum current for the constant rotation at 1 Hz is 150mA.

Design• Using the raw encoder data, we canestimate the HWP angle instability.

• The resistance of the single motor drive coil
Rcoil,300K = 5.5Ω, then the power of the heat dissipationfrom the 3-phase coil at 30K is estimated about 3 mW.
V. Summary and future works● We have developed LiteBIRD LFT PMU a bread model. We have carried out cryogenic testbed for the system.
● We spun the rotor at different frequencies, measured the Hall sensor signal, and the encoder signal.
● The BBMv2 tested results help to develop the design of the PMU flight model.
● The thermal model of the PMU and cryostat system is necessary for the power consumption estimation. Heat dissipation of the rotational mechanism is an on going research anddevelopment.
● In the future we will integrate 5-layer HWP with/without Anti-Reflecting coating, an aperture to the system and measure the optical modulated signal.
CMBxLSS Kyoto, 10-14th 2023