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Transcript
CRYOGENIC STEPPED ROTATION AND CHARACTERIZATION OF THE
SPIDER HALF WAVE PLATE FOR DETECTING THE COSMIC
GRAVITATIONAL WAVE BACKGROUND
Daniel Riley, Richard Bihary, Sean Bryan, John Ruhl, William Simmons:
Dept. Physics, CWRU 10900 Euclid Ave., Cleveland, OH 44106
HALF WAVE PLATE AND
POLARIZATION MEASUREMENTS
ABSTRACT
SPIDER, a balloon-borne experiment that will search for the inflationary cosmic
gravitational wave background by measuring the B-mode polarization of the CMB,
will use a cryogenic rotatable a half wave plate (HWP). The purpose of this research is
to design and develop a mechanism to provide stepped rotation and house a sapphire
HWP in a cryogenic environment and to characterize the optical properties of the
HWP. The rotation mechanism must be free of significant sources of friction. It must
also have a low heat output as well as an angular precision of 0.1˚. The basic design of
the rotation mechanism utilizes a modified stepper motor and worm gear to turn a large
spur gear concentric with the HWP housing. The HWP housing has a rounded edge
that rests on three metal spindles that allow it to rotate. A Fourier transform
spectrometer is being used to measure the polarization rotation properties of the HWP
at room temperature.
C
C
H
H
H
H
C
H
H
COSMIC GRAVITATIONAL WAVE
BACKGROUND
HWP ROTATOR GOALS
•Torque safety factor (TSF) of >20, TSF being the torque provided
divided by the minimum torque necessary to rotate the HWP
•Stepped angular rotation precision of <0.1°
•Heat production of <10 J per 22.5° rotation
•Mechanical compatibility with the SPIDER telescopes
•Safe housing of the sapphire HWP
•Burliness to prevent catastrophic failure during flight
•Feedback to track rotation of the HWP
•22.5° rotation in <20 seconds
•Operation at 4° K
Polarization sensitive detectors in
combination with a sapphire half wave
plate (HWP) will be used to make the
polarization measurements every 45° in
order to calculate the Stokes
parameters. Rather than rotating the
detector in 45° increments, we will be
rotating a HWP in 22.5° increments in
order to rotate the polarization
sensitivity of the detector beam in 45°
increments. This is done because the
detectors do not have symmetric beam
patterns. Asymmetric beam patterns
can give rise to false polarization
signals due to large temperature
variations in the CMB.
e-
The HWP rotator is shown in the photo mounted to the cryogenic tank. Significant
components of the rotator are labeled and listed.
1) Wheel and cradle – This is where the HWP is held in place by an aluminum ring
with small teeth that apply constant pressure. It has a rounded edge that interfaces
with the spinner bearings.
2) Spinner bearings – These are the three contact points that the wheel rests on. They
have a v-groove to hold the wheel, and they spin on stainless steel bearings that are
not lubricated.
3) Main gear and worm gear – The interface between these two gear allows the motor
to drive the rotation of the wheel.
4) Motor – This is a stepper motor that has been modified to allow operation at
cryogenic temperatures. The internal bearings were replaced with lubricant free
stainless steel bearings, and the size of the rotor was decreased to prevent the motor
from binding up when cooled down.
5) Optical LED encoder – This consists of an LED in front of a photodiode and a disc
with holes between them. We can track how much the shaft has rotated, thus
allowing us to track the rotation of the HWP. The signal output of the LED is
shown in Fig. 2.
6) Flexible Shaft Coupling – This flexible interface between the motor and the drive
shaft prevents stresses when the rotator is cooled down, thus allowing freedom of
rotation.
e-
Peak
Peak
Trough
SPIDER
CONCLUSIONS AND FUTURE WORK
Optical LED Encoder Signal Output
Motor Energy Dissipation vs. 22.5˚Rotation Time
140
A mechanism for rotating the HWP at cryogenic temperatures was successfully constructed. The heat
production and rotation speed goals of the project have been met. The mechanism has also been shown
to be mechanically compatible with the SPIDER telescopes. A system for precisely tracking the
rotation of the HWP was also successfully demonstrated. The torque safety factor goal has only been
met at 70° K but not at 4° K. There is either a significant source of friction in the motor assembly, or
some characteristic of the motor’s magnetic rotor is changing when cooled to 4° K. Angular precision
of the mechanism has been confirmed to be <0.1° at 70° K but has yet to be tested at 4° K. At this time
the angular precision can only be inferred to be <0.1° at 4° K. A test to determine if the rotator can
safely house a piece of glass (stand-in for the sapphire HWP) is currently being conducted. It has also
been determined that the rotator potentially isn’t burly enough for flight, and spinner bearings with
deeper grooves to provide a more secure fit on the wheel are being considered. The data on the optical
characteristics of the HWP collected by the Fourier transform spectrometer is also currently being
analyzed.
1.2
120
1
Phytron Motor - Microstep Driver - Room T
Phytron Motor - Chopper Driver - Room T
100
0.8
Phytron Motor - Linear Driver - Room T
80
Mycom Cryo Motor - Chopper Driver - Room T
0.6
Mycom Motor - Chopper Driver - Cryo T
Voltage
Energy Output (J)
Detecting the signature of the
CGB in the polarization of the
CMB is one of the
contemporary frontiers of
cosmological science and is the
goal of the SPIDER project.
This experiment is a
collaboration of Case Western,
Caltech, University of Toronto,
University of British
Columbia, NIST, JPL, Cardiff
University, and Imperial
College. It is a balloon-borne
experiment consisting of six
monochromatic telescopes
using polarization sensitive
detectors that are designed to
measure frequencies ranging
from 96 GHz to 275 GHz [1].
A Fourier transform spectrometer (FTS) was
used to characterize the optical properties of
the SPIDER HWP. The FTS is essentially a
Michelson interferometer with a moveable
mirror.
The mirror is moved and the
interferometer output is recorded as a function
of the mirror position. A Fourier transform is
then performed on the signal, and a power
spectrum is calculated. The HWP is placed at
the output of the FTS and a polarizing grid is
placed in front of the FTS detector. Readings
are taken with the HWP at varying
orientations to determine the polarization
rotation properties.
HWP ROTATOR DESIGN
Previous studies of the Cosmic Microwave Background (CMB) have measured its
temperature variations. These measurements provide information about the state of the
universe several hundred thousand years after the big bang, when the universe cooled
enough to become transparent to photons. A second theorized source of information
from even earlier moments of the universe is the Cosmic Gravitational Wave
Background (CGB). The universe was transparent to gravitational waves at an earlier
time period, and hence at a higher temperature, than photons. Studying the CGB can
provide us with information from the epoch of inflation. It leaves its signature on the
CMB in the form of polarization patterns free of divergence. Gravity waves produce
polarization in the CMB via tensor perturbations by distorting the metric of space and
creating quadrapole temperature anisotropies [1].
e-
HWP Characterization
60
0.4
40
0.2
REFERENCES
20
0
-0.05
-0.03
-0.01
0.01
0.03
0.05
0
0
5
10
15
20
25
-0.2
Time
22.5 Degree Rotation Time (s)
FIG. 1: Energy dissipation levels of different motors and driver used throughout
the year. The Phytron motor running on a microstepping driver had an energy
dissipation of over 120 J and was the first setup used. The Mycom motor running on a
chopper driver is the up-to-date setup and dissipates <10 J of energy for 22.5° of rotation
FIG. 2: Voltage output of an optical LED encoder on the motor drive shaft.
Triggering conditions will be applied to the output signal to track the movement of the
HWP. Each period is an angular rotation of 0.04°.
1. C. J. MacTavish, P. A. R. Ade, E. S. Battistelli, S. Benton, R. Bihary, J. J. Bock, J. R.
Bond, J. Brevik, S. Bryan, C. R. Contaldi, B. P. Crill, O. Doré, L. Fissel, S. R. Golwala,
M. Halpern, G. Hilton, W. Holmes, V. V. Hristov, K. Irwin, W. C. Jones, C. L. Kuo, A.
E. Lange, C. Lawrie, T. G. Martin, P. Mason, T. E. Montroy, C. B. Netterfield, D. Riley,
J. E. Ruhl, A. Trangsrud, C. Tucker, A. Turner, M. Viero, D. Wiebe. Spider
Optimization: Probing the Systematics of a Large Scale B-Mode Experiment. Submitted
to ApJ. arXiv:0710.0375