Upgraded University of Florida Torsion
Pendulum for Testing Key LISA Technology
Stephen Apple, Andrew Chilton, Taiwo Olatunde, Samantha Parry, Henri
Inchauspe, Anthony Davila, Peter J Wass, Guido Mueller,John Conklin
4/14/2019
April APS: Denver, Colorado 4/14/2019
1
Gravitational Reference Sensor (GRS)
• LISA GRS consists of
• Test mass
• Housing
• Actuation
• Sensing components
• 18 electrodes for 6 DOF capacitive
sensing and actuation
• Electronics
• Charge control
• Quiet thermal, EM, gravitational
environment with monitoring
LISA Pathfinder GRS
April APS: Denver, Colorado 4/14/2019
2
UF Torsion Pendulum
• LISA gravitational
reference sensor (GRS)
technology test bed
• Simplified GRS consists of:
• test mass
• corresponding
actuation
• Sensing
• 6 electrodes for 3 DOF
actuation and sensing
• charge control
• housing components
• Largely based on design
of University of Trento
torsion pendulum facility
April APS: Denver, Colorado 4/14/2019
1m
3
Capacitive Readout & Actuation
Preamp
Sensing electrodes
Diff. amp
PC
Demod/
processing
Preamp
DC Amp
(10/250 V)
ADC
DAC
TM
DC actuation
100 kHz injection
FPGA-side
NCO:
100 kHz
Injection electrode
φ+π/2
Host-side
Q
CIC
CIC
φ
R( )
I
Q’
I’
Output

To DAC
April APS: Denver, Colorado 4/14/2019
From ADC
4
IFO readout Design
• Polarization multiplexed Mach-Zehnder interferometer measures
differential displacement of two test masses
• Light delivered by fiber feedthrough
External
• Recombined beam sensed outside chamber (free-space)
bench
• Alignment insensitive to TM rotation
April APS: Denver, Colorado 4/14/2019
5
Equivalent LISA TM Acceleration Noise
April APS: Denver, Colorado 4/14/2019
6
Charge measurement/UV LED charge management
• Charge Control Motivation:
• Charge increases electrostatic stiffness
• Interacts w/stray electric fields and
induces force noise
• 2 V sinusoidal voltage at 17 mHz applied
across sensitive axis
• Response of the pendulum  TM charge
• Measurement made with IFO
• Analysis by heterodyne demodulation
April APS: Denver, Colorado 4/14/2019
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Pulsed Light UV LED Charge Management
• Driver modes: (a) DC, (b) PWM
referenced to external input
• Fiber Coupler: UV LED + TEC + lens
+ 600 µm fiber
April APS: Denver, Colorado 4/14/2019
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Torsion pendulum upgrades: LISA-like GRS
Motivation
• Make noise measurements more
representative of actual LISA GRS
• Test new LISA technology such as
pulsed UV LED charge management
April APS: Denver, Colorado 4/14/2019
•
•
•
•
•
•
Features
Design based on LISA Pathfinder GRS
18 electrodes (6 injection and 12 sensing)
6 degree of freedom actuation and sensing
Au coated aluminum with PEEK spacers
3 novel UV light injectors for UV LED charge management
Interiors faces and electrodes diamond turned to mirrorlike finish (12−16 nm roughness <10 μm flatness)
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Torsion pendulum upgrades/ Future work
Pendulum Upgrades
• New cross bar assembly with TMs diamond
turned to same specifications as LISA-like GRS
• Motorized rotation stage
• Newly designed and improved eddy current
damper
• Vacuum chamber bake-out/leak testing
• Heaters for temperature gradient experiments
• Improved interferometer design
Future Work
• Characterize noise of LISA-like GRS
• Test technology developed by UF Charge
Management System group including
electronics, hardware, software, charge
management techniques, etc.
April APS: Denver, Colorado 4/14/2019
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Acknowledgements
NASA N.G. Roman Tech Fellowship, grant number NNX15AF26G
NASA LISA CMS Project, grant number 80NSSC17K0277
April APS: Denver, Colorado 4/14/2019
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References
[1] Ciani, Giacomo, Andrew Chilton, Stephen Apple, Taiwo Olatunde, Michael Aitken,
Guido Mueller, and John W. Conklin. “A New Torsion Pendulum for Gravitational
Reference Sensor Technology Development.” Review of Scientific Instruments 88, no.
6 (June 1, 2017): 064502. https://doi.org/10.1063/1.4985543.
[2] Speake, C C, and S M Aston. “An Interferometric Sensor for Satellite Drag-Free
Control.” Classical and Quantum Gravity 22, no. 10 (May 21, 2005): S269–77.
https://doi.org/10.1088/0264-9381/22/10/019.
April APS: Denver, Colorado 4/14/2019
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Backup Slides
April APS: Denver, Colorado 4/14/2019
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LISA Requirements
Limited by GRS acceleration
noise
𝑚
1/2
−15
𝑆𝑎
≤ 3 × 10
𝑠 2 𝐻𝑧
April APS: Denver, Colorado 4/14/2019
Limited by interferometric readout with a
𝑝𝑚
displacement sensitivity on the order of 10 𝐻𝑧
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Torsion Pendulums
• Typically consist of a torsion member
suspended by a thin fiber and any
corresponding actuation and sensing
components
• Ideal for ground testing LISA GRS
technology/noise performance because:
• Allow for the measurement of very
small forces
• Suspension fibers naturally hang
parallel to the local gravitational
field
• Decouple measurements from
Earth’s local gravitational field
• Materials of suspension fiber can be
chosen to lower thermal torque
noise and increase sensitivity
• Gives a near free fall condition in
one degree of freedom
April APS: Denver, Colorado 4/14/2019
University of Trento pendulum torsion member [5]
15
AC Capacitive Readout & DC Actuation
Preamp
Diff. amp
PC
Demod/
processing
Preamp
DC Amp
(10/250 V)
ADC
DAC
TM
DC actuation
100 kHz injection
FPGA-side
NCO:
100 kHz
φ+π/2
Host-side
Q
CIC
CIC
φ
R( )
I
Q’
I’
Output

To DAC
April APS: Denver, Colorado 4/14/2019
From ADC
16
IFO readout Design
• Polarization multiplexed Mach-Zehnder interferometer measures differential displacement of two test masses
• Light delivered by fiber feedthrough
• Recombined beam sensed outside chamber (free-space)
• Alignment insensitive to TM rotation
• Electronics Noise 0.5 nm/Hz1/2 (10X worse below 1Hz, under investigation)
April APS: Denver, Colorado 4/14/2019
External
bench
17