Sesto Dolomiti 30 July 2010 - ego-gw.it · Sesto Dolomiti 30 July 2010. Laser InterferometerSpaceAntenna NASA / ESA Mission, tobelaunchedin 2020 •2 semi-independent 5 10 6km Michelson

LISA

William Joseph Weber

Dipartimento di Fisica, Università di Trento

LISA / LISA Pathfinder ProjectLISA / LISA Pathfinder Project

5th VESF School on Gravitational Waves

Sesto Dolomiti

30 July 2010

Laser Interferometer Space Antenna

NASA / ESA Mission, to be launched in 2020

• 2 semi-independent 5 106 km Michelson

interferometers with laser transponders

(measurement noise 40 pm/Hz1/2)

• 3 pairs of “free falling” test masses in 3

“Drag-Free” spacecraft shields

( acceleration noise < 3 10-15 m/s2/Hz1/2)

LISA: Laser Interferometer Space Antenna

( acceleration noise < 3 10-15 m/s2/Hz1/2)

LISA goals:

GW Band: 0.1 mHz – 1 Hz

Sensitivity: Sh1/2 ~ 10-20 Hz-1/2 at 1 mHz

∆∆∆∆(h) ~ 2 10-24 for 1 year integration

5 106 km

LISA Constellation

• 5 million km equilateral triangle

• 60°tilt with respect to ecliptic

• 1 AU from sun, 20°behind earth

Ground and Space GW Observatories Complementary

LISA Signals: mass, separation, chirp time, and distance

Keplerian orbit

frequency ( x 2)

[equal mass

binaries with

circular orbits]

( ) rf

ch~

GW 22τπ

Product of measured strain and measured

decay time gives distance to source!

Black hole

merger

Energy

decay

time τ

( )3

22

a

GMf TOT=π

LISA Gravitational Wave Astronomy:

Compact Object Mergers

Astronomers tell us ...

Most stars are in binary systems

Many stars “collapse” to compact

relativistic stars:

Neutron stars, White dwarfs, black holes

... but they are hard to ... but they are hard to

“see” electromagnetically

Only 5 merging NS-NS systems have been found

(need to be lucky to see the pulsar)

Only roughly 50 ultra-compact binaries observed (mostly WD-

WD)

LISA and Galaxial Binaries

• Known “calibration” signals

Signal “guaranteed” for a functioning LISA!

Verification of GR predictions for GW strain

Recent binary neutron star discovery

PSR J0737-3039

• 2 neutron stars (MTOT ~ 2.8 M◉ )

• 2.4 hour orbital period

• 3 times faster than HT, doubles

strain signal, easier detection at

higher frequency .25 mHz

• 2006 � orbital decay detected,

confirms GR at 1 % level

• possibly detectable by LISA (strain of order 10-21 at 0.25 mHz)?

• important for estimate of galaxial NS-NS binary merger rates (nearest system)

1 / 5000 years in our galaxy � 200 with τ < 1 million years (f > 2 mHz)

• LISA should provide a real measurement of populations of galactic binaries

• “only” 2000 light years away

• several times closer than HT

Stochastic GW noise: galaxial binaries and primordial backgrounds

1 year measurement: µHz 03.

year 1

1≈≈∆f

• 105 frequency bins up to 3 mHz

• many galactic white dwarf binaries (perhaps 108 ), lots per

frequency bin below 3 mHz, produces “noisy” background

Discrimination of noisy confusion limited galactic binary

“foreground”

• Sagnac variable to characterize instrument noise from noisy gravitational

foreground

• Annual modulation of noisy from galactic center

Sample data with instrument noise

Gravitational Wave Astronomy:

Massive Black Hole Mergers

Astronomers tell us ...

Many galaxies have massive black

hole at core

Most galaxies merge

... but we can’t “see” them... but we can’t “see” them

Our Milky Way appears to have

a 3 106 M◉ black hole at its core

Valtonen et al, Nature, 2008

• Observation of quasi-periodic (12 year) quasar light bursts since 1913, occuring in pairs

Quasar OJ287: gravitational radiation in a massive black hole system

Valtonen et al, Nature, 2008

September 2007 burst � without gravitational

radiation, burst would arrive 20 days later!

10 %-level validation of general relativity

description of gravitational radiation

• Optical bursts from an orbiting object

penetrating accretion disk of a massive black hole

• Mass – 18 109 M◉ — determined by geodesic

precession of eccentricity, 39°/ orbit

• orbit apogee roughly 10 RBH

The next major periodic outburst is expected in early January 2016, by which

time there may be methods to measure the gravitational waves directly.

-- Valtonen, et al, Nature, 2008

• Massive black hole binaries from cores of

merging galaxies (104 -108 M◉ )

• expect to see tens in a 5 year mission

• SNR up to 2000 in one year at z ≈1 – 3 �

observable anywhere in the universe

• visibility up to one year before merger

• chirp rate and amplitude combine to give

Coalescence of Massive Black Hole Pairs

• chirp rate and amplitude combine to give

the luminosity distance (0.2 % -1%

uncertainties)

• frequency and amplitude modulation

combine to give angular resolution (to within a

square degree)

�well calibrated source distances

� formation of MBH as function of

redshift

� with optical counterpart, measure

distance – redshift relationship

Simulated strain time series for a MBH merger at redshift z = 5

S/N ratio >> 1 even for single cycles near the end of a MBH merger

(2 105 M◉ at z = 5)

� High S/N observation of MBH mergers anywhere in the relevant universe!

Gravitational waves physics

• Gravitational wave observation (phase, polarization, amplitudes)

can probe general relativity in limit of strong gravitational fields,

near black hole event horizons

• Gravitational waves drive dynamics in such systems

• need compact “test particle” – NS or BH – not tidally disrupted

near MBH

Example: small “test

particle” black hole falling

into a massive black hole

LISA Sensitivity Curve

Sensitivity curve for 1 year integration and S/N=5

Photon shot

noise

Test mass

acceleration

noise

Decreased

interferometer

response

SEND

1 W

RECEIVE

~200 pW (< 100 pW final)

Telescope

D ~ 30 cmArriving Beam

~20 km

LISA Interferometry

Laser divergence:

YAG 1.06 µm

L1 L23 5 million km arms: 33 sec 2-way light time

(1st interferometry null at 30 mHz)

1 W ~200 pW (< 100 pW final)

1/2

4

222/1 pm/Hz 10

42≈==

D

L

P

c

P

cS

sentreceived

L

λ

η

λ

π

λ

πδ

hh

Goal: keep all optical path errors within 40 pm/Hz1/2

Shot Noise:

Laser transponding: outgoing light phase locked to incoming beam

LISA Interferometry: TM separation as 2 part measurement

� long interferometer and (2) short interferometer

*** pm precision requires subtracting nm spacecraft motion (thruster noise)

Gerhard Heinzel, AEI

LISA Optical Bench

� Astrium Germany design, ESA study

Light from 2 lasers

L1 � to remote SC (1 W), local TM

L2 (beam for 2nd arm) � local oscillator

for incoming beam and TM readout

3 interferometers

TM readout (L1, L2 as LO)

Remote beam readout (far laser, L2 as LO)

L1 – L2 measurement of relative phase noise

LISA Low Frequency Sensitivity:

Importance of free-fall

2

2/1

minmin

1

ωm

S

TLL

Lh

f≈

∆≈

Stray acceleration

noise (1/f2 ) for flat

spectrum

hmin ~ 10-23 at 1 mHz (S/N=5) requires Sf1/2/m ~ 3 10 -15 m/s2 / Hz1/2

Purity of free-fall critical to LISA science

Example: massive black hole (MBH) mergers

Integrated SNR at 1 week intervals for year before merger

Assuming LISA goal:

Sa1/2 < 3 fm/s2/Hz1/2

at 0.1 mHzat 0.1 mHz

Acceleration noise at and below 0.1 mHz determines how well, how far,

and how early we will see the most massive black hole mergers.

� do we see the merger for long enough to use orbital

modulation to pinpoint it? To search with optical telescopes?

LISA “differential accelerometry” performanceMeasurement of tidal accelerations between 2 or more

geodesic reference test masses

2

ha Lhω∆ ≈

2

measure na xω∆ ≈

f∆

effective “GW

acceleration”

distance

measurement noise

Stray force noise �

• LISA differential accelerometry represents a large leap in performance

� requires significant design changes

� requires experimental verification

• Free-fall at low frequencies difficult to test on ground

� Dedicated geodesic motion flight mission � LISA Pathfinder

force

fa

m

∆∆ ≈

Stray force noise �

imperfect geodesic

motion

Spacecraft shield

(mass M)

Stray forces and drag-free control

µNewton Thrusters

“Drag Free” loop gain

MωDF2

• Solar radiation pressure would give

10 nm / s2 acceleration to 1 kg test

mass

Springlike coupling to spacecraft

motion (“stiffness”) mωp2

“internal” stray forces fstr

Relative position

measurement xm

m

motion (“stiffness”) mωp

external forces on

satellite Fstr

Common problem for several precision space experiments: LISA,

GPB, STEP ...

++=

2

2

DF

strnp

strres

M

Fx

m

fa

ωω

Residual acceleration noise:

Relative spacecraft – TM

LISA Drag-free Control

Role of LISA drag-free control is to reduce test mass acceleration noise, with

respect to distant test mass

NOT to minimize relative spacecraft motion

NOT to produce most precise spacecraft orbit

Relative spacecraft – TM

motion

LISA control: spacecraft follows 2 masses at once




1: Move the spacecraft and centre the masses along laser beams








2: Re-center the masses along “orthogonal” axes using electrostatic forces






Need to sense all 6 degrees of freedom of the test mass

Need to apply (electrostatic) actuation forces on non-interferometry degrees of

freedom

Key LISA test mass acceleration noise sources

dx

Residual acceleration noise:

Springlike coupling to spacecraft:sensor readout stiffness (ωp

2xn ~ d)gravity gradients

10-6 N/m

External forces on

SC, finite control

loop bandwidth

Gap

++=

2

2

DF

strnp

strres

M

Fx

m

fa

ωω

gas damping

magnetic noise

readout back action (~ d-2)

Stray electric fields + charge/dielectric noise (~ d-1 ,d-2 )

∆T� radiation pressure, radiometric, outgassing effects

Local gravitational noise

Control force noise (leakage)

6 fN/Hz1/2

Sensor noise

Low frequency stability!

2.5 nm/Hz1/2

Gravitational Reference Sensor Design

• 46 mm cubic Au / Pt test mass (1-2 kg)

• 6 DOF “gap sensing” capacitive sensor

• Contact free sensing bias injection

• Resonant inductive bridge readout (100 kHz)

• Defines TM environment

• Provide nm/Hz1/2 measurement on all axes

• Provides electrostatic voltages (force, measurement)

VACT1

VACT2

VM

Cs1

Cs2

VAC

100 kHz L

L

Cp

Cp

• Resonant inductive bridge readout (100 kHz)

• ~ 1 nm/Hz1/2 thermal noise floor

• Audio frequency electrostatic force actuation

�avoid DC voltages

• Large gaps (2 – 4 mm)

� limit electrostatic disturbances

• High thermal conductivity metal (Mo) / sapphire

construction

� limit thermal gradients

Completing the GRS• Caging (2000 N load during launch)

• UV light photoelectric TM discharge system

• Vacuum chamber + getter pumps (10-5 Pa)

• Optical windows for IFO readout

What do we know about LISA free-fall performance ?

� experimental verification• LISA differential accelerometry represents a large

leap in performance

� requires significant design changes

� requires experimental verification

Two approaches ...

�Torsion pendulum small force testing

(Low frequency free-fall difficult to test on ground)

� Dedicated geodesic motion flight mission �

LISA Pathfinder

LISA

LPF is a single interferometry arm of LISA squeezed into a single spacecraft

LPF

LISA Pathfinder (2012): Einstein’s Geodesic Explorer

Mostly ESA test mission for free-

fall in LISA and other future

precision space missions

Shrink 1 LISA arm from 5 million km to 30 cm

Flight test of LISA free-fall at 30 fm/s2/Hz1/2 level at 1 mHz

Flight test of LISA local interferometry measurement at 10 pm/Hz1/2 level

Testing free-fall for LISA Measure differential acceleration between 2 free-falling TM

2TM in 2 spacecraft

(basically LISA)

2TM in 1 spacecraft

(LISA Pathfinder)

LISA

LPF is a single interferometry arm of LISA squeezed into a single spacecraft

LPF

(LISA Pathfinder)

LISA Pathfinder Mission• Launch 2012

• Roughly 2 month journey to Lagrange point 1

• Commissioning + 3 months LTP (ESA), 3 months DRS (NASA)

VEGA Launcher

Free-fall flight test: LISA Pathfinder

x

Xbase

~ 30 cm

TM1 TM2

drag-free

electrostatically

suspended

∆x12≡x2 - x1

• Compare relative noise in orbits of two “free-falling” test masses

• 1 spacecraft, 1 measurement axis (30 cm baseline)

• Relative displacement ∆x12 measured with interferometer to probe drag-free

performance

Optical interferometer Differential displacement ∆x12

LTP Goal: demonstrate ares < 30 10-15 m/s2/Hz1/2 for f > 1 mHz

(relaxed from LISA by factor 10 in both acceleration noise and frequency)

LPF primary measurement: stray force noise fstr

TM1 TM2

∆x12

• Satellite follows TM1 with sensor x1 and µN thrusters

• TM2 forced to follow TM1, using differential x1 – x2 IFO

and sensor 2 electrostatic actuation

• Open-loop differential acceleration in differential IFO

signal (calibrate transfer function)

21 2 2 22 ω2 2 , 22 2 2 ω

f∆x ω δxω x ω

IFO p p n opt pmω ω ω

ω

− = + + − − + − − +

Ff2str1 x

n1 1p2M

Baseline stability

(Zerodur)

Differential

force noise

++=

2

2

DF

strnp

strres

M

Fx

m

fa

ωωLISA

2

1

2 2 22 ω2 2 , 2LPF

f∆a x ω δxω x ω

p p n opt pmω

− = + ∆ − − + −

f211p

2 2 , 22 2 2 ω2

IFO p p n opt pmω ω ω

p ES

− +

n1 1p2MDF

Satellite coupling (can be

tuned to zero)IFO readout noise

LPF

LISA Pathfinder differential accelerometer “instrument performance”

LPF

LISA

• Not shown (order 0.1 fm/s2/Hz1/2): baseline thermal distortion and stiffness terms

• Actuation noise: order 6 ppm/Hz1/2 at 1 mHz (1/f in power), 0.65 nm/s2 DC acceleration

• Readout 9 pm/Hz1/2, relaxed as 1/f2 below 3 mHzLISA Symposium, Stanford, 29 June 2010

LPF Hardware: Interferometry Readout

• Mach Zehnder heterodyne interferometer on

Zerodur optical bench

• 10 pm/Hz1/2 precision

• Relative (x1 – x2) and x1 measurement IFO, +

frequency IFO, reference IFO, amplitude stabilization

• Quadrant photodiodes for wavefront sensing

angular readout

Testing LPF optical metrology: Overall performance test

+×

2

1/2 mHz 31pm/Hz 6

f

OMS performance achieved across entire LPF band• Performance achieved in harsher conditions – longer fiberlinks and pathlength differences, sub-optimum use of detector dynamic range LISA Symposium, Stanford, 29 June 2010

LISA Pathfinder: performance limited by 2 TM in 1 SC

� gravitational balancing and applied forces

• SC can only follow 1 TM along x (2 TM, 1 sensitive axis)

(UNLIKE LISA!!)

• Any differential DC acceleration must be balanced by

applied (electrostatic) forces

• Noise in applied voltage gives noisy force

g1g2

2

1/ 2 1/ 22

F V

S FS

∝

=

21.3 nm/sg∆ <

GRS compensation masses �

reduce 30 nm/s2 to 0.1 nm/s2

Modelling accuracy, positioning

1/ 2 1/ 2

/

1/ 2 1/ 2

/

2

2

F V V

a DC V V

S FS

S g S

δ

δ

=

= ∆1/ 2 6 1/2

/ 2 10 /HzV VSδ−< ×

2

1/ 2 1/ 2

/

1/ 2 1/ 2

/

2

2

F V V

a DC V V

F V

S FS

S a S

δ

δ

∝

=

= ∆

2

1/ 2 6 1/2

/

1.3 nm/s

2 10 /HzV V

g

Sδ−

∆ <

< ×

LISA Pathfinder: performance limited by 2 TM in 1 SC

� gravitational balancing and applied forces

• Actuation voltage carrier

amplitude measured to be stable to 3

roughly 8 ppm/Hz1/2 at 1 mHz

�Less than 10 fm/s2/Hz1/2

acceleration noise

(electronics Contraves Space,

test U. Trento / ETH Zurich)

LISA Pathfinder: avoiding actuation instabilities with free-fall mode

compensate average DC force imbalance by applying a large impulse

followed by free-fall (parabolic flight!)

x

Grynagier, CQG 26 (2009) 094007

• Example: Apply 300 x average needed force for 1s, followed by 300 s free-fall

• Keep displacement to 10 micron range

• Analyze force spectrum, without applied actuation forces, even to lower

frequencies, with windowed spectrum estimation

Grynagier, CQG 26 (2009) 094007

LPF stray force measurement:Given instrumental noise limit, what do we know aboutsources of test mass acceleration noise ?

Instrument limit

LISA Symposium, Stanford, 29 June 2010

Mo / Shapal EM

(4 mm gaps,

LPF geometry)

Mo / Sapphire LPF EM

UTN test campaign: free-fall inside LISA / LPF GRS prototypes

LPF FM-replica +

ELM electronics

1-mass torsion

pendulum

(torques)

4-mass torsion pendulum

(direct force sensitivity)

Lightweight TM � test surface forces

Bulk forces (gravitational, magnetic) tested

separately and with LISA PF


• With flight model-replica electronics

• Flight model TM polishing and coating

Torsion pendulum IS testing in Trento Hardware under test: Electrode Housing REPLICA

Torsion pendulum ground testing of LISA Free-fall

Measure stray surfaces forces as

deflections of pendulum angular

Light-weight test mass suspended as

inertial member of a low frequency

torsion pendulum, surrounded by

sensor housing

deflections of pendulum angular

rotation

to within < 100 LISAgoal

<10 LTP goal

Precision coherent measurement of

known disturbances

Multiple degree of freedom torsion pendulum for testing free-fall

Sensitive force measurements on (at least) 2 TM

degrees of freedom

• Can we achieve femto-g free fall in one axis

while actively controlling another TM along

another axis?

• How does control along some axes leak into

forces and force gradients along other axes?

Can we perform pm measurements with a TM • Can we perform pm measurements with a TM

that is moving in all 6 degrees of freedom?

PETER (pendolo roto-traslazionale)

In development at U. Firenze and U. Roma

Tor Vergata

“soft” torsional and translational degrees of

freedom

Single mass torsion pendulum for

LISA ground testing

• 110 gm TM + mirror (hollow Al, Au coated)

• 25 µm, 1 m long W fiber � 2 mHz

resonant frequency, Q ≈3000

Fused silica fiber, Q ≈ 800000 (τE ≈ 2 years)

• Passive magnetic damping of swing mode

(τ 100 s for swing mode)(τ 100 s for swing mode)

• Autocollimator and capacitive readouts

• On demand electrostatic damping

/actuation of swing mode

• Turbo vacuum pump 10-7 mB

• Thermally controlled foam room (50 mK

long term stability)

Sensor OFF

stray stiffness

Sensing

electrostatic

stiffness

Measure force gradient for satellite coupling

2strres p

fa x

mω= + ∆

Total LISA stiffness budget: ~ 1000 nN / m

Measured “stray” stiffness (sensor OFF) ~ 5 nN / m (DC bias? δVRMS ~ 90 mV)

• Unexpected stiffness likely not an issue for LISA (4 mm gaps!)

• LISA Pathfinder will perform full stiffness measurement (including gravity gradients)

Sensor stiffness (modeled): ~ 100 nN / m

LPF sensor bias

Order of magnitude estimate:

[ ]

2

0

0

2 0

imbalance thermal

rate of impacts momentum

x

B

B

F

k Tn L V m

m

mpL V V

k Tβ

≈ ×

≈ − ×

≈ − ≡ −

F⊥

x

Brownian force noise from residual gas damping

4F BS k T β= Viscous gas damping coefficient

F⊥

x

F⊥

x

2

04 4F B B

S k T pL m k Tβ= ≈⊥

||F

More accurate (normal + shear forces,

correlated incoming + reemission :

2

0

5121

8F B

S pL m k Tπ

π

= +

⊥

Enclosure inside sensor amplifies force noise

• Correlations between impulses from repeated impacts of

same molecule � slower averaging out of force noise

• Flow impedance of small channels around TM cause pressure

build-up with TM motion

⊥

||F

Brownian force noise from gas

damping: Numerical simulation• Maxwell-Boltzmann velocity distribution

• inelastic collisions, random cosine-law reemission

• calculation of force and torque spectra from

mean square momentum transfer

z

y x

Equal gaps

Actual sensor: dx = 4 mm, dy =

2.9 mm, dz = 3.5 mm, holes

Factor 13

increase in

Brownian acceleration noise: estimates for LISA

Infinite gap model

increase in

noise power

1/ 41/ 2

1/ 2 2 0

55 fm/s

10 Pa 30a

mpS

−

≈ × ×

Acceleration noise for LISA / LPF

� Need to improve to 10-6 Pa pressure with LISA

Gas damping: experimental results

Differential measurement (with and

without GRS)31

31

43 nm s

49 2 nm s

SIM

TM

EXP

TM

d

dp

d

dp

β

β

=

= ±

34

34

4.6 m s

5.7 0.3 m s

SIM

TM

EXP

TM

d

dp

d

dp

βµ

βµ

∆=

∆= ±

Simulation and experimental results consistent at 15% level

(pressure gauge calibration level)

Noise source: Brownian noise from residual gas

Gas damping

LISA

LPF

Instrument

• Depends on pressure (p1/2)

• 10-5 Pa for LPF (getter pumps)

• 10-6 Pa needed for LISA (vent to space)

• White noise, uncorrelated with any other external variable

• Possibility of identifying noise source (in case of excess) with DT measurement

LISA


Thermal gradient-induced forces

PRD, 76 102003 (2007)

38

3

27 pN/K

rad press RP

RP

ATF T

c

σκ

κ

= ∆ ×

≈ ×

4

18 pN/K

radiom RAD

RAD

TF AP

Tκ

κ

∆= ×

≈ ×1.25

RADκ ≈

0.3 ( =95%)RP

rκ ≈

T∆

radiometric

radiation

pressure

outgassingNumerical

2 ???outgas outgas

TF Q

T

∆∝ Θoutgassing

Numerical

simulations for

“finite size”

calculations

• dF/d∆T ~ 100 pN / K

� need S∆T1/2 < 10 µK / Hz1/2

• outgassing hard to predict � need a measurement

Thermal gradient-induced forces

� Direct measurement of force coupling dFx/d∆T

relevant to LISA force noise

� Much easier analysis of temperature distribution

Preliminary results:

• Verify radiometric model (10%)

309 K

298 K

• Verify radiometric model (10%)

• Outgassing observed (pre-bake)

� Zero pressure data

increase faster than

radiation pressure’s T3

• Measure roughly 100 pN/K at

10-5 Pa / 25 C

LISA pressure

10-5 Pa

LISA goal (100 pN / K)

• Looks OK for LISA

• Experiment to be repeated on LPF

• Will characterize thermal environment on-board

Noise source: Thermal gradient fluctuations

Thermal gradient

LISA

LPFInstrument

• Thermal analysis gives (worst case) 4 mK/Hz1/2 at 1 mHz

� certainly worse at lower frequency!

• Experimentally measured dF/dDT ≈ 100 pN / K (≈ half outgassing)

• LPF – in-flight calibration of dF/dDT (probe of gas pressure – gas damping)

• LPF – in-flight monitoring of noise in T, DT � verify thermal modelling for LISALISA Symposium, Stanford, 29 June 2010

q

(VM)

∆x / 2

δV1 δV2

xx

TOT

ii

TOT

x

C

C

q

Vx

C

C

qF

∆∂

∂≡

∂

∂= ∑ δ

TM charge Q

Stray electrostatic potentials δδδδV

Noise source: interaction between TM charge and stray electrostatic field

×

×

∆

×≈

∆∂

∂=

fN/Hz 7

2

2/1

1/2

2

2/1

f

x

C

C

eS

EFFx

x

T

EFF

F

λ

ω

λ Random charge noise

mixing with DC bias (Dx)

×

×

×≈mHz 1.0 /s300mV 100

fN/Hz 7

• Budget assumes 2000 e/s and 100 mV imbalance (uncompensated)

• compensation to <1 mV demonstrated, drift to order 5 mV in 1 month

×

×≈

∂

∂=

∆

∆

1/2

2/1

7

1/2

2/12/1

V/Hz 100 10fN/Hz 6.1

µx

x

S

e

q

Sx

C

C

qS

T

F

Noisy average “DC” bias (SDx)

mixing with mean charge

• non-stationary as charge drifts (2 106 -- 107 charge in one day)

• 100 mV / Hz1/2 current measurement resolutionLISA Symposium, Stanford, 29 June 2010

Experimental verification of stray potential compensation

δV1Α

δV2Α

δV1Β

δV2Β

V∆+VCOMP

+VCOMP

-VCOMP

-VCOMP

• Measurement (and FEE resolution) allow sub-mV compensation of Dx

• Modulation technique does not measure exact combination of DC bias relevant to TM

charging

o Observed to cause error of order 10 mV � possible need to verify

compensation with controlled TM charge variation

o Observed drifts small (order several mV in 1 month) � infrequent

compensation needed

δV2Β-VCOMP


LISA limit2 fN/Hz1/2

QTM = 107 e

Conservative

experimental

upper limit

Experimental upper limits on stray potential fluctuations

Two measurement

techniques:

(red) excess force noise with TM charged (4 108 e or 2 V)

(blue) coherent force detection with modulated TM potential

• Upper limit roughly 100 mV/Hz1/2 at 1 mHz (OK for LISA!)

• Consistent with a non-detection at 0.1 mHz, upper limit 350 mV/Hz1/2


Noise source: electrostatic noise

Fluctuations in ∆∆∆∆x

LISA

LPFInstrument

• Stray voltage fluctuations: upper limit 100 µV/Hz1/2 at 0.1 mHz

• random charge: assume balancing to 10 mV, effective charge rate 300 e/s

Random charging


Bx

mFx

rr

∂

∂= .

0

0µ

χ BVmm

rrr

+=

• Fluctuations in magnetic field:

• Fluctuations in gradient:

2/10i SVB

m

+χ

x

BS

V iBi ∂

∂ 02/1

0µ

χ

Force noise source: interaction of magnetic moment and B field

• Down conversion of field / gradient interaction:

(χ frequency dependent, Faraday and skin effect)

2/1

/

0

00 xB

ii i

SVB

m ∂∂

+

µ

χ

( ) ( ) ( ) ( ) ( )x

BB

i

is

x

tBtVBt ii

ii

∂

−∂

+−≈

∂

∂ ''

'1

'

0

3

0

ωωω

τω

τω

µµ

χ

( ) Hz 45024

22

1

0

21

≈

≈≈

−

− σµππτ

sfON

Onset (“superconductor limit”)

Shielding cutoff from

surrounding metal:kHz 1≈CUTfLISA Symposium, Stanford, 29 June 2010

LISA Pathfinder Test MassAu/Pt alloy for low susceptibility, low residual moment

TM Requirements: -5

8 2

0

10

10 Amm

χ−

<

<r

Field requirements:

T/m 5

T 1

0

0

<∂

<

B

B

r

r

µ

µ

Measured EM TM properties

Key noise sources:

• Fluctuating induced moment

(interplanetary field) interacting with

stable (spacecraft) field gradient

• AC Eddy current downconversion

kHz 1 tonT/Hz

T/m 5

1/2

0

<

<∂

∂

RMSACB

x

B

r

µ

5

2

0

102

nAm 20

−×≈

≈

χ

mr


Noise source: magnetic noise

AC Magnetic

(down conversion)

LISA

LPFInstrument

Interplanetary B fluctuations in band

• down-conversion of AC magnetic field (white)

� magnetic field of 0.2 nT RMS up to 1 kHz cutoff

� based on spacecraft AC magnetic field measurement

• assume field gradient of 12 mT/m, measured interplanetary fluctuations of 55 nT/Hz1/2

• in-flight measurement of static moment + susceptibility planned B fluctuations

(down conversion)


Torsion pendulum upper limits on GRS force noise:

Mo / sapphire sensor + EM FEE, 1TM pendulum

Angular deflection measurement with two readouts (GRS and autocollimator)

� distinguish true torque noise floor from background readout noise

{ },AC SN N NS S= ℜ

Recent upgrade of

torsion fiber from

Tungsten (Q =3000)

to Fused Silica (Q =

700000)

Upper limits on GRS force noise:

conversion from torque � force (acceleration)

• rule out large class of TM surface disturbances at level of 50 fm/s2/Hz1/2 at 1 mHz

• within factor 2 of LPF goal

• achieving similar acceleration noise levels with LISA would allow observation

of galactic binaries

Detecting force noise excess under different conditions:

Measurement resolutionLISA Pathfinder requirement

Noise difference between two groups of nominally identical noise runs

(roughly 30 25000 s cuts each) is zero to within the LPF noise goal

� can detect force noise excess of order LPF resolution

Examples: w / wo TM charge, w/wo sensor turned on, w/wo sensorLISA Symposium, Stanford, 29 June 2010

This

docum

en

t is

the p

rop

ert

y o

f A

str

ium

. It s

hall

not

be c

om

mun

icate

d t

o t

hird p

art

ies w

ithou

t prior

written a

gre

em

en

t. I

ts c

onte

nt

shall

not

be d

isclo

sed.

Astrium Satellites

This

docum

en

t is

the p

rop

ert

y o

f A

str

ium

. It s

hall

not

be c

om

mun

icate

d t

o t

hird p

art

ies w

ithou

t prior

written a

gre

em

en

t. I

ts

LTP Progress Meeting #12, ASD, 23/24 April 2008 p80

81

ESA LTP

Collaboration

Trento LISA Team

Stefano Vitale (LPF PI)

Federica Antonucci, Matteo Benedetti, Daniele

Bortoluzzi, Antonella Cavalleri, Rita Dolesi,

Mauro Hueller, Daniele Nicolodi, Antonio

Perreca, Bill Weber

Documents

Sesto Dolomiti 30 July 2010 - ego-gw.it · Sesto Dolomiti 30 July 2010. Laser InterferometerSpaceAntenna NASA / ESA Mission, tobelaunchedin 2020 •2 semi-independent 5 10 6km Michelson