LTP The LISA Technology Package - Institut national de...

XXXVIIIth RENCONTRES DE MORIOND

MORIOND WORKSHOP ON 22-29 marzo 2003 Gravitational Waves and Experimental Gravity

LTPThe LISA Technology Package:

goals and current baseline implementation

Rita DolesiDipartimento di Fisica, Università di Trento

INFN PD -Gruppo Collegato di Trento



Contents

• LISA’s sensitivity goal and the test mass “free fall” requirement

• The “drag free” control loop strategy

• LISA Technology Package goal

• LTP architecture and the gravitational sensor current baseline

• Testing on ground with a torsion pendulum



Sensitivity GOAL

GW at 0.1 mHz – 0.1 Hz

Sensitivity

4 10-21 Hz-1/2 @ 1 mHz

5 106 km

Spacecrafts

Test Masses

Telescopes

LISA

Importance of free-fall TM

for LISA sensitivity ?



separation between end-mirrors parasitic forces GW strain signal

2

2

2

2

F dd xdt

h Ldtm

+∆=δEquation of motion

“free fall” quality requirement LISA

LISA sensitivity at low freq

( )12

215

a 2

f m 1S f 3 10 13 mHz s Hz

− ≤ × +

0.1 mHz f 0.1 Hz≤ ≤

Level of parasitic forces acting on TM2 2

2

1 hL

x LFm

∆ω + ωω

=δ2 22

1 hL

x LFm

∆ω + ωω

=δ2 22

1 hL

x LFm

∆ω + ωω

=δ



Along the sensitive interferometer axis:

“drag free” control loop

The residual acceleration of the test mass will be

noisex

2parasiticmω

fb2Mω

mass S / CX −

extF

Requirements for max PSD of Fext, fparasitic/m, xnoise and for ωint and ωfb

parasiticf

mf

MFxa parasitic

fbCS

extnoiseparasiticresidual +

+≈ 2

/

2

ωω

S/C displacement wrt to test mass



10-15

10-14

10-13

10-12

FNSHz

Testing quality of free fall

LISA(ESA-NASA)

Torsion pendulumTrento

SMART-2

LTP (ESA) DRS (NASA)



2-implement the drag free control loopalong the alignment direction

S/C is controlled on TM1TM2 follows the spacecraft

subject to low frequency suspension loop

3- diagnostic Interferometerreads TMs mutual distance

measure the residual differentialacceleration of the 2 test masses

5 106 km

LTP basic idea:1- squeezing 1 LISA’s arm to 35 cm

and place it in a S/C

Control logic is necessarily different than in LISA!xBxA

TM1A TM1B

xBxA

TM1A TM1B

•2 drag-free axes xa, xb

•S/C follows TM1a along xa and TM2b along xb



DF LF

δxlaser interferometer readout

TM1 TM2

ares in LISA

Testing LISA Drag-Free with LTP

laser noise and baseline distortion<<

parasitic stiffnesses

low frequency suspension provides

sensitive to difference in stray forces BUT the lesser known (more dangerous!)

are likely to be uncorrelated



LTP goal

1 mHz f 30 mHz≤ ≤

In compliance with the following constraints:The sensor design to be tested has to be nominally identical to that to be used on

board LISA

Relaxation is allowed just with regard to stray effects due to the presence of the extra actuation and to the possibly more noise hostile environment

( )12

215

a 2

f m 1S f 3 10 13 mHz s Hz

− ≤ × +

0.1 mHz f 0.1 Hz≤ ≤

LISA goal

test of free-fall within 1 order of magnitude from LISA Goal

corresponds to LTP laser output measuring the residual differential acceleration

21 2 14n 2

f m 1S 4.5 10 13 mHz s Hz

− ≤ × +

Testing LISA Drag-Free with LTP



From other signals !isolate different contributions

• TM1 position position sensor dominated by the residual jitter of S/C due to external forces

we exctract (thrusters!!)

• the interferomer measures also the position of TM1 wrt position sensor (along x)

we calibrate the position sensor noise xn!

• Measurement of the stray stiffness of coupling between the test mass and the spacecraft.

2S C fbF Mω

2pω

Alternative control scheme: TM2 servoed to null the interferometry signal (rather than its position sensor )

tune to zero isolate it is very sensitive probe!



Test of the Charge Management proceduresTest of the Caging Mechanism: releasing and acquisition of the TMs on flight

Test of the low frequency suspension.......

Modulated electrostatic, magnetic and thermal disturbances can be purposely induced•measure of the charge onto the test masses (interaction with cosmic rays)

•characterization of force “feedtrough” of these disturbances

...diagnostic sensors•temperature transducer

•magnetometers

...moreover

Testing of other crucial aspects



LTP +DRSLISA Test Packages on board SMART-2

Remarkable added value•Coordinated tests

•Mutual rescue against failure of one sensor or laser•……



Noise budget for LISA and LTPwhich includes estimates of all known noise sources has been implemented with the aim of assessing the requirements on the performance and determining the feasibility of achieving

the noise specs for LTP and LISA

Main sources

work in progress

LISA



Gravitational Balancing

DC differential force limit:

Force gradient limit:

2gravgrav nm/s .

mF

a 75<=∆

2gravgrav /s

xF

m82 101 −<

∂∂

≡ω

• Required meshing accuracy for 2 kg electronics box at ~70 cm of 3 cm, positioning accuracy at 2 mm, 1° level• Optical bench placement at 100 micron, 5mrad degree level

Environmental Requirementsthat impact onto S/C and LTP design

• Magnetic

• Thermal

BBM ∇⋅

µχ+

00

Radiation pressure radiometric effects

1/2/B nT/m/Hz S 7021 <∇

T/m B µ<∇ 5

T B µ<10

1/2/T /HzS 421 10−<

1/2/T /HzS 521 10−

∆ <



Noisy Charging

Thermal stability related effects

Radiometer

Radiation Pressure

Outgassing

Sources inside the readout circuits

Noise sources in the gravitational sensor coreInteraction of the TM with the electrode housing

radS P Tam 4 T

∆=

3pr

Sa 10 T Tm c

σ≈ ∆

( )( )( )

hole channel holeoutoutgas sin g

channel hole channel hole

C 4C CP PAa TM T T 2C C 6C C

+−Θ= ∆+ +

L

L

n2L

Cp

Cp

VAC

100 kHzCfb

VACT2

VM

Csens1

Csens2

+ δVAC

+ δVACT

+ δVACT

+ δL

− δL

+ δCfb

vthL

L

n2L

Cp

Cp

Cp

Cp

VAC

100 kHzCfb

VACT2

VM

Csens1

Csens2

VMVM

Csens1

Csens2

+ δVAC

+ δVACT

+ δVACT

+ δL

− δL

+ δCfb

vthvth

( ) 1 21 216 V

12o eff

S m 15 mm 0.1mHz3.7 10m 5 mV 16 s fs Hz

−−

ω σ λ = × l

Stray forces and accelerations produced by position noise coupling

through the parasitic stiffness



• Cross talking, optical alignment, demand high sensitivity and relatively low stiffness also on the non-measurement axes

• yn kept low (1.8 nm/Hz1/2) to be used as second drag-free axis if one sensor is lost on LISA

• φn, θn, ψn all kept to 200 nrad/Hz1/2 (~ few nm/ Hz1/2 translational noise for ~4 cm mass)

• “off-axis” stiffnesses ω2z, ω2

φ, … held to ~ 10-6 /s2 on LISA, 4 10-6 /s2 on LTP

For the remaining degrees of freedomnot along the interferometer axis



Test-masses(gravitational sensor)

Interferometer

The LTP architecture and main subsystem



LTP Interferometer layout.

The laser metrology channels:Position of TM1 relative to TM2 along xPosition along x of TM1 relative to the OBPosition along x of TM2 relative to the OB Attitude of TM1and TM2 along φ and ηby means of quadrant photodiode

(Max Planck Inst.,Hannover, K.Danzmann, G. Heinzel)Heterodyne Mach-Zehnder interferometry



Carlo Gavazzi Space Ref. S1-038 ITT AO/1-3898/01/NL/PB

Inertial Sensor Definition for LISA

A proposal in response to ITT AO/1-3898/01/NL/PB

Vol. 1 Technical Proposal

August 2001

Carlo Gavazzi Space

ONERA

The University of Birmingham

The University of Trento

Gravitational sensordefinition for LISA

Carlo Gavazzi Space (prime)

Uni of Birmingham (control algorithm)

ONERA (FEE)

RAL (Caging Mechanism and supportfor gravitational sesnor definition)

Imperial College (Charge Manag.Syst.)

Uni of TRENTOtechnical coordination activities

responsibility for the gravitational sensor definition

will perform on ground GS testingby means of a torsion pendulum

based facility



The gravitational sensor

• GRAVITATIONAL SENSOR CORE – A free floating cubic, 27% Pt-73% Gold TM,2 kg– 6-DOF capacitive motion sensor– An electric field based TM actuation system

• CHARGE MANAGEMENT SYSTEM (ICL):– TM charge management control– UV light, photo electron extraction based,

• CAGING MECHANISM (RAL)– cages the mass via the action of a plunger that

pushes it against end-stops – prevents both translation and rotation– allows multiple operation including re-caging– releases the TM form the centre of the housing

• VACUUM ENCLOSURE



Capacitive position sensor electrodes configuration

• symmetric all gap-sensing• 6 sensing /actuation electrode pairs

• combinations of 6 2-channel pairs give 3 translational and 3 rotational TMdisplacement measurements

•large sensing gaps, 4 mm along x (to reduce unmodelled surface forces...)• 46 mm test mass (~2 kg)

Gap dx

x

y

φ

s

1

2

Gap dy

din = 4 mmΣCin = 4.40 pF

y

z

x

dy = 2.9 mmCy = .83 pF

dx = 4 mmCx = 1.15 pF

dz = 3.5 mmCz = .61 pF

Ctot = 25.6 pFVM0 ~ .6 VVINJ ~ 3.5 V



VACT2

Csens1

Csens2

+ δVACT

+ δVACT

vth

PSD

L

L

x

FEEE

VAC

100 kHz+ δVAC

z

ToComputer

VACT1

VACT2

Csens1

Csens2

+ δVACT

+ δVACT

vth

PSD

L

L

x

FEEE

VAC

100 kHz+ δVAC

z

ToComputer

VACT1

Position sensor with one channel connected to its readout circuit



Baseline for the implementation of the electrode housingMolybdenum –SHAPAL (sapphire)

high thermal conductance composite structure

•margin against thermal gradients •flexibility supporting the caging loads more securely and simply

geometrical tolerances of the order of 10 micron



10-4

10-3

10-2

10-1

100

101

10-8

10-7

10-6

10-5

10-4

10-3

SV1/

2 (V/H

z1/2 )

Frequency (Hz)

30000 s econd run230000 s econd runThermal Limit

10-1

100

101

102

103

Sx1/

2 (nm

/Hz1/

2 )

Top and Bottom

Lids

Central frame

Test Mass

position sensor noise measurements

Compliant with the LTP requirement!!!

Prototype already existing in Trento



Torsion Pendulum Ground Testing of Inertial Sensor• Test mass suspended as inertial member of a low frequency

torsion pendulum, surrounded by sensor housing

mHz 221

0 ≈Γ=I

fπ

• 25 µm W fiber, Q~1000• ~1 m long

• 100 gm test mass • 2 cm electrode separation

(“arm length”)

Twist angle

10-13 m/s2/sqrt(Hz) @ 1mHz

•Measure stray forces

thermal noise limit

•Measure coupling to sensor as deflections of pendulum rotation, with optical angular

readout

at level relevant for LISA

•Measure TM charge,electrode DC voltage imbalance,spurious TM magnetic moment...



3x10-13 m/s2/sqrt(Hz) @ 1mHz

Stray Force Upper Limit: Pendulum Angular Noise



Daniele BortoluzziPaolo Bosetti

Ludovico CarboneAntonella Cavalleri

Ilaria CristofoliniMauro Da Lio

Rita Dolesi

GiorgioFontanaVigilioFontanari

C.D. HoyleMauro HuellerStefano Vitale

J.W. Weber

Trento Team

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