LTP: The LISA Technology Package aboard LISA Pathfinder

Gerhard Heinzel,AEI Hannover

第 6回 DECIGOワークショップ2008年 4月 16日国立天文台，三鷹

using material from Paul McNamara, Stefano Vitale and EADS Astrium

Purpose of LTP

Test and verify the inertial sensor for LISA

Verify operation of the proof mass as reference mirror in a pm interferometer

Test drag-free operation and micro-Newton thrusters

side effect: Learn a lot about spacecraft design

Science Objectives

Terminology LISA Pathfinder (LPF) is the mission, managed by ESA

LPF used two have two scientific payloads: The European LISA Technology Package LTP The US Disturbance Reduction System DRS

DRS has been reduced to thrusters and a computer

The payload LTP consists of:

two inertial sensors with one test mass each the interferometer with laser, phasemeter etc.

The LPF mission includes:

micro-Newton thrusters drag-free control all standard spacecraft things

LPF/LTP participants

France:Laser modulator Germany: PI, LTP Architect (Astrium), LaserItaly: PI, Inertial Sensor (ISS), Caging MechanismNetherlands: ISS SCOESpain: Data Diagnostics System, Data Management UnitSwitzerland:ISS Front End ElectronicsUnited Kingdom: Optical Bench, Phase-meter, Charge Management

LTP team

LTP workshop in Trento (2005)

L

showing maybe ½ of people working on LTP

Orbit Lagrange-Point L1, about

1.5 million km from Earth, limits downlink data rate

constant orientation to Earth and Sun, stable thermal environment

Separation from propulsion module after several months cruising phase

stable without correction for the 3...6 month mission

Launch planned 2010 Baseline is new European launcher VEGA from Kourou

Alternative: using Rockot (former SS19 ICBM)

from Plesetsk, Russia (latitude 63°)

�

Max lift-off weight of S/C: 1910 kg

Operation

8 hours per day contact via ESA 15m/35m ground stations

science data downlink: average 10..20 kbit/s

program for 3 days in advance is uploaded and ready to run

interaction mainly via parameters of procedures

quasi real-time operation only in commissioning or emergency

Spacecraft

Test massx

Displacement sensor

Thrusters

High gain force feedback

Keeping the spacecraft with the proof-mass

Drag-free mode both test masses are

optically sensed

orientation is controlled to optimize interferometer contrast

TM nominal position is unstable (“negative stiffness”)

s

TM1 is drag-free reference, spacecraft follows TM1 with 65 mHz loop bandwidth

TM2 has suspension controller (electrostatic) with 3mHz bandwidth

LPF mission goal

LISA requires 3e-15 m/s2/sqrt(Hz) at 0.1mHz

LTP requires 3e-14 at 1mHz, but aims for LISA-like levels

LTP carries many diagnostic items to analyze and correlate any noise that occurs

Comparison with other missions

Key components

Inertial sensor: test mass: 2 kg of Au-Pt alloy, cubic form

capacitive sensor with front-end electronics, also works as actuator

vacuum enclosure with optical window

charge management system: fiber-coupled UV light

Drag-free system: micro-Newton thrusters

Software

mass balancing

Interferometer

Test mass 46mm cube of Gold-Platinum, 73% Au:27% Pt Mass = 1.96kg high density, low

magnetic suscept., but: hardness and magnetic

properties differ fromsmall samples tolarge piece!

Capacitive Sensor/Actuator large gaps (2...4 mm)

l

made from Mo/Al2O

3/Au

AC excitation (100 kHz)

A

front-end electronics challenging for noise and cabling redundancy

caging mechanism

needs to hold test mass at launch,Force = 3000N

release on orbit without “sticking”,velocity < 5µm/s !

TM gold coating must not be damaged

limited choice of materials

difficulty was severely underestimated

separation into “caging” (hydraulic) and “release” (piezos)

“

qualification tests ongoing

Charge management Test mass charges due to

cosmic radiation at ca. 50...100 e/s

Charge is measured either continuously or periodically via electrodes

discharge with UV light (Hg) shining from fibers on either test mass or housing

but: Au work function depends on contamination.254nm = 4.88eV, Au

contam. has up to 5.1eV !

Optical Window

needed for optical access of interferometer beams

like a flange of vacuum tank

optical pathlength in transmission 12mm/24mm

“athermal” glass S-PHM52 (Ohara) minimizes pathlength error dn/dT+(n-1)a

extensive testing at AEI for radiation hardness, pressure-dependent pathlength error and actual performance was successful.

Optical window electrostaticsan isolating window may accumulate charges and disturb the test mass

Solution: apply conductive ITO (In2O

3/SnO

2) layer to

optical window

micro-Newton thrusters

three systems under development:

Indium needle Cesium slit “colloidal” (organic ionic

liquid)

l

range about 100 µN, stepsize 0.1 µN, operated at a bias.

LTP will test two or three of them.

noise and frequency response (delay) are hard to predict.

issues are reliability and lifetime.

Interferometer

originally intended as a passive diagnostic tool with no feedback to test masses

Now a central part of the experiment, controlling the test masses

Using test masses as end mirrors has many complications and is a crucial experiment for LISA

Ifo requirements

Pathlength noise 9pm/sqrt(Hz) with freq. dependence

sufficient for LISA local readout

prototype fully meets requirement

Ifo principle

audio-frequency heterodyne Mach-Zehnder

independent of operating point

wide dynamic range (many fringes)

w

no lock acquisition needed, immediately ready after power-on.

Reference subtraction

Each photodiode measures Pathlength difference starting from first BS

External contributions must be subtracted via stable reference interferometer

subtraction is imperfect, hence phase of Ref. Ifo (“OPD”) must be stabilized

Interferometer noise

The usual suspects are below 1 pm each:

laser frequency is stabilized via auxiliary interferometer with unequal pathlength.

laser power is stabilized both atmHz for radiation pressure and at kHz for the beatnote phase measurement.

The OPD is stabilized via a Piezo in one of the arms.

Phasemeter electronic / digitization noise is below 1 pm.

Angle measurements

uses Differential Wavefront Sensing (DWS)

W

large amplification TM angle to audio phase difference (about 5000)

d

one quadrant diode is enough, no reference needed

immune to several noise sources

excellent sensitivity

Laser

Nd:YAG NPRO

35 mW output power

frequency and power actuators

flight heritage onTerra-SAR X

also useable as seed laser for high-power fiber amp (LISA)

a

Laser Modulator

contains beamsplitter,2 AOMs and Piezo OPD actuator

fiber coupled output:2 x 5mW

frequencies generated by 2 crystals in PLL arrangement

AOM also used for power control

challenge: spectral purity of output

Optical bench 20*20cm Zerodur baseplate hydroxycatalysis bonding about 30 components 4 interferometers challenges:

vertical alignment fiber launchers absolute alignment w.r.t. test mass

housing

4 interferometers TM1 position and

orientation w.r.t. optical bench

TM2 position w.r.t. TM1, TM2/TM1 orientation w.r.t. optical bench

Reference phase Frequency noise via

unequal pathlengths

coupling of unwanted d.o.f. A perfect interferometer would

sense only x An offset d will couple testmass

(or spacecraft) rotation into x An angle phi will couple y/z

motion into x. The coupling depends on the

precise interferometer layout and the beam parameters

Optical bench alignment bonding accuracy is roughly

10um or 50urad per component

extensive Monte-Carlo simulation of many possible misalignment combinations predict length error of about 10pm/sqrt(Hz)

1

mitigation: fitting coupling coefficient using natural fluctuations and subtraction of predicted contribution

subtraction has been experimentally verified

data flow

On-board computer (OBC) does not deliver its clock

LTP runs on nominally same frequency but not synchronized

drag-free requires continuous intimate interaction between them

non-synchronuous operation creates many problems

Phasemeter

One ADC in each channel

immediate single-bin DFT in FPGA hardware

AEI breadboard: 18bit/800kHz, 20 channels

FM: 16bit/100kHz

2*16 channels, fully redundant

breadboard noise <0.1µrad

interferometer postprocessing Phasemeter delivers DC, real and imaginary at 100Hz

longitudinal and alignment signals are derived by simple operation like arctan()

�

raw longitudinal signals are periodic with the wavelength

phasetracking algorithm removes jumps by 2π phasetracking needs to be reset at known test mass position

in order to provide absolute measurements

nominal signal handling is straightforward,80% of the effort goes into proper handling ofnon-nominal situations(loss of one quadrant, temporary loss of contrast etc.)

�

interferometer data

position and orientation of both test masses

summary status information

for debugging, many other channels exist:

contrast, power levels, intermediate results,...

several “menus” of data packets depending on application

Data analysis Software must be verified and delivered to ESA before

launch, since results are used to upload new parameters

Supplied by PI Institutes (Hannover and Trento)

S

Based on MATLAB with extensive own programming

Must have GUI interface for non-MATLAB experts

The package “LTPDA” (LTP Data Analysis) contains Time series tools (segmentation filtering, coloured noise generation, ...)

T

Frequency domain tools (spectra, cross-spectra, time-frequency analysis ...)

F

Arithmetic functions, data handling tools and auxiliary functions

Access to MATLAB internal functions via wrappers

LTPDA is also useful for other work,is freeware open-source freeware, and we welcome all new users:

http://www.lisa.aei-hannover.de/ltpda/index.html

Analysis objects A useful result is not a graph or

a file full of ASCII data

Each result must “know” how it was produced with all details

Each result can be reproduced by any user with access to the raw data, also with modified processing

All intermediate steps and final results are stored as MATLAB structures called “Analysis objects” (AO)

o

Each processing step appends its name, version and parameters to the history of the resulting AO

Mock data challenge

Test of the data analysis pipeline:

One team generates downlink data with a spectrum kept secret

Second team uses LTPDA to recover the spectrum

First round (simple model) successfully concluded

Lessons learned (my personal view)

�

Drag-free spacecraft require unusual architectures, the textbook-style clear separation between spacecraft/payload does not work well.

Asynchronous clocks are a bad idea.

Industry does not know or guess what scientists need, they work strictly on requirement documents.

You'll need more data channels for debugging than you might think now.

Start early to think about on-board data flow, software and computer tasks. Handling of errors and non-nominal situations is a major part of the software.

Resources on a spacecraft may be unbelievably limited (e.g. 20 MHz CPU with 256 kByte array for user data)!

Requirement documents must be over-complete. Even “self-evident” things must be spelled out in detail. It is very hard to add things later.

Wishing you good luck with DPF,Wishing you good luck with DPF,We are looking forward to an exciting time!We are looking forward to an exciting time!

Thank you for your attention!Thank you for your attention!