A. LoboSagamihara, 13 November 20081 LTP diagnostics Alberto Lobo ICE-CSIC & IEEC

A. Lobo Sagamihara, 13 November 2008 1

LTP diagnosticsLTP diagnostics

Alberto LoboICE-CSIC & IEEC


Why is diagnostics analysisneeded in the LTP?

for . This is so very demanding a previousLPF mission is planned, with a relaxed sensitivity requirement:

LISA’s top level sensitivity requirement is:

1/ 24

1/ 2 15 -1/22

0.1mHz m( ) 3 10 1 1 Hz

8mHz sa

fS f

f

0.1 mHz 0.1 Hzf

2

1/ 2 14 -1/22

( ) 3 10 1 Hz ,3 mHza

f mS f

s

1 mHz 30 mHzf

LISALISA::

LPFLPF::

Even if LPF works to top perfection, a fundamental queston remains:

How do we make it to LISA’s sensitivity?


DDS: Data Management& Diagnostics Subsystem

Diagnostics items:

• Purpose:– Noise split up

• Sensors for:– Temperature– Magnetic fields– Charged particles

• Calibration:– Heaters– Induction coils

DMU:

• Purpose:– LTP computer

• Hardware:

– Power Distribution Unit (PDU)– Data Acquisition Unit (DAU)– Data Processing Unit (DPU)

• Software:

– Boot SW– Application SW:

Diagnostics items Phase-meter Interfaces


Diagnostics items use

The methodology:

1. Split up total noise readout into parts –e.g., thermal, magnetic,...2. Identify origin of excess noise of each kind3. Orient future research in appropriate direction for improvement

1. Sensitive diagnostics hardware needs to be designed and built

2. a) Suitable places for monitoring must be identified. b) Algorithms for information extraction need to be set up.

3. Creative research activity thereafter...


Noise reduction philosophy

Problem: to assess the contribution of a given perturbation to the total noise force.

Approach: 1) Apply controlled perturbation to the system

2) Measure “feed-through” coefficient between force and perturbation:

int( )f

F

3) Measure actual with suitable sensors

4) Estimate contribution of by linear interpolation:

int ( ) ( )f F

6) Subtract out from total detected noise:

red int int ( )f f f

5) Check against a physical model of perturbations


Location NTCs Comments Req. No

Optical Bench 4 Near base corners 3.1

Optical Windows 4 2 per OW 3.2

Inertial Sensors 82 on each of the outer x-faces of the IS EH

3.3

LCA mounting struts 6 1 per strut, near centre 3.4

Total 22 --- ---

Sensor sensitivity requirement: 10-5 K/Hz , 1 mHz < f < 30 mHz

Global LTP stability requirement: 10-4 K/Hz , 1 mHz < f < 30 mHz

Thermal and sensor requirements


Thermal damper

1/ 2 1/ 2inside boundary, ,S r H r S

Transfer function

DMU test campaign


DMU test campaign

DMU EQM


DMU test campaign

Insulating anechoic chamber for the test

Open thermal damper,wiring and DMU


DMU test campaign

S/H A/DIntegrator

N M Z-1 2

1/2

+

-

Ax(t)+

n(t)SN(f)

+m(t)

y (t)PSDy(f)

PSD1(f) PSD3(f) PSDdiff(f)

Analog processing Digital processing

PSDint(f)

nA/D(t)SNA/D(f)

+

+

FEE Readout electronics PC data acquisition program

FEE Analog PCB FEE A/D PCB


DMU test campaign

Example run0.5 K/s

dT

dt


DMU test campaign

0.5 K/sdT

dt


DMU test campaign

0.5 K/sdT

dt


NTCs magnetic properties

In IEEC we eventually discovered the NTCs, used both astemeperature sensors and heaters in the EH, have magneticproperties:

Nickel parts areinside the device

Damaged sample!



First magnetisation curves

Damaged sample!



Summary of magnetic moments of the screened NTCs (Am2):



Assessment of magnetic noise generated by NTCs:

background

1/ 2 1/ 2

0Bx

F x

VS B S B B

B B

, background

1/ 2 1/ 2

0B xx

F BS S B BV

BB

These two terms add, and are due to the quadratic couplingof the magnetic field due to TM susceptibility. Independentcalculations done at IEEC and ESA reach similar conclusionsthat the noise is not very significant for the LTP budget,



Excerpt from S2-EST-TN-2026 (2-Oct-2008):


Thermal sensing summary

• Fully compliant with requirements

• Thermal gradients slightly distort performance at low frequency

• Can be improved by dithering the bridge voltage with a saw-tooth signal --not applicable to LPF, but useful for LISA.

• Magnetic properties of NTCs: not an issue of major concern, though some precautions are recommended.

• Research ongoing for improvements for LISA, encouraging first results


Magnetic disturbances in the LTP

Main problem is magnetic noise. This is due to various causes:

• Random fluctuations of magnetic field and its gradient• DC values of magnetic field and its gradient• Remnant magnetic moment of TM and its fluctuations• Residual high frequency magnetic fields

Test masses are a AuPt alloy

70% Au + 30% Pt

of low susceptibility

510

and low remnant magnetic moment:

8 20 10 A mm

a = 46 mmm = 1,96 kg


If a magnetic field B acts on a small volume d3x with remnant magnetisationM, and susceptibility , the force on this small volume is:

30 02

d

d x

FM B B M B B

Total force requires integration over TM volume, or:

00

V

BF Bm

Quantification of magnetic effects

Most salient feature is non-linearity of force dependence on B.


Magnitude Value

DC magnetic field T

DC magnetic field gradient T/m

Magnetic field fluctuation rms PSD 650 nT/Hz

Magnetic field gradient rms PSD 250 (nT/m) /Hz

Magnetic susceptibility 10-5

Remnant magnetic moment 10-8 Am2

Summary of magnetic requirements


Magnetic sensor requirements

Req 2: A minimum 4 magnetometers.Req 2-1: Resolution of 10 nT/sqrt(Hz) within MBW.Req 2-2: Two magnetometers located along x-axis, each as close as possible to the centre of one of the TM, not farther than 120 mm.Req 2-3: The other two may be offset from the x-axis by an amount not larger than 120 mm (TBC), their x coordinate should fall between the IS's at distances TBC.Req 2-4: Operation of magnetometers compatible with full science performance.Req 2-5: Final exact choice of magnetometer locations depends on final configuration of magnetic sources. Limited adjustment of magnetometer positions to within +/- 10 cm along x, y and z must be allowed until system CDR.


Magnetometers type:

• 4 Flux-gate, 3-axis magnetometers

Model is TFM100G2 fromBillingsley Magnetics.

Magnetometer layout

Areas for magnetometeraccommodation


Magnetometers’ accommodation


Magnetic field reconstruction

• Exact reconstruction not possible with 4 magnetometers and around 50 dipole sources (+solar panel)

• Tentative approaches attempted so far:

• Linear interpolation• Weighted interpolation –various schemes• Statistical simulation (“equivalent sources”)

• Best possible: Multipole field structure estimation:

• Only feasible up to quadrupole approximation, though, given only four magnetometers in LCA.


Multipole reconstruction theory

B x x

In vacuum,

A multipole expansion of B follows that corresponding to (x):

,

, , 1lm lml m

a r Y r x n x n n

The coefficients alm(r) depend on the magnetisation M(x). In anobvious notation, structure is:

)

,

lm

l m

(B x xB



Evaluation of multipole terms is based on some assumptions:

1. Magnetisation is due to magnetic dipoles only

2. Such dipoles are outside the LCA.

Somewhat legthy calculations lead to:

with

) 0

4 lll

mm

lmM r Y

(B x n

*

31

4

2 1lm

lm l

YM d x

l r

n

M x



Idea is now to fit measured field values to a limited multipoleexpansion model. Arithmetic sets such limit to quadrupole:

)

1model

lml

l m l

(B xB xL

Fit criterion is to minimise squared error:

4

2measur

2l

1m eed ods s

slmM

B xB x

2

0lmM



Arithmetics of reconstruction algorithm:

Data channels: 12

Mlm dipole: 3

Mlm quadrupole: 5

Mlm octupole: 7

3+5 = 8 < 12 => some redundancy, OKthese 3 are uniform field components

3+5+7 = 15, 3 unknowns too many!

Summing up:

• Full multipole structure up to quadrupole level.

• This is poor, only first order polynomial approximation

• Errors large --easily 300%, and rather unpredictable

• Weighting procedure seems better, but also unpredictable


Multipole reconstruction theoryS

imu

lati

on



Bo

tto

mli

ne

(qu

alit

ativ

e)


Ways to improve magnetic diagnostics accuracy:

• Fluxgates are:• Only 4• Far from TMs• Large size –long sensor heads, ~2 cm

We have recently started preliminary activites at IEECto assess feasibility of using AMR’s as an alternative tofluxgates. This kind of research is intended for LISA.


Summary:


Magnetometers tests

-metal enclosure

Billingsley TFM


Magnetometers tests

Billingsley TFM

LTP req.


Magnetometers comparative


SQUID test of AMR device


Magnetic diagnostics summary

• Fluxgate magnetometers are extremely sensitive• Sensor core is large => space resolution limits• Sensor core is permalloy => distance to TM constraints• Box is large and somewhat heavy => few sampling positions

• AMRs indicate good sensitivity• They are very tiny• Weakly magnetic –due to small mass• More thorough investigation needed –underway at IEEC


Ionising particles will hit the LTP, causing spurious signals in the IS.

These are mostly protons (~90%), but there are also He ions (~8%)and heavier nuclei (~2%).

Charging rates vary depending on whether

• Galactic Cosmic Rays (GCR), or• Solar Energetic Particles (SEP)

hit the detector, as they present different energy spectra.This has been shown by extensive simulation work at ICL.

Therefore a Radiation Monitor should provide the ability to distinguishGCR from SEP events.

This means RM needs to determine energies of detected particles.

Use of RM is to obtain frequent determinations of charging rates, andcorrelate them with IS charge management system.

Radiation Monitor


Radiation Monitor

ICL simulations,based on GEANT-4.

(Peter Wass andHenrique Araujo)


Radiation Monitor

It is a particle counter with somespecific capabilities:

• It counts particle hits

• Retrieves spectral information (coincident counts)

• Can (statistically) tell GCR from SEP events

• Electronics is space qualified


RM accommoation in S/C

Sun

Earth


In place for test at PSI, Nov-2005


In place for test at PSI, Nov-2005


Radiation Monitor data

Radiation Monitor data are formatted in a histogram-like form.A histogram is generated and sent (to OBC) every 614.4 sec.


Summary

Thermal diagnostics:• Sensors fully in place• Heaters: in place, and integrating in EMP• Improved sensitivity towards LISA in progress

Magnetic diagnostics:• Sensors fully in place, sub-optimum expected performance• Research in progress towards LISA• Coils fully in place, working on EMP

Radiation Monitor:• EQM built, green light to FM, PSI tests coming up• More on RM in Tim’s talk


End of PresentationEnd of Presentation

Documents

A. LoboSagamihara, 13 November 20081 LTP diagnostics Alberto Lobo ICE-CSIC & IEEC