1 paolo falferi - et wg2 meeting - glasgow, 22/7/2010 actuator magnetic noise measurement and...

1Paolo Falferi - ET WG2 meeting - Glasgow, 22/7/2010

Actuator magnetic noise measurement and possible developments

Paolo Falferi CNR-FBK Trento and INFN Sez. Padova

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In view of the realization of the third generation cryogenic interferometers some questions concerning the control actuators need answers from experimental activity

• Can present actuators (magnetic and electrostatic) work well down to cryogenic temperatures (~4K)?

• Are they consistent, in terms of noise and additive mechanical loss, with the expected sensitivity improvement (x100 the present detectors)?

• Can low temperature alternative techniques offer workable solutions for control actuators?

Coil-magnet actuatorWe started experimental activity on the SmCo permanent magnet:(Barkhausen) noise and magnetization at 4K

Paolo Falferi - ET WG2 meeting - Glasgow, 22/7/2010

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The Barkhausen noise is in the details of the hysteresis curve: when an external magnetic field is applied the response of the ferromagnetic material is dominated by a sequence of abrupt jumps

When we apply weak magnetic fields (and go along the hysteresis curve of the magnets) to control the interferometer mirrors, do we trigger a "dangerous" Barkhausen noise?

In a coil-magnet actuator magnetization jumps mean force jumps

Effect similar to the driver noise of the actuator coil but more insidious because the frequency up-conversion is generated in the magnet

Barkhausen Noise


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In the literature data mainly from soft ferromagnetic materials, at room T and around the coercive field (max )

The spectra have some general common characteristics*:

1. At high frequency typical shape 1/f1.7

÷ 2 and scales linearly with the average magnetization rate S(dI/dt)

2. Max at a frequency roughly proportional to (dI/dt)1/2

3. At lower frequencies scales as f0.61

Polycrystalline 7.8 % SiFe ribbon as a function of the magnetization rate dI/dt

Standard Barkhausen noise measurement: external homogeneous triangular field, pick-up coil around the sample, voltage peaks detection

*Stress of the sample (due for example to differential thermal contraction between mirror and magnet) may change the noise spectrum !

Barkhausen Noise in the literature


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Magnet

Pick-up =1mm

Teflon

Copper shieldBeCu springwasher

NbTi superconductingsolenoid

SnPb superconductingtube

Cryoperm shield

SQUID

"Old" Apparatus

A SQUID magnetometer is weakly coupled to the magnet (pick-up =1mm, flux transformer ratio T1/160) and operates in liquid helium (or vapors) at 4.2 K

• magnet (Sm-Co, 10 mm, h 4 mm) distant from the SQUID to avoid direct pick-up

• copper shield (instead of Nb) to avoid noise from flux creep and reduce the external magnetic noise at > s ≈ 6 Hz

• rigid assembly of magnet inside shield (to reduce relative displacements between magnet, shield and pick-up)

• external superconducting coil

• SQUID in cryoperm shield to reduce the SQUID trapped flux


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Solved problems:no direct pick-up magnet-SQUIDno flux creep in the magnet shield (of course)

but

The critical problem is still the vibrational noise "Thanks" to the high field of the magnet the system is a good displacement transducer: pick-up angular vibration ≈ 10-8 rad/√Hz is equivalent to the intrinsic SQUID noise

Transport Dewar metal Shield

LiquidHelium

"Old" Apparatus


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Noise measurements in different vibrational conditions

No external magnetic field applied

1 10 100 1000 10000 10000010-6

10-5

10-4

10-3

10-2

10-1

S1

/2 ( 0

/Hz1

/2)

Frequency (Hz)

in liquid, touching bottom in liquid in vapour

In theory, no applied H field, no Barkhausen noiseIn practice ambient field fluctuations, thermal activation and non-equilibrium condition could trigger Barkhausen noise


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In both cases no significant difference with respect to the noise spectra taken without external magnetic field

Noise measurements with external magnetic fieldTwo series of measurements1) field step and then noise measurement2) oscillating field during the noise measurement

0.5 mT step equivalent to 1 mN (~100 times max force on Virgo mirrors)

0.2 mTpp at 0.1 Hz equivalent to 400Npp


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Conclusions (6 months ago, Jena meeting)

1) The SmCo (Virgo marionetta) magnets remain magnetized going at low temperatures (only -7%)

2) The measured magnetic noise does not depend on the applied magnetic field

3) Most likely the measured noise is not Barkhausen noise but mainly vibrational noise. However, if considered Barkhausen noise, its level is "dangerous" for ET


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Magnet

Pick-up =1mm

Teflon

Copper shield

Suspensions

NbTi superconductingsolenoid

SnPb superconductingtube

Cryoperm shieldSQUID

Vacuum

To distinguish between vibrational noise and Barkhausen noise a cryogenic apparatus is needed that permits operation in vacuum and with adequate suspensions

"New" Apparatus


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• Al-7075 Ergal

• Compact design

• Electric Discharge Machining

• Effective in the range 100-800Hz

• Max attenuation 108 dB at 280 Hz

• Stress < 2/3 of the yield strength

100 mm

70 mm

"New" Apparatus - Suspensions (under construction)


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Results on new SmCo free samples from Audemars

Magnetization at 4.2K

1) Sm2Co17

cylindrical sample Φ 0.6 mm, h 0.65mmFrom Tamb to 4.2K +30%

2) Sm1Co5 cylindrical sample Φ 1.25 mm, h 1.5mmFrom Tamb to 4.2K +28%


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Experimental activity aimed at estimating the extra noise and the additional losses caused by the different actuation systems: electrostatic, coil-magnet and superconducting actuation methods

The effect is measured on a small mechanical resonator at cryogenic temperatures with a low-noise SQUID-based readout

Possible developments


14Paolo Falferi - ET WG2 meeting - Glasgow, 22/7/2010

Possible resonator design - Double Paddle Oscillator (DPO)

Niobium

Silicon

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Barkhausen Noise Measurements

Classical inductive Barkhausen noise measurement: an external solenoid to produce an homogeneous field H along the sample and a pick-up coil wound around the sample to detect the voltage signal induced by d/dt.

Voltage Signal d/dt = A0dH/dt+SdI/dt B = 0 (H+M)I = 0 MA=pick-up areaS=sample cross section

SQUID Barkhausen noise measurement: external field H around the sample and a superconducting pick-up coil wound around the sample connected to a SQUID magnetometer to detect the ac flux .

Output Signal = A0(H+M)


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Worst Case Scenario: the measured noise is entirely due to the Barkhausen noise

Is this noise negligible in ET?

Sp1/2/p=SF

1/2/F

Sp= flux noise spectrum at the pick-upp= dc flux at the pick-upSF= force noise spectrum of the actuatorF = force of the actuator

The force of the coil-magnet actuator is proportional to the magnetic moment of the magnet

"free mass" approximationmmir=20 kg

1st example: at 100 Hz Sp1/2/p=SF

1/2/F ≈ 6x10-10 Hz-1/2

In Virgo Mirror-RM actuators Fmax ≈ 10N

SF1/2 ≈ 10N 6x10-10 Hz-1/2= 6x10-15 N/Hz-1/2

Sx1/2 ≈ 8x10-22 m/Hz-1/2

Negligible in Virgo!

There are indications that the noise is largely due to vibrations


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3rd example: no mirror control, just marionette control ( better mirror Q) requested Fmax ≈ 5mN at 10 Hz SF

1/2 ≈ 2x10-11 N/Hz-1/2

"free mass" approximationfor mirror and marionettemmar= 300 kg mmir= 100 kg

Sx1/2 ≈ 1x10-19 m/Hz-1/2

Not Negligible in ET!

"free mass" approximationmmir=100 kg in ET

2nd example: at 10 Hz SF1/2 ≈ 5x10-14 N/Hz-1/2

Sx1/2 ≈ 1x10-19 m/Hz-1/2

Not Negligible in ET!


1 paolo falferi - et wg2 meeting - glasgow, 22/7/2010 actuator magnetic noise measurement and...

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