1. 1 Paolo Falferi - ET WG2 meeting - Glasgow, 22/7/2010 Actuator magnetic noise measurement and possible developments Paolo Falferi CNR-FBK Trento and INFN Sez. Padova

2. 2 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 actuator We started experimental activity on the SmCo permanent magnet: (Barkhausen) noise and magnetization at 4K Paolo Falferi - ET WG2 meeting - Glasgow, 22/7/2010

3. 3 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 Paolo Falferi - ET WG2 meeting - Glasgow, 22/7/2010

4. 4 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/f 1.7 ÷ 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 f 0.6  1 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 Paolo Falferi - ET WG2 meeting - Glasgow, 22/7/2010

5. 5 Magnet Pick-up  =1mm Teflon Copper shieldBeCu spring washer NbTi superconducting solenoid SnPb superconducting tube Cryoperm shield SQUID "Old" Apparatus A SQUID magnetometer is weakly coupled to the magnet (pick-up  =1mm, flux transformer ratio T  1/160) 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 Paolo Falferi - ET WG2 meeting - Glasgow, 22/7/2010

6. 6 Solved problems: no direct pick-up magnet-SQUID no 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 Liquid Helium "Old" Apparatus Paolo Falferi - ET WG2 meeting - Glasgow, 22/7/2010

7. 7 Noise measurements in different vibrational conditions No external magnetic field applied In theory, no applied H field, no Barkhausen noise In practice ambient field fluctuations, thermal activation and non- equilibrium condition could trigger Barkhausen noise Paolo Falferi - ET WG2 meeting - Glasgow, 22/7/2010

8. 8 In both cases no significant difference with respect to the noise spectra taken without external magnetic field Noise measurements with external magnetic field Two series of measurements 1) field step and then noise measurement 2) oscillating field during the noise measurement 0.5 mT step equivalent to 1 mN (~100 times max force on Virgo mirrors) 0.2 mT pp at 0.1 Hz equivalent to 400  N pp Paolo Falferi - ET WG2 meeting - Glasgow, 22/7/2010

9. 9 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 Paolo Falferi - ET WG2 meeting - Glasgow, 22/7/2010

10. 10 Magnet Pick-up  =1mm Teflon Copper shield Suspensions NbTi superconducting solenoid SnPb superconducting tube Cryoperm shield SQUID 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 Paolo Falferi - ET WG2 meeting - Glasgow, 22/7/2010

11. 11 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) Paolo Falferi - ET WG2 meeting - Glasgow, 22/7/2010

12. 12 Results on new SmCo free samples from Audemars Magnetization at 4.2K 1) Sm 2 Co 17 cylindrical sample Φ 0.6 mm, h 0.65mm From T amb to 4.2K +30% 2) Sm 1 Co 5 cylindrical sample Φ 1.25 mm, h 1.5mm From T amb to 4.2K +28% Paolo Falferi - ET WG2 meeting - Glasgow, 22/7/2010

13. 13 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 Paolo Falferi - ET WG2 meeting - Glasgow, 22/7/2010

14. 14 Paolo Falferi - ET WG2 meeting - Glasgow, 22/7/2010 Possible resonator design - Double Paddle Oscillator (DPO) Niobium Silicon

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16. 16 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 = A  0 dH/dt+SdI/dt B =  0 (H+M) I =  0 M A=pick-up area S=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   = A  0 (H+M) Paolo Falferi - ET WG2 meeting - Glasgow, 22/7/2010

17. 17 Worst Case Scenario: the measured noise is entirely due to the Barkhausen noise Is this noise negligible in ET? S  p 1/2 /  p =S F 1/2 /F S  p = flux noise spectrum at the pick-up  p = dc flux at the pick-up S F = force noise spectrum of the actuator F = force of the actuator The force of the coil-magnet actuator is proportional to the magnetic moment of the magnet "free mass" approximation m mir =20 kg 1st example: at 100 Hz S  p 1/2 /  p =S F 1/2 /F ≈ 6x10 -10 Hz -1/2 In Virgo Mirror-RM actuators F max ≈ 10  N S F 1/2 ≈ 10  N 6x10 -10 Hz -1/2 = 6x10 -15 N/Hz -1/2 S x 1/2 ≈ 8x10 -22 m/Hz -1/2 Negligible in Virgo! There are indications that the noise is largely due to vibrations Paolo Falferi - ET WG2 meeting - Glasgow, 22/7/2010

18. 18 3rd example: no mirror control, just marionette control (  better mirror Q) requested F max ≈ 5mN  at 10 Hz S F 1/2 ≈ 2x10 -11 N/Hz -1/2 "free mass" approximation for mirror and marionette m mar = 300 kg m mir = 100 kg S x 1/2 ≈ 1x10 -19 m/Hz -1/2 Not Negligible in ET! "free mass" approximation m mir =100 kg in ET 2nd example: at 10 Hz S F 1/2 ≈ 5x10 -14 N/Hz -1/2 S x 1/2 ≈ 1x10 -19 m/Hz -1/2 Not Negligible in ET! Paolo Falferi - ET WG2 meeting - Glasgow, 22/7/2010