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The control of the Virgo mirrors is realized using coil-magnet actuators

Actuator magnetic noise: preliminary study. Paolo Falferi CNR-FBK Trento and INFN Sez. Padova. The control of the Virgo mirrors is realized using coil-magnet actuators Can this technique be used in ET, from room to cryogenic temperatures?

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The control of the Virgo mirrors is realized using coil-magnet actuators

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  1. Actuator magnetic noise: preliminary study Paolo Falferi CNR-FBK Trento and INFN Sez. Padova The control of the Virgo mirrors is realized using coil-magnet actuators Can this technique be used in ET, from room to cryogenic temperatures? Is the Barkhausen noise of the control magnets negligible for ET? Paolo Falferi - ET WG2 meeting - Jena, 1-3/3/2010

  2. Barkhausen Noise 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 Paolo Falferi - ET WG2 meeting - Jena, 1-3/3/2010

  3. 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 dF/dt. B = m0 (H+M) I = m0 M A=pick-up area S=sample cross section Voltage Signal  dF/dt = Am0dH/dt+SdI/dt 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 F. Output Signal  F = Am0(H+M) Paolo Falferi - ET WG2 meeting - Jena, 1-3/3/2010

  4. Barkhausen Noise in the literature In the literature data mainly from soft ferromagnetic materials, at room T and around the coercive field (max m) 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 f0.61 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 ! Polycrystalline 7.8 % SiFe ribbon as a function of the magnetization rate dI/dt Paolo Falferi - ET WG2 meeting - Jena, 1-3/3/2010

  5. Status 6 months ago (Genova meeting) • The magnet (Sm-Co) does not deteriorate after many thermal cycles (room temperature - 4.2 K) and remains magnetized going at low temperatures (only -7%) • After the cooling the low frequency noise decreases (at least initially) with log(t/t0), a typical magnetic viscosity behaviour, then it stops • The noise of the magnet, if any, is buried under the flux creep noise of the magnet superconducting shield and the vibrational noise Paolo Falferi - ET WG2 meeting - Jena, 1-3/3/2010

  6. Cryoperm shield SnPb superconducting tube SQUID BeCu spring washer Copper shield Teflon NbTi superconducting solenoid Magnet Pick-up F=1mm • "New" Apparatus • As before, SQUID is weakly coupled to the magnet (pick-up F=1mm, flux transformer ratio T1/160) and operates in liquid helium (or vapors) at 4.2 K, but now • magnet (Sm-Co, F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 n > ns ≈ 6 Hz • more 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 - Jena, 1-3/3/2010

  7. mmetal Shield Transport Dewar Liquid Helium "New" Apparatus 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 Paolo Falferi - ET WG2 meeting - Jena, 1-3/3/2010

  8. 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 - Jena, 1-3/3/2010

  9. 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 mTpp at 0.1 Hz equivalent to 400mNpp In both cases no significant difference with respect to the noise spectra taken without external magnetic field Paolo Falferi - ET WG2 meeting - Jena, 1-3/3/2010

  10. 1st example: at 100 Hz SFp1/2/Fp=SF1/2/F ≈ 6x10-10 Hz-1/2 In Virgo Mirror-RM actuators Fmax≈ 10mN SF1/2≈ 10mN 6x10-10 Hz-1/2= 6x10-15 N/Hz-1/2 "free mass" approximation mmir=20 kg Sx1/2≈ 8x10-22 m/Hz-1/2 Negligible in Virgo! There are indications that the noise is largely due to vibrations Worst Case Scenario: the measured noise is entirely due to the Barkhausen noise Is this noise negligible in ET? SFp1/2/Fp=SF1/2/F SFp= flux noise spectrum at the pick-up Fp= dc flux at the pick-up SF= 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 Paolo Falferi - ET WG2 meeting - Jena, 1-3/3/2010

  11. 2nd example: at 10 Hz SF1/2≈ 5x10-14 N/Hz-1/2 "free mass" approximation mmir=100 kg in ET Sx1/2≈ 1x10-19 m/Hz-1/2 Not Negligible in ET! 3rd example: no mirror control, just marionette control ( better mirror Q) requested Fmax≈ 5mN  at 10 Hz SF1/2≈ 2x10-11 N/Hz-1/2 "free mass" approximation for mirror and marionette mmar= 300 kg mmir= 100 kg Sx1/2≈ 1x10-19 m/Hz-1/2 Not Negligible in ET! Paolo Falferi - ET WG2 meeting - Jena, 1-3/3/2010

  12. (Provisional) Conclusions 1) The SmCo magnets remain magnetized going at low temperatures (only -7%) 2) Their magnetic noise does not depend on the applied magnetic field 3) Most likely the measured noise is not Barkhausen noise but mainly vibrational noise (no contribution from flux creep in superconductors). However its level is "dangerous" for ET Paolo Falferi - ET WG2 meeting - Jena, 1-3/3/2010

  13. Things to do • Check the effect of the measured noise on the ET mirror displacement with a more realistic detector model • The evaluation of the intrinsic noise of the magnets is in any case interesting for the development of cryogenic SQUID-based accelerometers. Then, realize a cryogenic apparatus that permits operation in vacuum and with adequate suspensions • Check the effect of the mechanical stress due to the differential thermal contraction between magnet and mirror: stress can change the magnetization through the Inverse Magnetostrictive Effect • Check the effect of temperature fluctuations Paolo Falferi - ET WG2 meeting - Jena, 1-3/3/2010

  14. Low freq spectrum at 14 days from cooling Paolo Falferi - ET WG2 meeting - Jena, 1-3/3/2010

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