1 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 Actuator magnetic noise: preliminary study Paolo Falferi CNR-FBK Trento and INFN Sez. Padova

2 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 - 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 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 - Jena, 1-3/3/2010

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 - 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 K) and remains magnetized going at low temperatures (only -7%) After the cooling the low frequency noise decreases (at least initially) with log(t/t 0 ), 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 Magnet Pick-up  =1mm Teflon Copper shieldBeCu spring washer NbTi superconducting solenoid SnPb superconducting tube Cryoperm shield SQUID "New" Apparatus As before, SQUID 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, but now 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 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 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 ≈ rad/√Hz is equivalent to the intrinsic SQUID noise Transport Dewar  metal Shield Liquid Helium "New" Apparatus 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 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 - Jena, 1-3/3/2010

10 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 ≈ 6x Hz -1/2 In Virgo Mirror-RM actuators F max ≈ 10  N S F 1/2 ≈ 10  N 6x Hz -1/2 = 6x N/Hz -1/2 S x 1/2 ≈ 8x m/Hz -1/2 Negligible in Virgo! There are indications that the noise is largely due to vibrations Paolo Falferi - ET WG2 meeting - Jena, 1-3/3/2010

11 3rd example: no mirror control, just marionette control (  better mirror Q) requested F max ≈ 5mN  at 10 Hz S F 1/2 ≈ 2x N/Hz -1/2 "free mass" approximation for mirror and marionette m mar = 300 kg m mir = 100 kg S x 1/2 ≈ 1x 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 ≈ 5x N/Hz -1/2 S x 1/2 ≈ 1x 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