Perspectives about the capacitive-SQUID readout for the next run of AURIGAAndrea Vinante (University of Trento and INFN Padova)e-mail: vinante@science.unitn.it

Shh: 1st run (97-99) and 2th run (04-06) (FWHM) bandwidth enlarged: Dn=26 Hz (1st run Dn1 Hz) Teff=320 mK at T=4.5K (2th run Teff>1mK at T=0.1K)(L. Baggio et al, Phys Rev. Lett. (2005) )Shh

Main new featuresFirst operation of a resonant capacitive transducer with 2 effective modes (mechanical + electrical) tuned to the bar mode for a 3-modes operation

Increased coupling between bar and amplifier

The AURIGA readout: scheme

The AURIGA readout: compact assemblingMounted on the face of the bar Cryogenic switchDecoupling capacitor boxSuperconducting transformer and SQUID box Shield plateHigh voltage plateResonant transducerDecouplingspring

The shielded heavy transducerHV plateTeflon spacerMain resonatorEffective resonator mass (from Dn): 6.1 kg Mechanical resonance frequency: 898.4 Hzgap=40 mm, capacitance=8.5 nF

AURIGA output spectrumTypical spectrum averaged on 8 consecutive hoursMain modes = mixing of bar, transducer and electrical modes:865, 914, 955 Hz

Mechanical transfer function Experimental transfer function. Force applied by a capacitive force actuator. Spurious modes not considered. Work in progress to include them in model3-modes fit

AURIGA Shh: Noise budget Theoretical prediction by three-modes numerical model (spurious modes not included!). Most parameters of the model directly measured in situ. Transfer functions:Mechanical: 1 Zero (Bar mode)Electrical+BA: 2 Zeroes (2 mechanical modes at zero field)Additive: 3 Zeroes (3 modes)

Mechanical thermal noiseReferred to a force on the bar: In principle there is large reserve for increasing Qmec. Intrinsic Q of material (Al5056) is at least 1 order of magnitude higher (Ex. 2 spurious modes at 2.1 kHz exist, with Q=8107 !). Probably Q is degraded by clamping screws. Important issue to solve for next generation detectors. Simpler way to reduce thermal noise is cooling to ultracryogenic temperature. Target temperature is 100 mK (roughly obtained in AURIGA first run and NAUTILUS. Even lower in MINIGRAIL).

Ultracryogenic run scheduled for beginning 2007 (Not possible now because some modifications are needed to insert the dilution refrigerator)

T=4.5 KQmec=1.5-4 106

Electrical mode: the high Q transformer Primary coil: Lp=7.9 H Secondary coil: Ls=3.2 mH

High coupling factor k=0.86

Reduced stray capacitance Cp=10 pF (coil by sections)

Effective inductance (reduced by coupling to SQUID input):Leff=3.8 H

Electrical mode (transformer effective inductance + transducer capacitance) Resonance frequency nel=941 Hz Quality factor Qel=1.3x106 when not coupled to SQUID(Qel=4.7x105 when coupled to SQUID)

SQUID insertion adds dissipation!

Electrical thermal noise Two components of Qel: intrinsic electrical (dielectric and magnetic dissipation in transducer + superconducting transformer) and from SQUID input circuit (equivalent to back-action).

Qel=4.7105E=7.5 106 V/mQint 1.3106QSQ 7.4105

Seems difficult to increase significantly Qint .Recently tested a SQUID with QSQ factor >10 higher. Not yet demonstrated stability and reliability when coupled to a high Q resonant load. Within present experimental setup, referred to the bar: Seems much more effective to increase bias field E

Effect of increasing bias fieldMaximum realistic target for AURIGA ultracryogenic run: E x 2.5 (DUAL R&D target: factor >10 , Factor 6.5 already obtained on small samples)1) Mechanical unchanged2) Electrical + BA decreases (as 1/E)

Electrical thermal noise at ultracryogenic T Problem: poor thermalization (very low thermal conductivity of superconductors and polymers) at 100 mK. High Q LC resonator cooled by means of a dilution refrigerator. Inductance (NbTi coil in SnPb shield) and capacitance (Teflon capacitors) elements with similar technology with respect to AURIGA ones. 3He or 4He exchange gas.LC thermal noise scales down to 60 mK !

(A. Vinante et al, Rev. Sci. Instrum. 2005)

SQUID noiseTwo components:Back-action (Svv)Additive (SII) 1) Two-stage configurationNoise is close to the thermal limit in both components (TN T), down to T200 mK2) Damping network for stabilization of SQUID + high Q resonant loadsNoise can be optimized when the SQUID is operated on the real detector Main achievements of recent years:True energy sensitivity. (TN noise temperature)

History of improvements of Trento SQUID noise in recent years AURIGA-like SQUID (two-stage Quantum Design) coupled to a high Q LC resonator (11 kHz) e scales with T down to 200 mK as predicted. Best value 27 h.

AURIGA-SQUID at ultracryogenic T

AURIGA ultracryogenic: design sensitivityUltracryogenic operation (like AURIGA first run): T=100 mKAll noise components in AURIGA decrease WITH T!SQUID noise saturation at 200 mK taken into account.Mechanical and electrical noise assumed thermal (scale with T down to 100 mK).Quality factors assumed constant.

How far from quantum limit?Present run experimental (T=4.5 K)Minimum detectable energySQUID noise energyEmin 7000 hwkBTN 550 hw Ultracryo case much closer to the matching limit (factor 2). Crucial factor is the increase of bias field ENote: SQUID noise includes excess dissipation from input circuit and non-thermal 1/f noise contribution* Manufactured by IBM. kBTN estimated from bench tests (additive noise and excess dissipation), but not yet demonstrated operation on a high Q resonator and reliability.

Ultracryo with a better SQUID *Emin 62 hwkBTN 12 hw

Current R&D on SQUIDsBetter SQUIDs. In the near future plan to operate a SQUID with kBTN 10 hw coupled to a high Q resonator. Reduce (non-thermal) 1/f noise (Recent indications point to weak magnetic impurities near the SQUID loop).Reduce excess dissipation in the input circuit (i.e. increase electrical Q factor)Improve thermalization of shunt resistors, in order to avoid the saturation of electron temperature at 200 mK. Can be achieved by using cooling fins.

Collaborations:Giessen UniversityTwente University (MINIGRAIL)

Apparatus for High voltage studyR&D on voltage breakdown (DUAL R&D)M. Bonaldi, F. PenasaIFN Trento

Goal: 108 V/mAchieved: 50 MV/mMeasurement of V.B. of Al5056 polished surfaces of cylindrical samples

I will speak about the new readout of AURIGA. Experimental details, performance and mid-term prospectsHere you can see the strain noise spectrum of AURIGA in the present run in blue compared with the previous run (97-99).

The main achievement of the present run has been a significant enlargement of the sensitivity band.

The bandwidth is now 26 Hz according to the full width half maximum def

Shh