Cryoconference 2010 Millikelvin measurement platform for SQUIDs and cryogenic sensors M. Schmidt, J. Beyer, D. Drung, J.-H. Storm Physikalisch-Technische Bundesanstalt, Abbe Str. 2-22, 10587 Berlin, Germany Outline: Motivation Characterization Setup and Mounting Summary
Motivation Applications: SQUID characterization at low temperature (10mK - 500mK) Fast readout (dc - 100s MHz) for analysis of high-frequency SQUID performance Operation of TES/SQUID-based optical photon counters Requirement of the cryostat: Continuously cooling Temperature stability Large experimental volume dry 3 He/ 4 He dilution refrigerator
Cooling system dry 3 He/ 4 He dilution refrigerator 300 K 70 K 4 K 800 mk 100 mk 10 mk
Characterization of the Cooling power I R no fine tuning needed Heater base plate MFFT 400 350 P base plate =2050µW n& 3 T Still Q & n& 3 Requested cooling power 200 µw @ 100 mk T base plate (mk) 300 250 200 150 100 P base plate =1365µW P base plate =911µW P base plate =405µW P base plate =200µW T min =8.3mK 50 P base plate =50µW P base plate =0µW 0 0 2 4 6 8 10 12 14 16 P Still (mw)
Parasitic load and results of the characterization 2 Q & = 84n& 3T Q& 0 Q & 0 = 1,3µW P base plate (µw) 1000 100 10 Points Fit without offset Fit 1 0.1 1 10 100 1000 T base plate (mk) Sufficient cooling power No fine tuning of the still power necessary Typical parasitic load on the base plate Temperature stability: 30 µk rms --> no influence of the SQUIDs
Measurement inserts Sensor module for SQUID characterization 2 coaxial cables for fast readout Optical fiber feedthrough for TES/SQUID applications Requirements of the setup: Prevention of stress and vibrations Good thermal connections Increase of temperature so small as possible Electromagnetic shielding of the cables
Measurement module for characterization and low temperature detectors 5 SQUID channels 5x7 twisted pair dc wires - Conductive Material: CuNi (Alloy30), AWG32 - Shield: stainless steel low thermal conductivity Estimation of the head load Wiedemann-Franz law: Q& = A( T T ) 2 1 1 λ ( ) T T2 i i= T step λ = T L i L T ρ ρ (Ω m) 10-7 10-8 10-9 & dc = 900nW Q wires 10-10 Brass Alloy30 Copper 0 50 100 150 200 250 300 T (K)
Magnetic shielding High accurate characterization of SQUIDs Problem: low-frequency magnetic fields z a Superconductor magnetic shield Magnetic shield: Aluminium Tc: 1.2 K a=30 mm, z=100 mm SQUID positions trans: 0.61a axial: 0.15z and 0.89z zero points
SQUID mount Connectors (5 channels) PCB SQUIDs 100mm Magntic shield 60mm Base plate with SQUID module
High frequency SQUID measurement module 2 coaxial cable - Material: Brass - Dielectric: PTFE (Teflon) - Impedance: 50 Ohm - SMA connector Best possible electromagnetic shield Fast readout (dc-100s MHz) potential Problem: difficult coupling of the inner core Calculation: 300 K to 100 mk produce 200 µw heat flow to base Coupling on the 4 K-stage or decoupling from higher to lower stages very important Test coax: 106 mk base temperature 8.4 K inner core on the 4Kstage
Decoupling of the inner conductor Limiting the heat flow Thermal conductivity: - Brass: 35mW/cm*K @ 4 K - NbTi: 0.9mW/cm*K @ 4 K NbTi R PTFE 8.4K R OC/4K R NbTi/CuNi 4K 4K T base = 9.6 mk R PTFE Outer conductor Inner conductor Schematic diagramm (4K-stage) Mount on the 4K-plate
Fiber feedthrough for optical and near infrared TES applications Characterization of TES/SQUID Optical photon counter optical fiber I TES SQUID & TES electronics (analog) 300K <100mK 300K Two optical fibers from 300K to 10mK minimize scattered light
Mounting and thermal coupling fiber box thermal coupling Fiber TES SQUID TES/SQUID mount PCB Bonds
Summary Characterization 8.3 mk minimal temperature Sufficient cooling power for many experiments Temperature stability: 30 µk rms Sensor module for SQUID characterization Low head load to the base plate Magnetic shield High frequency SQUID measurement module Reduce heat flow to base plate ( T min =9.6 mk) Fiber feedthrough for optical and near infrared TES applications Simple and inexpensive method Compact SQUID/TES mount
Circuit of connections +F -F Feedback Coil 1 -INR R B,TES R TES V OUT 300K -V +V -INR +R Shunted 16-SQUID Array Bias Resistor +IN +R +F -F Feedback Coil 2 -INR -V +V -INR +R rf Filters Shunted 16-SQUID Array Bias Resistor +IN +R <100mK
Characterization of the cryostat 1000 precooling condensing continuouscycle 100 T (K) 10 70K plate (PT100) 4K plate (cernox) MC plate (RuO) 1 0.1 phase separation 0.01 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 t (hours) Precooling: PTC + circuit to precool lower stages Condensing: Joule-Thomson-Impedance to liquefy the mixture Continuous cycle: phase separation