Low-T c dc SQUID System

Transcription

1 2015 Winter School on HPSTR Low-T c dc SQUID System Yi Zhang 张懿 Peter Grünberg Institute (PGI-8), Forschungszentrum Jülich (FZJ), D Jülich, Germany Shanghai Institute of Microsystem and Information Technology (SIMIT), Chinese Academy of Sciences (CAS), Shanghai, P. R. China

2 Joint Research Institute Founded on

3 SQUID Applications SQUID 芯片读出电路低温技术

4 Contents What is a SQUID? SQUID Readout Electronics Noise Matching between SQUID and Preamplifier A Practical SQUID System Conclusion

5 What is a dc SQUID?

6 dc SQUID is... Superconducting QUantum Interference Device (SQUID) = Superconducting Loop + Josephson Junction I Wb I 1 J I 2 JJ 1 Φ JJ 2 V = Φ = nφ 0 + I dc SQUID Superconducting Loop: Flux Quantization Josephson Junction: Josephson Tunneling SQUID is a superconductive ring interrupted by two Josephson junctions. It combines two physical phenomena, flux quantization and Josephson tunneling.

7 Basic Knowledge Who understands Ohm s law, knows dc SQUID, too. Ohm s law: V = I R

8 I-V Characteristics -Two typical elements I I V V Resistors Diode

9 0 : Wb I-V Curve of SQUID I V I I Resistive region I c = n 0 Superconductive region V I-V curves of a dc SQUID depend on the magnetic flux and are divided into superconductive region and resistive region. Resistive region I c = (n+0.5) 0 Superconductive region V

10 Specificity of I-V Curve For example: = Wb. / 0 = I c I = n 0 = (2n+1)/4 0 ( ) 0 = = (n+0.5) 0 V (1/4) 0 (3/4) 0 (5/4) 0 (7/4) 0 (9/4) 0 (11/4) 0 (13/4) 0 0 (1/2) (3/2) (5/2) (7/2) 0 SQUID I-V Curves are limited by two states, =n 0 and =(n+0.5) 0, and periodically changed with increasing (decreasing) flux. The period denotes a flux quantum 0.

11 dc SQUID is... SQUID is a I I c = n 0 = (n+0.5) 0 SQUID is operated in the resistive region. V two terminal passive similar to a resistive sensor R d element. for measuring the change of a magnetic flux.

12 SQUID Bias Circuits -- On the Basis of Ohm s Law Current bias mode: I R d V An ideal current source connects SQUID to readout V( ) at I = constant. i V A R d Voltage bias mode: An ideal voltage source connects SQUID to readout i( ) at V = constant.

13 SQUID Signal Readout voltage bias mode working point -- Two Projections i I I swing V b V/ = ( i/ ) R V swing d 3 0 Relation between both bias modes: working point I b working point I b, V b : two bias lines V V working point Current bias mode

14 SQUID Readout Electronics V - characteristics will be linearized. V V out of SQUID system

15 Piezo-element Negative Feedback -- for linearization W 0 V V V/ W W 0 Working point W (jacking force) V V Jack V feedback W

16 Flux Locked Loop (FLL) -- in current bias mode I b V in =0 M - _ + G R f i V out V SQUID Flux Change Voltage Change V out = V in G V/ Current Vout Change i = V out / R f FLL G is automatically adjusted. Requirement: G open Compensation Flux - = i M FLL: V out is proportional to (linearized)

17 Flux Modulation Scheme (FMS) -- a standard readout technique C ac Swing 1:n FMS standardized in 1967 Input: Transformer + Output: Integrator Our recent analysis shows that FMS works in mixed bias mode, thus further reducing ac swing at transfomer primary winding.

18 Direct Readout Scheme (DRS) s : SQUID intrinsic noise M f V/ Φ V - preamp I b V + Amplifier R Integrator C V out SQUID R f 4.2 K I f Problem: preamp = [ V preamp / ( V/ )] > S

19 Noise Matching between SQUID and Preamplifier in Direct Readout System Integrator V/ Φ I b V + Amplifier V - R C V out M f SQUID R f I f

20 SQUID System Noise 2 system = 2 SQUID + 2 preamp. : SQUID intrinsic noise Our goal is to reach ( V/ Ф) the minimum of system V: Preamplifier noise Traditional notion: The readout electronics noise contribution should be suppressed below the SQUID intrinsic noise.

21 Noise Analysis in DRS δф system2 = δф s2 + δф preamp 2 2 V in δф s : SQUID intrinsic noise In current bias mode: δф preamp2 = [V n2 +(I n R d ) 2 ] /( V/ Ф) 2 Each preamplifier has two noise sources, V n and I n. The five parameters decide δф system.

22 Five Parameters -- influencing System Noise V n I n Preamplifier property Matching? V/ s R d SQUID property

23 Preamplifier Candidates V n 0.9 nv/ Hz V n 0.35 nv/ Hz V - V+ AD797 V out I n 2 pa/ Hz V - V + V+ 83 Ω V- T4 27 k T3-B T2-B T1-B T1-A T2-A T3-A 150 Ω 10 nf V- 1.5 k 1.5 k V out I n 5 pa/ Hz 6 SSM2220 Operational amplifier Parallel Connected Bipolar Transistors (PCBT) Preamplifier (V n, I n ): I n V n V n PCBT I n PCBT = (1/ 6) V n AD797 = 6 I n AD797 R s V n total voltage noise: V n2 = (I n R d ) 2 + V n 2 V out

24 Total Voltage Noise V n at different R s (R d ) AD 797 V n PCBT V n V n is dominated by V n. At R s = 50 Ω, I n R s influences V n in f < few Hz. Vn, preamp.=[v 2 n +(I n R s )2 ] 1/ Vn Vn, preamp. n 10 n Frequency [Hz] V n is dominated by I n R s. At R s = 50 Ω, I n R s completely determines V n in the whole frequency range.

25 Five Parameters -- influencing System Noise V n I n Preamplifier property Matching? V/ s SQUID property R d

26 RCSJ Model of SQUID R J C L s /2 L s /2 C R J Steward- McCumber parameter β c β c = 2 I c CR J2 / 0 In order to obtain different ß c, we change R I c C const.

27 Three Parameters at Different β c -measured R d, V/ and estimated noise s s, R d and V/ increase with increasing β c. β c R d [ ] V/ [μv/ф 0 ] s [µ 0 / Hz] (L s = 350 ph) Impossible Almost impossible Possible Easy V n [nv/ Hz] Required preamplifier noise preamp = 0.7 s V n = 0.7 s ( V/ ) Zeng J, Zhang Y, et al., Appl. Phys. Lett. 103, (2013)

28 Noise Matching c δф s2 Noise matching: δф preamp2 δф = [V n2 +(I n R d ) 2 ] /( V/ Ф ) 2 preamp2 = V n2 /( V/ Ф) 2 I n of AD797 can be neglected due to its large V n. s preamp Fortunately, the large I n contribution of PCBT can be PCBT reduces V n, but increases I n, suppressed by a Current Feedback Circuit (CFC). thus increasing I n R d and worsening Φ preamp. G.F. Zhang, Y. Zhang, et al., doi: /j.physc

29 Suggestion -- for selecting Preamplifier and SQUIDs β c V/ [μv/ф 0 ] s [µ 0 / Hz] (L s = 350 ph) Noise matching: strongly AD797 for c 3 Intermediately damped PCBT for 1 with weakly damped CFC damped c? PCBT AD 797 employed preamplifier

30 A Practical SQUID System

31 A Commercial Electronics -- in flux modulation scheme Front view Back view

32 Single Chip Readout Electronics Our concept: PI P, remove the integrator c 3 SCRE (DRS) AD797 P the simplest DRS Chang K, Zhang Y, et al, A simple SQUID system with one operational amplifier as readout electronics. Supercond. Sci. Technol. 27 (2014) (4pp)

33 Test of SCRE 94 kω 30 khz 6 μφ 0 Hz R f = 1 M Ω BW =3.8 khz 1.44 kω 2.7 MHz Test frequency (khz) Slew rate (Φ 0 /μs) High bandwidth and slew rate can be easily achieved with SCRE

34 Applications (I) Magnetocardiography (MCG) R f = 1 M Ω Demonstration for biomagnetism

35 Applications (II) --Unshielded ULF MRI SIMIT x y z 1.5 m Slider Sample LHe Cryostat G xy & G xz Electric Actuator Permanent Magnet(PM) pair B c G xx B applied B 1 SQUID PM pair Electric Actuator B z = B c +B earth (26 μt) = 0; B 0 = B earth// + B applied = 130 μt; 3D Gradient Coils B p = 0.65 T

36 Four-channel 3D MRI -- G = 47 T/m = 22 mm SQUID 50 mm 2 nd -order gradiometer 3 44 mm Pepper mm 4

37 Applications (III) Transient Electro-Magnetic (TEM) 15km 测区地理位置

38 Measurement Field 200 米 200 米 超导接收机 发射机 300 nt 80 ms Secondary field

39 Depth [m] TEM Profile Coil of Jilin University SQUID 0 17 km The SQUID detection depth is clearly larger than 1000 m.

40 TEM Profile of Shallow Layer Coil of EM 67 SQUID Depth [m] SQUID TEM 测量实用化之日, 便是我退出江湖 ( 金盆洗手 ) 之时 0 17 km Using SQUID, the shallow layer of < 100 m is detectable.

41 Conclusion (I) Three novel paradigms: 1) We expand the hysteresis-free range ( c >1). 2) We should aim at preamp s (not preamp < s ). 3) We change the feedback controller from PI to P (remove the integrator), thus obtaining the simplest SQUID system (SCRE). Concept for SQUID systems: 1) Intermediately damped SQUID ( c 1) + PCBT (preamplifier) + CFC yields a high performance. 2) Weakly damped SQUID ( c 3) + SCRE forms the simplest SQUID system with an acceptable system for many applications.

42 Conclusion (II) --a practical dc SQUID system Simplicity Stability Convenient to manufacture User-friendliness Robustness Acceptable system noise Single Chip Readout Electronics (SCRE) with a weakly damped SQUID the simplest SQUID system Albert Einstein: Everything should be made as simple as possible, but not simpler.

43 謝謝