Electromagnetic shielding of the SQUID in the nEDM Experiment: Bolts and Nuts

Andrei Matlashov, Michelle Espy, LANL, P-21

1. What happens when SQUID is exposed to RFI

2. Faraday rooms

3. Feed through filtration

4. Grounding problems

5. Johnson noise from conductive shields

6. 100% 0ptical insulation: The most effective supersensitive SQUID system design that works in LANL as well as in ABQ airport conditions.

1. What happens when SQUID is exposed to RFI

SQUID is a “flux to frequency to voltage” converter:Josephson V Frequency ratio 2e/h = 484 MHz/VAt 1 – 50 V voltage swing 0.5 – 25 GHzEven low-level external RF signal interferes with SQUID internal generation and

degrades its noise performances

High-level RFI (1 W radio transmitter at 10 m distance): • Depressed or even zeroed V- curve• SQUID can trap flux• Unpredictable jumps huge 1/f noise• Huge amount of “unknown” harmonics• White noise increased by 10 or more times

Medium-level RFI (can be a microwave oven or RF welding connected to the same power line):

• White noise increased up to 2-3 times• Some large “unknown” harmonics• Unacceptably high 1/f noise Low-level RFI (ULF MRI system in SM218):• A few low-level “unknown” harmonics• Not exactly flat noise below 100 Hz

2. Faraday rooms

The most effective method

Though it can add Johnson noise It can be either a distant thick metal enclosure or closely placed metal-plated mylar wrap

It needs very accurately designed feed through for all incoming and outgoing cables

RFI shielding factor can be tested using portable SW and FM radio receiver

Any possible sources of any kind of RF signals should be moved outside

It is not always clear how such shield should be grounded, it usually takes pretty long time to find it out by trying many different kinds of groundings. My best experience – it should not be grounded at all. Then it needs differential in and out signals, however that is not always acceptable.

3. Feed through filtration

It is very tricky to design, build and test them (some sort of “black magic recipe”)

It might not work after it was built using our best knowledge and common sense, but we will see it when it is too late and takes too much effort to rebuild

It limits what kind of signals can be used – no pulses with short fronts, no too high voltages, no large currents etc.

OR it should be designed very specifically for each kind of signal or supply too time consuming process

4. Grounding problems

One-point ground is a must

The grounding circuit diagram should look like a star with its center placed exactly at SQUID-electronics connector

All regular wall powered devices are grounded to the power line ground. Some times that ground is connected to the signal ground input or output connector. It can form a ground loop, if two different devices have such connection.

Desk top computer ground is the noisiest source of all kind of interference – it is from 1/f at below 100 Hz (jumps) up to 10-100 MHz signals. Data acquisition system as well as all other analog and digital interface boards should not be directly connected to a computer.

Right grounding – one more “black magic” trick

Even if we finally found one working grounding scheme, just one additional connection can completely ruin it

5. Johnson noise from conductive shields

High-resolution SQUID gradiometers can not work in open space without closely placed Faraday shield – gold-plated or aluminum-plated mylar

5-8 fT/sqrt(Hz) resolution can be reached with a pick-up coil wrapped with plated mylar

0.4 fT/sqrt(Hz) at 2 kHz was reached using a 120mm second order gradiometer placed inside a fiberglass dewar. The dewar was wrapped with one gold-plated mylar layer. Low-frequency noise was 1.5 fT/sqrt(Hz) at 10 Hz due to Johnson noise from dewar thermal shield.

Ten 1.5 mm diameter copper wires placed in parallel with 5 mm separation produce about 2 fT/sqrt(Hz) noise at 15 mm distance.

6. 100% optical insulation: The most effective supersensitive SQUID system design that works in LANL as well as in ABQ airport conditions.

• 7-channel SQUID system is placed inside two-layer magnetically shielded room (MSR).

• The system has 1.2 fT/sqrt(Hz) for the central channel (intrinsic SQUID noise) and 2.4 fT/sqrt(Hz) for all six side channels (thermal shield Johnson noise)

• All equipment inside MSR is battery powered including DAQ (24-bit, 40 kHz, 16 channel).

• The system is connected to outside world using ONLY fiber optic cables, including DAQ interface.

• DAQ has 50 MHz internal clock that was a problem until we used good RFI shielded power supply feed trough and separate batteries.

• MSR is not grounded

• One-channel SQUID gradiometer with 2.5 fT/sqrt(Hz) resolution was tested in ABQ airport. The system was placed inside Al foil shielded box and had autonomous battery power. NMR signal from water samples were recorded in all the noisiest airport locations. There was no magnetic shielding.

General view of the system Schematic of the coil system Imaging protocol

Magnetoencephalography

Auditory stimulus: train of 1.2 ms clicks with 14 ms intervals. Pre-stimulus interval: 50 ms

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Ch 1

Ch 2

Ch 3

Ch 4

Ch 5

Ch 6

Ch 7

time, s

Auditory evoked magnetic field