1. Progress report on the HV-SQUID Compatibility Test
Craig Huffer, Maciej Karcz, Chen-Yu Liu, Josh Long
Indiana University
Study the SQUID performance in a HV environment.
Study the HV breakdown
in superfluid under pressure.
using different materials for electrodes.

2. Instabilities in the SQUID sensor
Adding current bypass capacitors to the ground → filter the high frequency components from possible sparks → SQUID more stable.
SQUID feedback circuit.
A larger RC constant of the FB integrator helps(?) → increases the BW of the system → makes the SQUID operation less susceptible to frequent HV polarity switches.

3. Capability of the HV-SQUID probe
What can we test:
Without a SQUID sensor, measure
HV breakdown (dielectric strength) in liquid helium from 1.5K (1torr) - 5K (1500 torr), in a small gap (1mm ~ 1cm).
HV breakdown in superfluid helium under pressure.
HV breakdown with candidate electrode materials.
Leakage current through candidate cell materials.
With a SQUID sensor
Mitigate Radio Frequency Interference (RFI)
From different power supplies (SMPS vs Linear )
From micro-discharge (occurs below breakdown voltage) in dielectrics which makes the electrode, cell, etc...
Measure Johnson Noise
candidate material for electrode, RF shield, and ferromagnetic shield.
Investigate HV Breakdown vs SQUID survival rate

4. Pressurization in the test probe
Small size, top load probe allows easy access and short turn-over time for various changes.
We have changed the bellows scheme to a gas scheme.
Close the superfluid-tight needle valve to isolate the test probe from the helium reservoir.
2. P inside the test probe can be changed with pressurized helium gas introduced through a pressurization port.
3. T of the probe can be controlled by the helium reservoir, which can be pumped on.
to pump
17”
6.25”
2.7”

5. An Old Janis helium dewar Pressure manometers A dry pump
Baratron
A dry pump

6. Radiation Baffles Superfluid-tight needle valve Probe can
HV pumping port
The HV conductor is placed inside a SS tube (can be evacuated.)
This protects the HV line from breakdown in helium vapor.
Enough room below the ground electrode to accommodate
different SQUID sensors & pickup coils.
Superfluid-tight
needle valve
Probe can
(can be evacuated
or pressurized)

7. Electrode material Pressurization tube HV ground tube Squid wires
inside a 1/8” SS tube
Auxiliary port
HV feedthrough
(vacuum tight)
Electrode material
Cu, Al, SS, Ti
Graphite, MACOR/C,Torlon, Semitron, …
Pb foil or Pb/Sn solder tin (lining in the inside of the probe can) as a superconducting shield.
Ground Electrode
HV Electrode

8. SQUID Electronics Input coil Pickup coil
D. Drung, Supercond. Sci. Technol. 16 (2003) 1320

9. Why is RFI bad for SQUIDs?
1. SQUID feedback unlock with a high frequency input
Traditional flux lock loop (FLL) uses the modulation/PSD scheme to overcome the high low frequency noise from the pre-amp.
Does not track signal with frequencies higher than the ½ of the modulation frequency.
2. RF can heat up the SQUID component locally, changing the V- curve.
was applied to image microwave cavity.
3. Large transient induces big current in the SQUID thin film→trap flux→change V- curve.
RF shield the input coil,
use large BW, high slew rate electronics
RF shield the SQUID sensor

10. BW & Slew Rate Current system (StarCryoelectronics: PCI1000+ PFL 100)
modulation frequency: 256 kHz
BW: max=100kHz
BW is effected by
time constant of the integrator
the feedback coupling (the tighter the coupling, the lower the BW)
Magnicon + PTB Berlin
Additional Positive Feedback (APF) amplifies the voltage signal, allows for direct
coupling without modulation.
Fast electronics, large BW (6MHz, 20MHz), large slew rate ~ MPhi0/s, in a low
gain, should be able to track fast transients, keeping the SQUID locked under RFI.

11. Large, fast transients HV reversal Micro-discharge (random events)
Reset (in sync with field reversal)
shunt the integrator capacitor (flux lock mode off, SQUID sees all the field)
Micro-discharge (random events)
Shield the SQUID sensor (inside a Pb box)
RF shield the pickup coil
Mylar foil with Au, Al coating (superinsulation)
Enhance low pass filter across the input coil
RC filter in parallel to the input coil
could introduce a pole in the frequency response

12. RF Shield vs Johnson Noise
Skin depth:
Magnetic Johnson Noise:
Aluminized mylar superinsulation
t=1m, z=1cm, SB=1.13 fT/Hz
t=1m, z=10cm, SB=0.11 fT/Hz
t=300m, z=1cm, SB=19 fT/Hz
: conductivity
skin depth (m)
material
resistivity (Ohm-m)
conductivity mu
100kHz
1MHz
20MHz
1Hz Ag
1.47E-08
6.80E+07
E-06
E-05
1.3646E-05 Cu
1.72E-08
5.81E+07
E-05
1.4761E-05 Au
2.44E-08
4.10E+07
E-05
1.7581E-05 Al
2.82E-08
3.55E+07
E-05
1.89E-05
mils
t : thichness
z : separation
J. Nenonen, et al, Rev. Sci. Instrum. 67, 2397

13. Different SQUID sensors
Quantum Design Squid
Katchen type construction
SQUID loop consist of large area Nb film to enhance the coupling with the input coil
However, tends to trap flux easily
Jena Squid (Supracon)
Nb thin wire loops
Connected in parallel to keep the inductance small, but still has a large area for efficient coupling to the input coil.

14. HV power supply options
Switch mode power supply (SMPS)
Commonly available
Severe RFI
Linear power supply
Might be hard to find, probably needs to be
custom-made.
Array of dry cell batteries
Eveready (model 497), Zn/MnO2, 510V, 140 mAh, ~$75/each
For nEDM exp., we need 686 in series to supply 350kV
C~100pF, Q=CV=3.510-5C for each charge
The battery pack could provide 140mAh/Q = 1.4107 charges
(recharge every 1 hr, will last ~1620 years)
4.12 cm
14.29 cm
7.62 cm

15. Current limited battery array+ + +
C~100pF +

16. Progress Parts are machined, waiting for the final welding assembly.
Janis cryostat is cleaned up, leak checked and ready to go.
SQUID sensors (QD, CryoElectronics) and control electronics (CryoElectronics) have arrived; have to order Jena SQUID sensors.
Cool down in the next 1~2 weeks.
Results expected before the DOE review.