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Progress report on theHV-SQUID Compatibility Test
• Study the SQUID performance in a HV environment.
• Study the HV breakdown
– in superfluid under pressure.
– using different materials for electrodes.
Craig Huffer, Maciej Karcz, Chen-Yu Liu, Josh Long
Indiana University
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.
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
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.
1. 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”
The HV conductor is placed inside a SS tube (can be evacuated.) This protects the HV line from breakdown in helium vapor.
Radiation Baffles
Superfluid-tight needle valve
Enough room below the ground electrode to accommodate different SQUID sensors & pickup coils.
Probe can (can be evacuatedor pressurized)
HV pumping port
• 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.
HV feedthrough (vacuum tight)
Squid wiresinside a 1/8” SS tube
Auxiliary port
Pressurization tubeHV ground tube
HV Electrode
Ground Electrode
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 SQUID sensor
RF shield the input coil, use large BW, high slew rate electronics
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)
fbampdB R
MVf
int
mod3 2
Magnicon + PTB BerlinAdditional 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.
Large, fast transients
• HV reversal– 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
RF Shield vs Johnson Noise
• Skin depth:
• Magnetic Johnson Noise:Aluminized mylar superinsulation
– t=1m, z=1cm, SB=1.13 fT/Hz
– t=1m, z=10cm, SB=0.11 fT/Hz
– t=300m, z=1cm, SB=19 fT/Hz
2
d
skin depth (m)
material resistivity (Ohm-m) conductivity mu 100kHz 1MHz 20MHz 1Hz
Ag 1.47E-08 6.80E+07 1.25704E-06 0.000192983 6.10267E-05 1.3646E-05 0.061027
Cu 1.72E-08 5.81E+07 1.25704E-06 0.000208749 6.60124E-05 1.4761E-05 0.066012
Au 2.44E-08 4.10E+07 1.25704E-06 0.000248631 7.86242E-05 1.7581E-05 0.078624
Al 2.82E-08 3.55E+07 1.25704E-06 0.000267292 8.45251E-05 1.89E-05 0.084525
Al 10.5233006 3.327759838 mils
)(8)( 0, tzz
tTkBS B
ZnB
t : thichnessz : separation
J. Nenonen, et al, Rev. Sci. Instrum. 67, 2397
: conductivity
ztf dB 4
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.
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=3.510-5C for each charge– The battery pack could provide 140mAh/Q = 1.4107 charges (recharge every 1 hr, will last ~1620 years)
14.29 cm
7.62 cm
4.12 cm
Current limited battery array
+
7
8
2arg
52arg
max
102arg#
105.2
1000500)140(
5.122
1
105.2)(1000
5001
10005001000
1
esch
j
VmAhqVE
jCVE
jRCRIE
kRmA
VR
mAIset
storedbattery
echC
echR
+
+
+
C~100pF
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.