SQUID developments for the gravitational wave antenna MiniGRAIL

We designed two different sensor SQUIDs for the readout of the resonant mass gravitational wave detector MiniGRAIL. Both designs have integrated input inductors in the order of 1.5 muH and are planned for operation in the mK temperature range. Cooling fins were added to the shunt resistors. The fabricated SQUIDs show a behavior that differs from standard DC-SQUIDs. We were able to operate a design with a parallel configuration of washers at reasonable sensitivities. The flux noise saturated to a value of 0.84 muPhi0/radicHz below a temperature of 200 mK. The equivalent noise referred to the current through the input coil is 155 fA/radicHz and the energy resolution yields 62 h

Strongly coupled, low noise DC-SQUID amplifiers

The dc Superconducting Quantum Interference Device (dc-SQUID) is one of the most sensitive magnetic field sensors available. In this thesis we concentrate on its application as an amplifier. In this configuration, an input circuit of interest can be connected by means of a coupling coil. The intended application of our developed low-Tc SQUID amplifiers is the readout of the first spherical gravitational wave antenna MiniGRAIL. Using small-signal analysis as well as numerical simulations, we study the achievable signal-to-noise ratio of such sensors in practical measurements. Published theories and studies could be extended. Some interesting points treated are the influence of the often used negative feedback (flux-locked loop), an altered input impedance of the SQUID amplifier and the influence of back-action noise, which directly affects the object of interest. The altered operation of the SQUID due to the presence of the input circuit is studied in numerical experiments. Another important issue, the influence of integrated input coils on the dynamics of SQUIDs, is investigated theoretically and experimentally. The performance of our developed sensors is compared to numerical simulations on detailed models. The results give insights into the behavior, design and usage of dc-SQUIDs with integrated coils. For a SQUID with input inductances of 1.5 μH we reached a good coupled energy sensitivity of about 170 ¯h in a dilution refrigerator. Furthermore, we investigated the hot-electron effect. This effect typically limits the sensitivity of superconducting electronics at sub-Kelvin bath temperatures due to a weakened electron-phonon coupling. We performed heating experiments on thin-film resistors which are similar to the shunt resistors of the Josephson junctions used in our sensors. The suppression of the hot-electron effect via electronic thermal transport to attached cooling fins is investigated both experimentally, theoretically and numerically. The numerical technique turns out to be a useful tool for the thermal design of superconducting electronics

Numerical studies on dc-SQUID sensors with tightly coupled input coil

We investigated the behavior of two low-Tc direct current superconducting quantum interference device (dc-SQUID) sensors with integrated input coil. A model including the capacitance of the Josephson junctions, thermal noise of the integrated shunt- and damping-resistors as well as a frequency dependent inductance of the SQUID loop was determined and numerically simulated. The SQUID inductance is found to be mainly influenced by parasitic elements introduced by the integrated coils. The simulated characteristics of the examined SQUIDs show many features also seen in experiments, including a hysteresis due to the frequency dependent washer impedance. The measured sensitivity of one of the designs fits well to the simulated value

SQUID current amplifiers for sub-kelvin operation temperatures

Extremely sensitive current sensors are being developed by integrating input coils on superconducting quantum interference devices (SQUIDs) and optimizing them for being cooled to the mK temperature range. The most important effects for this type of sensors are investigated in this paper. SQUID characteristics were experimentally studied at mK temperatures and in numerical simulations, revealing a crucial hysteretic effect originating from parasitic elements introduced by the integrated coil. Furthermore, the cooling behavior of shunt resistors with attached cooling fins, dominated by the hot-electron effect, was investigated. A numerical model allows to reproduce the experimental data in a good way

Hot-electron effect in PdAu thin-film resistors with attached cooling fins

The sensitivity of superconducting electronics operated in the sub-Kelvin temperature range is usually limited by the hot-electron effect. Here, an increased thermal resistance due to a weakened electron–phonon coupling leads to a higher temperature of the electrons in the thin-film shunt resistors of the Josephson junctions. Cooling fins can be attached to weaken this effect. We characterized different configurations of resistors in PdAu with or without attached cooling fins by dissipating power and determining the effective electron temperature. This was done by directly measuring the Johnson noise with a SQUID amplifier. The results are compared to theory and numerical calculations on the electronic heat transport. The latter turns out to be a useful tool for the optimization of the thermal design of superconducting electronics