O. Usenko

Sensitivity of the spherical gravitational wave detector MiniGRAIL operating at 5 K

We present the performances and the strain sensitivity of the first spherical gravitational wave detector equipped with a capacitive transducer and readout by a low noise two-stage SQUID amplifier and operated at a temperature of 5 K. We characterized the detector performance in terms of thermal and electrical noise in the system output signal. We measured a peak strain sensitivity of 1.5x10{sup -20} Hz{sup -1/2} at 2942.9 Hz. A strain sensitivity of better than 5x10{sup -20} Hz{sup -1/2} has been obtained over a bandwidth of 30 Hz. We expect an improvement of more than 1 order of magnitude when the detector will operate at 50 mK. Our results represent the first step towards the development of an ultracryogenic omnidirectional detector sensitive to gravitational radiation in the 3 kHz range.

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.

Preparing for science run 1 of MiniGRAIL

We report on the eighth cryogenic run of MiniGRAIL, which is planned to be the first science run. Three new capacitive transducers were mounted on the sphere, all coupled to superconducting transformers and a double-stage SQUID amplifier. All SQUID modules had a noise level between 1 and 2 μΦ0 Hz−1/2 at 4 K in separate test runs. The detector was upgraded with a complete data acquisition and data storage system, and a data analysis program for millisecond burst signals was written using a matched filter. Two cosmic-ray detectors were built and installed on the roof above MiniGRAIL as a veto for cosmic rays. A test run was done, but failed because of bad electrical isolation of the transducer electrode at low temperatures. A new cool-down is currently going on. With an already obtained sphere temperature of 65 mK and a SQUID noise of 100, the expected sensitivity should be 4 × 10−22 Hz−1/2.

Optimization of a Low-Tc DC SQUID Amplifier With Tightly Coupled Input Coils

We optimized the design and operation of a low-Tc direct current superconducting quantum interference device (dc SQUID) with an integrated coupling coil of 1.5 muH inductance taking into account typical effects observed for similar devices. Numerical simulations were performed on a model including the capacitance of the Josephson junctions, thermal noise of the integrated shunt- and damping- resistors as well as a complex frequency dependent impedance of the SQUID loop originating from the integrated coils. The experimentally and numerically determined characteristics and sensitivity are in good agreement. A minimum additional coupled energy resolution of 700 (h/2p) and 250 (h/2p) was measured at a temperature of 4.2 K and 1.5 K, respectively.

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.

PRESENT STATUS OF MINIGRAIL

The latest results of the measurements on the vibration isolation system of MiniGRAIL at room temperature as well as an overview of the results of the ultra-cryogenic tests with the dilution refrigerator are presented. Two types of capacitive transducers have been developed and tested separately in a cryogenic set-up. The rosette-design transducers have been mounted on the sphere and tested at low temperature. We also report the progress in developing a two-mode inductive transducer with an Al5056 resonator as a second resonating mass and a Nb film coil as superconducting pick-up loop. Furthermore, we developed and tested two double-SQUID systems based on two types of dc SQUIDs as sensors and a DROS as the preamplifier stage.

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.