W. Vodel

Measurements of a low-temperature mechanical dissipation peak in a single layer of Ta2O5 doped with TiO2

Thermal noise arising from mechanical dissipation in oxide coatings is a major limitation to many precision measurement systems, including optical frequency standards, high-resolution optical spectroscopy and interferometric gravity wave detectors. Presented here are measurements of dissipation as a function of temperature between 7 K and 290 K in ion-beam-sputtered Ta2O5 doped with TiO2, showing a loss peak at 20 K. Analysis of the peak provides the first evidence of the source of dissipation in doped Ta2O5 coatings, leading to possibilities for the reduction of thermal noise effects.

Comparison of the temperature dependence of the mechanical dissipation in thin films of Ta2O5 and Ta2O5 doped with TiO2

Here we report the first results comparing the temperature dependence of the mechanical dissipation in thin films of Ta_2O_5 and Ta_2O_5 doped with TiO_2, of a type suitable for use in the multilayer optical coatings for advanced gravitational wave detectors. The results indicate that doping Ta_2O_5 with TiO_2 can significantly alter the distribution of activation energies associated with the low-temperature dissipation peak.

A cryogenic current comparator for the absolute measurement of nA beams

A new type of beam transformer, based on the principle of a Cryogenic Current Comparator (CCC), was built to measure extracted ion beams from the SIS, the heavy ion synchroton at GSI. A current resolution of 0.006−0.065 nA/Hz, depending on the frequency range, could be achieved allowing us to measure ion beams with intensities greater than 109 particles per second with high accuracy. Numerous investigations were carried out to study the zero drift of the system which shows a strong exponential slope with two time constants. In addition, the influence of external magnetic fields was measured. Furthermore the microphonic sensitivity of the system was studied by measuring noise spectra of the detector’s vibration and the output signal. Measurements with neon and argon beams will be presented and compared with signals emitted from Secondary Emission Monitors (SEM). Another measuring function of the CCC-detector aims at the analysis of the beam’s time structure to get information about beam spill fluctuations....

High Sensitive DC SQUID Based Position Detectors for Application in Gravitational Experiments at the Drop Tower Bremen

Free fall tests for proving the Weak Equivalence Principle (WEP) have been rarely be done in history. Although they seem to be the natural experiments to test the equivalence of inertial and gravitational mass, best results for proofs of the WEP could be attained with torsion pendulum tests to an accuracy of 10-12. Pendulum tests are long term periodic experiments, whereas free fall tests on Earth can be carried out only for seconds causing certain limitations in principle. Nevertheless, very precise fall tests in the 10-12 to 10-13 range are possible and under preparation to be carried out on the Drop Tower Bremen for a free fall over 110 m. These tests require position detectors with an extremely high resolution in order to measure tiny displacements of freely falling test masses. Using SQUID-based sensing technique, the displacements can be determined with an accuracy of 2 x 10-14 m/√Hz. The SQUID system, developed and manufactured at Jena University, provides high sensitivity and extremely low intrinsic noise, especially at low frequencies. Some recent results are discussed.

Dark current measurements on a superconducting cavity using a cryogenic current comparator

This paper presents nondestructive dark current measurements of tera electron volt energy superconducting linear accelerator cavities. The measurements were carried out in an extremely noisy accelerator environment using a low temperature dc superconducting quantum interference device based cryogenic current comparator. The overall current sensitivity under these rough conditions was measured to be 0.2 nA/Hz(1/2), which enables the detection of dark currents of 5 nA.

Measurement of the low-field Hall coefficient by d.c. SQUID technique

Abstract A d.c. SQUID voltmeter adapted for the measurement of the low-field Hall coefficient at 4.2 K is described. On the basis of this device Hall voltages of high purity metals of the magnitude of a few nanovolts produced by a current up to 1 A and magnetic induction up to 10 mT can be detected with a resolution of 1.5 pV Hz −1 2 . The reproducibility of the Hall voltage measured between different cooling cycles of the SQUID system is close to 0.1%. Results are presented for samples of high purity copper which contain dislocations.

Cryogenic current comparator for absolute measurements of the dark current of superconducting cavities for TESLA

A new SQUID based measurement system for detecting dark currents, generated by the TESLA (Tera Electron Volt Energy Superconducting Linear Accelerator) cavities is proposed. It makes use of the Cryogenic Current Comparator principle and senses dark currents in the nA range. To reach the maximum possible energy in the TESLA project is a strong motivation to push the gradients of the superconducting cavities closer to the physical limit of 50 MV/m. The field emission of electrons (dark current) of the cavities at strong fields may limit the maximum gradient. The absolute measurement of the dark current in correlation with the gradient will give a proper value to compare and classify the cavities. This contribution describes a Cryogenic Current Comparator (CCC) as an excellent tool for this purpose. The most important component of the CCC is a high performance DC SQUID system which is able to measure extremely low magnetic fields, e.g., caused by the extracted dark current. For this reason the SQUID input coil is connected across a special designed pick-up coil for the electron beam. Both the SQUID input coil and the pick-up coil form a closed superconducting loop so that the CCC is able to detect dc currents down to 1 nA//spl radic/Hz. Design issues and the application for the CHECHIA (horizontal test cryostat) cavity test stand at DESY are discussed.

Application of LTS-SQUIDs for testing the weak equivalence principle at the Drop Tower Bremen

Abstract Free fall tests to prove the weak equivalence principle were rarely be done in history. Presently, very precise fall tests in the 10−13 range are possible and under preparation to be carried out on Drop Tower Bremen during free fall over 109 m. A level of accuracy of 10−18 will be achieved in the current satellite test of the equivalence principle space mission of NASA/ESA. Both kinds of experiments require position detectors with an extremely high resolution to measure infinitesimal displacements of freely falling test masses. On the basis of the LTS SQUID system of the Jena University an experimental setup was developed containing a pair of superconducting levitated test masses installed in a vacuum chamber at 4.2 K. The resolution of the SQUID position detector was measured to be as high as 4×10−14 m/ Hz . This whole apparatus was successfully tested and dropped at the Drop Tower Bremen providing a free fall height of 109 m corresponding to a flight time of 4.7 s. Recent results of this measurements are described in this work.

Application of LTS-SQUIDs in Nuclear Measurement Techniques

Low temperature superconducting quantum interference devices (LTS SQUIDs) are used to make precision measurements of electromagnetic fields in applications ranging from biomedicine to high energy physics. We have previously described an LTS SQUID-based device for nuclear physics which employs the Cryogenic Current Comparator principle (CCC). The CCC consists of a high-performance LTS DC SQUID system, a toroidal pick-up coil, and a meander-shaped superconducting niobium shield. Theoretical investigations show that as external noise decreases, improvements in performance depend on the properties of the ferromagnetic core material embedded in the pick-up coil. Here we present the temperature- and frequency-dependence of several candidate ferromagnetic and nanocrystalline materials. We discuss these results in light of the optimization of the CCC sensor performance.

Cryogenic Q-factor measurement of optical substrates for optimization of gravitational wave detectors

Future generations of gravitational wave interferometers are likely to be operated at cryogenic temperatures because one of the sensitivity limiting factors of the present generation is the thermal noise of end mirrors and beam splitters that occurs in the optical substrates as well as in the dielectric coatings. A possible method for minimizing thermal noise is cooling to cryogenic temperatures, maximizing the mechanical quality factor Q, and maximizing the eigenfrequencies of the substrate. We present experimental details of a new cryogenic apparatus that is suitable for the measurement of the temperature-dependent Q-factor of reflective, transmissive as well as nano-structured grating optics down to 5 K. In particular, the SQUID-based and the optical interferometric approaches to the measurement of the amplitude of vibrating test bodies are compared and the method of ring-down recording is described.