Torsten Koettig

Operation of a high-Tc DC-SQUID-gradiometer on a non-metallic pulse tube refrigerator

A planar high-Tc DC-SQUID gradiometer has been operated with a specially developed low noise pulse tube refrigerator. The cold finger of the refrigerator consists only of non-metallic and non-magnetic materials. The high sensitive SQUID-gradiometer with a base length of 4 mm has a field gradient resolution of about 3 pT/(cm Hz1/2) at 1 Hz and 0.7 pT/(cm Hz1/2) in the white noise region in magnetically shielded environment. During the operation of both, the sensor and the refrigerator, the noise generated by the cryocooler is below the noise level of the SQUID-gradiometer. We demonstrate the potential of this non-metallic pulse tube refrigerator by measuring the magnetic field originating from a human heart (magnetocardiogram), without additional suppression of the intrinsic refrigerator noise.

Cryogenic resonant acoustic spectroscopy of bulk materials (CRA spectroscopy)

The capability to measure Q factors at cryogenic temperatures enhances the ability to study relaxation processes in solids. Here we present a high-precision cryogenic setup with the ability to measure Q factors of at least 109. This level of sensitivity offers new potential for analyzing relaxation processes in solids and for correlating mode shape and relaxation strength. Our improved method of mechanical spectroscopy, cryogenic resonant acoustic spectroscopy of bulk materials, is verified by identifying relaxation processes in low-loss quartz crystals. For the first time, we observe additional damping peaks. The mechanical Q factors of different modes of cylindrical crystalline quartz substrates were measured from 300 down to 6K. Resonant modes with frequencies between 10 and 325kHz were excited without contact to the substrates and the ring down of the amplitudes was recorded using an interferometric vibration readout.

Mechanical losses in low loss materials studied by cryogenic resonant acoustic spectroscopy of bulk materials (CRA spectroscopy)

The capability to measure Q factors at cryogenic temperatures enhances the ability to study relaxation processes in solids. Here we present a high-precision cryogenic setup with the ability to measure Q factors of at least 10(9). This level of sensitivity offers new potential for analyzing relaxation processes in solids and for correlating mode shape and relaxation strength. Our improved method of mechanical spectroscopy, cryogenic resonant acoustic spectroscopy of bulk materials, is verified by identifying relaxation processes in low-loss quartz crystals. For the first time, we observe additional damping peaks. The mechanical Q factors of different modes of cylindrical crystalline quartz substrates were measured from 300 down to 6 K. Resonant modes with frequencies between 10 and 325 kHz were excited without contact to the substrates and the ring down of the amplitudes was recorded using an interferometric vibration readout.

APPLICATION OF NOVEL REGENERATOR MATERIAL WITHIN A COAXIAL TWO-STAGE PULSE TUBE REFRIGERATOR

We have developed a lead-based regenerator material which is suitable for working temperatures below 20 K. This material can be used as a substitute for the state of the art non-magnetic materials used today within very low-temperature regenerators. This high efficient regenerator matrix combines technological advantages with the possibility to vary the thermodynamic and flow characteristics over a wide range, continuously. Hence, our self-made electroplated screen material is compared with standard regenerator materials (commercially available woven screens and packed spheres). We present the pressure drop and heat transfer characteristics of the new material for matrix porosities between 0.55 and 0.38 which corresponds to the porosities of screens and packed spheres. The design parameters and the influencing variables to optimise the regenerator performance will be discussed. The utility of the shop-made lead coated regenerator material is demonstrated in a coaxial two-staged pulse tube refrigerator wit...

High-sensitivity tool for studying phonon related mechanical losses in low loss materials

Fundamental mechanical loss mechanisms exist even in very pure materials, for instance, due to the interactions of excited acoustic waves with thermal phonons. A reduction of these losses in a certain frequency range is desired in high precision instruments like gravitational wave detectors. Systematic analyses of the mechanical losses in those low loss materials are essential for this aim, performed in a highly sensitive experimental set-up. Our novel method of mechanical spectroscopy, cryogenic resonant acoustic spectroscopy of bulk materials (CRA spectroscopy), is well suited to systematically determine losses at the resonant frequencies of the samples of less than 10-9 in the wide temperature range from 5 to 300 K. A high precision set-up in a specially built cryostat allows contactless excitation and readout of the oscillations of the sample. The experimental set-up and measuring procedure are described. Limitations to our experiment due to external loss mechanisms are analysed. The influence of the suspension system as well as the sample preparation is explained.

Design of a fibre reinforced plastic anticryostat for magnetorelaxometric measurements

Publisher Summary This chapter presents a new method for the characterization of magnetic nanoparticles, which is based on the analysis of the temperature dependence of the Neel relaxation sample signal. The presented cryostat extends the investigated temperature range from 300 K to 77 K down to the boiling point of liquid helium at 4.2 K. An anticryostat was designed to use only one liquid helium cryostat for the sample as well as for the Low-Tc SQUID. The permeation process was studied through the used fibre reinforced plastic (FRP) material and the adhesive joints between the components. To improve the lifetime of FRP helium cryostats other coating materials and methods have to be tested. The mechanical properties especially the adhesive strength down to 4.2 K limits the material variety. There are eligible candidates among these, like lead borate glasses and lead borosilicate glasses. The films will be sputtered as surface layers with a thickness of more than 5 μm.

Operation of a Cryogenic Current Comparator with Nanoampere Resolution for Continuous Beam Intensity Measurements in the Antiproton Decelerator at CERN

Low-intensity charged particle beams are particularly challenging for non-perturbative beam diagnostics due to the small amplitude of induced electromagnetic fields. The Antiproton Decelerator (AD) and Extra Low ENergy Antiproton (ELENA) rings at CERN decelerate beams containing ∼ 107 antiprotons. An absolute intensity measurement of the circulating beam is essential to monitor the operational efficiency and to provide important calibration data for the antimatter experiments. This paper reviews the design of an operational Cryogenic Current Comparator (CCC) based on Superconducting QUantum Interference Device (SQUID) for current and intensity monitoring in the AD. Such a system has been operational throughout 2017, relying on a stand-alone cryogenic infrastructure based on a pulse-tube cryocooler. System performance is presented and correlated with different working environments, confirming a resolution in the nanoampere range. INTRODUCTION DC Current Transformers resolution is limited to 1 μA [1]. Other monitors, such as AC Current Transformers or Schottky monitors are able to measure low-intensity beam currents, but neither can simultaneously provide an absolute measurement, with a high current and time resolution, which at the same time is independent of the trajectory and energy. At CERN’s low-energy antiproton (p̄) decelerators, the AD and ELENA (currently being commissioned) rings, both bunched and coasting beams of antiprotons circulate with average currents ranging from 300 nA to 12 μA [2]. The AD cycle, shown in Fig. 1, consists of alternate phases of deceleration, when the beam is bunched, and beam cooling, when the beam is debunched and its velocity is kept constant. The beam is also bunched at injection and extraction. In each cycle 5 × 107 pbar (design value) are injected with a momentum of 3.5 GeV corresponding to a revolution frequency of frev = 1.59 MHz, and are extracted with frev = 100 MeV and frev = 174 kHz. The biggest change in beam current happens at beam injection, when four bunches of length 4σt = 30 ns (assuming a Gaussian shape) generate a current slew rate of 8.6 kA s−1. Superconducting QUantum Interference Devices (SQUIDs) based Cryogenic Current Comparator (CCC) ∗ miguel.fernandes@cern.ch Figure 1: AD cycle. CERN image. monitors have been used to measure DC and slowly extracted beams with resolutions in the nA range by [3, 4]. This project is a collaboration between CERN, GSI, Jena University and Helmholtz Institute Jena. FUNCTIONING PRINCIPLE OF THE CCC SQUIDs are highly sensitive magnetic flux sensors that permit the measurement of the weak fields created by the beam. The CCC (schematic shown in Fig. 2) works by measuring the magnetic field induced by a charged particle beam. This field is concentrated in a high-permeability ferromagnetic pickup core, from which it is coupled into the SQUID sensor via a superconducting flux transformer. The measured coupling factor of this circuit SIb = Φ/Ib , where Ib is the beam current and Φin is the magnetic flux coupled to the SQUID through Mi , in units of magnetic flux quantum φ0 = 2.068 × 10−15 Wb, was SIb = 10.49(1) φ0/μA. The superconducting magnetic shield structure around the pickup-core renders the coupled magnetic field nearly independent of the beam position and also shields the system against external magnetic field perturbations [5,6]. The feedback loop in the SQUID read-out implements a so called Flux Locked Loop (FLL), increasing the dynamic range of the SQUID, but imposing a stability limit on the maximum slew-rate of the input signals [7, 8]. The FLL electronics is configured with a gain of 43 mV/φ0. The used SQUID/FLL system is supplied by Magnicon [9]. In order to reduce the slew-rate of the signal coupled to the SQUID, a 2nd order RLC low-pass filter has been implemented in the coupling circuit [10]. The coupling function obtained from the current calibration of the monitor after SIb (s) = Φ/Ib has a bandwidth from DC to 1 kHz and a low frequency gain of SIb = 10.49 φ0/μA. 9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW Publishing ISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-THPML044 06 Beam Instrumentation, Controls, Feedback, and Operational Aspects T03 Beam Diagnostics and Instrumentation THPML044 4741 Co nt en tf ro m th is w or k m ay be us ed un de rt he te rm so ft he CC BY 3. 0 lic en ce (© 20 18 ). A ny di str ib ut io n of th is w or k m us tm ai nt ai n at tri bu tio n to th e au th or (s ), tit le of th e w or k, pu bl ish er ,a nd D O I.

Pulse tube refrigerator cryostat with an intrinsic top-loading system

The authors have undertaken basic research and prototype developments of four-valve pulse-tube refrigerators (FVPTR) since several years. Two single-stage FVPTR in coaxial and U-shaped arrangement have been designed for maximum refrigeration power at cooling temperatures below 30 K. The heat flow through a thermal link between the pulse tube and the regenerator is the determining difference between the U-shaped and the coaxial configuration. The intrinsic heat flux has a complex influence on the cooler performance, e.g. the cooling power, the temperature distribution within the pulse tube and the dynamic losses. Based on these results we propose a design study of a coaxial FVPTR with an intrinsic top-loading system. This cryostat is most suitable for measurements and analytical applications with even low maintenance costs. The main advantage for the operator is the rapid sample exchange while the refrigerator is operating. The process takes only a matter of minutes. Thus the time to cool successive samples is greatly reduced over cold finger cryostats.