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An improved cryogenic current comparator for fair
Abstract
Online monitoring of low intensity (below 1 μA) charged particle beams without disturbing the beam and its environment is crucial for any accelerator facility. For the upcoming FAIR project a beam monitor based on the Cryogenic Current Comparator principle with an enhanced resolution was developed. The main focus of research was on the low temperature properties of the ferromagnetic core material of the superconducting pickup coil. The pick-up coil transforms the magnetic field of the beam into a current that is detected by a high performance low temperature dc Superconducting QUantum Interference Device (LTS-DC-SQUID). The penetration of the pick-up coil by interfering magnetic fields is highly attenuated by a meander shaped superconducting shielding. The Cryogenic Current Comparator is able to measure DC beam currents, e.g. as required for slow extraction from a synchrotron, as well as bunched beams. In this contribution we present first results of the improved Cryogenic Current Comparator working up to now in a laboratory environment.
... About 4 × 10 10 particles per machine cycle were accelerated in the SIS and were extracted to the beam diagnostics test bench with a transmission of about 50 %. The worldwide first measurement of high energy ions using a CCC is shown in Fig. 4. For the upcoming FAIR project a beam monitor based on the CCC with an enhanced resolution was developed (see Fig.5) [4]. Therefore we focused our investigations on the low temperature properties of the ferromagnetic core material of the superconducting pick-up coil. ...
An absolute and exact measurement of the intensity of charged particle beams - extracted from an accelerator or circulating in a Storage Ring - is one of the major problems of beam diagnostics. Also the measurement of socalled dark currents, generated by superconductive RF accelerator cavities at high voltage gradients to characterize the quality of these components becomes more and more important for the commissioning of new accelerators (XFEL at DESY). The Cryogenic Current Comparator (CCC) based on high precision LTS SQUIDs is an excellent tool to solve these problems. This contribution gives an overview of the development of highly sensitive SQUID-based Cryogenic Current Comparators (CCC) for nuclear physics from the first successful demonstration of its performance at GSI Darmstadt through the latest improved version for FAIR and the Cryogenic Storage Ring at MPI Heidelberg.
This paper presents a new shield configuration for cryogenic current comparators (CCCs), namely the folded coaxial geometry. An analytical model describing its shielding performance is first developed, and then validated by means of finite element simulations. Thanks to this model, the fundamental properties of the new shield are highlighted. Additionally, this paper compares the volumetric performance of the folded coaxial shield to the one of a ring shield, the latter being installed in many CCCs for measuring particle beam currents in accelerator facilities.
Superconducting quantum interference devices SQUIDs have been a key factor in the development and commercialization of ultrasensitive electric and magnetic measurement systems. In many cases, SQUID instrumentation offers the ability to make measurements where no other methodology is possible. We review the main aspects of designing, fabricating, and operating a number of SQUID measurement systems. While this article is not intended to be an exhaustive review on the principles of SQUID sensors and the underlying concepts behind the Josephson effect, a qualitative description of the operating principles of SQUID sensors and the properties of materials used to fabricate SQUID sensors is presented. The difference between low and high temperature SQUIDs and their suitability for specific applications is discussed. Although SQUID electronics have the capability to operate well above 1 MHz, most applications tend to be at lower frequencies. Specific examples of input circuits and detection coil configuration for different applications and environments, along with expected performance, are described. In particular, anticipated signal strength, magnetic field environment applied field and external noise, and cryogenic requirements are discussed. Finally, a variety of applications with specific examples in the areas of electromagnetic, material property, nondestructive test and evaluation, and geophysical and biomedical measurements are reviewed.
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Polycrystalline cadmium telluride layers were implanted with phosphorus (P) in order to obtain an enhanced p-type doping close to the back contact of CdTe solar cells. The implantation parameters were adjusted based on computer simulations using SRIM. While the implantation profile was kept constant, different CdTe layer thicknesses were investigated. Furthermore, different annealing and activation processes and their influence on the P distribution in the device as well as ion beam induced damage were investigated. The P level was identified by photoluminescence measurements, the effective doping was investigated by means of capacitance–voltage measurements, and the effect on the solar cell properties was analyzed by current voltage characteristics. The results show the P distribution in the CdTe layer to depend strongly on the thermal and chemical post-implantation treatment. The effect of the P-doping on the solar cell properties becomes obvious by an increase of the open-circuit voltage due to the implantation.
Within the last years many groups have realized and investigated different types of intrinsic Josephson junction (IJJ) arrays out of high-temperature superconducting single crystals or thin films. We tried to improve the synchronization between the junctions by external shunts. Mesa structures as well as microbridges on vicinal cut substrates showed multi-branch behaviour in their IV characteristics and random switching between branches. Theoretical modelling was done investigating phase dynamics and stability numerically as well as analytically. Branch structure in current voltage characteristics of IJJ is studied in the framework of different models, particularly, in capacitevely coupled Josephson junctions (CCJJ) model and CCJJ model with diffusion current. Results of modelling of return current in IV characteristics for stacks with different number of IJJ are presented. We discussed the possible mechanisms of synchronization and the ranges of stability. Conclusions with respect to application of such arrays such as radiation sources were given.
In Part 1 the connexion between the error of a cryo-current comparator and the magnetic field attenuation in the gap region of its superconducting shield has been treated for the special but very common case of co-axial gap regions. This paper extends the error analysis on gaps forming a ring cavity. Such gap elements occur in toroidal cryo-current comparators as a link between two adjacent co-axial cavities. The results for these comparators are: the only field component which may penetrate the gap without attenuation is the one containing the useful signal about the current linkage of the comparator. All other field components contribute to the error of the comparator. They are attenuated according to their degree of symmetry. As the exact way the error functions is known it will be possible to construct cryo-current comparators with a predictable upper limit to their dc error.
A ultra low noise DC SQUID magnetometer designed and constructed for high resolution magnetocardiographic measurements is presented. Details of the design considerations and the construction of the single-channel system are given, focusing in particular on the matching of the DC SQUID with the gradiometer and the room temperature electronics. Using a first-order gradiometer as antenna for biomagnetic fields, a white noise level of 2 fT Hz-1/2 has been attained with a 1/f corner frequency at 3 Hz. The intrinsic magnetic flux noise of the SQUID system of 2*10-6 Phi 0 Hz-1/2 corresponds to a noise limited sensitivity of 700 aT Hz-1/2.
A prototype low temperature dc ratio transformer is described. This transformer, which uses a new technique for the precise equalization of current ratios, provides dc ratios which have accuracies of the order of 0.01 ppm and may be calibrated to 0.002 ppm. The ratio transformer may be applied in low temperature current and resistance ratio measurements of high accuracy and may, without modification, be used as a sensitive current detector.
Monitoring of beam currents in particle accelerators without affecting the beam guiding elements, interrupting the beam or influencing its profile is a major challenge in accelerator technology. A solution to this problem is the detection of the magnetic field generated by the moving charged particles. We present a non-destructive beam monitoring system for particle beams in accelerators based on the Cryogenic Current Comparator (CCC) principle. The CCC consists of a high-performance low-temperature DC superconducting quantum interference device (LTS DC-SQUID) system, a toroidal pick-up coil, and a meander-shaped superconducting niobium shield. This device allows the measurement of continuous as well as pulsed beam currents in the nA-range. The resolution and the frequency response of the detector strongly depend on the toroidal pick-up coil and its embedded ferromagnetic core. Investigations of both the temperature and frequency dependence of the relative permeability and the noise contribution of several nanocrystalline ferromagnetic core materials are crucial to optimize the CCC with respect to an improved signal-to-noise ratio and extended transfer bandwidth.
Inductive filters to reduce electromagnetic interference (EMI) are commonly made by winding coils around a magnetic material with high permeability. This permeability should remain high at high frequencies, so that the impedance of the inductor made from it can be large to block off any high frequency EMI signals. Room temperature magnetic material like ferrite loses most of its permeability at low temperatures. Some low temperature materials like Cryoperm-10 are known to have high magnetic permeability. However, the permeability of all of them decreases to a low value at frequencies higher than a few hundred hertz due to eddy current shielding of the magnetic field. We have measured the complex relative permeability of Metglas 2714A (also known as Magnaperm) at 4.2 K, and found that its magnitude is larger than 10,000 at frequencies up to 100 kHz. We also measured the magnetization noise density from this material using a SQUID magnetometer. We find that the noise density agrees well with the predictions of the fluctuation dissipation theorem. This implies that low temperature inductors and transformers with predictable noise characteristics can be designed for applications where the lowest noise is not a limiting factor of performance. For very low noise applications, a common mode filter can be made with this material, where the two input leads to a pre-amplifier are wound as a pair around the material, so that any magnetization noise is cancelled out.