Theo Schneider

High field magnet facilities and projects at the Forschungszentrum Karlsruhe

Three experimental facilities JUMBO, HOMER I, and MTA exist in the high field magnet group of the Institute for Technical Physics for investigations in high magnetic fields up to 20 T. All setups are based on advanced superconducting magnets. The facility Homer II is presently under construction in order to achieve magnetic fields up to 26 T. Our current projects focus on the development of solenoids for NMR (nuclear magnetic resonance) spectrometers. Together with our industrial partner Bruker Analytik GmbH, we introduced in 1991 the worlds first 750 MHz (17.6 T) and in 1995 the worlds first 800 MHz NMR spectrometer (18.8 T) on the market. At present a prototype magnet system for a 900 MHz spectrometer (21.1 T) is in progress. To further improve the resolution of the NMR spectrometers, a national project for the development of a 1000 MHz spectrometer (23.5 T) has been started together with Bruker Analytik GmbH and Vacuumschmelze GmbH.

Manufacture and test of a 5 T bi-2223 insert coil

With the goal of obtaining a magnetic field of 25 T in our facility HOMER II with a superconducting LTS-HTS hybrid magnet, a first prototype 5 T high temperature superconducting (HTS) insert coil has been constructed and tested. The HTS insert consists of 16 double pancakes made of stainless steel reinforced Bi-2223 tapes manufactured by American Superconductor. The HTS coil was operated at 1.8 K and produced 5.4 T at a current of 151.2 A. In a background field of 11.5 T provided by our facility HOMER I, a total field of 16.9 T was obtained several times. No training or quench of the coil was observed during the test, but after warming up a defect in the winding of one double pancake was detected, presumably due to a ballooning of the tape. The design of the coil and the results of the test are presented and discussed.

Superconductive undulators with variable polarization direction

In the past planar superconductive undulators have been successfully developed and tested with beam. They produce linearly polarized light (X-rays) and allow to tune the emitted wavelength electrically. In this paper a novel type of superconductive undulators is introduced which allows to tune electrically in addition to the wavelength the polarization direction. A short prototype was built and tested in a LHe bath.

Angle-Dependent U(I) Measurements of HTS Coated Conductors

Increasing magnetic fields generated by pure superconducting high field magnets to greater than 25 T is not possible with the LTS NbTi and NbSn conductors that are currently used. Therefore, the next step is to use the HTS REBCO-CC, which has a greater critical current density, Jc, at higher magnetic fields. Because of the anisotropy of this type of superconductor, its critical properties change according to its orientation with respect to an external magnetic field and therefore further analysis is necessary before these REBCO conductors can be used for a high field insert coil within the experimental facility HOMER II. To study the effect of orientation and magnetic field strength, U(I) measurements of several commercial CCs were carried out in our JUMBO facility in a free bore of 100 mm and magnetic field up to 10 T at 4.2 K and 77 K. The angle between the applied field and the normal vector to the tape can be adjusted between 0 and 180°. The resulting U(I) curves were approximated by various mathematical models using Matlab software, and from these the parameters describing the angular dependence of the critical current, Ic, were determined. Three-dimensional surface plots of Ic as a function of angle and field strength were produced and a suitable fit found. The fitted function was then extrapolated up to 30 T to enable the determination of the behavior of the superconductor in the high field region.

Fiber Bragg Grating Sensors for Strain Measurement at Multiple Points in an NbTi Superconducting Sample Coil

Multipoint stress/strain measurement in a simple single layered Niobium Titanium wire wounded superconducting sample coil (SSC) was carried out using fiber Bragg grating sensors (FBGs) at low temperature (4.2 K) and at high background magnetic field (8 T) excited by power supply. Ten gratings with different spatial period were fabricated at various positions along a single mode fibre (SMF). The grating location in the SMF sensing array was then mounted at the measuring points on the SSC using stycast 2850 FT. The change in the strain in the SSC. This induced strain, changes the Bragg wavelength of the mounted FBG sensor, which was then recorded using a FS5100 Bragg meter. Experimental results showed that the FBG sensor array was highly reproducible with error of ±0.8 μm/m for high external magnetic field and high current. Also the Youngs modulus of the SSC at 900 A, 8 T, and 4.2 K was calculated to be ~ 117 GPa, which is below to the average Youngs modulus of the NbTi/Cu superconducting composite wire, which is in the range of 130 GPa at 4.2 K. The loading and unloading characteristic curves of the SSC exhibit reversible behavior without any hysteresis in the operated range. This ensures that the SSC operates linearly within the elastic limit for the applied current and magnetic field. In this paper, we report the behavior of each single turn of the single layered SSC and its corresponding strain due to Lorentz force. The FBG sensing principle is validated by executing various modes of SSC excitation.

Magnetic Field Test Facility for Superconductive Undulator Coils

Superconducting undulators and wigglers are developed for synchrotron light sources, damping rings for linear colliders and polarized positron sources. In an undulator the emitted photons along the trajectory have to interfere. In order to do so the magnetic field in all periods has to be almost identical. The field strength over one or several hundred periods is not allowed to deviate by more than 1%. Translated into mechanical accuracy the position of the wire and the poles has to be more accurate than about 5 over 1 to 2 m. High quality measurement of the field is an essential requirement. In this paper we present two field measuring systems, one is under construction and another one is under design phase at the Forschungszentrum Karlsruhe.

Usage of Bi-HTS in high field magnets

At present, superconducting high field magnets built up of metallic low temperature superconductors (LTS) like NbTi and ternary/quaternary Nb/sub 3/Sn is near to the upper limit of achievable field strength. Fields above approx. 23 T seem to be only reachable with LTS-HTS hybrid configurations consisting of an outer LTS section and a high temperature superconductor (HTS) insert. Commercially available Bi-HTS wires were investigated for their application in high field facilities like the HOMER II system with the goal of 25 T and in new generations of NMR magnets of 1000 MHz and above. Therefore the superconducting properties of the HTS wires were examined at 4.2 K in magnetic fields up to 10 T. The voltage-current relation was examined resistively using a high resolution four-point measurement technique. The dependence of the critical current and the n-value on the winding diameter, on the field alteration (increasing/decreasing), and on the field orientation to the wire is presented and discussed.

Construction and Test of MgB 2 Mock-Up Coils for LIQHYSMES

One of the major challenges that come with an increasing contribution of renewable energies is the storage of the produced energy to balance the temporal fluctuations of power generation and consumption. Therefore, a novel multifunctional hybrid energy storage concept, i.e., LIQHYSMES, has recently been proposed. LIQHYSMES combines the advantages of the energy carrier LIQHYSMES (LH2) and the Superconducting Magnetic Energy Storage (SMES). With its critical temperature of 39 K, magnesium diboride (MgB2) is a promising superconductor for this application. In this paper, we report on the construction and test of the first demonstrator LIQHYSMES solenoid mock-up coils made of MgB2 wires with lengths up to 1000 m. The results of cycling (charging and discharging) of the coils with different load curves and ramp rates up to 50 A/s are presented.

Critical Current Distribution in Composite Superconductors

It has long been known that for the description of the E(I)-characteristics of composite superconductors a distribution function can be used instead of a power law relation. With this approach, the conductor is represented by a serial connection of short subsections where the electrical resistance of each subsection is given by a parallel connection of the flux flow resistance of the superconductor and the resistance of the normal conducting matrix. Furthermore the local critical currents are assumed to be normally distributed around a mean value mu with a standard deviation sigma. If the local critical current in a subsection is exceeded a voltage is generated. The current distribution function is then given by the second derivative of the E (I)-characteristics divided by R/L with R being the overall resistance and L the measuring length. In general only the lower part of the distribution function is apparent. By soldering the conductor in a copper bar the whole distribution can be made visible. In this paper we will give examples of the suitability of the description with Gaussian distribution functions for the low temperature superconductors NbTi and Nb3Sn as well as for a Bi2223 tape. A comparison will be made between measurements with and without additional copper.

Superconducting In-Vacuum Undulators

During the last years the Research Center Karlsruhe developed novel high-field superconducting in-vacuum undulators with period lengths of 3.8 and 14 mm. The undulators were tested with beam, both in linac type accelerators (Mainzer Microtron MAMI) and storage rings (ANKA). Based on this experience a new generation of superconducting undulators is planned: undulators capable of electrical field error compensation, undulators with electrically variable polarization direction and high brilliance for various accelerators