Bitter Magnet Research Articles

Dielectric Fabry–Pérot interferometer for temperature dependent far-infrared reflectivity measurements of heavy-fermion metals in high magnetic fields

A far-infrared reflectivity set up has been developed for spectroscopy on highly reflective materials in a 20 T-class resistive Bitter magnet. As a first application, far-infrared reflectivity measurements on the heavy-fermion compound URu2Si2 have been performed using a silicon reflection Fabry–Pérot interferometer as a multiple reflection device. In a resonance, this Fabry–Pérot technique is one order of magnitude more sensitive than a single reflection measurement. Changes in the reflectivity as a function of magnetic field are resolved with an accuracy of 0.2% and the absolute value of the reflectivity can be obtained with an accuracy of 0.5%. With this interferometer, an excitation at about 40 cm−1 in the heavy-fermion system URu2Si2 is investigated at temperatures between 2 and 20 Kelvin and in magnetic fields up to 20 T. The excitation appears to extend to lower energies under influence of a large magnetic field.

Cu-Ag alloy Bitter type magnet for repeating pulsed field

Cu-Ag alloy is used for the repeating pulsed field magnets. It is found that fields up to 22 T or more will be available for this purpose instead of 16 T which is obtained with normal copper magnet used at present. This result is a big advantage for neutron diffraction experiments.

The world's first 27 T and 30 T resistive magnets

We describe in detail a 30 Tesla, 32 mm warm bore, 15 MW resistive magnet which was put into operation at the National High Magnetic Field Laboratory in Tallahassee, FL in March 1995. The magnet consists of three concentric axially-cooled Bitter stacks connected electrically in series. This magnet employs a substantial new development in Bitter magnet technology which allows high current densities without the usually accompanying high stresses. Details of magnet optimization, design, construction, testing and operation are presented. We also report on operating experience with the 27 T magnets.

New high-strength, high-conductivity Cu-Ag alloy sheets

A sheet-conductor fabrication method has been developed for Cu-Ag alloys containing 6–24 wt% Ag in which high-strength and high-conductivity are obtained by coldworking combined with intermediate heat treatments. The intermediate heat treatments were repeated three times at 400–450°C for 1–2 h at appropriate stages of cold-rolling. The optimized Cu-24wt% Ag alloy sheet with a 96% reduction ratio shows an ultimate tensile strength of 1050 MPa and an electrical conductivity of 75% IACS at room temperature. Anisotropy in the strength with respect to the rolling direction is less than 10%, and no anisotropy in the electrical conductivity occurs. We demonstrated to be able to manufacture the Cu-Ag sheets for Bitter magnet on a commercial basis. The sheets fabricated by this method are promising as conductors for high-field Bitter magnet coils.

Ａｇ繊維分散強化Ｃｕ合金とその超強磁場発生技術への応用

Recently, we successfully developed a new Cu-Ag microcomposite alloy with a promising combination of high mechanical strength and high electrical conductivity. When a Cu-16 at% Ag alloy ingot was cold-worked into a wire or a sheet with several times of intermediate annealing at 350-450°C, it shows high conductivity of 75-83% IACS and a high tensile strength of 0.7-1.1GPa at room temperature. These values are superior to those of Cu-Nb microcomposite alloy. The Cu-Ag microcomposite alloy shows excellent mechanical strength with cold work of over 93% areal reduction ratio, while a very heavy cold work of more than 99.97% areal reduction ratio is necessary for realizing such mechanical strength in the Cu-Nb microcomposite. A further advantage of the Cu-Ag microcomposite is easy casting of the alloy ingot, resulting in excellent homogeneity of the microstructure and, therefore, the properties in the alloy wire and sheet. We wound the Cu-Ag microcomposite alloy wire into several pulsed magnets. One of them generated non-destructively 73.4T with duration time of 5msec in a 10mm bore. An other one generated non-destructively 65.3T with duration time of 100msec in a 16mm bore. The feasibility study of the Cu-Ag microcomposite alloy sheet as Bitter magnet sheets is progressing now in collaboration with the Francis Bitter National Magnet Laboratory and the National High Magnetic Field Laboratory. A steady field of 35.2T could be generated by inserting the Cu-Ag microcomposite alloy Bitter sheets into the highest-field region in Hybrid III of FBNML. These pulsed fields and the steady one are world records as of this writing.

The Francis Bitter national magnet laboratory at MIT: An update

The high DC field facility of the FBNML marked its 30th year of operation in its present location. This year's group of users once again included more than 300 experimenters involved in ≈ 100 programs. They exploited the advantages provided by the greatest number (32) and models (17) of high field DC magnets available anywhere in the world - and all at no charge. Bitter magnets with five different bore diameters provide a wide range of tradeoffs between bore size and maximum field. There are also both vertical and horizontal bore perpendicular access magnets. The operating characteristics of all these types will be presented.

A new 18-T resistive magnet with radial bores

The new 20 MW installation has been running in our laboratory since the end of 1991. In order to fit the new characteristics of the power supplies, we have developed some new solenoids, especially for high homogeneous fields or special configurations with large bores. We describe in detail the new 10 MW, 5 cm room temperature bore, Bitter type magnet with horizontal axis, delivering a continuous field higher than 18 T for a power consumption of 9.5 MW. In the middle plane, 4 cylindrical 5 mm diameter tubes provide optical accesses for laser beam perpendicular to the main field orientation. The results of the tests are presented and the major problems due to the high pressure of the cooling water are discussed. This facility is mainly used for magneto-optical studies such as Cotton-Mouton effect and Faraday effect on complex fluids and heterogeneous solids. The corresponding instrumentation is briefly described.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High-strength and high-conductivity Cu-Ag alloy sheets: new promising conductor for high-fieId Bitter coils

A sheet-conductor fabrication method has been developed for Cu-Ag alloys containing 6-24 mass% Ag. Both the strength and the conductivity of the sheet were improved remarkably by the repetition of heat treatment combined with cold working. The intermediate heat treatments were repeated 3 times at 400-450/spl deg/C for 1-2 h at appropriate stages of cold rolling. The optimized Cu-24 mass% Ag alloy sheet with a 96% reduction ratio showed a ultimate tensile strength of 1050 MPa and an electrical conductivity of 75% IACS at room temperature. Anisotropy in the strength with respect to the rolling direction was less than 10% and no anisotropy in the electrical conductivity were observed. The sheets fabricated by this method are promising as conductors for high-field Bitter magnet coils.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High Tc Bitter magnets

Bitter magnets offer some advantages to the construction of high temperature superconducting magnets. The superconductor is manufactured in the required shape. This avoids the need to develop ductile materials. This method can be utilized for manufacturing solenoidal, toroidal, saddle, and other types of magnets. The issues facing this type of magnet construction will be discussed, including quench protection, materials compatibility, stability, and cooling. The technique is especially suited to resolve some of the compatibility problems between the brittle superconductor and the structure. Novel methods to improve the compatibility between the superconductor and the structure will be presented. The results of FEM modelling and preliminary testing of the superconductor composite will be shown.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Anisotropy of the high-field magnetization of cubic Zn1-xFexSe.

The magnetization of cubic ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Fe}}_{\mathit{x}}$Se, with x=0.003 and 0.007, was measured at temperatures 2\ensuremath{\le}T\ensuremath{\lesssim}50 K in magnetic fields H along the [100] and [111] directions. Data for H\ensuremath{\le}55 kOe were taken with a superconducting quantum interference device magnetometer. Other data in fields up to 200 kOe were taken with a vibrating sample magnetometer operating in a Bitter magnet. At the lowest temperatures the magnetization M at high magnetic fields is anisotropic: at a given T and for the same magnitude H of the magnetic field, M for H\ensuremath{\parallel}[100] is larger than for H\ensuremath{\parallel}[111]. The difference between the values of M for these directions reaches a maximum value of 19% at the lowest temperatures and when H\ensuremath{\simeq}150 kOe. As the temperature rises, the anisotropy of the high-field magnetization decreases gradually, becoming very small above 40 K. In low fields (H\ensuremath{\lesssim}10 kOe) the anisotropy is very small at all temperatures. These experimental results are generally in good agreement with theoretical calculations based on crystal field theory and on the assumption that only isolated ${\mathrm{Fe}}^{++}$ ions (singles) contribute to the magnetization. Minor discrepancies between experiment and theory remain, however.

Magnetic properties of the Tb4+ ion in Li2TbF6

Both oxygen and fluorine have a partiality towards the stabilization of high oxidation states of rare earths such as Ce, Pr, and Tb. During the study of the MF-M′F4 (M=alkaline metal; M′ rare ion) the compound Li2TbF6 was found to constitute the only representative obtained under ambient pressure of a structural type, namely α-Li2ZrF6, known until now only through the high pressure form. Magnetic measurements were carried out over the 1.4–300 K temperature range in continuous magnetic fields up to 20 T produced by a Bitter magnet. At low temperature and in H higher than 15 T, the magnetization M was observed to be field independent and very close to the calculated Tb4+ free ion value, i.e., 7μB; on the other hand, the data do not present any departure from the Brillouin function of the pure 8S7/2 state. The reciprocal magnetic susceptibility obeys a Curie–Weiss law leading to an effective moment of 7.86μB. A very unusual behavior of the terbium ion which presents a 4+ oxidation state is concluded: to the best of our knowledge, it is the first observation of this form. The so-found spherical 4f7 electronic configuration is the most compatible with the high coordination number of eight. Finally, the discussion within the series Li2MIVF6 compounds to understand the influence of both electronic configuration and size effects is presented.

Changes of FIR Reflectivity of HgSe:Cr in a Magnetic Field

Diluted magnetic semiconduction are crystals in which one of the non-magnetic components is substituted in a controlled way by magnetic ions [1]. Currently, the main interest has shifted from manganese based compounds to crystals in which the cations are substituted by other transition metal ions, see for example, see for example [2]. In HgSe:Cr the Cr 2+ ion substitute Hg atoms in the zinc blende type host lattice. HgSe, having an inverted band structure is always n-type. The only possible way of investigating low-energy excitations in such a material is through reflectivity measurements. The considerable number of competing processes such as plasma-, phononand electron-related excitations can make it difficult to identify the origin of observed stuctures. However, for energy levels which can be tuned by an applied magnetic field, magnetospectroscopy can help to clarify the nature of the observed spectral features. The investigated samples had low concentrations of Cr, not exceeding several times 1018 cm-3 . Therefore for these crystals we do not expect to observe effects due to local vibrations and exchange interaction. The magnetoreflectivity measurements were performed in the spectral range 50-700 cm -1 in magnetic fields up to 20 T using a Fourier Transform Spectrometer which was connected via an outcoupling optic to either a superconducting coil or a Bitter magnet [3]. All measurements were done in the Faraday geometry with nonpolarized light at 2 K.

Velocity ratio measurements of a gyrotron electron beam

A 140 GHz high-power gyrotron has recently been operated in a 14 T Bitter magnet to characterize emission as a function of magnetic field and beam current. The velocity ratio (or pitch angle) α=〈v⊥〉/〈v∥〉 of the beam electrons is a critical parameter for high-efficiency gyrotron operation and was measured using a capacitive probe located in the beam tunnel before the cavity. The observed velocity ratio decreased as the beam current increased while the beam voltage and magnetic fields were held fixed. This decrease in α partially explains the reduced gyrotron efficiency observed at high-beam currents. The velocity ratio exhibited saturation effects as a function of both the beam current and the control-anode voltage, at low cathode magnetic field values. Particle code results show a decrease in α as a function of beam current that is consistent in magnitude with the observed values.

Eddy current shielding and heating: Reduction of dissipation for very low-temperature experiments in the presence of magnetic field ripple

We have studied eddy current heating and shielding for conducting materials in a cylindrical geometry, and we have performed numerical calculations as a function of the important parameters. Results are presented in graphical form. Eddy current heating in OFHC copper at low temperatures and means of suppressing it have been studied in a Bitter magnet in a field of 15 T. Results show that it is possible to use conducting materials for experiments in high magnetic fields at temperatures below 1 K.

Intermediate scale industrially processed Nb/sub 3/Sn(Ti) by powder metallurgy

Powder metallurgy processing of Nb/sub 3/Sn(Ti) wire was scaled up to 75-mm-diameter billets, each containing 4 kg of pressed powder composite. Prealloyed Nb-1.5-wt.% Ti powder was used for optimum high-field properties. An initial hot extrusion step provided complete consolidation and excellent integrity of the powder composite. Using the extruded bars, various single and multiple tin core billets were prepared and hydrostatically extruded to 12-mm-diameter bars. Long lengths of internal tin core wire were drawn without any breakage to less than 1 mm diameter. To evaluate the performance of long-length wires, small magnets were fabricated from these wires and tested in background fields of water-cooled Bitter magnets up to 20 T. The processing, performance, and microstructure of these small magnets are discussed. This intermediate-scale process used conventional metal-forming techniques that can be scaled to larger scale industrial processing. >

Calorimetric measurement of AC loss in ICCS conductors subjected to pulsed magnetic fields

A calorimetric apparatus was developed to measure the AC loss in internally-cooled cabled superconductors (ICCS) which were subjected to pulsed magnetic fields. The purpose of the measurement was to compare the losses of several superconducting wires being considered for use in ohmic heating coils for tokamak fusion devices. The apparatus consisted of a vacuum chamber containing two samples of ICCS, and a pulse coil set surrounded by a Bitter magnet which provided a steady background field. First, the composite enthalpies of the samples were determined, and then the response of each sample to pulsed fields was recorded. A plot of energy versus pulse rate was then constructed. Results are presented for several types of Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn wires (bronze process, internal tin, and jelly roll) at a 10 tesla background field and for pulse rates up to 150 tesla/second.

Finite element analysis of friction joints

A simplified finite element technique for analyzing friction joints in a laminated structure is presented. It is shown that by substituting the effects of adjacent layers by a set of proper nonlinear springs one can reduce a large three-dimensional analysis to an iterative solution of a two-dimensional problem which is much more efficient. The application of the technique to the analysis of high-field Bitter plate solenoids is presented.

Finite element analysis of friction joints

A simplified finite element technique for analyzing friction joints in a laminated structure is presented. It is shown that by substituting the effects of adjacent layers by a set of proper nonlinear springs one can reduce a large three-dimensional analysis to an iterative solution of a two-dimensional problem which is much more efficient. The application of the technique to the analysis of high-field Bitter plate solenoids is presented.

SHIELDING THE a.c. MAGNETIC FIELDS IN BITTER-TYPE MAGNETS

We present the results of shielding efficiency investigations of various shields for a.c. component of magnetic field generated by Bitter magnets. The best effects were obtained for shields made of In with RRR ≃ 105 and for single-turn V3Ga solenoid shorted by Cu wire. In both cases the a.c. component was decreased about ten times in field of 14 T.

Transient nonlinear solutions of coupled electromagnetic and thermal problems in bitter plate Tokamaks

An interactive package of computer programs interfacing a general purpose finite element system for the solution of coupled transient nonlinear electromagnetic, thermal, and structural problems in Bitter plate tokamaks is discussed. Taken into account are the variations in material, thermal, and electrical properties with temperature and the magnetoresistivity. The transient analysis is incremental. At a given moment of the toroidal field pulse it generates the magnetic induction penetrations, the current density and temperature distributions, and the Lorentz body forces in the Bitter plates.